To satisfy the stringent requirements of emerging broadband services in home networks, a novel Centralized optical and Wi-Fi Access Network (C-WAN) has been proposed within the context of Fiber-to-The-Room (FTTR). In C-WAN, centralized management and control of multiple Wi-Fi access points (APs) deployed in each room are facilitated by relocating portions of Wi-Fi protocols from the APs to a centralized entity. This approach significantly enhances network performance, including throughput and roaming capabilities. However, C-WAN also imposes strict demands on the fronthaul networks, specifically requiring high bandwidth and ultra-low latency. In this context, orthogonal frequency division multiplexing passive optical network (OFDM-PON) emerges as a promising solution to support the C-WAN fronthaul network by allocating dedicated subcarriers to each AP. In C-WAN over OFDM-PON, Wi-Fi stations still contend for access to the wireless channel based on existing Wi-Fi protocols, which may result in prolonged wireless access delays. Consequently, the Quality of Service (QoS) requirements for time-sensitive (TS) services may not be met. Additionally, the variation in maximum Wi-Fi throughput due to the contention-based access mechanism presents a significant challenge for the efficient allocation of optical network resources under stringent delay constraints. To address these issues, we propose a priority-based access mechanism that assigns higher priority to TS services for accessing Wi-Fi channels and obtaining wireless resources. Building on this mechanism, we further develop a Wi-Fi throughput prediction model, which is used to optimize the allocation of optical network resources. Simulation results demonstrate that the proposed scheme can effectively reduce wireless access delay and jitter for TS services, meeting their performance requirements while also improving the utilization of optical network resources.

The networking industry is offering new services leveraging recent technological advances in connectivity, storage, and computing such as mobile communications and edge computing. In this regard, extended reality, a term encompassing virtual reality, augmented reality, and mixed reality, can provide unprecedented user experience and pioneering service opportunities such as: live concerts, sports, and other events; interactive gaming and entertainment; immersive education, training, and demos. These services require high-bandwidth, low-latency, and reliable connections, and are supported by next-generation ultra-reliable and low-latency communications in the vision of 6G mobile communication systems. In this work, we devise a novel scheme, called backup from different data centers with multicast and adaptive bandwidth provisioning, to admit reliable, low-latency, and high-bandwidth extended reality live streams in next-generation networks. We consider network services where contents are non-cacheable and investigate how backup services can be offered by different data centers with multicast and adaptive bandwidth provisioning. Our proposed service-provisioning scheme provides protection not only against link failures in the physical network but also against computing and storage failures in data centers. We develop scalable algorithms for the service-provisioning scheme and evaluate their performance on various complex network instances in a dynamic environment. Numerical results show that, compared to conventional service-provisioning schemes such as those seeking backup services from the same data center, our proposed service-provisioning scheme efficiently utilizes network resources, ensures higher reliability, and guarantees low latency; hence, it is highly suitable for extended reality live streams.

Emerging machine learning techniques enable Internet-connected devices to generate service function chain (SFC) requests for reliability-sensitive applications. To facilitate reliable SFC provisioning, prior works have proposed the SFC dedicated protection approach for fog and cloud networks, which generally have abundant resources and connectivity. However, with limited resources and connectivity, it might be inefficient to directly employ these approaches at the network edge. In this work, we study how to efficiently embed and provide dedicated protection when accommodating SFCs at the network edge. We formally define the problem of SFC embedding with dedicated protection (SFCE-DP) to minimize the bandwidth resources usage at the network edge. Based on the proposed technique of augmenting path in a layered graph, we construct an efficient algorithm, called augmenting-path-based SFC embedding with dedicated protection (AP-SDP), to optimize SFCE-DP. When the network resources are limited, our results show that AP-SDP significantly outperforms the benchmark that is directly extended from the state-of-the-art.

Edge computing provides computing capability at close-user proximity to reduce service latency for end users. To improve the efficiency of edge computing infrastructures, geographically-distributed edge datacenters can co-work with each other and with cloud datacenters, forming a new paradigm referred to as cooperative edge-cloud computing. In this context, applications typically run on a virtual machine (VM) that can be replicated at multiple sites, and thus user traffic can be served at all the sites where corresponding VMs reside. For the performance of many applications, latency is a critical parameter. In this work, taking applications’ latencies as the primary constraint, we model the problem of “VM placement and workload assignment” as a mixed integer linear program and develop heuristic algorithms accordingly. The goal is to minimize the consumption of information technology (IT) infrastructures for placing VMs in cooperative edge-cloud computing, while meeting the heterogeneous latency demands of different applications. Some preliminary results indicate that edge datacenter's resource efficiency can be optimized by proper cross-site VM placement and workload re-direction.

With the adoption of edge computing, several data centers are available within the footprint of an optical metro network, and contents are replicated in multiple locations. Such a wide content replication offers a unique opportunity to provide better services to users, especially for content-based services, e.g., video delivery. Thus, a service-provisioning scheme can embrace this opportunity to optimize network resource utilization, improve reliability, and achieve lower latency. In this study, we propose a reliable service-provisioning scheme that selects the optimal subset of data centers hosting the desired content and inversely multiplexes a content request over multiple link-disjoint paths. We formulate an integer linear program and develop heuristics for the problem, and use them to solve various complex and realistic network instances. Numerical data show that, compared to conventional service-provisioning schemes such as multipath routing from a single data center or dedicated-path protection, our proposed scheme efficiently utilizes network resources, improves reliability, and reduces latency; hence, it is suitable for the above-mentioned services.

In the 5G-and-beyond era, ultra-reliable low latency communication (URLLC) services are ubiquitous in edge networks. To enhance the performance metrics and the quality of service (QoS), URLLC services are delivered via a sequence of software-based network functions, also known as a service function chain (SFC). Towards reliable SFC delivery, it is imperative to incorporate fault-tolerance during SFC deployments. However, deploying an SFC with fault-tolerance is challenging because the protection mechanism needs to jointly consider multiple concurrent physical/virtual network failures and hardware/software failures. Considering these concurrent heterogeneous failures, this work investigates how to effectively deliver an SFC in edge networks with the objective of minimizing bandwidth resource consumption. First, we introduce the concept of${k}$-heterogeneous-faults-tolerance and propose an augmented protection graph, called${k}$-connected service function slices layered graph (KC-SLG). Based on the KC-SLG, we formulate a novel problem called${k}$-heterogeneous-faults-tolerant SFC embedding and propose an effective algorithm, called fault-tolerant service function graph embedding (FT-SFGE). FT-SFGE employs two proposed techniques:${k}$-connected network slicing (KC-NS) and${k}$-connected function slicing (KC-FS). Via thorough mathematical proofs, we show that KC-NS is 2-approximate. Extensive simulations show that KC-FS has the best average cost-efficiency when${k}$= 2, and FT-SFGE outperforms the schemes directly extended from the state-of-the-art.

Cooperation among datacenter providers (DCPs) and network carriers is necessary to support today's ubiquitous cloud services. However, such cooperation can be constrained by limited visibility as confidential information, such as network topology, resource availability, etc., may not be disclosed among these entities due to regulatory policies. We study a DCP-carrier cooperation-based service restoration scheme during a disaster with the aid of a third-party mediator, namely a Provider Neutral Exchange (PNE). We propose a novel resource-driven demand-matching strategy to restore DCP services. When multiple DCPs compete for network resources (due to post-disaster resource crunch), resource balancing by PNE can achieve fair and efficient service restoration. To allow flexibility in demand-resource matching, DCPs generate multiple sets of connection requests and define varying priorities and bandwidth degradations for each request. Carriers evaluate the DCP requests and provide feedback (e.g., whether a request can be satisfied or not) based on their available resources. We present an eight-phase DCP-carrier cooperation framework, with each phase employing individual sub-tasks carried out by DCPs, carriers, and PNE. Results under different disaster scenarios show that our strategy significantly improves DCP service restoration, incurring less restoration time.

Optical networks can support service degradation by providing bandwidth lower than that required to adapt the network provisioning when optical resources are insufficient. This paper proposes a service-degradation algorithm that is aware of application characteristics in Elastic Optical Networks (EONs). The algorithm considers a proportional Quality-of-Service (QoS) model and cross-layer information to decide which lightpath to be degraded, and it aims to reduce the impact of resource unavailability on delay and bandwidth sensitive applications. Results show that the proposed strategy can decrease blocking probability by up to 93% and reduce the number of applications penalized by service degradation by 100% compared to other approaches unaware of application characteristics.

Emerging 5G technologies can significantly reduce end-to-end service latency for applications requiring strict quality of service (QoS). With network function virtualization (NFV), to complete a client’s request from those applications, the client’s data can sequentially go through multiple service functions (SFs) for processing/analysis but introduce additional processing delay. To reduce the processing delay from the serially-running SFs, network function parallelism (NFP) that allows multiple SFs to run in parallel is introduced. In this work, we study how to apply NFP into the SF chaining and embedding process such that the latency, including processing and propagation delays, can be jointly minimized. We introduce a novel augmented graph to address the parallel relationship constraint among the required SFs. Considering parallel relationship constraints, we propose a novel problem called parallelism-aware service function chaining and embedding (PSFCE). For this problem, we propose a near-optimal maximum parallel block gain (MPBG) first optimization algorithm when computing resources at each physical node are enough to host the required SFs. When computing resources are limited, we propose a logarithm-approximate algorithm, called parallelism-aware SFs deployment (PSFD), to jointly optimize processing and propagation delays. We conduct extensive simulations on multiple network scenarios to evaluate the performances of our schemes. Accordingly, we find that (i) MPBG is near-optimal, (ii) the optimization of end-to-end service latency largely depends on the processing delay in small networks and is impacted more by the propagation delay in large networks, and (iii) PSFD outperforms the schemes directly extended from existing works regarding end-to-end latency.

Following successful adoption of cloud computing, many service providers (SPs) are now using high-performance Virtual Machines (VMs) located in large datacenters owned by public cloud infrastructure providers to deploy their virtual network functions (VNFs). Since using these VMs has a cost depending on utilization time, a complex problem of VNF placement and scheduling (VPS) must be addressed to achieve satisfactory network performance (e.g., latency) while minimizing the cost paid to lease VMs. In this study, a cost-efficient VPS scheme (CE-VPS) is proposed to address the VPS problem in public cloud networks considering dynamic requests of ordered sequences of VNFs. Our CE-VPS scheme goes beyond existing solutions as it models some important practical aspects such as an additional latency incurred by booting a VM and installing a VNF instance. Also, CE-VPS considers that VNFs can be multi-threaded or single-threaded, and that their throughput as a function of allocated computing resources must be modeled differently. CE-VPS is formulated as a mixed inter linear program (MILP) and also as an efficient heuristic algorithm. CE-VPS achieves lower cost and latency than conventional Best-Availability and Cost-Efficient Proactive VNF Placement schemes, and a better trade-off between resource consumption and latency performance than a conventional Low-Latency scheme.

Large-scale disasters affecting both network and datacenter (DC) infrastructures can cause severe disruptions in cloud-based services. During post-disaster recovery, repairs are usually carried out in stages in a progressive manner due to limited repair resource availability. The order in which network elements and DCs are repaired can significantly impact users' reachability to important contents/services. We investigate joint progressive network and DC recovery in which network recovery and DC recovery are conducted in a coordinated manner such that users have access to the maximum possible amount of contents/services at each repair stage. We first solve the optimization problem of joint progressive recovery to find the optimal sequence of network element and DC repairs with the objective to maximize cumulative weighted content reachability in the network. We then propose a scalable heuristic for scheduling the sequential repair of network nodes/links and DCs. Our model assumes that, at each repair stage, one network node with adjacent links and one DC can be fully repaired; however, full recovery may not be guaranteed due to limited resource availability. Hence, we also propose a “resource-aware” approach (with two resource-allocation strategies, namely “selective allocation” and “adaptive allocation”), which considers both full and partial recovery of elements based on available resources at each stage. We show that, compared to disjoint progressive recovery approach, in which network recovery and DC recovery plans are independent, our joint progressive recovery approach provides significantly higher per-stage content reachability in the network.

Network Function Virtualization (NFV) enables Network Operators (NOs) to efficiently respond to the increasing dynamicity of network services. Virtual Network Functions (VNFs) running on commercial off-the-shelf servers are easy to deploy, update, monitor, and manage. Such virtualized services are often deployed as Service Chains (SCs), which require in-sequence placement of computing and memory resources as well as routing of traffic flows. Due to the ongoing migration towards cloudification of networks, the concept of auto-scaling which originated in Cloud Computing, is now receiving attention from networks professionals too. Prior studies on auto-scaling use measured load to dynamically react to traffic changes. Moreover, they often focus on only one of the resources (e.g., compute only, or network capacity only). In this study, we consider three different resource types: compute, memory, and network bandwidth. In prior studies, NO takes auto-scaling decisions, assuming tenants are always willing to auto-scale, and Quality of Service (QoS) requirements are homogeneous. Our study proposes a negotiation-game-based auto-scaling method where tenants and NO both engage in the auto-scaling decision, based on their willingness to participate, heterogeneous QoS requirements, and financial gain (e.g., cost savings). In addition, we propose a proactive Machine Learning (ML) based prediction method to perform SC auto-scaling in dynamic traffic scenario. Numerical examples show that our proposed SC auto-scaling methods powered by ML present a win-win situation for both NO and tenants (in terms of cost savings).

Next-generation optical metro-access networks are expected to support end-to-end virtual network slices for critical 5G services. However, disasters affecting physical infrastructures upon which network slices are mapped can cause significant disruption in these services. Operators can deploy recovery units or trucks to restore services based on slice requirements. In this study, we investigate the problem of slice-aware service restoration in metro-access networks with specialized recovery trucks to restore services after a disaster failure. We model the problem based on classical vehicle-routing problem to find optimal routes for recovery trucks to failure sites to provide temporary backup service until the network components are repaired. Our proposed slice-aware service-restoration approach is formulated as a mixed integer linear program with the objective to minimize penalty of service disruption across different network slices. We compare our slice-aware approach with a slice-unaware approach and show that our proposed approach can achieve significant reduction in service-disruption penalty.

Cloud-Fog Radio Access Networks (CF-RAN) were proposed as an alternative network architecture to alleviate the high fronthaul capacity requested in traditional Cloud RAN (CRAN) by moving some BaseBand Units (BBUs) from the cloud nodes to fog nodes closer to users. However, when BBU processing is moved into fog nodes, OPEX and CAPEX will increase, and the cost and energy savings introduced by CRAN will also reduce. Moreover, mobile traffic fluctuations may lead to an unbalanced resource utilization and energy- inefficient operation in fog nodes. To address this problem, processing functions in fog nodes could be activated and deactivated in function of network traffic and BBUs placed on fog nodes could be migrated to cloud nodes when network traffic is low. In this paper, we propose an Integer Linear Programming (ILP) formulation to address this dynamic resource allocation problem. By means of Network Functions Virtualization (NFV), virtualized BBUs (vBBUs) can be dynamically allocated and deallocated in fog nodes. Furthermore, considering the availability of cloud nodes and the optical fronthaul, vBBUs can be migrated from fog nodes to cloud nodes in order to balance processing loads and save energy. Compared to a baseline incremental algorithm without vBBU migration, our proposal reduces blocking probability in 89% and achieves power savings of 38%, while providing a very small rate of service interruption due to vBBUs migration.

Transient traffic spikes are becoming a crucial challenge for network operators from both user-experience and network-maintenance perspectives. Different from long-term traffic growth, the bursty nature of short-term traffic fluctuations makes it difficult to be provisioned effectively. Luckily, next-generation elastic optical networks (EONs) provide an economical way to deal with such short-term traffic fluctuations. In this paper, we go beyond conventional network reconfiguration approaches by proposing the novel lightpath-splitting scheme in EONs. In lightpath splitting, we introduce the concept of SplitPoints to describe how lightpath splitting is performed. Lightpaths traversing multiple nodes in the optical layer can be split into shorter ones by SplitPoints to serve more traffic demands by raising signal modulation levels of lightpaths accordingly. We formulate the problem into a mathematical optimization model and linearize it into an integer linear program (ILP). We solve the optimization model on a small network instance and design scalable heuristic algorithms based on greedy and simulated annealing approaches. Numerical results show the tradeoff between throughput gain and negative impacts like traffic interruptions. Especially, by selecting SplitPoints wisely, operators can achieve almost twice as much throughput as conventional schemes without lightpath splitting.

Virtualization of network functions (as virtual routers, virtual firewalls, etc.) enables network owners to efficiently respond to the increasing dynamicity of network services. Virtual Network Functions (VNFs) are easy to deploy, update, monitor, and manage. The number of VNF instances, similar to generic computing resources in cloud, can be easily scaled based on load. Auto-scaling (of resources without human intervention) has been investigated in academia and industry. Prior studies on auto-scaling use measured network traffic load to dynamically react to traffic changes. In this study, we propose a proactive Machine Learning (ML) based approach to perform auto-scaling of VNFs in response to dynamic traffic changes. Our proposed ML classifier learns from past VNF scaling decisions and seasonal/spatial behavior of network traffic load to generate scaling decisions ahead of time. Compared to existing approaches for ML-based auto- scaling, our study explores how the properties (e.g., start-up time) of underlying virtualization technology impacts QoS and cost savings. We consider four different virtualization technologies: Xen and KVM, based on hypervisor virtualization, and Docker and LXC, based on container virtualization. Our results show promising accuracy of the ML classifier. We also demonstrate using realistic traffic load traces and optical backbone network that our ML method improves QoS and saves significant cost for network owners as well as leasers.

Elastic Optical Networks (EONs) represent a new approach for dealing with the enormous traffic demand in core networks as they can offer bandwidth granularities closer to those requested by the user and hence improve spectral utilization. In current literature there is a lack of dynamic strategies for service degradation which is a possible measure to address problems related to network congestion and consists in reducing the amount of resources provided. Since services of different classes can be requested, we propose in this paper an online strategy for service degradation using proportional Quality of Service (QoS). Our proposed strategy aims at minimizing the number of blocked requests due to lack of resources while provides throughput and delay guarantees for provisioned lightpaths. Thus, in order to quantify the impact of the degradation on the lightpaths we modeled source-destination pairs in an EON as a queuing system working under the Generalized Processor Sharing (GPS) service discipline with admission control of Leaky Bucket policy. The obtained results show that the proposed algorithm can reduce the blocking probability and give network operators more control between different degraded service classes.

Network function virtualization (NFV) aims to simplify service deployment using virtual network functions (VNFs). Service deployment involves the placement of VNFs and in-sequence routing of traffic flows through VNFs comprising a service chain (SC). The joint VNF placement and traffic routing is called SC mapping. In a wide-area network (WAN), where several traffic flows, generated by many distributed node pairs, require the same SC; a single instance (or occurrence) of that SC might not be enough. SC mapping with multiple SC instances for same SC is a very complex problem, since sequential traversal of VNFs has to be maintained while accounting for traffic flows in various directions. This paper is the first to deal with the problem of SC mapping with multiple SC instances to minimize network resource consumption. We propose an integer linear program (ILP), a column-generation-based ILP (CG-ILP), and a two-phase column-generation-based model (2PhMod) to solve this problem. ILP does not scale to large networks and CG-ILP scalability is limited by quadratic constraints. So, to get results over large network topologies within reasonable computational times, we propose 2PhMod. Using such an approach, we observe that an appropriate choice of only a small set of SC instances leads to a solution very close to minimum bandwidth consumption. Furthermore, this approach also helps us to analyze effects of number of VNF replicas and number of NFV nodes on bandwidth consumption when deploying these minimum number of SC instances.

Physical resources are highly virtualized in todays datacenter-based cloud-computing networks. Servers, for example, are virtualized as Virtual Machines (VMs). Through abstraction of physical resources, server virtualization enables migration of VMs over the interconnecting network. VM migration can be used for load balancing, energy conservation, disaster protection, etc. Migration of a VM involves iterative memory copy and network re-configuration. Memory states are transferred in multiple phases to keep the VM alive during the migration process, with a small downtime for switchover. Significant network resources are consumed during this process. Migration also results in undesirable performance impacts. Suboptimal network bandwidth assignment, inaccurate pre-copy iterations, and high end-to-end network delay in wide-area networks (WAN) can exacerbate the performance degradation. In this study, we devise strategies to find suitable bandwidth and pre-copy iteration count to optimize different performance metrics of VM migration over a WAN. First, we formulate models to measure network resource consumption, migration duration, and migration downtime. Then, we propose a strategy to determine appropriate migration bandwidth and number of pre-copy iterations, and perform numerical experiments in multiple cloud environments with large number of migration requests. Results show that our approach consumes less network resources when compared with maximum and minimum-bandwidth provisioning strategies while using an order of magnitude less bandwidth than maximum-bandwidth strategy. It also achieves significantly lower migration duration than minimum-bandwidth scheme.

Traditionally, networks operate at a small fraction of their capacities; however, recent technologies, such as software-defined networking, may let operators run their networks harder (i.e., at higher utilization levels). Higher utilization can increase the network operator's revenue, but this gain comes at a cost: daily traffic fluctuations and failures might occasionally overload the network. We call such situations Resource Crunch. Dealing with Resource Crunch requires certain types of flexibility in the system. We focus on scenarios with flexible bandwidth requirements, e.g., some connections can tolerate reducing their bandwidth allocation. This may free capacity to provision new requests that would otherwise be blocked. For that, the network operator needs to make an informed decision, since reducing the bandwidth of a high-paying connection to allocate a low-value connection is not sensible. We propose a strategy to decide whether or not to provision a request (and which other connections to degrade) focusing on maximizing profits during Resource Crunch. To address this problem, we use an abstraction of the network state, called a connection adjacency graph (CAG). We propose an algorithm, called PROVISIONER, which integrates our CAG solution with an efficient linear program (LP). We compare our method to existing greedy approaches and to LP-only solutions, and show that our method outperforms them during Resource Crunch.

Decoupling control and data planes in a software-defined network (SDN) has its advantages along with its challenges. Especially, resilient communication between elements in the data plane (switches) and in the control plane (controllers) is key to SDN's success as disruption of this communication after a failure can severely affect data-plane functions. After a failure, simultaneous recovery of all switch-controller communication paths (control paths) may not be possible, and multiple recovery stages may be required. Since restoration of disrupted data paths depends on the recovery of disrupted control paths feeding control information to switches, the performance of control-path recovery seriously affects data-path recovery performance. The assignment of controller to switches and the routing of controller-switch control paths are what determines the control-plane recovery performance, and hence should be performed in conjunction with a recovery plan after failures. This study proposes an algorithm for recovery-aware switch-controller assignment and routing (RASCAR), which enables fast data-path recovery after a set of failures (e.g., single point of failures and disasters). We formulate the problem as an integer linear program and propose an efficient heuristic algorithm to solve large problem instances. Our illustrative numerical studies show that RASCAR significantly reduces the data-path restoration times after any failure with a minor increase in resource consumption of control paths.

Network Function Virtualization (NFV) aims to simplify deployment of network services by running Virtual Network Functions (VNFs) on commercial off-the-shelf servers. Service deployment involves placement of VNFs and in-sequence routing of traffic flows through VNFs comprising a Service Chain (SC). The joint VNF placement and traffic routing is usually referred as SC mapping. In a Wide Area Network (WAN), a situation may arise where several traffic flows, generated by many distributed node pairs, require the same SC, one single instance (or occurrence) of that SC might not be enough. SC mapping with multiple SC instances for the same SC turns out to be a very complex problem, since the sequential traversal of VNFs has to be maintained while accounting for traffic flows in various directions. Our study is the first to deal with SC mapping with multiple SC instances to minimize network resource consumption. Exact mathematical modeling of this problem results in a quadratic formulation. We propose a two-phase column-generation-based model and solution in order to get results over large network topologies within reasonable computational times. Using such an approach, we observe that an appropriate choice of only a small set of SC instances can lead to solution very close to the minimum bandwidth consumption.

In the context of future Cloud Radio Access Networks (CRAN), optical networks will play an important role to provide the required transport capacity between cell-sites and processing pools, especially for future 5G scenarios. For instance, using CPRI fronthaul technologies a single antenna element can generate data up to 24.3Gbps even with current configurations of radio transmissions, and it is expected to generate up to Tbps with the advance of technology. So, the transport segment of a 5G network needs to be accurately planned to accommodate all the generated traffic. In this work, we propose the use of a Passive Optical Network (PON) jointly with the emergent paradigms of Fog Computing and Network Function Virtualization (NFV) to energy-efficiently support the high traffic transported in emergent mobile networks in an hybrid architecture called Cloud/Fog RAN (CF-RAN) that allows local and remote baseband processing. We introduce an Integer Linear Programming (ILP) model to schedule the processing of CPRI demands among the processing nodes of the network and turn on or off processing functions on demand. Our approach is able to accommodate demands on the nodes of the network in the most energy efficient way. We compare our results with CRAN and distributed architectures (DRAN) and show that an energy efficient planning can achieve considerable gains in power consumption.

Mobile Fronthaul (MF) is defined as the connection between Remote Radio Head (RRH) and Baseband Processing Unit (BBU) in a Cloud Radio Access Network (C-RAN). Dedicated MF connections between RRH and BBU would be very costly. Thus, Time Division Multiplexing Ethernet Passive Optical Network (TDM EPON) is a promising solution to reduce cost as it can enable multiplexing gain. Note that, even though mobile users transmit intermittently, in the upstream channel RRH is sampling radio signal all the time, limiting the achievable multiplexing gain. Our study enhances the conventional TDM EPON architecture by introducing traffic classification, sifting of useless data to avoid the transmission of unnecessary EPON frames, and hence increasing the multiplexing gain in the upstream channel. We also propose a Hybrid Bandwidth Allocation (HBA) scheme to exploit the traffic usefulness classification information. Simulation results show significant improvements in terms of load and number of connected RRHs that can be supported by same EPON, while keeping the end-to-end delay under 100 μs.

Today's communication networks require special redundancy to overcome severe multiple element failures. These severe failures can isolate entire sub-components of terrestrial networks - e.g., possible aftermath of natural disasters and threats such as a High-Altitude Electromagnetic Pulse (HEMP). An integrated use of all communication systems available is important to help lessen the impact over distressed areas. This work investigates the use of aerial platforms with well-defined trajectories (such as LEO satellites) to evacuate data from systems within the affected regions. We propose an algorithm capable of generating an evacuation plan for data located in terrestrial isolated systems, such as Data Centers, through the satellite network, towards final destinations in the main network. Our method works whether the satellite network is damaged or not. The evacuation plan is a node-to-node transmission schedule that maximizes the amount of evacuated data. Post-disaster scenarios are used to analyze how our method performs under different impact sizes and satellite network configurations. The results show that the proposed algorithm produces node-to-node transmission schedules that maximize evacuated data while maintaining fairness among disconnected components.

As more organizations rapidly adopt cloud services, energy consumption in data centers (DCs) is increasing such that today Information and Communication Technology (ICT) has become a major consumer of energy. A large portion of ICT energy consumption is used to power servers running in DCs and the network they use to communicate. In this study, we consider that, often, energy cost at a particular DC is related to the electricity price regulated by Independent System Operators / Regional Transmission Organizations (ISOs/RTOs). As these prices vary in time and depend on the geographical locations of the DCs, recent studies have shown that the spatio-temporal variations of electricity price can be exploited to reduce electricity cost. While most prior works consider a quasi-static scenario with known workload patterns, our study proposes a dynamic workload-aware algorithm that exploits the spatio-temporal variations of electricity costs with the goal to minimize the energy cost in ICT. Our algorithm uses dynamic request rerouting and live virtual machine (VM) migration to move workloads to DCs with lower electricity cost. We consider VM migration cost (including electricity cost at optical backbone network nodes), bandwidth constraints for migration, VM consolidation, constraints from Service Level Agreement (SLA), and administrative overhead of VM migration. Our simulation studies show that the proposed algorithm reduces operational cost and improves energy efficiency of data centers significantly.

Cloud radio access network (CRAN) has been proposed as a promising evolution of mobile network architecture where baseband processing functions of a base station are split/decoupled from the radio unit (RU) and centralized. However, rigid bandwidth and latency requirements are incurred by fronthaul, i.e., transport link, which connects RU to the central cloud. Therefore, new functional splits are discussed for CRAN with dual-site processing, which we call Hybrid-RAN (H-RAN), where some functions remain distributed while others are centralized. In this work, from a perspective of minimizing the total cost of ownership (TCO) for H-RAN, we present a techno-economic study to find the optimal functional splits for a base station (BS), with a given configuration. A configuration of a BS represents frequency layers, carrier bandwidths, and MIMO schemes, associated with different frequency bands. For each functional split, we present a model to calculate the requirement of computational resources and fronthaul bandwidth. We formulate a TCO minimization model using constraint programming. Numerical results show that the optimal functional split depends on BS configuration, fiber ownership, and data transmission direction. H-RAN with optimal functional split can achieve lower TCO than both classical Distributed RAN and CRAN.

5G Radio Access Networks (RANs) are supposed to increase their capacity by 1000x to handle growing number of connected devices and increasing data rates. The concept of cloud-RAN (CRAN) has been recently proposed to decouple digital units (DUs) and radio units (RUs) of base stations (BSs), and centralize DUs into central offices. CRAN can ease the implementation of advanced radio coordination techniques, e.g., Coordinated Multi-Point (CoMP) Transmission/Reception, to enhance its system throughput. However, separating DUs and RUs, and implementing CoMP in CRAN require low-latency and high-bandwidth connectivity links, called "fronthaul". Today, consensus has not yet been achieved on how BSs, fronthaul, and central offices will be orchestrated to enhance the system throughput. In this study, we present a CRAN over Passive Optical Network (PON) architecture called virtualized-CRAN (V-CRAN). V-CRAN leverages the concept of virtualized PON (VPON) that can dynamically associate any RU to any DU so that several RUs can be coordinated by the same DU, and the concept of virtualized BS (V-BS) that can jointly transmit common signals from multiple RUs to a user. We propose a novel mathematical model based on constraint programming for joint allocation of radio, optical network, and baseband processing resources to enhance RAN throughput, and we solve it by optimally forming VPONs and V-BSs. Comprehensive simulations show that V-CRAN can enhance the system throughput and the efficiency of resource utilization.

Degraded provisioning provides an effective solution to flexibly allocate resources in various dimensions to reduce blocking for differentiated demands when network congestion occurs. In this work, we investigate the novel problem of online degraded provisioning in service-differentiated multi-layer networks with optical elasticity. Quality of Service (QoS) is assured by service-holding-time prolongation and immediate access as soon as the service arrives without set-up delay. We decompose the problem into degraded routing and degraded resource allocation stages, and design polynomial-time algorithms with the enhanced multi-layer architecture to exploit network flexibility in temporal and spectral dimensions. Numerical results verify that we can achieve significant blocking reduction, especially for requests with higher priorities. They also indicate that degradation in optical layer can increase the network capacity, while degradation in electric layer provides flexible time-bandwidth exchange.

In emerging cellular systems, optical fronthaul is expected to play a major role to support many control operations, e.g., Coordinated Multipoint (CoMP). CoMP is a promising technique for interference mitigation as it can transform interfing signals into joint transmission (reception) in which signals from adjacent cell sites are simultaneously transmitted (received) to (from) mobile terminals. But the exchange of information required by CoMP demands high flexibility and capacity. This paper proposes a new architecture for supporting CoMP operations in emerging cellular systems. It is based on a time-and-wavelength-division-multiplexed passive optical network (TWDM-PON) fronthaul, using virtualized base stations and a cloud radio access network (C-RAN) architecture. We also propose techniques to distribute the load on controllers to minimize the coordination delay. Results show that, for a typical setting, our methods can save up to 37% on the time required to distribute channel state information among multiple base stations.

In network virtualization, when a disaster hits a physical network infrastructure, it is likely to break multiple virtual network connections. So, after a disaster occurs, the network operator has to schedule multiple teams of repairmen to fix the failed components, by considering that these elements may be geographically dispersed. An effective schedule is very important as different schedules may result in very different amounts of time needed to restore a failure. In this study, we introduce the multiple traveling repairmen problem (MTRP) for post-disaster resilience, i.e., to reduce the impact of a disaster. Re-provisioning of failed virtual links is also considered. We first formally state the problem, where our objective is to find an optimal schedule for multiple teams of repairmen to restore the failed components in physical network, maximizing the traffic in restored virtual network and with minimum damage cost. Then, we propose a greedy (GR) and a simulated annealing (SA) algorithm, and we measure the damage caused by a disaster in terms of disconnected virtual networks (DVN), failed virtual links (FVL), and failed physical links (FPL). Numerical result shows that both proposed algorithms can make good schedules for multiple repairmen teams, and SA leads to significantly lower damage in terms of DVN, FVL, and FPL than GR.

With the rapid growth of content-based network services, there is increasing interest in reducing the emissions associated with brown-energy consumption in Content Delivery Networks (CDNs). At the same time, content needs to be placed in safe Data Center (DC) locations, which are unlikely to be hit by disasters. Further risk reduction is achieved using content replication which provides inter-DC content redundancy. Unfortunately, there is contention between the objectives of brown-energy minimization and risk reduction since replicating content increases brown-energy consumption. To address these contradictory issues, we leverage the concept of content fragmentation used inside DCs and propose an inter-DC Content Fragmentation (CF) scheme which aims to achieve brown-energy saving by reducing storage overhead while maintaining low risk compared to basic replication schemes. We also propose a Green and Low-Risk Content Placement approach (GR-CP) to address the tradeoff between brown-energy consumption and disaster risk. Both CF and replication schemes are implemented in GR-CP and evaluated over a range of content popularity and redundancy levels. Our results show that CF outperforms replication scheme except when content popularity is high and the risk constraint is stringent. When popularity and risk are both low, CF can save more brown energy by using low-redundancy fragmentation techniques.

In recent years, the increasing traffic demand in radio access networks (RANs) has led to considerable growth in the number of base stations (BSs), posing a serious scalability issue, including the energy consumption of BSs. Optical-access-enabled Cloud-RAN (CRAN) has been recently proposed as a next-generation access network. In CRAN, the digital unit (DU) of a conventional cell site is separated from the radio unit (RU) and moved to the “cloud” (DU cloud) for centralized signal processing and management. Each DU/RU pair exchanges bandwidth-intensive digitized baseband signals through an optical access network (fronthaul). Time-wavelength division multiplexing (TWDM) passive optical network (PON) is a promising fronthaul solution due to its low energy consumption and high capacity. In this paper, we propose and leverage the concept of a virtual base station (VBS), which is dynamically formed for each cell by assigning virtualized network resources, i.e., a virtualized fronthaul link connecting the DU and RU, and virtualized functional entities performing baseband processing in DU cloud. We formulate and solve the VBS formation (VF) optimization problem using an integer linear program (ILP). We propose novel energy-saving schemes exploiting VF for both the network planning stage and traffic engineering stage. Extensive simulations show that CRAN with our proposed VF schemes achieves significant energy savings compared to traditional RAN and CRAN without VF.

Cloud computing uses cloud networks (CNs) that integrate and virtualize computing servers and communication networks. In a CN, virtual machines (VMs) are interconnected through virtual networks (VNs) provisioned over a physical optical network. A disaster event is a serious threat to cloud computing infrastructure, not only for CN disconnections caused by multiple infrastructure failures, but by subsequent and unpredictable CN disconnections induced by cascading failures. Studies on disaster protection for CNs suggest large pre-provisioning of additional capacity before a possible disaster, with limited protection for later cascading failures. In this work, we propose an adaptive and cascading- failure-aware CN disaster recovery scheme that (re-)acts after the disaster, and uses risk modeling to reduce the capacity required for the recovery and minimize the post-disaster disconnection of CNs. Major power grid outages could cause cascading failures on cloud infrastructure operation. Thus, in this study, propagation patterns of power grid failures are used to estimate the location of cascading failures. Simulation results based on human-made disasters, e.g., weapon of mass destruction (WMD) attacks, show that our approach can lead to significant reduction in the risk of CN disconnections due to cascading failures, while reducing up to 50% of the capacity re-provisioning required for the recovery.

Communication networks, such as core optical networks, heavily depend on their physical infrastructure, and hence they are vulnerable to man-made disasters, such as Electromagnetic Pulse (EMP) or Weapons of Mass Destruction (WMD) attacks, as well as to natural disasters. Large-scale disasters may cause huge data loss and connectivity disruption in these networks. As our dependence on network services increases, the need for novel survivability methods to mitigate the effects of disasters on communication networks becomes a major concern. Software-Defined Networking (SDN), by centralizing control logic and separating it from physical equipment, facilitates network programmability and opens up new ways to design disaster-resilient networks. On the other hand, to fully exploit the potential of SDN, along with data-plane survivability, we also need to design the control plane to be resilient enough to survive network failures caused by disasters. Several distributed SDN controller architectures have been proposed to mitigate the risks of overload and failure, but they are optimized for limited faults without addressing the extent of large-scale disaster failures. For disaster resiliency of the control plane, we propose to design it as a virtual network, which can be solved using Virtual Network Mapping techniques. We select appropriate mapping of the controllers over the physical network such that the connectivity among the controllers (controller-to-controller) and between the switches to the controllers (switch-to-controllers) is not compromised by physical infrastructure failures caused by disasters. We formally model this disaster-aware control-plane design and mapping problem, and demonstrate a significant reduction in the disruption of controller-to-controller and switch-to-controller communication channels using our approach.

In recent years, the increasing traffic demand in radio access networks (RAN) has led to considerable growth of the number of base stations (BS), posing a serious scalability issue with respect to the energy consumption of BSs. Optical-access-enabled Cloud RAN (CRAN) has been recently proposed as a next-generation access network, where the digital unit (DU) of a conventional cell site is separated from the radio unit (RU), by an optical access network (fronthaul), and moved to the “cloud” (DU pool) for centralized signal processing and management. Time-Wavelength Division Multiplexing (TWDM) Passive Optical Network (PON) is a promising fronthaul solution due to its low energy consumption and high capacity. In this study, we propose the concept of Virtual Base Station (VBS), which is dynamically formed for each cell by assigning virtualized network resources, including i) a virtualized PON link connecting the DU and RU and ii) virtualized functional entities performing baseband processing in DU pool. We propose a novel energy-saving scheme exploiting VBS formation for CRAN and compare its performance with the optimal results of an Integer Linear Program for VBS formation optimization problem. Numerical evaluation shows that CRAN with VBS formation achieves significant energy savings compared to traditional RAN and CRAN without VBS formation.

Connections provisioned in a backbone network are usually protected. A “good” protection scheme can decrease the downtime experienced by a connection, which can reduce (or eliminate) penalties for the violation of the Service Level Agreement (SLA) between the network operator and its customer. Although “good” protection schemes can guarantee high availability to connections, they usually require high capacity (e.g., bandwidth). However, backbone networks usually have some excess capacity (EC) to accommodate traffic fluctuations and growth, and when there is enough EC, the high capacity requirement of protection schemes can be tolerated. However, under traffic growth, the network operator has to add more bandwidth to avoid capacity exhaustion, which increases upgrade costs. In this study, we show that, in case of connections supporting differentiated services, where connections' tolerable downtimes are diverse, efficient exploitation of EC can decrease both SLA violations and upgrade costs. We develop a novel EC management (ECM) approach that provides high-availability high-capacity protection schemes when EC is available, and reprovisions backup resources with multiple protection schemes so that SLAs are still respected, but network upgrade costs are kept under control. We formulate this problem as an integer linear program (ILP) and develop an efficient heuristic as the ILP is intractable for large problems. We present several alternatives of our ECM approach to show its compatibility with different protection-scheme combinations. Numerical examples are presented to illustrate how the proposed ECM technique finds a tradeoff between upgrade costs and penalties paid for SLA violations while reducing the total cost significantly.

Time- and wavelength-division multiplexed passive optical network allows wavelength sharing by optical network units (ONUs) in a time-division multiplexing fashion. When ONUs are lightly loaded, they can share fewer wavelengths to reduce energy consumption. In such a dynamic system, the configuration of wavelength sharing should adapt to the traffic changes for energy saving and load balancing (which affects the quality of service). However, going from one configuration to another is nontrivial, as it usually involves wavelength reassignment and potential service disruptions. The optimization and algorithm design have to account for such reconfiguration (along with other aspects). In this study, we have developed optimization models and online algorithms that incorporate the reconfiguration dimension. Various underlying tradeoffs (e.g., energy saving versus reconfiguration and load balancing versus reconfiguration) are investigated for both static planning and dynamic operations.

Traffic in optical backbone networks is increasing and becoming more heterogeneous with respect to bandwidth and QoS requirements due to the popularity of high-bandwidth services (such as cloud computing, e-science, telemedicine, etc.), which need to coexist with traditional services (HTTP, etc.). Mixed-line-rate (MLR) networks that support lightpaths of different rates such as 10, 40, 100 Gb/s, etc., are being studied to better support the heterogeneous traffic demands. Here, we study the important topic of degraded services in MLR networks, where a service can accept some degradation (i.e., reduction) in bandwidth in case of a failure in exchange for a lower cost, a concept called partial protection. Network operators may wish to support degraded services to optimize network resources and reduce cost. We propose using multipath routing to support degraded services in MLR networks, a problem that has not been studied before and is significantly more challenging than in single-line-rate (SLR) networks. We consider minimum-cost MLR network design (i.e., choosing which transponder rates to use at each node), considering the opportunity to exploit multipath routes to support degraded services. We propose a mixed-integer-linear-program (MILP) solution and a computationally efficient heuristic, and consider two partial-protection models. Our illustrative numerical results show that significant cost savings can be achieved due to partial protection versus full protection and is highly beneficial for network operators. We also note that multipath routing in MLR networks exploits volume discount of higher-line-rate transponders by cost-effectively grooming requests over appropriate line rates to maximize transponder reuse versus SLR.

Cloud computing is evolving as a major priority for many enterprises. Better wide-area networks with low latency and cost are needed to interconnect geographically-distributed data centers and offices using virtual private lines (VPLs). We propose novel network architectures based on sliceable optical networks to implement future VPLs. Optimal designs of the new network architectures and the traditional packet-over-optical network architecture are proposed and compared. It is found that the new architectures can achieve the `lowest-possible' latency with potentially lower cost than traditional architecture.

Spectrum elastic optical networks support flexible central frequency and spectrum assignment for lightpaths. When provisioning a new connection in an elastic optical network that allows traffic grooming, the control plane has to solve two problems: the electrical-layer routing and optical-layer routing and spectrum assignment (RSA). The electrical-layer routing determines how to route the new connection through a combination of new and existing lightpaths, while the optical-layer RSA decides how to establish new lightpaths under the spectrum-continuity constraint. The flexibility (e.g., bandwidth variability of lightpaths) provided by elastic optical networks makes it suitable for accommodating dynamic traffic. It is important and challenging to exploit the full potential of the flexibility when dealing with the above two problems. In this study, we propose a multi-layer auxiliary graph to jointly solve the electrical-layer routing and optical-layer RSA. Various traffic-grooming policies (objectives) can be achieved by properly adjusting the edge weights in the auxiliary graph. Also, we propose a spectrum reservation scheme that can efficiently utilize the bandwidth variability of lightpaths by reserving bandwidth for non-fully utilized lightpaths and grooming future connections onto them. We show that there is a tradeoff among different traffic-grooming policies, and the spectrum reservation scheme can be easily incorporated into various traffic-grooming policies and lead to a significant reduction in operational expenditure (OPEX) and better spectrum efficiency.

Operational networks get gradually upgraded to avoid capacity exhaustion and create resources to handle traffic growth. These upgrades require installation of new physical resources (e.g., line cards) and might be costly to network operators. The additional capital expenditure (CapEx) to upgrade the network can be reduced or eliminated by relocating excess resources which were deployed into the network, but are unused (e.g., due to optimistic forecasts or excessive overprovisioning). We investigate a cost-effective scheme to relocate excess capacity in an optical WDM backbone network to (i) reduce or eliminate upgrade costs, (ii) avoid capacity exhaustion (by migrating resources from underutilized links to overutilized links), and (iii) improve network robustness (by migrating resources from unreliable links to reliable links). We propose a novel generic relocation scheme that network operators can use based on their objectives to determine how to relocate excess resources. We also provide a solution which maximizes various benefits (e.g., network robustness and capacity exhaustion avoidance) of network operators by using game theory for our relocation scheme. Our results show that network operator can save a lot of CapEx while upgrading/improving the network by using our relocation scheme.

As new Internet applications are emerging, traffic in optical backbone networks is increasing with heterogeneity in bandwidth and QoS requirements. Higher bit-rate wavelength channels of 40 Gbps, 100 Gbps, and beyond are being deployed in WDM backbone networks to meet these growing traffic needs. Mixed-line-rate (MLR) networks that support various line rates on the same fiber are becoming popular to address the heterogeneous traffic growth. We study the important problem of long-term cost-effective dimensioning of WDM backbone networks with MLR. Our study should enable operators to dimension their networks with deployment of MLR transponders and optical cross-connect (OXC) ports over multiple time periods. We study two multi-period dimensioning approaches: all-periods planning and incremental planning. Our approaches predict the network equipment to be deployed at various nodes in the network during each time period while exploiting the cost reduction of network equipment over time to support traffic growth over multiple time periods. We propose mixed-inter-linear-program (MILP) solutions and a computationally-efficient heuristic. Our illustrative examples show that significant cost savings can be achieved by our long-term planning approaches.

As optical network continues to evolve, lightpaths will take on different spectrum spaces as opposed to current uniform 50-GHz grid. While lightpaths of heterogeneous bandwidths co-exist, two factors emerge that will degrade the provisioning efficiency and negatively impact its sustainable evolution: 1) unfairness of access among different bandwidth connections, and 2) spectrum fragmentation caused by bandwidth mismatch. Through analysis and simulations, we show that a spectrum management method that partitions resources for dedicated usage by different bandwidth lightpaths achieves better provisioning efficiency by resolving these two problems.

This study investigates the survivable traffic grooming problem for elastic optical networks with flexible grid employing new transmission technologies, e.g., orthogonal frequency-division multiplexing (OFDM). In such networks, the strict ITU-T wavelength grid is not followed. Instead, optical transponders are developed to be capable of properly tuning their rates. Equipping the network with gridless and elastic optical paths allows us to efficiently use the optical spectrum. In this paper, we propose a novel elastic shared path protection (ESPP), which does not only provide the traditional backup sharing of shared path protection (SPP) (i.e., the backup capacity of one optical path (i.e., lightpath) can be shared among multiple backup paths, provided that their corresponding working paths are link-disjoint), but also explores a new opportunity of sharing enabled by the tunability of the transponders: in fact, the backup spectrum can be shared between two adjacent lightpaths on a link, if their corresponding working paths are link-disjoint. The elasticity of the transponder enables the expansion and contraction of the lightpaths, so that at one time, the backup spectrum is used by only one of the adjacent lightpaths. Note that in traditional wavelength-division-multiplexing (WDM) networks, the lightpaths are fixed-grid, rigid-bandwidth, nor can they overlap each other. Our results show that ESPP is more spectrum efficient than traditional SPP.

Spectrum elastic optical networks support flexible central frequency and spectrum assignment for lightpaths. In this paper, we investigate energy-efficient dynamic provisioning for such networks. When provisioning a connection, the routing problems in both electrical layer (routing over multiple lightpaths) and optical layer (routing over multiple fibers) have to be addressed. Also, the control plane has to determine the optical-layer spectrum assignment considering the spectrum-continuity constraint as well as the bandwidth variability of transponders. We adopt a novel auxiliary graph based on which a new dynamic provisioning policy called Time-Aware Provisioning with Bandwidth Reservation (TAP-BR) is proposed. TAP-BR incorporates two important factors to facilitate energy-efficient provisioning: time awareness and bandwidth reservation. We compare TAP-BR with previously-proposed dynamic provisioning policies and show that TAP-BR can save significant amount of energy and make efficient use of spectrum resources.

In mixed-line-rate (MLR) networks, different line rates can coexist on the same fiber on different wavelengths. MLR brings flexibility to handle diverse demands. High line rates require advanced modulation techniques, such as DQPSK and DP-QPSK. On the other hand, signals being propagated over transparent paths are exposed to detrimental effects of physical layer impairments (PLI). Advanced modulation techniques are more susceptible to PLI, especially to the cross phase modulation (XPM) induced by intensity modulated channels. Inverse multiplexing, in MLR networks, is a technique which tries to exploit the advantage of transmitting the signals with low line rates where the high line rate is not possible due to impairments. In this study, we investigate the performance of employing inverse multiplexing technique in MLR networks, for dynamic RWA problem considering physical-layer impairments.

C3 - Journal Articles Unrefereed Letters or Notes

Excess capacity (EC) is the unused capacity in a network. We propose EC management techniques to improve network performance. Our techniques exploit the EC in two ways. First, a connection preprovisioning algorithm is used to reduce the connection setup time. Second, whenever possible, we use protection schemes that have higher availability and shorter protection switching time. Specifically, depending on the amount of EC available in the network, our proposed EC management techniques dynamically migrate connections between high-availability, high-backup-capacity protection schemes and low-availability, low-backup-capacity protection schemes. Thus, multiple protection schemes can coexist in the network. The four EC management techniques studied in this paper differ in two respects: when the connections are migrated from one protection scheme to another, and which connections are migrated. Specifically, Lazy techniques migrate connections only when necessary, whereas Proactive techniques migrate connections to free up capacity in advance. Partial Backup Reprovisioning (PBR) techniques try to migrate a minimal set of connections, whereas Global Backup Reprovisioning (GBR) techniques migrate all connections. We develop integer linear program (ILP) formulations and heuristic algorithms for the EC management techniques. We then present numerical examples to illustrate how the EC management techniques improve network performance by exploiting the EC in wavelength-division-multiplexing (WDM) mesh networks.

Any operational network has some excess capacity (EC) to avoid early exhaustion of resources. We propose to exploit the EC in optical WDM backbone networks to support efficient and survivable traffic grooming where connection requests are of sub-wavelength granularity and each provisioned request has to be protected from a single link failure. Our novel EC management techniques can improve network performance, at no additional cost to the operator, since excess capacity is normally unutilized. Our techniques exploit EC such that a connection can use a protection scheme which provides high reliability but may consume more resources when traffic is low, but it switches to another protection scheme which provides lower reliability but is resource efficient by reprovisioning backup resources. As a complement, we propose hold-p-lightpath scheme to exploit EC by preventing the termination of pre-established (but unused) resources. The backup reprovisioning problem is split it into three subproblems: when, how, and what to reprovision; and we propose our solutions for each subproblem. For the what to reprovision subproblem, we design three methods with different reliability and resource-efficiency performance. We compare our approaches with traditional protection schemes for typical daily fluctuating traffic, and show that significant improvements in performance and cost can be achieved.

We propose an orthogonal frequency division multiplexing (OFDM) based backbone optical network design. We focus on minimizing the total energy consumption of the network i.e., to make the network green. OFDM is a promising technology for next-generation optical networks with per-wavelength capacities higher than or equal to 100 Gbps. In addition, it can support heterogeneity in network traffic by having flexible bandwidth allocation per wavelength. The flexibility comes through the multiple subcarriers in an OFDM signal which can be modulated with the client data signals. Another paradigm for supporting traffic heterogeneity and high bandwidth demands is mixed-line-rate (MLR) networks where wavelengths can have discrete capacities of 10/40/100 Gbps which are single carrier based. In this study, we compare the energy efficiency of an OFDM-based network versus a MLR network. Our results show that OFDM outperforms MLR in terms of energy efficiency.

Today's access networks are increasingly shaped by the services that they provide to the end users. In a hybrid wireless-optical broadband access network (WOBAN), to access any service, connection requests require multi-hop communications over the wireless mesh network (WMN) and the Passive Optical Network (PON), and subsequently over the Internet to some server in the service provider's domain. To improve the delivery of such services over WOBAN, we can design a Cloud-Integrated WOBAN (CIW) by deploying a few cloud components (CCs) in the WOBAN itself and serve local services from these local CCs. In this paper, we propose a novel energy-saving routing mechanism, called Green Routing for CIW (GRC), that manages the activation of network components, namely ONUs and CCs, to minimize the overall energy consumption of CIW. It performs load-balanced anycast routing across active devices. Our performance evaluation shows that GRC operates with low average packet delay and achieves significant energy savings by turning off about 50% of the ONUs and about 20% of the CCs.

By utilizing Layer-1 Virtual Private Networks (L1VPN), a single physical network, e.g., optical backbone networks, can support multiple virtual networks, which is the basic infrastructure for cloud computing and other enterprise networks. The L1VPN hose model is an elegant and flexible way to specify the customers' bandwidth requirements, by defining the total incoming and outgoing demand for each endpoint. Furthermore, multi-domain physical infrastructures are common in L1VPNs, since these are usually deployed on a global scale. Thus, high-performance Routing for Multi-domain VPN Provisioning (RMVP) for the hose model is an important problem to efficiently support a global virtual infrastructure. In this paper, we formulate the RMVP problem as a Mixed Integer Linear Program (MILP). Also, we propose a Top-Down Routing (TDR) strategy to compute the optimal routing for the hose-model L1VPN in multi-domain backbone networks. Results indicate that TDR approaches the minimum routing cost when compared to ideal case of single-domain routing.

Many telecom, customers have applications with time varying reliability requirements. However, current for a service-level agreement (SLA) frameworks provide the same reliability during the entire holding time, rather than providing high reliability only when required. Recent papers describe the need for time-differentiated resilience and establish an SLA framework to support this using Critical Windows (CW). CWs are scheduled, regularly repeating time periods where additional resilience is required. We expand the previous work to a more general and dynamic network setting and further investigate the structural properties of critical windows. We propose and evaluate first an algorithm for CW-aware path assignment in mesh telecom networks, and then a CW-aware algorithm that also employs Shared-Path Protection (SPP). Our study shows: 1) that CWs provide a substantial performance improvement in mesh networks; 2) CW performance gain is insensitive to several network parameters; and 3) CWs can be implemented on top of generic SPP to provide even better performance.

An optical access network should offer low-cost reliable services to its end users. To address this problem, an optimal solution is needed which can turn an optical access architecture into a self-healing system. Hence, we propose the Self-Healing Optical Access Network (SHOAN), in which two or more optical access architectures are partners of each other, and they are interconnected by elementary optical crossbar switches into a simple mesh network. In SHOAN, the crossbar switches can keep each access architecture as an independent and closed system for only serving its own end users in normal state. But the crossbars become open in fault scenarios. Whenever a failure occurs in the network, the fault can be monitored and affected services can be recovered by the partner of the access architecture that is affected. Such an interconnected optical access network can withstand failures in its transmission paths, and recover network services in a self-healing way. Compared to existing solutions (e.g., dual-home architecture), illustrative examples demonstrate that SHOAN has many desirable properties: (1) it is robust because risks are disjointed, (2) it is reliable because service recovery is given top priority, and (3) it has low cost because redundant backup components are not necessary since the partner's resources act as backup resources. Analysis results show that SHOAN can minimize disruption duration and network cost for broadband access services.

Ethernet's success in local area networks (LANs) is fueling the efforts to extend its reach to cover metro and long-haul networks. This new Ethernet is refereed to as Carrier Ethernet. Among the various transport infrastructures for realizing Carrier Ethernet, wavelength-division multiplexing (WDM) optical network is a strong candidate for this purpose. Optical transmission rates per channel are increasing from 10 to 40 Gb/s and even 100 Gb/s, and they can also coexist in the same fiber. Along with the flexibility associated with such a network with mixed-line rates (MLR), signal-related constraints at high rates become a challenge for cost-efficient routing. Among these issues is the maximum nonregenerated optical distance that a signal can travel before its quality degrades or maximum transmission range (TR). TR is rate-dependent: The higher the rate, the shorter the range. While high-rate pipes may require signal regeneration to restore the signal's quality, they support more traffic and, hence, can save resources. We study the problem of cost-efficient routing of multi-bit-rate (1/10/40/100 Gb/s) Ethernet tunnels using MLR over a carrier's WDM optical network with signal-transmission-range constraints. We studied the effect of TR for mixed-rate signals (10/40/100 Gb/s) on the network's cost to determine the optimal TR of each bit rate. We present an analytical model based on a mixed-integer linear program (MILP) to determine the optimal TR of a small network. Since MILP has scalability constraints that makes it hard or sometimes impossible to solve for real network topologies, we propose a graph-based solution that constructs a mixed-line-rate auxiliary (MLR-AUX) graph to capture the network's heterogeneity and a weight-assignment approach that allows the routing to be cost-efficient. Our algorithms were tested on a U.S. nationwide network topology. We found that it is possible to reduce the network's cost by using short TR and that the optimal TR depends strongly on traffic characteristics and on the TR values of different bit-rate signals.

Survivability is a critical concern in modern telecom mesh networks because the failure of a network element may cause tremendous data and revenue loss in such networks using high-capacity optical fibers employing wavelength-division multiplexing (WDM). Multipath provisioning is a key feature of next-generation SONET/SDH networks (which can be used on top of optical WDM), and they can support virtual concatenation (VCAT); thus, multipath provisioning can significantly outperform single-path provisioning in resource efficiency, service resilience, and flexibility. However, in multipath provisioning, differential delay is an important constraint that should be considered. We investigate survivability of service paths based on differential-delay constraint (DDC) and multipath provisioning together in telecom backbone mesh networks. We propose the Shared Protection of the Largest Individual Traversed link (SPLIT) method for survivable multipath provisioning and present a DDC-based algorithm for multipath routing subject to DDC. We also compare the DDC-based algorithm with theKshortest link-disjoint paths (KDP) algorithm, using SPLIT, under dynamic service requests. We find that exploiting link-disjoint paths is very efficient for survivable multipath provisioning, and our algorithm is resource-efficient, has low signaling overhead, and has fast fault recovery for survivable multipath provisioning with DDC. For a 5-ms DDC, our algorithm can decrease the bandwidth blocking ratio (BBR) significantly in typical U.S. backbone networks.

A service-level agreement (SLA) typically specifies, among other metrics, the availability a service provider (SP) promises to a customer. In an optical wavelength division multiplexing (WDM) network, connection-oriented provisioning is commonly based on whether the path's statistical availability complies with the SLA-requested availability. Because of the stochastic nature of network failures, the actually provisioned availability over a specific time period is subject to uncertainty, and hence the SLA is usually at risk. We consider this uncertainty and study provisioning to minimize SLA violations. We show that the SLA Violation Risk is affected by a number of factors (e.g., failure profiles, availability target, and penalty period), and hence cannot simply be characterized by statistical path availability. We formulate the problem of risk-aware provisioning in WDM mesh networks, where path selection is dictated by SLA Violation Risk. In particular, we focus on devising an efficient scheme capable of computing path(s) that are likely to successfully accommodate the SLA-requested availability. A novel technique is applied to convert links with heterogeneous failure profiles to reference links that capture the main risk features in a relative manner. Based on the “reference link” concept, our Risk-Aware Provisioning scheme uses only limited failure information. We also extend our Risk-Aware Provisioning to use shared-path protection (SPP) for connections with strict availability requirements. We evaluate the performance and demonstrate the effectiveness of our schemes in terms of SLA violation ratio compared to the generic availability-aware approaches.

Passive Optical Networks (PONs) are experiencing their first evolutionary steps in order to support higher capacity. As high-bandwidth applications and services continue to emerge, it is expected that capacity upgrades for existing PON infrastructure will occur in the near future. In this paper, we address the upgrading problem of existing PONs that need to increase their capacity, in an “as-needed” fashion and at different points in time. We propose and investigate the characteristics of a method that upgrades network line-rates and enables migration of network services towards new wavelength channels based on increasing traffic demands and cost constraints. Our method is intended to minimize capital expenses and system disruptions, while ensuring optimal resource usage. To do so, we have designed a multi-step model based on Mixed Integer Linear Programming (MILP) and pricing policies. We consider a typical case study for this problem, which is solved using CPLEX. Results from our illustrative numerical examples demonstrate the aforementioned attractive properties of our method.

Internet customers may have strict resilience requirements for specific times periods. However, these time-differentiated resilience requirements are not effectively addressed by current SLA frameworks. To satisfy the resilience-sensitive periods, a generic SLA framework typically provides upgraded protection over the entire service duration, which is unnecessary and expensive. In this study, we propose a novel SLA framework, which allows customers to specify Critical Windows (CW) to address time-differentiated resilience demands. CWs correspond to the periods that require extra protection, and are backed up using path-level pre-cross-connected protection. Considering the complementary profiles of CWs, we identify the opportunity for backup resource sharing in the time domain. To exploit backup sharing, a heuristic scheme is proposed (Globally-CW-Aware connection assignment) to reduce backup resources. Our study shows that, using the proposed SLA framework: 1) low operational complexity can be achieved; 2) backup resources can be significantly reduced; and 3) high resilience (Service Level) can be achieved for CWs.

Since telecom networks consume a large (and increasing) amount of energy, "green" strategies are desirable to help Service Providers (SP) operate their networks and provision services more energy-efficiently. In this study, we focus on operating optical backbone networks with green strategies. We consider a typical optical backbone network architecture, and minimize the Operational Power for service provisioning following a Traffic Engineering (TE) approach. Service provisioning is schematically decomposed as multiple serial operations, and power efficiency is analyzed for both optical bypass and traffic grooming. We propose a novel auxiliary graph, which can capture the flow of operations and their associated power. Based on the auxiliary graph, we present a Power-Aware scheme that minimizes the Operational Power. Simulation results show reduced power consumption by our scheme, in comparison to a generic traffic grooming approach.

A Service Level Agreement (SLA) typically specifies the availability a Service Provider (SP) promises to a customer. In an Optical Transport Network, finding a lightpath for a connection is commonly based on whether the availability of a lightpath availability complies with the connection's SLA-requested availability. Because of the stochastic nature of network failures, the actual availability of a lightpath over a specific time period is subject to uncertainty, and the SLA is usually at risk. We consider the network uncertainty, and study routing to minimize the probability of SLA violation. First, we use a single-link model to study SLA Violation Risk (i.e., the probability of SLA violation) under different settings. We show that SLA Violation Risk may vary by paths and is affected by other factors (e.g., failure rate, connection holding time, etc.), and hence cannot be simply described by path availability. We then formulate the problem of risk-aware routing in mesh networks, in which routing decisions are dictated by SLA Violation Risk. In particular, we focus on devising a scheme capable of computing lightpath(s) that are likely to successfully accommodate a connection's SLA-requested availability. A novel technique is applied to convert links with heterogeneous failure profiles to reference links which capture the main risk features in a relative manner. Based on the "reference link" concept, we present a polynomial Risk-Aware Routing scheme using only limited failure information. In addition, we extend our Risk-Aware Routing scheme to incorporate shared path protection (SPP) when protection is needed. We evaluate the performance and demonstrate the effectiveness of our schemes in terms of SLA violation ratio and, more generally, contrast them with the generic availability-aware approaches.

Many networking applications require distribution of data from a central point to multiple destinations; this distribution can be efficiently achieved by the means of multicasting. Traditionally, multicasting has been considered for on-demand applications such as HDTV, Video-on-Demand (VoD), IPTV, which usually require to start data transmission immediately. However, in the case of emerging e-Science and high-performance applications (which frequently need to replicate large datasets to multiple locations), the data distribution does not necessarily need to take place instantaneously; instead, the multicast session can be accommodated considering a flexible start time for the large data transfer. We study the efficient provisioning of Multicast Data-Distribution Requests (MDDRs) with flexible scheduling over WDM networks. We consider the practical case of multicast sessions that may require less than the entire capacity of a wavelength; hence the multicast sessions need to be "traffic-groomed". Our first multicast provisioning approach (named Rand) generates randomized alternate multicast trees on which we try to provision the multicast session, and then attempts to assign wavelengths and schedule the session's start time. In our second approach (named AllSlots), for each available start time S, we dynamically generate trees depending on the network state at time S. In our next approach (named Break), for the cases when provisioning an entire multicast tree fails, we enable the possibility of "breaking" the tree into subtrees (with independent start times) serving subsets of destinations. Moreover, we study the impact of partitioning the datasets into pieces on our multicast provisioning approaches, and also compare our multicast algorithms with an unicast approach.

Most previous researches on patching scheme mainly focused on efficient patch stream generation to minimize the required bandwidth by proposing the optimum patching window. Another typical aspect is that they assumed that all the streaming speed is playback speed without deeply analyzing the effects of streaming speed. However, higher streaming speed, especially higher multicast streaming speed, may give better results in some aspects. Therefore, in this study, we analyze the effects of multicast streaming speed in the aspect of bandwidth usage, when VCR function is supported. First, we reanalyze the previous optimum patching window and total required bandwidth of the patching scheme, since the previous result is too approximate and give inaccurate results in some cases. So, we propose a modified optimum patching window to generate more accurate results when the speed of multicast streams is playback speed. Second, we calculate the optimum patching window and required bandwidth when the multicast streaming speed is multiple times the playback speed. The result shows that optimum patching window size reduces and required bandwidth increases when the speed of multicast stream increases. Finally, we show that the performance patching scheme degrades when VCR action is generated. The playback speed streaming of multicast streams gives good performance in bandwidth usage if VCR function is not considered. However, our analysis shows that the required bandwidth sharply increases when we consider VCR function support. The streaming speed of the multicast stream, which is equal to fast search (FS) streaming speed, gives the upper bound for the required bandwidth when FS function is considered.

Recently, new solutions for automatized management in optical networks promise to allow customers to specify on-demand the terms of the service level agreement (SLA) to be guaranteed by the service provider. In this paper we show that is possible to design a highly efficient load balancing algorithm, called RABBIT, for the dynamic provisioning of connections exploiting the knowledge, among the other service level specifications (SLS), of the connections duration. The core idea of RABBIT consists in routing connections based on the transient probability of future-link congestion, that can be estimated with higher precision when the knowledge of connections durations is given. So, we introduce a time-dependent link-weight assignment that evaluates future link congestions probability based on the transient analysis of the Markovian model of the link, making it computationally feasible by means of an effective approximation technique. By means of an extensive set of simulative experiments, we compare our approach to other traditional holding-time agnostic, yet efficient, dynamic routing algorithms. We consider different performance metrics, among which the blocking probability (BP), in a wavelength-convertible WDM mesh network scenario. For a typical US nationwide network, RABBIT obtains savings on BP of up to 20% for practical scenarios.

We study the problem of reliably provisioning traffic using multipath routing in a mesh network. Traditional approaches handled reliability requirements using full-protection schemes. Although full-protection approaches offer high assurance, this assurance can be costly. We take a less expensive approach to maintain reliability by offering partial-protection. Specifically, our approach guarantees part of the requested bandwidth, rather than the full amount, in the event of a link failure. We first show that the amount of partial-protection that can be guaranteed is limited by the topology of the network and the bandwidth requirement of a connection request. We then propose an effective multipath algorithm that attempts to provision bandwidth requests while guaranteeing the maximum partial-protection possible. Results show that by effectively selecting paths that limit edge overuse, our algorithm achieves very low bandwidth blocking probability. Our algorithm also serves significantly more requested bandwidth than the protection approach.

The hybrid wireless-optical broadband access network (WOBAN) architecture provides a new and promising architecture for access networks by combining the beneficial properties of wireless and optical technologies. Thus it can achieve low deployment costs (as no cable infrastructure is necessary for the last mile) and provide a mobile yet economically viable end-user access. However, the integration of these technologies (optical and wireless) also has its challenges: While optical links have high availabilities (especially with modern protection techniques), the performance of wireless links depends on a variety of external parameters, which in many cases can be only described statistically. In this work, we evaluate the availability performance of a WOBAN in different demand scenarios and study the influence of various routing strategies (shortest paths with different link metrics, specialised routing algorithms for a WOBAN and multi-path routing). Our results show that we can significantly improve availability for end-users by using shortest path routing with link unavailability as the link cost metric. Using multi-path routing, further gains can be achieved.

A service level agreement (SLA) typically specifies the availability a service provider (SP) promises to a customer. Current schemes usually employ backup resources to achieve high SLA satisfaction. We propose a new provisioning framework, called service cluster (SC), which uses no explicit backup resources. By grouping several services with (typically) different availability specifications, an SC can dynamically re-allocate resources to avoid SLA violations. Services that can tolerate additional down time lend resources to services that need to be kept running. We first analyze a condition for admission control. We then propose a dynamic resource allocation scheme (ADT1balancing scheme, SC-ABS) and apply it to the SC framework. The dynamic management of SC-ABS is realized by novel SLA-violation estimation in an event-driven manner without continuous monitoring. We compare SC-ABS with shared-path protection, and numerically show its various advantages in terms of: 1) higher SLA satisfaction (up to 30% more); 2) lower service blocking ratio; 3) higher tolerance of failures; 4) more balanced SLA satisfaction; and 5) consuming no explicit backup (or standby) resources. Our scheme can meet the SLA of significantly more services than protection-based schemes, thus providing more profit for the SP and lower cost for the customer.

Wireless communication provides network access free of the limitations of wired networks. However, some emerging applications with high bandwidth requirements and delay constraints, such as real-time video-on-demand and IPTV, suffer poor performance due to the high compression rate of video frames and/or high packet loss rate in the wireless access networks. In this paper, we propose a novel optimization algorithm referred to as the network state dependent video compression rate (NSDVCR) algorithm, which determines the compression rates depending on the video characteristics and the network condition. The proposed NSDVCR algorithm is able to achieve optimal video transmission rate for a given network state characterized by packet loss rate. The simulation results of the proposed mechanism show that significant improvement of the video quality measured in terms of peak-signal-to-noise ratio (PSNR), is achieved compared with standard compression mechanisms.

With the advances in optical technology, the span of a broadband access network using passive optical network (PON) technology can be increased from today's standard of 20 km to 100 km or higher, and thereby serve a lot more users. Such an extended-reach PON is known as SuperPON in the literature, and we call it a long-reach PON (LR-PON). A major challenge in LR-PON is that the propagation delay (for data as well as control signals) between the telecom central office (CO) and the end user is increased by a very significant amount. Now, traditional PON algorithms for scheduling the upstream transmission, such as dynamic bandwidth allocation (DBA) algorithms, may not be sufficient; actually, they may lead to degraded performance because of the long delay of the CO-to- Users "control loop." This challenge motivates us to propose and study a multi-thread polling algorithm to effectively and fairly distribute the upstream bandwidth dynamically. This algorithm exploits the benefits of having multiple polling processes running simultaneously and enabling users to send bandwidth requests before receiving acknowledgement from the CO. We compare the proposed algorithm with traditional DBA, and show its advantage on average packet delay. We then analyze and optimize key parameters of the algorithm, such as initiating and tuning multiple threads, inter-thread scheduling, and fairness among users. Numerical results demonstrate the algorithm's advantage to decrease the average packet delay and improve network throughput under varying offered loads.

Many large-scale scientific applications need to aggregate large amounts of data from multiple distributed sites to a centralized facility. We call such a request as a data-aggregation request (DAR). In this study, we investigate the novel problem of on-demand DAR provisioning over a wavelength-division multiplexing (WDM) backbone network. We provide a mathematical formulation for our problem as a mixed integer linear program (MILP). To solve large versions of our problem, we propose a DAR provisioning heuristic (called DARP). We use the MILP with various objectives as a benchmark for studying the performance of DARP.

We propose an efficient video adaptive streaming (VAST) Algorithm which can be used in FTTx broadband access networks. To serve video-on-demand (VoD) users, the algorithm uses a patching scheme to save on network bandwidth for video streaming, and increases the efficiency by adaptively changing the speed of patch streams and patching window based on time of the day and video popularity. To analyze the algorithm, we consider a 2.488-Gbps GPON network providing VoD service to 512 households. We use a daily video request model and a video popularity model to determine the adaptive streaming speed and dynamic patching window. Numerical results show that, when the available network bandwidth is reduced below the required level due to background traffic, the algorithm can considerably reduce the average user waiting time and the number of waiting requests.

We study the performance of TCP (transmission control protocol) over a wireless fading link using MIMO (multiple input multiple output) technology. MIMO technology using multiple transmit and receive antennas can potentially mitigate the effect of fading by providing diversity gain and/or increase the throughput by providing multiplexing gain. Given the number of transmit and receive antennas, there exists a tradeoff between the diversity and multiplexing gains. We study the effect of the so-called diversity-multiplexing tradeoff on the throughput of TCP. We demonstrate by cross-layer simulations involving the physical and transport layers that diversity gain is more useful to enhance TCP throughput when the signal-to-noise ratio (SNR) is low, and multiplexing gain is more useful when SNR is high. Our results indicate that there exist SNR levels at which switching from diversity-providing schemes to multiplexing schemes would provide enhanced TCP throughput. We propose a cross-layer rate-switching scheme to enhance TCP throughput over a wide range of SNR values.

We address the problem of ldquocapacity allocationrdquo in wireless mesh networks (WMNs). In the ldquocapacity allocationrdquo problem, the network topology (namely the desired wireless links) and routing are given. The problem then is to assign capacities to different links (which are carved out of the radio capacity of a wireless node) in such a way that the average network delay is minimized. This classic problem has been solved by Kleinrock for wired networks subject to a cost constraint. The problem of capacity allocation in wireless networks is different because of the following reasons: i) the capacity of a wireless radio is limited by the physical-layer technology; ii) wireless channel is a shared medium; and iii) the overall capacity of the wireless network is limited by interference. These unique features of a wireless network necessitates a cross-layer approach, involving the physical and the network layers, to solve this problem as opposed to a traditional network planning problem. We use queuing theory along with wireless interference modeling to present a cross-layer solution to this problem.

Noting that the optical part of a hybrid wireless- optical broadband access network (WOBAN) has high capacity, we need to enhance the capacity for wireless access using a low-cost solution. Our prior work developed a solution where each wireless node was equipped with a single radio. Deploying multiple radios, say two, at each node will improve the performance of wireless access, but this will also increase the cost of the solution. However, deploying multiple radios at only a few nodes, especially those that are overloaded with traffic, can lead to a less-costly solution, possibly without sacrificing performance. Hence, we study how to optimally place a limited number of additional radios at the wireless nodes to save the overall network cost. We formulate this problem as an Integer Linear Program (ILP) and solve it using a standard solver such as CPLEX. As expected, by deploying multiple radios at bottleneck wireless nodes, we can obtain almost the same performance as a WOBAN with multi-radios at all nodes.

Service level agreement (SLA) is a contract between the service provider (SP) and the customer. Among various parameters, SLA imposes penalties on the SP when the customer's quality-of-service (QoS) requirements are violated. Hence, it is desirable to avoid SLA violations and such penalties. SLA also states the maximum amount of downtime a connection may tolerate during its lifetime. In this study, we present a dynamic provisioning scheme called Preemption-Oriented SLA-Aware Provisioning (POSAP) for telecom mesh networks, which considers a connection's (1) availability requirements, (2) penalty, and (3) state. In addition to the connection's various parameters (availability, penalty, etc.), the amount of affordable downtime is used as a metric to determine the risk of violating the connection's SLA, and hence, its priority. Connection's priority is denoted by urgency level. Our scheme allows resource preemption among connections, i.e., high-priority connections may preempt backup resources from low-priority connections. Also, new connection requests may preempt resources from existing connections. Urgency level determines how resources can be preempted. Our scheme is applied to a US-wide network. Our results show the improved performance in terms of (1) reduced SLA violations, (2) increased network utilization, and (3) increased network revenue, compared to a dedicated primary-backup (P-B) scheme.

We investigate dynamic bandwidth allocation with flexible transmission rates for deadline-driven requests (DDRs) in wavelength-division multiplexing (WDM) mesh networks. DDRs provide more scheduling flexibility to the network operator compared with typical requests with no deadline and requiring a fixed bandwidth. By choosing the bandwidth and adapting the transmission rate depending on network state, the network's performance can be improved. We investigate several bandwidth allocation policies and study their benefits on network performance. Our investigation shows that an adaptive policy generally performs the best. Further improvement can be achieved by using dynamic readjustment of the allocated bandwidth.

With emerging applications in packet-transport networks such as IP-over-WDM networks (e.g., eBanking), packets need to be delivered within a bounded delay (i.e., given deadlines) with high probability. To satisfy such requirement, we propose an Inbuilt-Burstification Urgency-driven Scheduling (iBUS) Algorithm in IP-over-WDM networks. In this approach, packets are assembled into bursts according to their destinations by a preset timer. The timing burstification can reduce the amount of contentions, and support deadline constraints. Then all assembled bursts need to be scheduled efficiently with the deadline constraint. We introduce the concept of urgency degree as a scheduling metric. Bursts are scheduled according to their urgency degrees so that utilization of available network resources can be maximized under the constraint of deadlines. We also study a high-level variant of our algorithm,beta-iBUS, where the parameterbetais used to truncate the acceptable range of urgency degrees [0, 1] into [0,beta] (wherebetales 1) so that invalid scheduling caused by high urgency degrees can be minimized. Simulation experiments demonstrate that our proposal can achieve limited loss probabilities with the constraint of bounded delay.

We investigate the problem of adaptive reliable multipath provisioning in next-generation backbone mesh networks employing optical wavelength-division multiplexing (WDM) and channelization techniques such as SONET/SDH, and supporting virtual concatenation (VCAT). With VCAT, a connection request can be split, diversely routed, and inversely multiplexed on to multiple paths, a feature that has many advantages over conventional single-path provisioning, such as improved reliability, load balancing, etc. However, these diversely routed traffic components are subject to the differential delay constraints (DDC), which could be limited by the destination node's delay-compensation capability, network operations, administration, maintenance, and provisioning (OAM&P), or connection quality-of-service (QoS). We introduce a new notation M : N (m) for multipath provisioning where a service path for a connection is set up with M primary paths and N backup paths, where each path may have a fraction of the bandwidth of the connection, and (m) in this notation denotes "multipath'. We present the flexibility and benefits of multipath provisioning and develop an analytical model to analyze the connection availability under M : N (m) provisioning schemes. We propose two types of bandwidth migration methods, which can be implemented by link- capacity adjustment scheme (LCAS) protocol of next-generation SONET/SDH, to optimize resource usage. We develop two adaptive heuristic algorithms to provision a connection subject to DDC while satisfying its service-level agreement (SLA). We show that, for end-to-end connection-availability-guaranteed service, multi- path provisioning can achieve better network performance than traditional single-path provisioning. With bandwidth migration, we can further improve the performance.

We study how to achieve a desired connectivity in a wireless mesh network using beamforming antennas. We show that there is no unique solution to this problem. We propose a simple algorithm, directionality as needed (DAN), which strikes a good balance between the dual goals of minimum network design cost and minimum interference among different links.

Directional antennas are very attractive in wireless mesh networks (WMN). We study the problem of link scheduling and power control in a time-division multiple access (TDMA) WMN where the nodes use directional antennas. This is a cross- layer design problem spanning the physical and the link layers. Link scheduling in WMNs requires careful modeling of interference. Interference models used for omni-directional antennas cannot be used for directional antennas. We develop a generalized interference model applicable to directional antennas. Then, we use this model to formulate the link scheduling and power control problem as a Mixed Integer Linear Program. We also propose a heuristic algorithm to solve the problem efficiently.

Hybrid wireless-optical broadband access network (WOBAN) is a combination of wireless and optical networks to optimize the cost and performance of an access network. Wireless nodes collect traffic from end users and carry them to the optical part of a WOBAN using multiple hops, accumulating delay at each wireless node. Moreover, the radio capacity on each wireless link limits the capacity on each outgoing link from the node in a single-radio wireless mesh network (WMN) of a WOBAN. Thus, delay and capacity limitation in the WMN of a WOBAN is a major bottleneck. We design a capacity and delay aware routing scheme, CaDAR, to minimize the delay and increase network support in the WMN of a WOBAN. Our analysis shows that CaDAR is an efficient routing scheme for a single-radio WMN for a WOBAN that can support much higher load and has lower system delay than other approaches because of better load balanced routing.

In this paper, we investigate delay constrained multicast routing for supporting QoS guaranteed point to multi-point communications in IP over WDM networks. To achieve high bandwidth utilization, packets coming from different multicast connections are groomed and carried together over a single wavelength. Lightpath scheme is adopted in this paper that unicast lightpath is provisioned to support the multicast traffic in the IP network. Hop count constraint is introduced to deal with and queueing delay from traffic grooming. The challenge of the problem comes not only from considering delay constrained multicast routing but also WDM lightpath routing and wavelength assignment (RWA). We formulated the problem as an integer optimization problem in which the revenue from admitting multicast groups is to be maximized. The problem constraints include hop count constraint for end-to-end QoS requirements, tree constraint for multicast routing, IP link capacity and WDM fiber link capacity constraints, and wavelength continuity constraint. We apply Lagrangean relaxation technique to perform constraint relaxation and propose optimization-based heuristics (LGR) to tackle this problem. We draw performance comparisons between the LGR and the minimum hop (MH) heuristics. Numerical results demonstrate that LGR outperforms MH algorithm under all experimental cases.

Telecom mesh networks require periodic upgrades to support increasing traffic. Such upgrades, which are known as network engineering (NE), determine the network resources that must be provided to meet the network performance while minimizing the network cost. To handle traffic growth, we introduce a new parameter namely network-cut exhaustion probability. We develop a method to calculate this parameter and evaluate specific upgrade requirements of telecom mesh network.

Multipath provisioning is a key feature of next-generation SONET/SDH networks (which can be used on top of optical WDM) and they can support virtual concatenation (VCAT); thus, multipath provisioning can significantly outperform single-path provisioning in resource efficiency, service resilience, and flexibility. However, in multipath provisioning, differential delay is an important constraint which should not be ignored. We investigate survivability of service paths based on differential-delay constraint (DDC) and multipath provisioning together in a telecom backbone mesh network. We present a DDC-based K link-disjoint paths algorithm (DDCKDP) for multipath provisioning subject to DDC. We also compare it with the minimum-cost-flow (MCF) and K shortest link-disjoint paths (KDP) algorithm, using Shared Protection of the Largest Individual Traversed link (SPLIT), under dynamic service request with several different DDCs. We find that (1) exploiting link-disjoint paths is very efficient for survivable multipath provisioning; and (2) SPLIT-DDCKDP is resource efficient, has low signaling overhead, and has fast fault-recovery for survivable multipath provisioning with DDC. For a 5 ms DDC, DDCKDP can decrease the Bandwidth Blocking Ratio (BBR) by more than 100% compared with KDP in a typical US backbone network.

In the challenging social/geographical environment in Africa, how to design a robust network while tolerating the traffic imbalance is studied in this paper. We propose the Africa TWO network which connects all countries in Africa by employing two concentric rings. We investigate an Integer Linear Programming model and develop heuristic algorithms to deal with some special issues in Africa, such as a large desert in North Africa, high-speed network connectivity to the rest of the world through a few existing undersea fiber cables, and traffic imbalance caused by the differential populations of the various countries. To further exploit the properties of the designed network, we propose Little-Arc-First (LAF) routing algorithm, First-Matching (FM) wavelength assignment, and Sharing Peer Protection (SPP) scheme. The proposed algorithms LAF and FM are more efficient than existing algorithms. Sharing Peer Protection (SPP) scheme achieves high reliability in a cost- effective manner. We also address how to upgrade the network to achieve longer network lifetime for overall economic benefit. Our simulation experiments demonstrate that the concentric two-ring network is a natural candidate not only for the African continent, but can also be extended to other countries and regions.

SONET/SDH is historically the dominant telecom transport infrastructure for backbone networks, and it is optimized for reliable delivery of voice and private-line services. Efficient utilization of the existing infrastructure through novel methods and algorithms can lead to higher revenue, which is attractive for network operators. In this study, we examine a network of inter-connected SONET/SDH rings that use dual-node interconnection employing the drop-and-continue facility, which is the de-facto standard for interworking SONET/SDH protection architectures. We develop an efficient algorithm for provisioning in such a network while keeping fragmentation of capacity low and taking into account the various constraints imposed by the underlying physical layer. We examine the case of dual-node-interconnected ring networks employing both contiguous and virtual concatenation (a next-generation SONET/SDH architecture), and as expected, we discover that provisioning higher-bandwidth connections with virtual concatenation offers significant improvement. The stringent time-slot alignment and contiguity constraints imposed by contiguous concatenation are further compounded by the constraints imposed by the ring interworking architecture, and virtual concatenation allows capacity to be used more efficiently.

In a WOBAN, the back end is a wired optical network, the front end is managed by wireless connectivity, and, in between, the tail ends of the optical part [known as optical network unit (ONU)] communicate directly with wireless access points (AP). We study a WOBAN deployment scenario and investigate an algorithm to optimize the placement of multiple ONUs. To obtain some representative data on locations of typical wireless users, we have conducted a survey on the distribution and types of wireless routers in the Wildhorse residential neighborhood of North Davis, CA. We also formulate the multiple-ONU deployment problem using a combinatorial optimizer, viz., simulated annealing. Having found the suitable locations for ONUs, we compare the expenditures of a WOBAN vs. a wired access solution, namely Passive Optical Network (PON). To capture the challenges behind a complete WOBAN setup, we propose and investigate a joint optimization algorithm, which considers design aspects of both the wireless front end, such as avoiding interference among neighboring APs, and the optical back end, such as minimizing expensive fiber layout.

Survivability is a very important issue in telecom networks. Many previous works discuss availability-guaranteed and service-differentiated provisioning for survivable network design. Our work makes two new contributions. First, our work proposes a novel provisioning algorithm to explore the effect of link sharing among primary and backup paths. Second, the solution approach applies multi-backup protection scheme for those services which have extremely high availability requirements (relative to the network component availabilities). We develop a mathematical model to quantify a connection's availability with N (Nges1) backup paths with k-link (kges1) sharing between primary and backup paths. Based on the model, we propose a provisioning algorithm for dynamic connections with differentiated availability requirements. A connection can be either unprotected, or protected with N backup paths. The algorithm intelligently explores link sharing among primary and backup paths of a connection to target availability guarantee and resource efficiency. The illustrative numerical examples show that our scheme achieves better performance in both availability satisfaction and network resource utilization, compared to a traditional availability-constrained 1+1 provisioning strategy.

Progress in network technologies and protocols is paving the road towards flexible optical transport networks, in which dynamic leasable circuits could be set up and released on a short-term basis according to customers requirements. Recently, new solutions for automated network management promise to allow customers to dinamically specify the terms of the Service Level Agreement (SLA) to be guaranteed by the service provider. Since this new information is made available, we propose to exploit the knowledge of connection holding time, among the other Service Level Specifications (SLS), to improve the routing efficiency. In this work, we consider that a typical electronic-layer (e.g., SDH or MPLS) demand requires only a fraction of the capacity of the single wavelength bandwidth and we investigate a new algorithm for traffic grooming of sub-wavelength connections in an optical mesh network. We rely on the knowledge of the holding time of connection requests to exploit lightpath capacity and hence to achieve significant reduction in blocking probability for the traffic grooming problem. Our new methodology is applied on a typical US nation-wide network and results are compared with those given by previous known approaches.

In many e-science applications, there exists an important need to aggregate information from data repositories distributed around the world. In an effort to better link these resources in a unified manner, many lambda-grid networks, which provide end-to-end dedicated optical-circuit-switched connections, have been investigated. In this context, we consider the problem of aggregating files from distributed databases at a (grid) computing node over a lambda grid. The challenge is (1) to identify routes (that is, circuits) in the lambda-grid network, along which files should be transmitted, and (2) to schedule the transfers of these files over their respective circuits. To address this challenge, we propose a hybrid approach that combines offline and online scheduling. We define the Time-Path Scheduling Problem (TPSP) for offline scheduling. We prove that TPSP is NP-complete, develop a Mixed Integer Linear Program (MILP) formulation for TPSP, and then propose a greedy approach to solve TPSP because the MILP does not scale well. We compare the performance of the greedy approach on a few representative lambda-grid network topologies. One key input to the offline schedule is the file transfer time. Due to dynamics at the receiving end host, which is hard to model precisely, the actual file transfer time may vary. We first propose a model for estimating the file transfer time. Then, we propose online reconfiguration algorithms so that as files are transferred, the offline schedule may be modified online, depending on the amount of time that it actually took to transfer the file. This helps in reducing the total time to transfer all the files, which is an important metric. To demonstrate the effectiveness of our approach, we present results on an emulated lambda-grid network testbed.

Ethernet is a success story in Local Area Networks (LAN). Efforts for extending its boundaries beyond LAN to the carriers' backbone networks are in progress. We study the problem of designing reliable and cost-efficient high-rate (100 Gbit/s) carrier-grade Ethernet in a multi-line-rate telecom network under signal transmission-range constraints. Reliability is achieved using shared-path protection at two levels: (1) Protection-at-Connection (PAC) level, or (2) Protection-at-Lightpath (PAL) level. We study the two cases for their impact on network cost and other performance parameters. We construct a graph, called Mixed Topology (MT), using which it is possible to: (1) identify traffic grooming possibilities, (2) select a path which requires the minimum amount of 3R regeneration, and (3) effectively choose the data rate of a lightpath to be established. Our algorithms, tested on the 17-node German network, lead to the following findings: (1) for both PAL and PAC, our MT-based algorithm resulted in lower network cost and higher lightpath utilization compared with other schemes; and (2) in general, PAL incurs slightly higher cost than PAC.

We study the problem of reliably provisioning traffic in high-capacity backbone mesh networks supporting virtual concatenation (VCAT). VCAT enables a connection to be inversely multiplexed on to multiple paths, a feature that has many advantages over conventional single-path provisioning. We propose improved routing algorithms which use minimum- cost flow to find efficient collections of paths that satisfy the traffic requests. We first investigate the performance of our scheme under a uniform setting with symmetric traffic distribution and equal link capacities. We then apply our algorithm in a more realistic setting with asymmetric traffic and differing link capacities. Our algorithm is effective in both the uniform and non-uniform settings, and is much more effective than previously proposed schemes. Our study in the non-uniform setting is significant as it gives insight into the performance of our algorithm under more realistic scenarios.

With the development of optical technology, the span of a broadband access network using passive optical network (PON) technology can be increased from today's standard of 20 km to 100 km or higher. As a result, we have the long-reach (LR) PON which not only has extended reach, but which can also support a large base of users by employing wavelength-division multiplexing (WDM) for its data transmissions. However, a major challenge in the LR-PON is that the propagation delay (for data as well as control signals) between the telecom central office (CO) and the end user is increased by a very significant amount. Now, traditional PON algorithms for scheduling the upstream transmission, such as dynamic bandwidth allocation (DBA) algorithms, may not be sufficient; actually, they may lead to degraded performance because of the long delay of the "control loop" between the CO and the users. This challenge motivates us to investigate a multi-thread polling algorithm to distribute the upstream bandwidth dynamically in the LR-PON. In this study, we analyze key parameters of the algorithm, such as initiating and tuning multiple threads. We then demonstrate the algorithm's advantage to decrease the average packet delay under varying offered loads.

Hybrid wireless-optical broadband access network (WOBAN) is a promising architecture for future network operations. Recently, the wireless part of WOBAN has been gaining increasing attention and early versions are being deployed as a municipal access solution to eliminate the wired backhaul to every wireless router. This architecture saves on network deployment costs because fiber (or wiring) does not need to extend to the end user, and it extends the reach of emerging optical access solutions, e.g., passive optical network (PON)-based access solutions. However, a major research opportunity exists in developing an efficient routing algorithm for the wireless front end of WOBAN. We propose and investigate the characteristics of "delay- aware routing algorithm (DARA)" that minimizes the average packet delay in the wireless front end of a WOBAN. We model wireless routers as queues and predict wireless link states periodically. Our simulation experiments show that DARA achieves better load balancing and less congestion compared to tradional approaches such as minimum-hop routing algorithm (MHRA) and shortest- path routing algorithm (SPRA). In addition to minimizing the delay, DARA also improves the average hop count compared to the predictive throughput routing algorithm (PTRA), a popular protocol used in several deployments for the wireless front end of a WOBAN.

Multigranular optical WDM network aims to reduce network cost by grouping multiple wavelengths and then switching those wavelengths together at waveband or fiber levels. To configure such multi-granular network, we have two choices-homogeneous network and heterogeneous network. The former applies only a single type of optical cross connect (OXC) while the latter allows different types of OXCs. Due to the demands varies and change asymmetrically geographically, networks with heterogeneous OXC nodes is especially suitable for placing best switching types at different location. In this paper, we aim at the design of an algorithm to solve the network planning problem in optical network with heterogeneous multi- granularity OXCs. The planning program determines not only the switching granularity for each node but also determine the routing and wavelength assignments to satisfy the given demands. The contributions of the paper are four folded. First, we propose a graph model to represent the OXC node. The transformed graph simplifies the representation of node structure and helps to model the problem. Secondly, we propose a mathematical formulation to model the network planning problem as an ILP problem. Thirdly, a Lagrangean relaxation based heuristic algorithm is proposed to obtain a near optimal solution. Fourthly, we studied the impact of waveband size on network cost. This work reveals that the waveband size plays a crucial factor. The proposed algorithms can determine the optimal number of waveband size.

The concept of a hybrid wireless-optical broadband access network (referred to as WOBAN here) is a very attractive one. This is because it may be costly in several situations to run fiber to every home from the telecom central office (CO); also, providing wireless access from the CO to every end user may not be possible because of limited spectrum. Thus, running fiber as far as possible from the CO towards the end user and then having wireless access technologies take over may be an excellent compromise. How far should fiber penetrate before wireless takes over is an interesting engineering design and optimization problem, which our study is focussing on. We propose and investigate the characteristics of a "mixed integer programming (MIP)" model for optimum placements of base stations (BS) and optical network units (ONU) in a WOBAN (primal problem). We develop several constraints to be satisfied: BS and ONU installation, user and channel assignment, and signal-quality and interference constraints. To solve this MIP with reasonable accuracy, we use "Lagrangean relaxation" to obtain the corresponding "Lagrangean dual" problem. Via simulation experiments, we verify how sensitive is the placement problem with respect to a set of chosen metrics.

The 15 papers in this special issue focus on traffic engineering for multi-layer networks. The papers can be roughly divided into the following three groups: 1) New Approaches to Traffic Engineering, 2) Multi-layer Survivability, and 3) Specific Issues. Some of the specific issues include optimal topology design, multicast flow aggregation, load balancing, traffic grooming, and Ethernet-based technology.

Survivability is a key concern in modern network design in order to achieve fast service restorability against network failures. This paper investigates the problem of survivable dynamic connection provisioning in general telecom backbone networks, which are mesh structured. These networks employ optical fibers, which may fail due to network outages such as fiber cuts, etc. Our study applies to survivability of optical wavelength-division multiplexing (WDM) as well as multi-protocol label switching (MPLS) networks. For survivability study, we assume differentiated services where connections may have different availability requirements, so they may be provisioned differently with protection (if needed) based on their availability requirements and current network state. Therefore, it may be possible that connections with the same source, destination, and availability requirement are provisioned differently (unprotected, shared-path protected, or dedicated-path protected) at different times based on current network state. Such differentiated provisioning can provide diverse levels of service performance and achieve network resource optimization flexibly. Our main contributions are as follows. First, we develop an analytical model to quantify the availabilities of connections with various protection modes, i.e., unprotected, dedicated-path protected, and shared-path protected. Particular emphasis is placed on computing a connection's availability with shared-path protection by employing the link-vector technique, because this technique can maximally explore the sharing potential among backup paths and achieve bandwidth-assignment flexibility. Based on the mathematical model, we then present a novel provisioning strategy for dynamic connection requests in which multiple levels of services are provided and different protection schemes may be applied to different connections. The strategy jointly considers both connection availability satisfaction and resource optimization. Numerical results show very good accuracy of our model and high effectiveness of our provisioning strategy

Optical burst switching (OBS) is a promising technique for supporting high-capacity, bursty data traffic over optical wavelength-division-multiplexed (WDM) networks. An optical link failure may result in a huge amount of data (and revenue) loss, and it has been an important survivability concern in optical networks. In this paper, we study the fault- management issues related with a link failure in an OBS network. We propose to use pre-planned global rerouting to balance network load and to reroute bursts after a link fails. We apply optimization techniques to pre-plan explicit backup routes for failure scenarios. Our objective is to achieve optimal load balancing both before and after a failure such that the network state can still remain stable with minimum burst-loss probability when a failure occurs. We apply the pre-planned normal and backup routing tables to an OBS network, and study the network performance after a failure occur using illustrative numerical examples. The results show that the average burst-loss probability can be significantly reduced by 60% - from an average of 0.10 to 0.04 (when the normalized link load is less than 0.5) using globally-rerouted backup routes, when compared with the scheme without global rerouting. We also observe that the burst-loss probability is reduced by 43% - from an average of 0.07 to 0.04 (when the link load is less than 0.5) if the rerouting is done using optimization techniques, when compared with shortest-path routing.for Fault Management

Cost is a major concern in telecom industry. Cost-efficient network design has been attracting the attention of network operators (NO). Wavelength-division multiplexing (WDM) optical technology is being widely deployed in telecom networks for their high capacity. Backbone telecom networks, which are mesh-connected, employ optical crossconnects (OXC) at switching nodes; and OXC deployments today are of the opaque (optical-electronic-optical, OEO) variety. In such a network, the number of OXC ports is a major portion of the cost of the switching equipment (not counting the cost of the transmission equipment, which is needed anyway independent of the switching equipment). In this work, we focus on a practical situation in which a bundle of ports are fabricated on a single blade, called a line card (LC). If we are restricted to using only bundles of ports per LC, this will impose an extra constraint on cost-efficient network design, especially for asymmetric traffic between node pairs and arbitrary network topologies. Our design relies on a two-step approach using two algorithms whose objective is to reduce the number of LCs (ports). First, we route traffic connections on low-cost paths where cost depends on how much resources are needed to support the connection. The second step is a reassignment step, which tries to re-optimize the outcome of the first step. Simulation was conducted on a sample backbone network. Results show a reduction in cost (in terms of the number of deployed LCs) by nearly 6%.

In the challenging social/geographical environment in Africa, how to design a robust network while tolerating the traffic imbalance is the major goal in the paper. To achieve the goal, we propose and analyze the design of a symmetrical concentric two-ring network to connect all countries in Africa. We develop optimization schemes while dealing with some special issues in Africa, such as a large desert in North Africa, high-speed network connectivity to the rest of the world through a few undersea fiber cables, and traffic imbalance caused by the differential populations of the various countries. Our simulation experiments demonstrate that the concentric two-ring network design is a natural candidate for the African continent.

Survivability is a very important problem in telecom networks. Many previous works discuss availability-guaranteed and service-differentiated provisioning. Our work makes two new contributions. First, we consider link sharing between a primary path and backup path(s). In order to achieve better capacity utilization, a connection can share some highly reliable links in common among its primary and backup paths. Second, we consider 1+N (Nges2) protection schemes for those connections which have extremely high availability requirements or have always-on service quality demands. Multiple backup paths are provided to an availability-stringent connection, compared to the traditional 1+1 protection scheme which may have to block some high-availability connections. We develop a mathematical model to quantify a connection's availability with 1+N (Nges2) protection with k-link (kges1) sharing. Based on the model, we propose a provisioning algorithm for dynamic connections with differentiated availability requirements. A connection can be either unprotected, 1+1 protected, or 1+2 protected using our current model with the 1+N extension being an open problem. One-link sharing is allowed among primary and backup paths. The illustrative numerical results show that our approach achieves better performance in both blocking probability and resource utilization, compared to a traditional 1+1 provisioning strategy.

Progress in network technologies and protocols is paving the road towards flexible optical transport networks, in which leasable circuits could be set up and released on a short-term basis. Thus we consider it reasonable that, at least for some types of services, the holding time of connection requests can be known in advance. In this paper, we propose to exploit the knowledge of connection-holding time to improve the performance of an algorithm for shared-segment protection (SSP). For a typical US nationwide network, we compared our approach to an holding-time-unaware, but otherwise shared segmented efficient, approach, obtaining savings on resource overbuild of up to 7% for practical scenarios.

A cross-layer approach is investigated to improve data security in Mobile Ad Hoc Networks (MANETs). The use of directional antennas and intelligent multipath routing is proposed to enhance end-to-end data confidentiality and data availability with respect to outsider attacks. The goal is to impede rogue attempts to gain unauthorized access to classified information or disrupt the information flow. The interplay between the physical, link, and network layers is considered. A novel simulator is developed to accurately quantify the data confidentiality benefits of these approaches. This study leverages the existence of multiple paths between end-nodes to statistically improve data confidentiality and data availability in hostile MANET environments, where both insider and outsider adversaries may be present. Simulation results show that the proposed mechanisms can greatly improve data confidentiality as compared to existing schemes. These mechanisms can also improve data availability.

Survivability is a key concern in modern telecom mesh networks because of the enormous capacity of a telecom link which is usually an optical fiber employing wavelength-division multiplexing (WDM). Backup bandwidth reprovisioning has been shown to be an effective approach for improving network survivability as well as preventing existing services from unnecessary interruption. We investigate the advantages of reprovisioning new backup paths for connections when a previous failure is repaired (as well as when a network failure occurs). We consider reprovisioning of backup paths either (1) for unprotected or vulnerable connections that lose their primary or backup paths due to a previous failure and fail to be reprovisioned when the failure happens due to resource limits, or (2) for all existing connections in the network. The pros and cons of the two policies are investigated. A wavelength-convertible network and shared-path protection are assumed in this study. We compare the performance of our dynamic reprovisioning approach with a conventional scheme which reprovisions backup paths for connections only when a network failure occurs. The simulation results demonstrate that our approach achieves more network robustness and better backup capacity optimization.

A control-plane architecture for supporting advance reservation of dedicated bandwidth channels on a switched network infrastructure is described including the front-end web interface, user and token management scheme, bandwidth scheduler, and signaling daemon. A path computation algorithm for bandwidth scheduling is proposed based on an extension of Bellman-Ford algorithm to an algebraic structure on sequences of disjoint non-negative real intervals. An implementation of this architecture for UltraScience Net is briefly described.

Next-generation e-science applications will require the ability to transfer information at high data rates between distributed computing centers and data repositories. To support such applications, lambda grid networks have been built to provide large, on-demand bandwidth between end-points that are interconnected via optical circuit-switched lambdas. It is extremely important to develop an efficient transport protocol over such high-capacity, dedicated circuits. Because lambdas provide dedicated bandwidth between endpoints, they obviate the need for network congestion control. Consequently, past research has demonstrated that rate-based transport protocols, such as RBUDP, are more effective than TCP in transferring data over lambdas. However, while lambdas eliminate congestion in the network, they ultimately push the congestion to the endpoints - congestion that current rate-based transport protocols are ill-suited to handle. In this paper we introduce a "rate-adaptive protocol for intelligent delivery (RAPID)" of data that is lightweight and end-system performance-aware, so as to maximize end-to-end throughput while minimizing packet loss. Based on self monitoring of the dynamic task-priority at the receiving end-system, our protocol enables the receiver to proactively deliver feedback to the sender, so that the sender may adapt its sending rate to avoid congestion at the receiving end-system. This avoids large bursts of packet losses typically observed in current rate-based transport protocols. Over a 10-Gigabit link emulation of an optical circuit, RAPID reduces file-transfer time, and hence improves end-to-end throughput by as much as 25%.

The Passive Optical Network (PON) is an attractive solution for high-bandwidth access networks. In the context of a broadband access network, the term open access implies the ability of multiple service providers to share the deployed access network infrastructure to make services available to the end users. Multiple services may thereby be delivered over a shared access channel. Open access requires fairness in terms of throughput, delay, jitter, and other network parameters in the access channel among the sharing entities, namely service providers and end users. Since the traffic in an access network is very bursty, an access network may be frequently subjected to high loads for certain durations of time. Meeting the above fairness requirements under such conditions is therefore very challenging. In this study, we first motivate the problem of meeting fairness requirements simultaneously to both service providers and users, which are located at opposite ends of an access channel. We then investigate the importance of two different sets of Service-Level Agreements (SLAs), which we call Dual SLAs. After formulating a mathematical model, we propose an efficient scheduling algorithm to meet Dual SLAs which is based on the well-known concept of max-min fairness. We then demonstrate the effectiveness of our proposed algorithm through simulations using a discrete-event-simulator-based PON set-up, which compares the fairness of the Dual-SLA scheduling algorithm with that of other traditional fair queuing algorithms such as Deficit Round Robin (DRR)

As networks grow in size and complexity, both the probability and the impact of failures increase. The pre-allocated backup bandwidth, which has been widely investigated in the literature, may not be able to provide full protection guarantee when multiple failures occur in a network. In this study, we consider multiple concurrent failures where concurrent means that a new failure occurs before a previous failure is repaired. To combat the effect of multiple concurrent failures, new backups can be reprovisioned after one failure such that the next potential failure can be handled effectively and efficiently. We consider dynamic traffic where a pair of link-disjoint primary and backup paths is provisioned when a new connection request arrives. After a failure occurs, the affected connections switch traffic from their primary paths to backup paths. To protect against next potential failure, we reprovision new backups for connections that become unprotected or vulnerable because of losing their primary or their backup due to the previous failure or due to backup resource sharing. This approach is called Minimal Backup Reprovisioning (MBR). An alternative approach is to globally rearrange backups for all connections after one failure occurs, which is called Global Backup Reprovisioning (GBR). Backup reprovisioning can be performed whenever the network's state changes, e.g., (1) when a new request arrives, (2) when an existing connection terminates, (3) when a network failure occurs, (4) when a failed link/node is repaired, etc., to utilize the available resources more efficiently or to recover quickly from the next failure. In this study, we perform MBR or GBR after one network failure occurs to protect against the next potential failure in a wavelength-convertible WDM mesh network. The link-vector network model which can maximally explore the backup-sharing potential is assumed in this study. We then analyze the complexity of MBR and GBR under such a network model. A reprovisioning algorithm is proposed for MBR which can significantly reduce the connection vulnerability without the knowledge of the location of the next failure. In GBR, both integer linear program (ILP) and heuristic-based approaches are proposed. We compare capacity requirement and computational complexity of MBR to that of GBR through numerical examples. MBR demonstrates a good tradeoff between complexity and capacity efficiency to handle multiple concurrent failures

Location update/paging strategies have been widely studied in the traditional single-tier cellular networks. We propose and evaluate a novel crossing-tier location update/paging scheme that can be used in a hierarchical macrocell/microcell cellular network. Location update is proceeded only in the macrocell tier, where a location area (LA) is made up by larger macrocells. A mobile user will stay in such a LA for longer time. Therefore, the cost on location update can be reduced due to the decreased frequency of location update. To reduce the paging delay, the paged mobile user will be searched in the macrocell tier only when the paging load is not high. Otherwise, it will be searched in the microcell tier, where a sequential searching method is applied. The operation for the scheme is simple, as the macrocell/microcell cellular network has the advantage because a mobile user can receive a signal from both a microcell and the overlaid macrocell. Analytical models have been built for cost and delay evaluation. Numerical results show that, at relatively low cost, the crossing-tier scheme also achieves low paging delay.

Target tracking in wireless sensor networks requires efficient coordination among sensor nodes. Existing methods have focused on tree-based collaboration, selective activation, and group clustering. This paper presents a prediction-based adaptive algorithm for tracking mobile targets. We use adaptive Kalman filtering to predict the future location and velocity of the target. This location prediction is used to determine the active tracking region which corresponds to the set of sensors that needs to be "lighted". The velocity prediction is used to adaptively determine the size of the active tracking region, and to modulate the sampling rate as well. In this paper, we quantify the benefits of our approach in terms of energy consumed and accuracy of tracking for different mobility patterns. Our simulation results show that advance resource reservation coupled with adaptively changing the size of the active tracking region and the sampling rate reduces the overall energy consumed for tracking without affecting the accuracy in tracking.

Optical burst switching (OBS) is a promising solution to implement the optical Internet backbone. However, the lack of adequate congestion-control mechanisms may result in high burst loss. Schemes such as fiber delay line (FDL), wavelength conversion, and deflection routing to reduce burst collision are unable to prevent the network congestion effectively. To address this problem, we propose and investigate a global solution, called integrated congestion-control mechanism (ICCM), for OBS networks. ICCM, which combines congestion avoidance with recovery mechanism, restricts the amount of burst flows entering the network according to the feedback information from core routers to edge routers to prevent network congestion. Also, a flow-policing scheme is proposed to intentionally drop the overloaded traffic with a certain probability at a core router to support fairness among flows. Moreover, the transmission rate of each flow is controlled to achieve optimized performance such as maximizing throughput or minimizing loss probability using a two-step rate controller at the edge router. Simulation results show that ICCM effectively eliminates congestion within the network and that, when combined with a flow-policing mechanism, the fairness for competing flows can be supported while maintaining effective network performance.

Ethernet passive optical networks (EPONs) are an attractive solution for meeting the broadband access requirements of residential users. Open access is a regulatory requirement in many countries which mandates that the access infrastructure must be open to various service providers for free competition. Thus, different service providers may cater services to same or different users using the same shared access channel. Open access demands fairness in the use of network resources among sharing entities, namely service providers and end users. In this study, we investigate scheduling algorithms for fair bandwidth sharing in the context of open access for EPONs.

This study considers reducing cost to ISPs by allowing them to dynamically lease only the required amount of bandwidth from network operators in order to satisfy their customers' traffic needs. Network operators can also generate additional revenue from this approach by leasing out the spare bandwidth to time-of-the-day-insensitive applications. We study a virtual-topology adaptation model for the ISP based on minimizing its total dollar cost for running its network. Relative to the flat-rate model (where the ISP's topology is fixed based on worst-case traffic), our minimum-cost virtual-topology adaptation method can yield significant cost savings of over 21% (based on a US ISP network employing realistic traffic and bandwidth costs). Our optimization approach is based on a mixed integer linear program (MILP) which nominally adds or deletes one link (lightpath) in the ISP's virtual topology at a time after a certain observation period, which is a few hundreds of seconds.

We investigate the problem of reliable multi-path provisioning of traffic in high-capacity optical backbone mesh networks, e.g., next-generation SONET/SDH networks supporting virtual concatenation (VCAT). VCAT enables a connection to be inversely multiplexed on to multiple paths. As a result, a connection can be provisioned more flexibly, and such a multi-path provisioning approach may lead to significantly improved performance over the conventional single-path provisioning approach. This multi-path provisioning approach may also be applicable to other mesh networks such as those employing optical wavelength-division multiplexing (WDM) and multi-protocol label switching (MPLS), where the bandwidth of a connection to be provisioned exceeds the available capacity on any path based on current network state. We propose effective multi-path bandwidth as the metric to provision a connection on to multiple paths while satisfying its reliability requirements (measured in terms of availability). We show that effective multi-path bandwidth provides more flexibility and lower blocking probability without the cost and complexity associated with traditional protection schemes developed for optical WDM and MPLS networks.

Optical burst switching (OBS) is a promising technique for supporting high-capacity bursty data traffic over optical wavelength-division-multiplexed (WDM) networks. A label-switched path can be established to forward a burst control packet (BCP) if each OBS node is augmented with an IP/MPLS controller. Such a network is called a labeled OBS (LOBS) network, and it can exploit the explicit routing and constraint-based routing properties supported in the MPLS framework to perform traffic and resource engineering. However, the burst-loss probability (denoted as BLP) can be large if the traffic is not properly engineered in a LOBS network, especially after a failure occurs. In this paper, we propose to use pre-planned global rerouting to balance network load and to restore bursts after a link fails. We apply optimization techniques to pre-plan explicit backup routes for failure scenarios. Our objective is to achieve optimal load balancing both before and after a failure such that the network state can still remain stable with minimum BLP when failure occurs. We apply the pre-planned global rerouting method in a LOBS network and study the performance of different rerouting schemes on some typical network topologies. Our illustrative numerical examples show that the BLP can be significantly reduced by 25%-99% (when the average link load is less than 0.5) using globally rerouted backup routes, when compared with the scheme without global rerouting. We also observe that the BLP is reduced by 20%-65% (when the average link load is less than 0.5) if the rerouting is done using optimization techniques, when compared with shortest-path routing.

The problem of aggregating large data files from distributed databases and address the corresponding challenges involved from a network architecture perspective is considered. We model this problem as one of identifying a time-path schedule (TPS) in a graph representation of the network. We prove that the TPS problem (TPSP) is NP-complete. We then propose a mixed integer linear programming (MILP)-based approach and three heuristics longest-file-first (LFF), disjoint-paths (DP), and most-distant-file-first (MDFF) - to solve TPSP.

This paper investigates a generalized protection framework for availability-guaranteed connection provisioning in an optical wavelength-division multiplexing (WDM) mesh network. We develop a link-state-modeling mechanism to form a dynamic link-state parameter, called link and resource availability (LRA). Such link-state information can be used by a standard link-state routing protocol to efficiently provision reliable connections. Based on the LRA parameter, we then propose a connection-provisioning algorithm which can guarantee customers' availability requirements. A new generalized protection model is developed through our dynamic LRA-based provisioning. Numerical results demonstrate the performance of the proposed provisioning approach to be promising.

As networks grow in size and complexity, both the probability and the impact of failures increase. The preallocated backup bandwidth cannot provide 100% protection guarantee when multiple failures occur in a network. In this study, we consider multiple concurrent failures where concurrent means that a failure occurs before the previous failure is physically repaired, and we present a comprehensive study on backup reprovisioning to combat the effect of multiple-link failures. The basic idea is to reprovision new backups for connections that become unprotected or vulnerable for the next possible failure, due to losing the primary or the backup in the first failure or due to backup resource sharing. The pros and cons of the backup-reprovisioning approach are extensively discussed. A generalized network model which can maximally explore the backup-sharing potential is assumed in this study. We then discuss the complexity of backup reprovisioning under such a network model. A reprovisioning algorithm is proposed which can significantly reduce the connection vulnerability without the knowledge of the location of next failure. The effectiveness of our reprovisioning algorithm is demonstrated through numerical examples.

Reconfigurable OADMs (ROADMs) provide flexibility and enables fast provisioning of dynamic traffic in ring-based metro optical WDM networks. We investigate the online connection provisioning and propose heuristics to combat various constraints in this network.

Localization and clustering of sensor nodes are important services in a sensor network since the nodes are typically deployed in an ad-hoc manner into an infrastructure-less terrain. When beacons are used for localization, there are two critical design issues: 1) to maximize the lifetime of the beacons and 2) to maximize the coverage area. With clustering, the goal is to minimize the energy dissipation of the sensor network. In this paper, we consider the placement of beacons and data loggers (that act as cluster heads) in the Cosumnes River Preserve, which is a joint collaborative restoration project between the Cosumnes Research Consortium at University of California at Davis (UCD) and The Nature Conservancy. Currently, there are many types of sensors deployed in the preserve which are wired to data loggers. Our objective is to determine the minimum number and placement of beacons and data loggers for wireless sensors deployed in the preserve. We formulated an optimization problem which is solved by integer linear program (ILP).

In this paper, we propose and investigate the characteristics of a fair queueing with service envelopes (FQSE) algorithm-a hierarchical fair-share scheduling algorithm for access networks based on a remote scheduling system such as Ethernet passive optical networks (EPON) or cable TV network. FQSE is designed to overcome the limiting factors of a typical remote scheduling system such as large control-plane delay, limited control-plane bandwidth, and significant queue switch-over overhead. The algorithm is based on a concept of service envelope-a function representing the fair allocation of resources based on a global network condition called satisfiability parameter (SP). We define properties of cousin-fairness and sibling-fairness and show the FQSE to be cousin-fair. FQSE is unique in that it is the only hierarchical algorithm that is simultaneously cousin-fair. Furthermore, we show the necessary techniques to adapt FQSE to variable-sized packet-based networks. We analyze FQSE performance in EPON serving 1024 independent queues and demonstrate FQSE's ability to provide guaranteed bandwidth to each queue and to share the excess bandwidth fairly.

This paper investigates survivable lightpath provisioning and fast protection switching for generic mesh-based optical networks employing wavelength-division multiplexing (WDM). We propose subpath protection, which is a generalization of shared-path protection. The main ideas of subpath protection are: 1) to partition a large optical network into smaller domains and 2) to apply shared-path protection to the optical network such that an intradomain lightpath does not use resources of other domains and the primary/backup paths of an interdomain lightpath exit a domain (and enter another domain) through a common domain-border node. We mathematically formulate the routing and wavelength-assignment (RWA) problem under subpath protection for a given set of lightpath requests, prove that the problem is NP-complete, and develop a heuristic to find efficient solutions. Comparisons between subpath protection and shared-path protection on a nationwide network with dozens of wavelengths per fiber show that, for a modest sacrifice in resource utilization, subpath protection achieves improved survivability, much higher scalability, and significantly reduced fault-recovery time.

The problem of how to design a survivable optical network under single and multiple link failures has been studied quite extensively; however, there are few studies on the design of optical networks that could survive single node failures. We design a dynamic wavelength routing scheme that will set up a primary path as well as a node-disjoint shared protection path for each connection using as few wavelength channels as possible by allowing multiple protection paths to share wavelength channels. Sharing full link state information among all network nodes will incur heavy processing overhead and increase routing traffic significantly. We propose a novel scheme that uses three vectors to convey incomplete information of link state. The scheme can be implemented by extending the OSPF protocol. As it is important to network service provider, we also compare the capacity requirement between link failure protection scheme and node failure protection scheme.

We propose a hybrid optical-electrical switch architecture which integrates all-optical waveband switching and electrical time-division-multiplexed (TDM) switching. By grouping pass-through traffic into wavebands and switching them all-optically, the hybrid architecture can significantly reduce the size of electrical switch and number of associated transponders. As a result, it provides tens of terabits per second switch throughput with a small footprint. The proposed architecture is capable of TDM switching, wavelength conversion, and multicasting. In this paper, we investigate the network-optimization problem with static traffic to minimize the overall network switch cost. A mathematical formulation of the optimization problem and its solutions are presented. Fast heuristic approaches for near-optimal solutions are also proposed and evaluated.

In this paper, we present a connection-provisioning framework to satisfy customers' availability requirements using appropriate protection schemes. The framework contains two pats: (a) WDM mesh network service availability analysis; and (b) a connection-provisioning approach using the analysis. We present the availability analysis for connections with different protection schemes (i.e., unprotected, dedicated, or shared protected) and propose an integer linear program (LIP) based provisioning approach for static traffic. We verify the theoretical availability analysis through simulations, and demonstrate the effectiveness s of our provisioning approach using numerical examples.

This paper investigates the problem of dynamic shared-path-protected lightpath provisioning in optical mesh networks employing wavelength-division multiplexing (WDM). We prove that the problem of finding an eligible pair of working and backup paths for a new lightpath request requiring shared-path protection under the current network state is NP-complete. We develop a heuristic, called CAFES, to compute a feasible solution and an algorithm, called OPT, to optimize resource consumption for a given solution. The merits of our approaches are that they capture the essence of shared-path protection and approach to optimal solutions without enumerating paths. We evaluate the effectiveness of our heuristics and the results are found to be promising.

In this report, we investigate approaches and algorithms for establishing a multicast session in a mesh network while protecting the session against any single link failure, e.g., a fiber cut in an optical network. We propose two new and efficient approaches for protecting a multicast session: 1) segment protection in which we protect each segment in the primary tree separately (rather than the entire tree) and allow these backup segments to share arcs with the other existing primary and backup segments; and 2) path-pair protection in which we protect a path between each source-destination pair by finding a disjoint backup path. Unlike previous schemes such as finding link-disjoint trees and arc-disjoint trees, our new schemes 1) guarantee a solution where previous schemes fail and 2) find efficient solution requiring less network resources. Our algorithm, based on the path-pair protection scheme, called optimal path-pair-based shared disjoint paths (OPP-SDP) algorithm, finds a solution if such a solution exists and outperforms all the other schemes in terms of network cost. We also show that OPP-SDP performs close to the optimal solution obtained by solving a mathematical formulation of the problem expressed as an integer linear program (ILP).

We investigate the survivable traffic-grooming problem for optical mesh networks employing wavelength-division multiplexing (WDM). In the dynamic provisioning context, a typical connection request may require bandwidth less than that of a wavelength channel, and it may also require protection from network failures, typically fiber cuts. Based on a generic grooming-node architecture, we propose three approaches for grooming a connection request with shared protection: protection-at-lightpath level (PAL); mixed protection-at-connection level (MPAC); separate protection-at-connection level (SPAC). In shared-mesh protection, backup paths can share resources as long as their corresponding working paths are unlikely to fail simultaneously. These three schemes explore different ways of backup sharing, and they trade-off between wavelengths and grooming ports. Since the existing version of the problem for provisioning one connection request with shared protection is NP-complete, we propose effective heuristics. Under today's typical connection-bandwidth distribution where lower bandwidth connections outnumber higher bandwidth connections, we find the following: 1) it is beneficial to groom working paths and backup paths separately, as in PAL and SPAC; 2) separately protecting each individual connection, i.e., SPAC, yields the best performance when the number of grooming ports is sufficient; 3) protecting each specific lightpath, i.e., PAL, achieves the best performance when the number of grooming ports is moderate or small.

This paper investigates the characteristics and performance of different optical grooming switches, i.e., optical cross-connects (OXCs) capable of traffic grooming, under a dynamic traffic environment. We present four optical grooming-OXC architectures, namely, single-hop grooming OXC, multihop partial-grooming OXC, multihop full-grooming OXC, and light-tree-based source-node grooming OXC. After exploring their grooming capabilities, we propose three grooming schemes and two corresponding algorithms, grooming using auxiliary graph and grooming using light-tree. Through the algorithms, we evaluate the performance of different optical grooming OXCs in a dynamic traffic environment under different connection bandwidth-granularity distributions. Our investigation uncovers the following results: 1) the multihop full-grooming OXC can achieve the best network performance, but it may encounter cost and scalability constraints; 2) by using significantly less low-granularity electronic processing and intelligent traffic-grooming algorithms, the multihop partial-grooming OXC shows reasonable network performance and, hence, can be viewed as a cost-effective alternative when a network node does not require full-grooming capability; 3) the single-hop grooming OXC may cause a large amount of capacity waste and lead to poor network performance; and 4) through its multicast capability, a light-tree-based source-node grooming OXC can significantly out-perform the performance of a single-hop grooming OXC in terms of network throughput and network resource efficiency. From our results, we also observe that the connection bandwidth-granularity distribution has a large impact on network throughput and network resource efficiency and, therefore, should be carefully considered for network design and traffic provisioning.

We investigate the design of a WDM backbone network with optical cross-connects (OXCs) of different switching granularities to reduce the network-wide OXC port cost. We enhance our proposed graph model (Zhu, H. et al., IEEE/ACM Trans. Networking, vol.11, p.285-99, 2003), and the extended graph model can represent different node architectures in which a node may have multiple OXCs with different switching granularities simultaneously. Based on this model, we propose a provisioning algorithm for a single connection and a framework for network design, which can intelligently determine the type of OXCs at each node according to the traffic so that the benefit of different types of OXCs can be utilized. Numerical examples are presented showing that granularity-heterogeneous networks are more cost-effective than granularity-homogeneous networks.

We present a new approach to the virtual-topology reconfiguration problem for a wavelength-division-multiplexing- based optical wide-area mesh network under dynamic traffic demand. By utilizing the measured Internet backbone traffic characteristics, we propose an adaptation mechanism to follow the changes in traffic without a priori knowledge of the future traffic pattern. Our work differs from most previous studies on this subject which redesign the virtual topology according to an expected (or known) traffic pattern, and then modify the connectivity to reach the target topology. The key idea of our approach is to adapt the underlying optical connectivity by measuring the actual traffic load on lightpaths continuously (periodically based on a measurement period) and reacting promptly to the load imbalances caused by fluctuations on the traffic, by either adding or deleting one or more lightpath at a time. When a load imbalance is encountered, it is corrected either by tearing down a lightpath that is lightly loaded or by setting up a new lightpath when congestion occurs. We introduce high and low watermark parameters on lightpath loads to detect any over- or underutilized lightpath, and to trigger an adaptation step. We formulate an optimization problem which determines whether or not to add or delete lightpaths at the end of a measurement period, one lightpath at a time, as well as which lightpath to add or delete. This optimization problem turns out to be a mixed-integer linear program. Simulation experiments employing the adaptation algorithm on realistic network scenarios reveal interesting effects of the various system parameters (high and low watermarks, length of the measurement period, etc.). Specifically, we find that this method adapts very well to the changes in the offered traffic.

This study investigates the problem of fault management in a wavelength-division multiplexing (WDM)-based optical mesh network in which failures occur due to fiber cuts. In reality, bundles of fibers often get cut at the same time due to construction or destructive natural events, such as earthquakes. Fibers laid down in the same duct have a significant probability to fail at the same time. When path protection is employed, we require the primary path and the backup path to be duct-disjoint, so that the network is survivable under single-duct failures. Moreover, if two primary paths go through any common duct, their backup paths cannot share wavelengths on common links. This study addresses the routing and wavelength-assignment problem in a network with path protection under duct-layer constraints. Off-line algorithms for static traffic is developed to combat single-duct failures. The objective is to minimize total number of wavelengths used on all the links in the network. Both integer linear programs and a heuristic algorithm are presented and their performance is compared through numerical examples.

As the operation of our fiber-optic backbone networks migrates from interconnected SONET rings to arbitrary mesh topology, traffic grooming on wavelength-division multiplexing (WDM) mesh networks becomes an extremely important research problem. To address this problem, we propose a new generic graph model for traffic grooming in heterogeneous WDM mesh networks. The novelty of our model is that, by only manipulating the edges of the auxiliary graph created by our model and the weights of these edges, our model can achieve various objectives using different grooming policies, while taking into account various constraints such as transceivers, wavelengths, wavelength-conversion capabilities, and grooming capabilities. Based on the auxiliary graph, we develop an integrated traffic-grooming algorithm (IGABAG) and an integrated grooming procedure (INGPROC) which jointly solve several traffic-grooming subproblems by simply applying the shortest-path computation method. Different grooming policies can be represented by different weight-assignment functions, and the performance of these grooming policies are compared under both nonblocking scenario and blocking scenario. The IGABAG can be applied to both static and dynamic traffic grooming. In static grooming, the traffic-selection scheme is key to achieving good network performance. We propose several traffic-selection schemes based on this model and we evaluate their performance for different network topologies.

In a WDM optical network, the bandwidth requirement of a customer's connection can vary over a wide range, and many of these connections could have a capacity that is much lower than the capacity of a wavelength channel. Efficiently grooming low-speed connections onto high-capacity wavelength channels can significantly improve the bandwidth utilization and minimize the network cost. Our research shows that it is not necessary to have traffic-grooming capability at every network node. We call a network which has only a few grooming nodes to be a sparse-grooming network. Through proper network design and traffic engineering, it is possible for a sparse-grooming network to achieve similar network performance as a network which has grooming capability at every node. We investigate the problem of designing such a sparse-grooming WDM mesh network. The problem is mathematically formulated and several design schemes are proposed. Illustrative numerical results from the mathematical formulation as well as heuristics show that, by properly choosing the grooming nodes, a network with sparse-grooming capability can achieve good network performance and the network cost can be significantly reduced.

We employ a new, generic graph model for dynamic traffic grooming in WDM mesh networks. The novelty of this model is that, by only manipulating the edges of an auxiliary graph created by the model and the weights of these edges, the model can achieve various objectives using different grooming policies, while taking into account various constraints. Based on the auxiliary graph, we develop a dynamic traffic-grooming algorithm. Different grooming policies can be implemented by different weight functions assigned to the edges in the auxiliary graph. We propose four fixed grooming policies and an adaptive grooming policy (AGP), and our results show that AGP outperforms the fixed grooming policies.

Summary form only given. Today, the telecommunication industry is experiencing "a nuclear winter". It is a challenging proposition right now to debate research and development (R and D) priorities for optical networking. However, even though the telecommunication market is unsettled today, we need to be ready with appropriate technological solutions to meet the growing bandwidth needs of our information society. Optical networking - using wavelength-division multiplexing (WDM) - is the technology for meeting these demands. While there is a glut of dark fiber and WDM transmission capacity today, we believe that there will be a tremendous need for optical switching equipment for managing high-capacity optical signals. This talk first makes the case for the important role software plays in bringing cost-effective and intelligent optical networking to the marketplace. After providing an overview of telecommunication networks, we discuss promising R and D challenges for optical networking in access and metropolitan areas. Then, we examine the near-term R and D challenges for backbone optical networks: namely, dynamic provisioning of high-capacity connections of different bandwidths, fault management, and topology engineering. Finally, we state R and D priorities over a longer-term horizon.

We present a new approach to the virtual-topology reconfiguration problem for wavelength-routed, optical wide-area networks under dynamic traffic demand. By utilizing the measured Internet backbone traffic characteristics, an adaptation mechanism is proposed to follow the changes in traffic without assuming that the future traffic pattern is known. In that sense, our work differs from previous studies which redesign the virtual-topology according to an expected (or known) traffic pattern, and then modify the connectivity to reach the target topology. The key idea of our approach is to adapt the underlying optical connectivity by measuring the actual traffic load on lightpaths continuously (periodically based on a measurement period) and reacting promptly to the imbalances caused by fluctuations in the traffic by adding or deleting one lightpath at a time. We aim to correct the encountered load imbalance directly, either by tearing down a lightpath that is lightly loaded or by setting up a new lightpath when congestion occurs. We introduce high and low watermark parameters on lightpath loads to detect any over/underutilized lightpath, and to trigger an adaptation step. The adaptation method is evaluated through simulations and the effect of system parameters (high and low watermarks, length of the measurement period) are investigated.

We consider an IP-over-WDM network in which network nodes employ optical crossconnects and IP routers. Nodes are connected by fibers to form a mesh topology. Any two IP routers in this network can be connected together by an all-optical wavelength-division multiplexing (WDM) channel, called a lightpath, and the collection of lightpaths that are set up form a virtual topology. In this paper, we concentrate on single fiber failures, since they are the predominant form of failures in optical networks. Since each lightpath is expected to operate at a rate of few gigabits per second, a fiber failure can cause a significant loss of bandwidth and revenue. Thus, the network designer must provide a fault-management technique that combats fiber failures. We consider two fault-management techniques in an IP-over-WDM network: (1) provide protection at the WDM layer (i.e., set up a backup lightpath for every primary lightpath) or (2) provide restoration at the IP layer (i.e., overprovision the network so that after a fiber failure, the network should still be able to carry all the traffic it was carrying before the fiber failure). We formulate these fault-management problems mathematically, develop heuristics to find efficient solutions in typical networks, and analyze their characteristics (e.g., maximum guaranteed network capacity in the event of a fiber failure and the recovery time) relative to each other.

In wavelength-division multiplexing (WDM) optical networks, the bandwidth request of a traffic stream can be much lower than the capacity of a lightpath. Efficiently grooming low-speed connections onto high-capacity lightpaths will improve the network throughput and reduce the network cost. In WDM/SONET ring networks, it has been shown in the optical network literature that by carefully grooming the low-speed connection and using wavelength-division multiplexer (OADM) to perform the optical bypass at intermediate nodes, electronic ADMs can be saved and network cost will be reduced. In this study, we investigate the traffic-grooming problem in a WDM-based optical mesh topology network. Our objective is to improve the network throughput. We study the node architecture for a WDM mesh network with traffic-grooming capability. A mathematical formulation of the traffic-grooming problem is presented in this study and several fast heuristics are also proposed and evaluated.

Consider an optical network which employs wavelength-routing crossconnects that enable the establishment of wavelength-division-multiplexed (WDM) connections between node pairs. In such a network, when there is no wavelength conversion, a connection is constrained to be on the same wavelength channel along its route. Alternate routing can improve the blocking performance of such a network by providing multiple possible paths between node pairs. Wavelength conversion can also improve the blocking performance of such a network by allowing a connection to use different wavelengths along its route. This work proposes an approximate analytical model that incorporates alternate routing and sparse wavelength conversion. We perform simulation studies of the relationships between alternate routing and wavelength conversion on three representative network topologies. We demonstrate that alternate routing generally provides significant benefits, and that it is important to design alternate routes between node pairs in an optimized fashion to exploit the connectivity of the network topology. The empirical results also indicate that fixed-alternate routing with a small number of alternate routes asymptotically approaches adaptive routing in blocking performance.

We propose medium-access control protocols in multi-wavelength ring networks where each node is equipped with a DWADM (Dynamic Wavelength Add-Drop Multiplexer) (and a SONET ADM) instead of tunable or fixed transceivers. The property of a DWADM dictates that, at any given time, the input channel wavelength must be the same as the output channel wavelength. The DWADM is an emerging device which could turn out to be very inexpensive, so we investigate how to exploit its usage in a multi-wavelength ring network for dynamic packet transport. Our results show that the proposed protocols can solve the fairness problem inherent in such a system and achieve good performance.

In near video-on-demand (near-VoD), requests for a video title are grouped together (i.e. batched) and are served with a single multicast stream, thereby increasing the number of concurrent users which can be supported by the system. Since users may not be able to tolerate the delay incurred by batching and hence cancel their requests, a batching policy should be designed so as to achieve low user loss and high revenue (given by the total pay-per-view collected over a long period of time across all movies). We propose an adaptive batching policy which offers users low delay at low arrival rate, and gates the allocation of the channels at high rate. Such adaptivity is achieved by the use of a simple "token-tray" (TT) scheme which governs when a stream may be allocated to a movie. In assigning a movie to a stream, we propose a weight function which depends on the user queuing-time and its pay-per-view (hence the term "weighted queuing-time") (WQT). By comparing our batching policy (TT/WQT) with a number of traditional ones (FCFS, forced-wait, batch-size-based scheme, etc.), our scheme is shown to achieve the highest revenue and lowest loss rate even when the arrival rate changes, with the user loss rate across the movies being fairly uniform, and the user delay being fairly low even at high arrival rate.

We investigate the problem of grooming low-speed traffic streams into high-capacity lightpaths in a WDM-based optical mesh topology network. We study the node architecture for a WDM mesh network with traffic grooming capability. A mathematical formulation of the traffic grooming problem is presented and some results from mathematical formulation are also shown.

We propose a WDM client-server network architecture based on a passive-star-coupler-based broadcast-and-select network. In this architecture, all down-stream data traffic uses WDM data channels and all up-stream requests, up-stream control messages, and down-stream control messages use an Ethernet channel (control channel). This architecture is broadcast and multicast capable. We propose a detailed point-to-point connection-setup procedure for this architecture. The system's performance is analyzed in terms of whether the control channel or the data channels are the bottleneck. We also analyze the request delay, request's channel-holding time and system throughput following the model description. We conclude that the control channel is not a bottleneck in the system. When the user request rate is high, the server-channel scheduler is the most important adjustable factor to reduce the user response time. An illustrative analysis of FCFS scheduling policy for the system is provided.

In multi-tier cellular networks a set of contiguous microcells are overlayed with a macrocell. Such architectures provide a higher system capacity because of the small frequency reuse distance of the microcell layer. However, handoff control is a key issue in such architectures; decrease in the cell dwelling time causes higher number of handoffs and hence higher handoff blocking probability. Queueing handoff calls when there are no idle channels, in the target cell, can improve handoff blocking probability without significantly increasing the new call blocking probability. We first present an analytical model to study the performance of a single-tier cellular network with two types of users, namely, high-speed users and low-speed users. We then use this model to analyze a multi-tier network with a queue in only one tier. Finally, we use a simulation model to examine the multi-tier network with different queue strategies. Our results show that by using a priority queue in the multi-tier network, the blocking probability of handoff calls can be significantly improved at the same time supporting a large system load.

In a cellular network, a fixed number of channels is normally assigned to each cell. However, under this scheme, the channel usage may not be efficient because of the variability in the offered traffic. Different approaches such as channel borrowing (CB) and dynamic channel allocation (DCA) have been proposed to accommodate variable traffic. In this paper, we expand on the CB scheme and propose a new channel allocation scheme-mobile-assisted connection-admission (MACA) algorithm-to achieve load balancing in a cellular network. In this scheme, some special channels are used to connect mobile units from different cells; thus, a mobile unit, which is unable to connect to its own base station because it is in a heavily-loaded "hot" cell, may be able to get connected to its neighboring cold cell's base station through a two-hop link. We find that MACA can greatly improve the performance of a cellular network.

The SONET ring is the most widely used optical network infrastructure today. While deploying the WDM/SONET ring, traffic grooming is an important network-design problem. SONET allows each wavelength to carry several lower-rate independent traffic channels in TDM fashion. For each logical connection that is established on one TDM time slot of a wavelength, traffic needs to be added and dropped only at the two end nodes of the connection. It is possible to have some nodes on some wavelength where no add/drop is needed on any time slot, thus resulting in savings of electronic equipment cost. By carefully arranging the connections on the network, the savings can be maximized. In the WDM/SONET ring, the equipment cost is predominantly high, so efficient traffic grooming can greatly reduce the network cost. In this paper, we first present a comprehensive mathematical definition of the problem, which turns out to be an integer linear program (ILP). Then, we propose a simulated-annealing-based heuristic algorithm for traffic grooming. A simple heuristic is also provided for the case where a hub node is used to bridge traffic from different wavelengths (called the multihop approach). We find the following main results. The simulated-annealing approach provides very good results in most cases. In general, multihop approaches can achieve better equipment savings when the grooming ratio is large but it consumes more bandwidth. Single-hop approaches will do better in all aspects when the grooming ratio is small. This paper focuses on nonuniform traffic and both unidirectional and bidirectional rings.

Layered video is a video compression technique to encode video data in multiple layers. It typically consists of a base layer and additional layers that provide enhanced video quality. The multicasting operation of a layered video may need to satisfy: (i) bounded end-to-end delay from a source to each receiver, (ii) minimum total cost, and (iii) minimum delay jitter between the various video streams received by the receivers. Because different nodes may request different video quality and because of limited bandwidth on the network's links, different layers of video data may reach their destinations over different distribution trees, and not all receivers may receive all of their requested layers. The problem of computing such data distribution paths is NP-complete, which means that no optimal solution method is available. This paper presents a new heuristic algorithm called LVMSR. With O(Rn/sup 2/) time complexity and O(R/sup 2/) message complexity, where n is the number of nodes in the network and R is the receiver group size. Our simulation results show that the multicast data paths computed by our algorithm can always satisfy the delay constraint with reasonably small total cost.

In a cellular system, if there are too many data users in a cell, which we refer to as a hot cell, data may suffer large delay, and system's quality of service (QoS) may degrade. A dynamic channel-allocation scheme assigns more channels to those hot cells under centralized control (CC). In mobile-assisted data forwarding (MADF), we add an ad-hoc overlay to the fixed cellular infrastructure and special channels-called forwarding channels-are used to connect users in a hot and its surrounding cold cells without going through the hot cell's base station. The forwarding-channel management in MADF is done by the mobile units themselves to relieve the load on the CC. We find that, using MADF, under a certain delay requirement, the system performance can be greatly improved.

While optical-transmission techniques have been researched for quite some time, optical "networking" studies have been conducted only over the past dozen years or so. The field has matured enormously over this time: many papers and Ph.D. dissertations have been produced, a number of prototypes and testbeds have been built, several books have been written, a large number of startups have been formed, and optical WDM technology is being deployed in the marketplace at a very rapid rate. The objective of this paper is to summarize the basic optical networking approaches, report on the WDM deployment strategies of two major US carriers, and outline the current research and development trends on WDM optical networks.

We present algorithms for the design of optimal virtual topologies embedded on wide-area wavelength-routed optical networks. The physical network architecture employs wavelength-conversion-enabled wavelength-routing switches (WRS) at the routing nodes, which allow the establishment of circuit-switched all-optical wavelength-division multiplexed (WDM) channels, called lightpaths. We assume packet-based traffic in the network, such that a packet travelling from its source to its destination may have to multihop through one or more such lightpaths. We present an exact integer linear programming (ILP) formulation for the complete virtual topology design, including choice of the constituent lightpaths, routes for these lightpaths, and intensity of packet flows through these lightpaths. By minimizing the average packet hop distance in our objective function and by relaxing the wavelength-continuity constraints, we demonstrate that the entire optical network design problem can be considerably simplified and made computationally tractable. Although an ILP may take an exponential amount of time to obtain an exact optimal solution, we demonstrate that terminating the optimization within the first few iterations of the branch-and-bound method provides high-quality solutions. We ran experiments using the CPLEX optimization package on the NSFNET topology, a subset of the PACBELL network topology, as well as a third random topology to substantiate this conjecture. Minimizing the average packet hop distance is equivalent to maximizing the total network throughput under balanced flows through the lightpaths. The problem formulation can be used to design a balanced network, such that the utilizations of both transceivers and wavelengths in the network are maximized, thus reducing the cost of the network equipment. We analyze the trade-offs in budgeting of resources (transceivers and switch sizes) in the optical network, and demonstrate how an improperly designed network may have low utilization of any one of these resources. We also use the problem formulation to provide a reconfiguration methodology in order to adapt the virtual topology to changing traffic conditions.

Wavelength-division multiplexing (WDM) local-area networks based on the optical passive-star coupler have traditionally been classified as being either single-hop or multihop. A single-hop network provides a direct connection between the source and the destination of a packet during the packet transfer duration, but may require some amount of coordination between the nodes which may involve tuning of the transmitters or receivers at each node. Since the time required to tune a tunable optical transmitter or receiver may be high, a single-hop network may incur significant overhead. On the other hand, a typical multihop network requires little or no tuning, but a packet may traverse a number of intermediate nodes between the source and destination nodes. Each hop incurs additional queueing delays at each node and also increases the overall load on each link and on the network. In this paper, we propose a new class of multiconfiguration multihop protocols (MMPs) which use tunable transmitters and receivers to cycle through a number of configurations which together make up a multihop logical topology. This class of protocols offers a trade-off between the tuning required in a single-hop network and the number of hops required in a multihop network. We present a generalized framework for comparing the proposed protocols with existing single-hop and multihop protocols, and we show that these protocols may offer significant performance gains for systems with high tuning delays and a limited number of transmitters and receivers at each node.

Node architectures which are being considered for wavelength-routed WDM optical networks do not provide a high degree of scalability with respect to the number of wavelengths in the system. In this investigation, we propose a new node architecture which allows new wavelengths to be added to the network without significant additional costs. We study the performance of such nodes in a network environment, and we develop a simple cost model for the network. We show that the proposed architecture provides a higher degree of scalability as well as significant cost benefits when compared to a traditional node architecture, such as the wavelength-routing switch (WRS) or a wavelength-selective cross-connect (WSXC), while also maintaining comparable performance in terms of blocking probability.

This investigation considers optical networks which employ wavelength cross-connects that enable the establishment of wavelength-division-multiplexed (WDM) channels, between node-pairs. In such and other networks, the failure of a network element may cause the failure of several optical channels, thereby leading to large data losses. Ramamurthy and Mukherjee formulated integer linear programs to determine the capacity requirements for different protection schemes for a static traffic demand. In this paper, we formulate a model of protection switching times for the different protection schemes, and propose distributed control protocols for path and link restoration, assuming a fully distributed control network. Based on our assumptions, we find that when the cross connect configuration time is low (/spl les/10 /spl mu/s), the protection schemes in increasing order of average protection-switching times are as follows: (a) shared-link, (b) dedicated-path, and (c) shared-path. When the cross-connect configuration time is high (/spl ges/500 /spl mu/s), the protection schemes in increasing order of average protection-switching times are as follows: (a) dedicated-path, (c) shared-link, and (d) shared-path. Numerical results obtained by simulating the distributed restoration protocols indicate that, for a representative network topology, path restoration has a better restoration efficiency than link restoration, and link restoration has a better restoration time compared to path restoration.

Fair allocation of available bandwidth to competing flows is a simple form of quality of service (QoS) that can be provided to customers in packet-switched networks. A number of packet-scheduling and buffer-management techniques have been proposed in the literature to achieve this goal efficiently. However, the complexity of the existing algorithms prevents a high-speed implementation with the current state of router technology. We propose a computationally simple mechanism based on token bucket policing to achieve almost equal bandwidth allocation for a set of competing flows. The proposed method adjusts the token bucket threshold dynamically and measures the instantaneous arrival rate of flows. It uses this information to decide whether or not to admit a packet arriving at the network edge. With minor modifications, our framework can be used in the Internet and frame relay based virtual private networks (VPNs). We present a detailed simulation study that evaluates the performance of our algorithm. The simulation results indicate that DTB is fair, efficient, and robust.

This investigation considers optical networks which employ wavelength cross-connects that enable the establishment of wavelength-division-multiplexed (WDM) channels, between node-pairs. In such and other networks, the failure of a network element (e.g., fiber link, cross-connect, etc.) may cause the failure of several optical channels, thereby leading to large data losses. This study examines different approaches to protect mesh based WDM optical networks from single-link failures. These approaches are based on two basic survivability paradigms: (a) path protection/restoration, and (b) link protection/restoration. In path- and link-protection schemes, backup paths and wavelengths are reserved in advance at the time of call setup. Path- and link-restoration schemes are dynamic schemes in which backup paths are discovered (from the spare capacity in the network) upon the occurrence of a failure. In part 1 of this study presented in this paper, we formulated integer linear programs to determine the capacity requirements for the above protection schemes for a static traffic demand.

Wavelength-division multiplexing (WDM) local-area networks based on the optical passive-star coupler have traditionally been classified as being either single-hop or multihop. A single-hop network provides a direct connection between the source and the destination of a packet during the packet transfer duration, but may require some amount of coordination between the nodes which may involve tuning of the transmitters or receivers at each node. Since the time required to tune a tunable optical transmitter or receiver may be high, a single-hop network may incur significant overhead. On the other hand, a typical multihop network requires little or no tuning, but a packet may traverse a number of intermediate nodes between the source and destination nodes. Each hop incurs additional queuing delays at each node and also increases the overall load on each link and on the network. We propose a new class of multiconfiguration multihop protocols (MMPs) which use tunable transmitters and receivers to cycle through a number of configurations which together make up. A multihop logical topology. This class of protocols offers a trade-off between the tuning required in a single-hop network and the number of hops required in a multihop network. We present a generalized framework for comparing the proposed protocols with existing single-hop and multihop protocols, and we show that these protocols may offer significant performance gains for systems with high tuning delays and a limited number of transmitters and receivers at each node.

An attractive target for a computer system attacker is the router. An attacker in control of a router can disrupt communication by dropping or misrouting packets passing through the router. We present a protocol called WATCHERS that detects and reacts to routers that drop or misroute packets. WATCHERS is based on the principle of conservation of flow in a network: all data bytes sent into a node, and not destined for that node, are expected to exit the node. WATCHERS tracks this flow, and detects routers that violate the conservation principle. We show that WATCHERS has several advantages over existing network monitoring techniques. We argue that WATCHERS' impact on router performance and WATCHERS' memory requirements are reasonable for many environments. We demonstrate that in ideal conditions WATCHERS makes no false-positive diagnoses. We also describe how WATCHERS can be tuned to perform nearly as well in realistic conditions.

We explore an optical network architecture which employs dense wavelength division multiplexing (WDM) technology and passive waveguide grating routers (WGRs) to establish a virtual topology based on lightpath communication. We examine the motivation and the technical challenges involved in this approach, propose and examine the characteristics of a network design algorithm, and provide some illustrative performance results.

Wavelength conversion has been proposed for use in wavelength-division multiplexed networks to improve efficiency. This study highlights systems challenges and performance issues which need to be addressed in order to incorporate wavelength conversion effectively. A review/survey of the enabling technologies, design methods, and analytical models used in wavelength-convertible networks is provided.

Optical networks based on passive-star couplers and employing WDM have been proposed for deployment in local and metropolitan areas. These networks suffer from splitting, coupling, and attenuation losses. Since there is an upper bound on transmitter power and a lower bound on receiver sensitivity, optical amplifiers are usually required to compensate for the power losses mentioned above. Due to the high cost of amplifiers, it is desirable to minimize their total number in the network. However, an optical amplifier has constraints on the maximum gain and the maximum output power it can supply; thus, optical amplifier placement becomes a challenging problem. In fact, the general problem of minimizing the total amplifier count is a mixed-integer nonlinear problem. Previous studies have attacked the amplifier-placement problem by adding the "artificial" constraint that all wavelengths, which are present at a particular point in a fiber, be at the same power level. This constraint simplifies the problem into a solvable mixed-integer linear program. Unfortunately, this artificial constraint can miss feasible solutions that have a lower amplifier count but do not have the equally powered wavelengths constraint. In this paper, we present a method to solve the minimum-amplifier-placement problem, while avoiding the equally powered wavelength constraint. We demonstrate that, by allowing signals to operate at different power levels, our method can reduce the number of amplifiers required.

In a single-hop WDM optical network, a straightforward approach to implementing multicasting is to schedule a single transmission to multiple destinations so that all of the destinations' receivers must tune to the same channel at the same time. Although scheduling a single transmission in this manner reduces the amount of transmitter and channel resources being used, it may also place a burden on the receivers in the network. If all receivers do not become available at the same time, then some receivers may have to wait (and be idle) for significantly long periods of time before receiving the message. In this paper, we investigate methods for partitioning a multicast group into a number of smaller subgroups and for scheduling a separate transmission for each of these subgroups. We show that this approach more effectively conserves and balances the usage of transmitter and receiver resources in the network and may lead to significantly improved system performance over the conventional single-transmission multicast approach.

Wavelength-division multiplexing (WDM) has the ability to utilize the enormous bandwidth offered by optical networks using today's electronics. We present an approximate analytical solution for the average packet delay in a local light-wave network, which employs the multicast scheduling algorithm (MSA) as its medium-access control (MAC) protocol. First, we develop an approximate analytical solution for the probability distribution of the receiver busy time. We demonstrate and explain some interesting and unobvious behavior of this distribution. Then, using the receiver busy time distribution, we calculate the maximum receiver throughput and the average packet delay. Results from the analytical solution match very well with those obtained from simulation.

We consider a wavelength-routed optical network operated as a lightpath-based virtual topology. We present an exact linear programming formulation for the complete virtual topology design, including choice of constituent lightpaths, routes for these lightpaths, and intensity of packet flows through these lightpaths. By making a shift in the objective function to minimal hop distance and by relaxing the wavelength-continuity constraints (i.e., assuming wavelength converters at all nodes), we demonstrate that the entire optical network design problem can be linearized and hence solved optimally. The linear formulation can be used to design a balanced network, such that the utilizations of both transceivers and wavelengths are high, i.e., neither of these expensive resources are under-utilized. We also use the linear formulation to provide a reconfiguration methodology in order to adapt the virtual topology to changing traffic conditions.

Optical networks based on passive star couplers and employing wavelength-division multiplexing (WDM) have been proposed for deployment in local and metropolitan areas. Amplifiers are required in such networks to compensate for the power losses due to splitting and attenuation. However, an optical amplifier has constraints on the maximum gain and the maximum output power it can supply; thus optical amplifier placement becomes a challenging problem. The general problem of minimizing the total amplifier count, subject to the device constraints, is a mixed-integer nonlinear problem. Previous studies have attacked the amplifier-placement problem by adding the "artificial" constraint that all wavelengths, which are present at a particular point in a fiber, be at the same power level. In this paper, we present a method to solve the minimum-amplifier-placement problem while avoiding the equally-powered-wavelength constraint. We demonstrate-that, by allowing signals to operate at different power levels, our method can reduce the number of amplifiers required in several small to medium-sized networks.

A local lightwave network can be constructed by employing two-way fibers to connect nodes in a passive-star physical topology, and the available optical bandwidth may be effectively accessed by the nodal transmitters and receivers at electronic rates using wavelength division multiplexing (WDM). The number of channels, /spl omega/, in a WDM network is limited by technology and is usually less than the number of nodes, N, in the network. We provide a general method using channel sharing to construct practical multi-hop networks under this limitation. Channel sharing may be achieved through time division multiplexing. The method is applied to a generalized shuffle-exchange-based multi-hop architecture, called GEMNET. Multicasting-the ability to transmit information from a single source node to multiple destination nodes-is becoming an important requirement in high-performance networks. Multicasting, if improperly implemented, can be bandwidth-abusive. Channel sharing is one approach toward efficient management of multicast traffic. We develop a general modeling procedure for the analysis of multicast (point-to-multipoint) traffic in shared-channel, multihop WDM networks. The analysis is comprehensive in that it considers all components of delay that packets in the network experience-namely, synchronization, queuing, transmission, and propagation. The results show that, in the presence of multicast traffic, WDM networks with /spl omega/

A local lightwave network can be constructed by employing two-way fibers to connect nodes in a passive-star physical topology, and the available optical bandwidth may be accessed by the nodal transmitters and receivers at electronic rates using wavelength-division multiplexing (WDM). The number of WDM channels, w, in such a network is technology-limited and is less than the number of network nodes, N, especially if the network should support a scalable number of nodes. We describe a general and practical channel sharing method, which requires each node to be equipped with only one transmitter-receiver pair, and in which each WDM channel is shared in a time-division multiplexed fashion; optical fiber LANs are discussed in particular. We also develop a general model for analyzing such a shared-channel, multi-hop, WDM network. Our analysis yields a counterintuitive result: it is sometimes better to employ fewer channels than a larger number of channels. We explore bounds on the ranges of w which admit queueing stability-using too few or too many channels can lead to instability. We also obtain an estimate for the optimal number of channels that minimizes network-wide queueing delay.

The transmission control protocol/Internet protocol (TCP/IP) suite is widely used to interconnect computing facilities in modern network environments. However, there exist several security vulnerabilities in the TCP specification and additional weaknesses in a number of its implementations. These vulnerabilities may enable an intruder to "attack" TCP-based systems, allowing him/her to "hijack" a TCP connection or cause denial of service to legitimate users. We analyze TCP code via a "reverse engineering" technique called "slicing" to identify several of these vulnerabilities, especially those that are related to the TCP state-transition diagram. We discuss many of the flaws present in the TCP implementation of many widely used operating systems, such as SUNOS 4.1.3, SVR4, and ULTRIX 4.3. We describe the corresponding TCP attack "signatures" (including the well-known 1994 Christmas Day Mitnick Attack) and provide recommendations to improve the security state of a TCP-based system, e.g., incorporation of a "timer escape route" from every TCP state.

A local lightwave network can be constructed by employing two-way fibers to connect nodes in a passive-star physical topology, and the available optical bandwidth may be effectively accessed by the nodal transmitters and receivers at electronic rates using wavelength division multiplexing (WDM). The number of channels, w, in a WDM network is limited by technology and is usually less than the number of nodes, N, in the network. Channel sharing, achievable via time-division-multiplexing, may be used to construct practical multi-hop networks under this limitation. Multicasting-the ability to transmit information from a single source node to multiple destination nodes-is becoming an important requirement in high-performance networks. Multicasting, if improperly implemented, can be bandwidth-abusive. Channel sharing is one approach towards efficient management of multicast traffic. We develop a general modeling procedure for the analysis of both unicast (point-to-point) and multicast (point-to-multipoint) traffic in shared-channel, multi-hop WDM networks. The analysis is comprehensive in that it considers all components of delay that packets in the network experience-namely, synchronization, queueing, transmission, and propagation. The results show that, in the presence of multicast traffic, WDM networks with w

We consider large optical networks in which nodes employ wavelength-routing switches which enable the establishment of wavelength-division-multiplexed (WDM) channels, called lightpaths, between node pairs. We propose a practical approach to solve routing and wavelength assignment (RWA) of lightpaths in such networks. A large RWA problem is partitioned into several smaller subproblems, each of which may be solved independently and efficiently using well-known approximation techniques. A multicommodity flow formulation combined with randomized rounding is employed to calculate the routes for lightpaths. Wavelength assignments for lightpaths are performed based on graph-coloring techniques. Representative numerical examples indicate the accuracy of our algorithms.

A passive-star-based, broadcast-and-select, local lightwave network which can support a limited number of wavelength-division multiplexed (WDM) channels, but serve a much larger number of nodes, is considered. Each node is equipped with one tunable transmitter and one fixed receiver, and each WDM channel is operated in a time-division multiplexed (TDM) fashion for carrying packet traffic. Bandwidth is allocated to the node pairs when traffic flow between them is nonuniform, while also accommodating transceiver tuning latency. Our approach exploits well-known results from scheduling theory to create efficient transmission schedules. Multiprocessor task scheduling heuristics that can be applied to load balancing in a multichannel network is also examined.

Rainbow is a prototype optical metropolitan area network (MAN) developed at IBM. It employs wavelength-division multiplexing (WDM) on a fiber-optic, passive-star network topology, with each station equipped with a laser, which is fixed tuned to its own unique wavelength, and a Fabry-Perot filter, which is tunable across all wavelengths. This paper presents a model and analysis of the protocol used in the Rainbow prototype using the equilibrium point analysis (EPA) technique. We examine the system's throughput and how it is affected by various system parameters such as message arrival rate, message length, and timeout duration. We show that, for a given arrival rate, there is a timeout duration that will yield the optimal throughput. The analytical results are verified by simulation.

We explore design principles for next-generation optical wide-area networks, employing wavelength-division multiplexing (WDM) and targeted to nationwide coverage. This optical network exploits wavelength multiplexers and optical switches in routing nodes, so that an arbitrary virtual topology may be embedded on a given physical fiber network. The virtual topology, which is used as a packet-switched network and which consists of a set of all-optical "lightpaths", is set up to exploit the relative strengths of both optics and electronics-viz. packets of information are carried by the virtual topology "as far as possible" in the optical domain, but packet forwarding from lightpath to lightpath is performed via electronic switching, whenever required. We formulate the virtual topology design problem as an optimization problem with one of two possible objective functions: (1) for a given traffic matrix, minimize the network-wide average packet delay (corresponding to a solution for present traffic demands), or (2) maximize the scale factor by which the traffic matrix can be scaled up (to provide the maximum capacity upgrade for future traffic demands). Since simpler versions of this problem have been shown to be NP-hard, we resort to heuristic approaches. Specifically, we employ an iterative approach which combines "simulated annealing" (to search for a good virtual topology) and "flow deviation" (to optimally route the traffic-and possibly bifurcate its components-on the virtual topology). We do not consider the number of available wavelengths to be a constraint, i.e., we ignore the routing of lightpaths and wavelength assignment for these lightpaths. We illustrate our approaches by employing experimental traffic statistics collected from NSFNET.

Intrusion detection systems (IDSs) attempt to identify unauthorized use, misuse, and abuse of computer systems. In response to the growth in the use and development of IDSs, the authors have developed a methodology for testing IDSs. The methodology consists of techniques from the field of software testing which they have adapted for the specific purpose of testing IDSs. They identify a set of general IDS performance objectives which is the basis for the methodology. They present the details of the methodology, including strategies for test-case selection and specific testing procedures. They include quantitative results from testing experiments on the Network Security Monitor (NSM), an IDS developed at UC Davis. They present an overview of the software platform that has been used to create user-simulation scripts for testing experiments. The platform consists of the UNIX tool expect and enhancements that they have developed, including mechanisms for concurrent scripts and a record-and-replay feature. They also provide background information on intrusions and IDSs to motivate their work.

A passive-star-based, broadcast-and-select, local lightwave network which can support a limited number of WDM channels (on the order of ten), but serve a much larger number of nodes (a few tens or hundreds), is considered. Each node is equipped with one tunable transmitter and one fixed receiver, and each WDM channel is operated in a TDM fashion for carrying packet traffic. Bandwidth is allocated to the node pairs when traffic flow between them is non-uniform, while also accommodating transceiver tuning latency. Our approach exploits well-known results from scheduling theory to create efficient transmission schedules.

GEMNET is a generalization of shuffle-exchange networks and it can represent a family of network structures (including ShuffleNet and de Bruijn graph) for an arbitrary number of nodes. GEMNET employs a regular interconnection graph with highly desirable properties such as small nodal degree, simple routing, small diameter, and growth capability (viz. scalability). GEMNET can serve as a logical (virtual), packet-switched, multihop topology which can be employed for constructing the next generation of lightwave networks using wavelength-division multiplexing (WDM). Various properties of GEMNET are studied.>

The design of a medium access control scheme for a single-hop, wavelength-division-multiplexing-(WDM) multichannel local lightwave network poses two major difficulties: relatively large transmitter/receiver tuning overhead and large ratio of propagation delay to packet transmission time. Most schemes proposed so far have ignored the tuning overhead, and they can only schedule fixed-length packet transmissions. To overcome these two difficulties, the authors propose several scheduling algorithms which can reduce the negative impact of tuning overhead and schedule variable-length messages. A separate channel (control channel) is employed for transmission of control packets, and a distributed scheduling algorithm is invoked at each node every time it receives a control packet. By allowing the length of messages to be variable, a long message can be scheduled with a single control packet transmission, instead of fragmenting it into many fixed-length packets, thereby significantly reducing the overhead of control packet transmissions and improving the overall system performance. Three novel scheduling algorithms are proposed, varying in the amount of global information and processing time they need. Two approximate analytical models are formulated to study the effect of tuning time and the effect of having a limited number of data channels. Extensive simulations are conducted. Average message delays are compared for all of the algorithms.>

Two major difficulties in designing a single-hop multichannel local lightwave network are: relatively large transmitter/receiver tuning overhead and large ratio of propagation delay to packet transmission time. The authors propose several scheduling algorithms which can reduce the negative impact of tuning overhead and schedule variable-length messages. Thus, a long message can be scheduled with a single control packet transmission, instead of being segmented into many fixed-length packets, thereby significantly increasing the system's efficiency. Three novel scheduling algorithms are proposed, varying in the amount of global information and processing time. Two approximate analytical models are formulated to study the effect of tuning time and the effect of having a limited number of data channels. The system's performance is found to improve (1) if a simple mechanism is employed to avoid unnecessary transceiver tuning and/or (2) if a predictive transmitter tuning strategy is adopted.>

Explores design principles for next generation optical wide-area networks, employing wavelength-division multiplexing (WDM), and targeted to nationwide coverage. This almost-all-optical network will exploit wavelength multiplexers and optical switches in routing nodes, so that arbitrary virtual topologies may be imbedded on a given physical network. The virtual topology, which is packet switched and which consists of a set of all-optical lightpaths, is set up to exploit the relative strengths of both optics and electronics viz. packets of information are carried by the virtual topology "as far as possible" in the optical domain, but packet forwarding from lightpath to lightpath is performed via electronic switching, whenever required. Algorithms are developed so that the WDM-based network architecture will (a) provide a high aggregate system capacity due to spatial reuse of wavelengths, and (b) support a large and scalable number of users, given a limited number of wavelengths. The authors illustrate their approaches by employing experimental traffic statistics collected from NSFNET.>

The increasing popularity of local area networks (LANs) and the limitations of a single LAN (on its geographical coverage, information-carrying capacity, and number of stations) have triggered tremendous interest in the study and implementation of interconnected LANs. The authors address the problem of interconnecting the widely deployed CSMA/CD LANs (or LAN segments) via a transparent bridge. A transparent bridge is so called because it "learns" about the network structure and organization during its operation, i.e., it learns about the location of the network stations relative to the bridge in order to do the forwarding of frames selectively; also, the stations need not be aware of the presence of the bridge. The authors make two important contributions. First, they present an approximate analytical model of a transparent CSMA/CD bridge which can be either of the normal store-and-forward type or of the novel cut-through variety (introduced by Kwok and Mukherjee). Using the analytical model, which is also verified via simulation, the performance of the cut-through bridge is compared with (and found to be superior than) that of the normal bridge. Secondly they present the design of a cut-through bridge which can be easily implemented using off-the-shelf components.>

The authors exploit the capabilities of lightwave technology in order to construct photonic implementations of "adaptive" and "optimized" distributed queue dual bus (DQDB) structures. These (virtual) structures are linear and multihop in nature, and they can be constructed on any physical topology by exploiting the broadcast-and-select property of WDM lightwave networks. The present study is important since it will allow DQDB (IEEE 802.6) networks to scale up by taking advantage of the various attractive properties of lightwave technology when they become available. The specific problem is on topological design, and it can be stated as follows: given that the network nodes must be connected linearly and that the node positions in the network can be adjusted by properly tuning their (optical) transmitters and receivers, what is the best pattern for interconnecting them? Two sets of heuristic optimization algorithms are formulated. The first set is concerned with minimizing the maximum flow in any link. The second set of heuristics requires the knowledge of not only the traffic matrix but also the distance matrix, and these heuristics are aimed at minimizing the network-wide mean packet delay. A dynamic node migration heuristic is also formulated under which neighbouring nodes swap their positions based on local information in order to preserve the optimality criterion in effect when the offered traffic changes. The performance of these heuristics are compared, some of their properties are analyzed, while their other attractive properties are highlighted via numerical examples.>

Computer and network systems are vulnerable to attacks. Abandoning the existing huge infrastructure of possibly-insecure computer and network systems is impossible, and replacing them by totally secure systems may not be feasible or cost effective. A common element in many attacks is that a single user will often attempt to intrude upon multiple resources throughout a network. Detecting the attack can become significantly easier by compiling and integrating evidence of such intrusion attempts across the network rather than attempting to assess the situation from the vantage point of only a single host. To solve this problem, we suggest an approach for distributed recognition and accountability (DRA), which consists of algorithms which “process”, at a central location, distributed and asynchronous “reports” generated by computers (or a subset thereof) throughout the network. Our highest-priority objectives are to observe ways by which an individual moves around in a network of computers, including changing user names to possibly hide his/her true identity, and to associate all activities of multiple instances of the same individual to the same networkwide user. We present the DRA algorithm and a sketch of its proof under an initial set of simplifying albeit realistic assumptions. Later, we relax these assumptions to accommodate pragmatic aspects such as missing or delayed “reports”, clock skew, tampered “reports”, etc. We believe that such algorithms will have widespread applications in the future, particularly in intrusion-detection systems.

The unique capabilities of lightwave technology are exploited in order to construct optimized regular multihop networks when the traffic flow among the network nodes is asymmetric. The specific problem addressed is as follows: given that the network nodes must be connected in a regular interconnection pattern and that the node positions in the regular network can be adjusted by properly tuning their (optical) transceivers, what is the best node placement in a given regular topology? In particular, the ShuffleNet-based regular topology is examined. ShuffleNet has the property of producing large connected graphs with small degree and diameter. In order words, it can interconnect a large number of nodes with a small number of transceivers per node such that information from a source can reach its destination in a small number of hops. Since finding the optimal node placement is a computationally hard problem, efficient heuristic algorithms are formulated to design optimized ShuffleNet structures for a given traffic matrix.>

The unfairness problem of the distributed queue dual bus (DQDB) (IEEE Std 802.6) network is addressed, and several alternative solutions that can improve the network's fairness are proposed. Implementation methods that require simple additional hardware on top of the regular DQDB interface are outlined. Simulation examples are employed to compare the performance of the schemes and to gain insight into their characteristics. The performance is also compared with that of the original DQDB and the bandwidth-balancing DQDB. An analytical model of the DQDB network is developed. Some constrained assumptions for analytical tractability are used to obtain a Markov chain model for (an earlier version of) the entire DQDB network, the corresponding state-space explosion problem is highlighted. For reasonably small systems, the analytical model can predict an individual station's throughput and mean segment delay for known (possibly asymmetric) loading patterns. The model is verified via simulation.>

The goal of the study described is to exploit the capabilities of emerging lightwave technology and the fact that the IEEE 802.6 MAN is a linear network, to construct near optimal linear multihop lightwave networks. Heuristic algorithms are proposed for constructing photonic implementations of near optimal distributed queue dual bus (DQDB) structures. Two sets of heuristic optimization algorithms are formulated. The first set is concerned with minimizing the maximum flow in any link in the network, while the second set of heuristics is aimed at minimizing the network-wide mean packet delay. Important properties of these algorithms are analyzed and their performance is demonstrated with several representative numerical examples.>

This study deals with the distributed queue dual bus (DQDB) (IEEE 802.6) network, and makes two independent contributions. First, the unfairness problem of DQDB is addressed, and several alternative solutions that can improve the network's fairness are proposed. They include (1) the proportional assignment scheme (PR); (2) the (multiple-request) FCFS (first come first served)-message-queue-based DQDB scheme (MD); and (3) a combination of MD and PR, denoted by MP. Implementation methods that require simple hardware in addition to the regular DQDB interface are outlined. The schemes are compared through simulation, and insights into their characteristics are gained. The performance of these schemes is also compared with that of regular DQDB and bandwidth balancing DQDB. The second contribution is the development of an analytical model of the DQDB network. By employing some constrained assumptions for analytical tractability, a Markov chain model for (an earlier version of) the entire DQDB network is formulated. The model is verified via simulation.>

A strategy, called the continuation-bit approach, for scheduling the transmission of variable-length (multi-packet) messages on slotted high-speed LAN/MAN (local/metropolitan area networks) is studied. Its overhead is analyzed and compared with that of a conventional slotted system. Then, the authors apply this approach to the p/sub i/-persistent protocol, which is an efficient unity-capacity protocol proposed for high-speed LAN/MAN. Specifically, approximate analytical models for light and for heavy traffic loads are formulated, and from these models the proper network operating parameters p/sub i/ are determined. The authors consider several examples to study the characteristics of the continuation-bit approach, and verify the accuracy of the approximations via simulation.>

The open-ring/active-bus network structure for packet-switched, multiple-access communication over high-speed fiber-optic networks is studied. The structure is shown to suffer from fewer synchronization constraints than a closed ring structure and to provide a capacity greater than unity because it allows reuse of channel bandwidth. Various access mechanisms on this structure are discussed, and their channel capacities are analyzed.>

The p/sub i/-persistent protocol is based on a probabilistic scheduling mechanism (see Mukherjee and Meditch, 1988). The authors further develop the protocol to make it easily implementable, by allowing it to be sensitive to changing load conditions. They study various properties of a simple algorithm which stations execute independently by using channel feedback information. This results in a fully distributed control mechanism that continuously adjusts the station probabilities p/sub i/ at their proper levels as governed by the offered traffic. An extensive simulation model has been developed to study properties of this control mechanism such as p/sub i/ settling time and accuracy, behavior under step changes in traffic load, effect of injection of additional packets, and effect of various parameters associated with the underlying algorithm. These experiments indicate that this algorithm is suitable for implementing the protocol.>

A token-ring local area network capable of operating in multiple parallel segments which are interconnected via a central switch (called a bridge) is the focus of this study. It is shown that while it is beneficial to operate the network as a single token ring at light loads, the single-ring mode also has a limited channel capacity, and that it is preferable to operate the ring in multiple partitions for heavier loads. In particular, it is found that if the switch processing delay is insignificant compared to the mean packet transmission time, it is preferable to operate the network either as a single large ring or as a star network. An approximate analytical model is used. The results are verified via simulation.>

This study concentrates on the security-related issues in a single broadcast LAN (local area network) such as Ethernet. The authors formalize various possible network attacks. Their basic strategy is to develop profiles of usage of network resources and then compare current usage patterns with the historical profile to determine possible security violations. Thus, the work is similar to the host-based intrusion-detection systems. Different from such systems, however, is the use of a hierarchical model to refine the focus of the intrusion-detection mechanism. The authors also report on the development of an experimental LAN monitor currently under implementation. Several network attacks have been simulated, and results on how the monitor has been able to detect these attacks are analyzed. Initial results demonstrate that many network attacks are detectable with the authors' monitor, although it can be defeated.>

A novel bridging architecture, called a cut-through bridge, which can be used to interconnect existing carrier-sense multiple access with collision detection (CSMA/CD) LANs is proposed. This bridge uses the cut-through switching concept. After providing a design of this bridge, the authors examine its performance via a detailed simulation model. They demonstrate the performance enhancement achievable by the cut-through bridge over a normal bridge, namely lower average frame delay while attaining the same maximum throughput as the latter. Since the cut-through bridge can be readily used to replace passive repeaters while achieving superior performance, it has promise for immediate application in existing CSMA/CD LANs.>

In the above-titled paper by Takine et al. (see ibid., vol.COM-36, no.10, p.1119-27, Oct. 1988) an exact analysis of a nonsymmetric polling system with single message buffers was reported. The commenters provide an alternate method for analyzing the exact mean waiting times of the individual stations in the same system by extending an exact analysis for the two-station case. Some corrections to the numerical results provided in the paper are made.>

The authors investigate the delay-throughput performance of this protocol operating under either equal or nonuniform load conditions based on the fairness criterion of equal average packet delay. Because of the complexity of the system, an approximate analytical model for selecting the p/sub i/ for this fairness criterion is developed. The p/sub i/ that satisfy the fairness criterion are determined from a Markov chain model for the protocol. The authors begin by considering the finite-buffer case and then proceed to the infinite-buffer one. They find that the performance of the protocol is insensitive to bus length, data rate, and the number of stations on the network. Hence, the network is suitable for long-distance, high-bandwidth applications as in fiber-optic local and metropolitan area systems. Simulation studies which serve to establish the range of validity of the analytical model are presented.>

The p/sub i/-persistent protocol is an excellent candidate for multiaccess communication over very long and very high-speed (unidirectional) fiber-optic bus networks because it does not suffer from the distance and bandwidth limitations of round-robin-type access mechanisms. The authors develop this protocol further to make it easily implementable, by allowing it to be sensitive to changing load conditions. In particular, they provide and study various properties of a simple algorithm which stations execute independently by using channel feedback information. This results in a fully distributed control mechanism that continuously adjusts the station probabilities p/sub i/ at their proper levels as governed by the ordered traffic, where the p/sub i/ are parameters of the p/sub i/-persistent protocol.>

Recent advances in fiber optic technology (viz. its promise to provide information-carrying capacity in the Gpbs range over long repeater-free distances) has triggered tremendous activity in the study of unidirectional bus networks (because signal flow in the fiber is unidirectional). A popular network structure that has received significant attention is the Dual-bus Unidirectional Broadcast System (DUBS) network topology. Most of the access mechanism studied on this structure are based on round-robin scheduling (or some variation thereof). However since round-robin schemes suffer a loss of channel capacity because of their inter-round overhead (which can be significant for long high-speed buses), a probabilistic scheduling strategy, called pi-persistent protocol, has recently been proposed and studied for single channel unidirectional bus systems. Our concern here is to apply this probabilistic scheduling strategy to each bus in DUBS, and study the corresponding network performance. In so doing, we allow stations to buffer multiple packets, represent a station's queue size by a Markov chain model, and employ an independence assumption. We find that the average packet delay is bounded and the maximum network throughput approaches two pkt/slot with increasing buffer size. Further, the protocol's performance is insensitive to bus characteristics, and it appears to be a particularly well suited for fiber-optic network application requiring long distances and high bandwidth. Simulation results, which verify the analytical model, are also included.

The authors extend the p/sub i/-persistent protocol previously proposed to include voice for multiaccess communication over unidirectional broadcast bus networks. They use a framed approach for integrating these two traffic types. Further, they use not only speech detectors for modeling speech sources, but also a movable-boundary scheme for their protocol in which the voice subframe size is estimated from the number of voice packets transmitted in the previous frame. The authors determine the fraction of speech loss and the fraction of wasted allocated bandwidth due to the estimator by formulating a Markov chain for the number of ready voice stations at the frame boundaries. To analyze data performance, a nonhomogeneous Markov chain is constructed for a data station's buffer content just before the data slots visit that station. Then, these models are converted to homogeneous chains from which the authors determine the optimum p/sub i/ for each data slot.>

The authors introduce the concept of partitioning a token-ring network, and demonstrate quantitatively the corresponding performance advantage. Attention is given to a two-partition token-ring network which can operate in either the single-ring mode or the two-partition mode. It is found that for reasonable values of system parameters, while the single-ring mode performs close to a conventional token ring, the two-partition mode results in substantial performance improvement such as higher channel capacity and better delays vs. throughput behavior when traffic is more local. It is demonstrated that there exists a certain threshold load above which the two-partition token ring performs better in the two-partition mode, while, below this load, it is preferable to operate the ring in the single-ring mode.>

A protocol for multiaccess communication over unidirectional bus networks is proposed, and its performance capabilities are determined. Under this protocol, time is slotted with a slot equaling a packet's transmission time. A station with a packet to send persists in transmitting its packet in an empty slot with probability p/sub i/ until it is successful. Three criteria for fairness in selection of the p/sub i/ are modeled using Markov chains, which are solved to obtain the proper p/sub i/ that satisfy each fairness criterion. Unlike previous studies of unidirectional bus networks, stations are allowed to buffer more than one packet. The average packet delay for this protocol is bounded, and the maximum achievable throughput approaches unity with increasing buffer size. Further, the protocol provides better delay versus throughput behavior for fixed packet lengths than previous round-robin schemes, its performance is insensitive to bus characteristics, and it appears to be particularly well suited for fiber-optic network applications requiring long distances and high bandwidths. Simulation results that confirm the predicted performance are included.>

A previously proposed protocol is extended to include voice for multiaccess communication over unidirectional broadcast bus networks. The authors use a framed approach for integrating these two traffic types where a frame, which is repeated, consists of a voice subframe followed by a data subframe. They use speech detectors for modeling speech sources in order to prevent the loss of about 60% of the bandwidth that would otherwise be required, and they use a moveable-boundary scheme in which the voice subframe size is estimated from the number of voice packets transmitted in the previous frame. The authors determine the fraction of speech loss and the fraction of wasted allocated bandwidth due to the estimator by formulating Markov chain for the number of ready voice stations at the frame boundaries. To analyze data performance, they use a nonhomogeneous Markov chain or a data station's buffer content just before the data slots visit that station. By proper control of the estimator function, they can guarantee an upper bound on the average speech loss at any voice station, and a combined voice-data throughput close to unity is achieved. Results from the analysis and from a simulation match closely.>