Byzantine Fault Tolerant (BFT) system designs have evolved to withstand increasingly challenging threat scenarios. In particular, network-attack-resilient BFT systems have developed frameworks for distributing replicas among multiple geographical sites to withstand sophisticated network denial of service attacks that can isolate an entire site. To make such systems economically feasible, data centers can be used in addition to on-premises sites to reduce the overall cost. Prior work focuses on developing system designs that tolerate a fixed number of replica failures and site disconnections. However, it has not considered the trade-offs between these designs in terms of their availability and cost. In this paper, we develop a framework to assess network-attack-resilient BFT systems in terms of their cost and availability, while accounting for differences between on-premises and data center sites. We compare five system designs proposed in the literature and identify the most cost-effective designs for a given availability target.

Societal dependence on the digital domain has resulted in governments increasingly concerned with digital sovereignty. The major digital sovereignty issues revolve around how to ensure the security, privacy and availability of government and citizens digital information in any form, from sensing/creation of information to communication through networks to processing on computing platforms to storage in the cloud. While there is clearly interest in the digital sovereignty topic by governments, academics, network operators, equipment vendors and policy organizations, it is unclear if there is corresponding citizen engagement. This paper seeks to gauge public interest in the digital sovereignty using social sensor data from platforms such as X and Google. The findings suggest that while governments and industry stakeholders may prioritize digital sovereignty, the general public exhibits limited interest and online engagements in the topic.

The advent of Next Generation (NextG) Radio Access Networks (RANs) necessitates the strategic geographical placement of edge computing infrastructure to meet the demands for low latency, high bandwidth, reliable network connectivity and support for Artificial Intelligence (AI) based applications. This paper investigates the use of solar powered micro edge data centers in NextG RANs, which will reduce the carbon emissions of network operators and can ensure continuous operation during power outages. We adopt a nanogrid power architecture that integrates solar power generation and energy storage co-location with small edge data centers. A novel methodology is proposed that uses data driven techniques to identify the power outage prone sections of a city and the solar generation potential of rooftops in that part of the city. Given a set of rooftops that meet or exceed the edge data center energy requirements, unsupervised machine learning is used to determine the geographic locations to place micro edge data centers. We illustrate our methodology using New Your City as a case study. This work provides insights for urban planners, network operators, and policymakers aiming to develop sustainable and resilient NextG RANs for societal good.

Byzantine Fault Tolerant (BFT) systems are used in applications that need to maintain high availability even in the presence of failures or compromises. BFT systems are typically designed to tolerate a preconfigured threshold number of faulty replicas (f), where the$f$faulty replicas may behave arbitrarily (potentially maliciously). However, as BFT system designs become more complex, it is important to evaluate the impact of such designs on the overall system availability with respect to standard crash faults. In this paper, we analyze three BFT systems that are designed to withstand both system compromises and network attacks: Spire, Confidential Spire, and Decoupled Spire. These network-attack-resilient BFT systems utilize a combination of on-premises and data center replicas to reduce cost and simplify deployment for system operators. We develop an availability model that accounts for differences between the availability of on- premises and data center replicas and analyze Spire, Confidential Spire, and Decoupled Spire under this model. Key system design insights from the analysis are that network-attack-resilient BFT configurations can meet the availability requirements of critical systems, as they provide higher availability than Classical BFT with less available servers. We show also that network-attack- resilient BFT configurations with fewer geographic sites provide higher availability, and that Confidential Spire has the highest availability, followed by Spire and then Decoupled Spire.

Next generation cellular networks are expected to enable a wide range of new applications, increasing societal dependence on the network infrastructure and requiring a higher level of resilience than current networks. In this paper, we consider the challenges network operators face in providing end-to-end connections across the backhaul part of the cellular network in the face of equipment failures and power outages. In particular, we discuss the impact of the move to commodity hardware, disaggregation of the radio access network, edge computing, densification of the network, and the increased electric power requirements on resilience. Techniques and research directions for overcoming the challenges are presented. This includes thinking beyond methods for a single network operator including cooperative operator techniques and extending resilient overlays to the wireless edge.

The growth of wind power generation is accelerating worldwide. However, wind turbines need to be carefully controlled to maintain structural stability and optimal power generation. The state of the turbine and power generated is digitized and transmitted over communication networks for physical component monitoring, control, and data management. Motivated by creating power outages, financial loss, and physical wind turbine damage, attackers could craft and inject falsified network traffic to maliciously influence control actions. In this paper, we propose a physics-informed machine learning anomaly detection approach. The mechanism is utilized at each wind turbine, where generated power signals are validated based on laws of physics and incorporated into machine learning algorithm to detect anomalies. We show that the proposed combined physics and machine learning approach improves the accuracy of detecting falsification attacks compared with using machine learning alone.

An inherent problem in Software-Defined Networking (SDN) is the Controller Placement Problem, which addresses how many controllers to deploy in the network, and where to place them. Several variants of this problem have been addressed and researched to find the placements that adapt best to different contexts. In this article, we address a more complex variant of this problem, to satisfy QoS requirements and to offer robustness against disaster-based failures. We address the joint optimization problem of controller placement and finding a tree subgraph which can be upgraded to have enhanced availability, in order to satisfy delay and availability constraints. Additionally, we consider geodiversity constraints as a way to enhance robustness to disaster-based failures.

In this paper, we propose a framework to leverage electrical microgrids and cellular networks to support post-disaster communications for the public, government and critical infrastructure operation. The framework involves both policy and technical components. The proposed approach is an integration of electrical microgrids to provide power together with self con-figuring wireless mesh communication networks and local edge computing infrastructure to support critical communications and smart infrastructure services/applications in a specific geographic area. Hence, geographic zones which are resilient safe havens are created in a city. We outline the basic components of our approach and discuss open challenges to realizing the vision.

The increased connectivity introduced in Internet of Things (IoT) applications makes such systems vulnerable to serious security threats. In this paper, we consider one of the most challenging threats in IoT networks, where devices manipulate (maliciously or unintentionally) the data transmitted in information packets as they are being forwarded from the source to the destination. We propose unsupervised learning that exploits network diversity to detect and identify suspicious networked elements. Our proposed method can identify suspicious nodes along multihop transmission paths and under variable attack levels within the network. More specifically, we formulate a contribution metric for each networked element, which is used as a feature to cluster the nodes based on their behavior. We proposed two detection approaches, namely hard detection and soft detection. In the former, nodes are clustered into malicious or benign group; while in the latter, nodes are clustered into three groups based on their suspicious level, then highly suspicious nodes are discarded and more accurate contribution features are evaluated for the remaining nodes. Soft detection has higher detection accuracy provided that there is sufficient network diversity. Simulation results show that the proposed methods achieve high detection accuracy under different percentages of malicious nodes in the network and in the existence of channel errors.

Future Smart Cities require new ways to manage services that benefit the end users. An important issue is how to connect homes in a community and create an alert management system in Smart Cities with coordination among different entities. In this paper, we present an architectural vision of a software defined home alert management system for Smart Cities. This alert management system would make the residents aware of any incidents in the neighborhood such as fire. In this work, we use the features of software defined networking to design a manageable and flexible smart home for a smart community to provide services such as smart alarm systems.

A new recursive heuristic is proposed to calculate a shortest simple path, from a source node to a destination node, that visits a specified set of nodes in a network. To provide survivability to failures along the path, the proposed heuristic is modified to ensure that the calculated path can be protected by a node‐disjoint backup path. Additionally, the case when both paths in the disjoint path pair are required to visit specific sets of nodes is studied and effective heuristics are proposed. An evaluation of the solutions of the heuristics is conducted by comparing with results from an integer linear programming (ILP) formulation for each of the considered problems, and also with previous heuristics. The ILP solver may require a significant amount of time to obtain a solution, especially in large networks, which justifies the need for effective, computationally efficient heuristics for solving these problems. © 2017 Wiley Periodicals, Inc. NETWORKS, Vol. 70(4), 292–307 2017

Demand response (DR) plays a key role for optimum asset utilization and to avoid or delay the need of new infrastructure investment. However, coordinated execution of multiple DRs is desired to maximize the DR benefits. In this study, we propose a hierarchical DR architecture (HDRA) to control and coordinate the performance of various DR categories such that the operation of every DR category is backed-up by time delayed action of the others. A reliable, cost-effective communication infrastructure based on ZigBee, WiMAX, and fibers is designed to facilitate the HDRA execution. The performance of the proposed HDRA is demonstrated from the power system and communication perspectives in a cosimulation environment applied to a 0.4 kV/400 kVA real distribution network considering electric vehicles as a potential DR resource (DRR). The power simulation is performed employing a real time digital simulator whereas the communication simulation is performed using OMNeT++. The HDRA performance demonstrated the maximum utilization of available DR potential by facilitating simultaneous execution of multiple DRs and enabling participation of single DRR for multiple grid applications.

To cope with the unprecedented growth of mobile data traffic, we investigate the performance gains obtained from unifying coverage-centric 4G mobile networks and capacity-centric fiber-wireless (FiWi) broadband access networks based on data-centric Ethernet technologies with resulting fiber backhaul sharing and WiFi offloading capabilities. Despite recent progress on backhaul-aware 4G studies with capacity-limited backhaul links, the performance-limiting impact of backhaul latency and reliability has not been examined in sufficient detail previously. In this paper, we evaluate the maximum aggregate throughput, offloading efficiency, and in particular, the delay performance of FiWi enhanced LTE-Advanced (LTE-A) heterogeneous networks (HetNets), including the beneficial impact of various localized fiber-lean backhaul redundancy and wireless protection techniques, by means of probabilistic analysis and verifying simulation, paying close attention to fiber backhaul reliability issues and WiFi offloading limitations due to WiFi mesh node failures as well as temporal and spatial WiFi coverage constraints. We use recent and comprehensive smartphone traces of the PhoneLab data set to verify whether the previously reported assumption that the complementary cumulative distribution function of both WiFi connection and interconnection times fit a truncated Pareto distribution is still valid. In this paper, we put a particular focus on the 5G key attributes of very low latency and ultra-high reliability and investigate how they can be achieved in FiWi enhanced LTE-A HetNets. Furthermore, given the growing interest in decentralization of future 5G networks (e.g., user equipment assisted mobility), we develop a decentralized routing algorithm for FiWi enhanced LTE-A HetNets.

Providing encrypted communications among power grid components is expected to be a basic requirement of smart grid systems in the future. Here, we propose a key management architecture and associated protocols tailored to support encrypted smart grid communications. The architecture consists of two levels structured around the grid control system hierarchy. At the top level, which consist of control centers and regional coordinators, a bottom-up key structure is adopted using hash chaining and a logical key hierarchy. The lower level of the architecture consists of the regional coordinators (i.e., substations and distribution systems) and remote ends (e.g., meters and pole-top sensors) and utilizes a top-down key management approach built on an inverse element method. The proposed key management schema supports the hierarchical structure of the smart grid control mechanisms, and it takes the resource and electronic/physical security differences of the control levels into account. We define a set of protocols utilizing the architecture to provide secure unicast, multicast, and broadcast communications. Furthermore, we illustrate how the architecture is flexible enough to easily handle power grid nodes joining and leaving the system at the different levels. Lastly, we compare the proposed schema with existing ones and show that our architecture can achieve efficient key management to provide secure communications. Copyright © 2016 John Wiley & Sons, Ltd.

Cellular network virtualization is being considered as a key trend in future mobile networks towards improved resource utilization. However, virtualization scenarios need investigation to understand the considerations which should be taken into account when deploying virtualized wireless networks in practice. Towards this, we address the performance of a virtualized network in the presence of heterogeneous classes of traffic. In previous cellular network virtualization literature, both Real time (RT) and Non-Real time (NRT) traffic requests have been included without distinction. Both types are provisioned using the same algorithm for allocation of resources specified by the Network Scheduler [1]. However, different types of traffic have different characteristics [2], e.g., RT requests are delay sensitive but may need fixed bandwidth, and hence should be treated differently, especially when wireless channel conditions are factored into the scheduling. We recognize this difference and in this paper, we propose a new approach to improve scheduling of resources for RT and NRT traffic. In particular, we prioritize the traffic belonging to different virtual slices from all service providers (SP/VEs) at the Network Scheduler before allocating resources to different SP/VEs, i.e., We form a Virtual Prioritized Slice (VPS). The virtual prioritized slice is forwarded to the VPS scheduler to serve all RT requests first. Only after the RT traffic is scheduled, the NRT traffic is provisioned using proportional fairness (PF) scheduling. We show by simulation results that this new VPS approach helps outperform recently proposed resource allocation schemes.

The exact calculation of all‐terminal reliability is not feasible in large networks. Hence estimation techniques and lower and upper bounds for all‐terminal reliability have been utilized. Here, we propose using an ordered subset of the mincuts and an ordered subset of the minpaths to calculate an all‐terminal reliability upper and lower bound, respectively. The advantage of the proposed new approach results from the fact that it does not require the enumeration of all mincuts or all minpaths as required by other bounds. The proposed algorithm uses maximally disjoint minpaths, prior to their sequential generation, and also uses a binary decision diagram for the calculation of their union probability. The numerical results show that the proposed approach is computationally feasible, reasonably accurate and much faster than the previous version of the algorithm. This allows one to obtain tight bounds when it not possible to enumerate all mincuts or all minpaths as revealed by extensive tests on real‐world networks. © 2015 Wiley Periodicals, Inc. NETWORKS, Vol. 66(4), 282–295 2015

Using IEEE 802.15.4 and Zigbee for home area networks (HANs) in the Smart Grid is becoming an increasingly prominent topic in the research area. As the standard designed for low data rate and low cost wireless personal area networks, IEEE 802.15.4 is widely employed in the construction of home sensor networks to assist with real-time environment information. For the purposes of Smart Grid the Zigbee Alliance has defined new Smart Energy Profile Protocol that leverages the existing TCP and HTTP protocols. In this paper, we provide an overview of the Smart Grid's Advanced Metering Infrastructure (AMI) and Demand Response (DR) functionalities, and the communication requirement they pose for the new SEP protocol. The discussion is followed by an evaluation of the theoretical performance bounds of the new architecture based on a analytical model. We conclude, by extending the model to account for WiFi interference which is expected to be present in home and office environments.

Virtualizing wireless networks has the potential to improve resource usage efficiency (system capacity) through spectrum sharing while allowing for isolation between users and customization of applications {9}. In most work related to wireless network virtualization, the sharing of spectrum is considered at the level of chunks of frequency that do not interfere. Such spectrum sharing, where service provider SPA can use the spectrum allocated to SPB when SPB does not use it, results in multiplexing gains improving the resource usage (see for example, [12]). We argue that sharing radio resources that are a function of geography and signal strength, rather than slices of spectrum is also possible. When we consider sharing of radio resources, the transmit power, the interference, and the usage scenario (capabilities/needs of devices) become important in determining what can be shared. In this paper, the potential gain from sharing such radio resources while using MIMO for combating interference and exploiting spatial degrees of freedom is investigated in a two service provider collaboration scenario. The metric used is the capacity of the system (with a large cell and a small cell) as a function of separation distance, transmit power, cell range, and various MIMO settings. We show that radio resource sharing is feasible, but it has implications on isolation between users of different SPs and MIMO settings are an important factor.

An approach advocated in the recent literature for reducing energy consumption in cellular networks is to put base stations to sleep when traffic loads are low. However, several practical considerations are ignored in these studies. In this paper, we aim to raise questions on the feasibility and benefits of base station sleeping. Specifically we analyze the interference and capacity of a coverage-based energy reduction system in CDMA based cellular networks using a simple analytical model and show that sleeping may not be a feasible solution to reduce energy consumption in many scenarios.

Heterogeneous conditions can occur in multi-hop wireless networks due to a variety of factors such as variations in transmission power and signal propagation environments. Directed links can occur when the environment and/or the nodes are heterogeneous. In this paper, we examine the network connectivity for heterogeneous multi-hop wireless networks and propose an algorithm to identify the connectivity of the network. We follow this with a numerical study of the connectivity in random topologies. Lastly, we propose two schemes for constructing additional links to enhance the connectivity of the network. Our proposed schemes identify the links to be improved or created via a cluster based approach.

Abstract The identity and/or location of communicating entities in wireless ad hocnetworks is extremely important due to the potential of their being identified and subsequently subjected to cyber or physical attacks. In this paper, we show that a global attacker who can eavesdrop on the overall data transmissions and count them can simply visualize the transmissions and infer contextual information. Current approaches to obfuscate the locations of source and destinations do not provide protection against such attacks. We propose two novel techniques (1) SECLOUD: Source and Destination Seclusion using Clouds to obfuscate the true source/destination nodes and make them indistinguishable among a group of neighbor nodes, and (2) ANONYRING: Anonymous Ring which hides the source/destination nodes within a group of nodes that form a ring. Both proposed techniques work well even under network‐wide traffic visualization by a global attacker. Furthermore the proposed techniques are shown viasimulation to be superior to existing schemes in the literature. Copyright © 2010 John Wiley & Sons, Ltd.

Recent work on cooperative communications has demonstrated benefits in terms of improving the reliability of links through diversity and/or increasing the reach of a link compared to a single transmitter transmitting to a single receiver (single-input single-output or SISO). In one form of cooperati

We propose a novel technique called dimming to improve the energy efficiency of cellular networks by reducing the capacity, services, and energy consumption of cells without turning off the cells. We define three basic methods to dim the network: coverage, frequency, and service dimming. We construct a multi-time period optimization problem to implement frequency dimming and extend it to implement both frequency and service dimming together. We illustrate the ability of dimming techniques to adapt the capacity and network services in proportion to the dynamic spatial and temporal load resulting in significant energy savings through numerical results for a sample network.

In this paper, we develop a performance modeling technique for analyzing the time varying network layer queueing behavior of multihop wireless networks with constant bit rate traffic. Our approach is a hybrid of fluid flow queueing modeling and a time varying connectivity matrix. Network queues are modeled using fluid-flow based differential equation models which are solved using numerical methods, while node mobility is modeled using deterministic or stochastic modeling of adjacency matrix elements. Numerical and simulation experiments show that the new approach can provide reasonably accurate results with significant improvements in the computation time compared to standard simulation tools.

The ability to correctly determine their neighborhood is a fundamental requirement for nodes in ad hoc and sensor networks. Many applications, protocols, and system functionality rely on neighborhood discovery. Malicious nodes that taint neighborhood information using wormholes can significantly dis

Detecting malicious packet dropping is important in ad hoc networks to combat a variety of security attacks such as blackhole, greyhole, and wormhole attacks. We consider the detection of malicious packet drops in the presence of collisions and channel errors and describe a method to distinguish between these types. We present a simple analytical model for packet loss that helps a monitoring node to detect malicious packet dropping attacks. The model is analyzed and evaluated using simulations. The results show that it is possible to detect malicious packet drops in the presence of collisions and channel errors.

The survivability of backbone networks to failures is an on-going concern. This paper investigates survivability strategies for IP-over-WDM networks in a multi-layer framework where traffic originates at each layer. We present an optimization- based formulation of performing recovery mechanisms at the bottom layer for both layers of traffic in two cases: With capacity sharing between backup paths of the traffic in two layers and without. We then study and compare spare capacity requirements under multi-layer traffic ratios and the impact of network connectivity. Numerical results indicate that, in such a wavelength-based optical network, implementing survivability of all traffic at the bottom layer can be a viable solution with significant advantages.

The privacy of communicating entities in wireless ad hoc networks is extremely important due to the potential of their being identified and subsequently subjected to attacks (e.g., in military networks). Previously, random walk and fractal propagation schemes have been proposed to address privacy of source and destination nodes in ad hoc or sensor networks. Entropy of packet transmissions has been used as the metric for comparison. In this paper, we show that under a global attacker that can eavesdrop on the overall data transmissions and count them, neither of these approaches provide sufficient privacy when the attacker can visualize the transmissions and infer contextual information. Moreover, we show that the entropy is not a useful metric in such a case. We propose SECLOUD: Source and Destination Seclusion using Clouds to obfuscate the true source/destination nodes and make them indistinguishable among a group of neighbor nodes which works well even under network-wide traffic visualization by a global attacker.

A well-known approach to increase the resilience of mobile ad hoc networks (MANETs) and unstructured sensor networks is to ensure a network topology where there are at least k disjoint routes in the network between each pair of network nodes (usually called k-connectivity). Asymptotic analyses of node density requirements for k-connectivity have been considered in the literature. In this paper, we present the results of a simulation study investigating the relationship between asymptotic results in the literature and k-connectivity under varying nodal density and nodal degree. The numerical results illustrate where the asymptotic approximations breakdown and we show that this largely due to the existence of critical connectivity points in the topology. Using a critical point identification algorithm we examine how the number of critical points varies with nodal degree, nodal density and node mobility. In addition, critical point is evaluated its effectiveness on the network caused by failure.

Jamming attacks are considered one of the most devastating attacks as they are difficult to prevent and sometimes hard to detect. In this paper we consider the impact of the placement and range of limited-range jammers on ad hoc networks. Limited range jammers are more difficult to detect as they use transmission powers similar to that of regular nodes (or perhaps even smaller transmit powers). The attacker can locate his jammer(s) randomly in the network. Alternatively, jammers can be placed at strategic locations. For instance, intuitively, this can be nodes with the highest traffic inputs/outputs (discovered by sensing the traffic flow in the network). Using OPNET, we perform extensive simulations to show how significant such strategically placed attacks can be compared to random placement of limited-range jammers on both TCP and UDP traffic.

The wormhole attack is considered to be a serious security attack in multihop ad hoc and sensor networks. We propose "DeWorm", a simple protocol to effectively detect wormhole attacks without the need for special hardware and/or strict location or synchronization requirements. DeWorm makes use of discrepancies in routing information between neighbors to detect wormholes. A simulation based analysis of DeWorm for a variety of scenarios shows that the proposed protocol can detect wormhole attacks with a high detection rate, a low false positive rate and low overhead. Further, in comparison to other wormhole detection protocols, the proposed protocol is simple, localized, and capable of detecting a variety of types of wormhole attacks including physical layer wormholes.

Radio resources in third generation (3G) wireless cellular networks (WCNs) such as the universal mobile telecommunications system (UMTS) network are limited in terms of soft capacity. The quality of a signaling service transmission depends on various factors (i.e., a user's location, speed, and data rate requirement), and has an impact on the quality of user data communications where the opposite is also true. In this paper, we provide the first step to evaluate the impact that various signaling service types have on call blocking and ongoing call dropping in UMTS systems. The radio resource's acquisition time for various signaling services is calculated according to the specifications in UMTS standards. The maximum number of sessions that a signaling service type can transmit simultaneously is estimated along with the impact when other signaling service types are transmitted. Our analysis reduces the computational complexity in the call admission control (CAC) and allows the preservation of classes of services. An example traffic scenario is given illustrating the benefit of our study.

Abstract In this paper, we investigate optimal key management design for secure and survivable heterogeneous wireless sensor networks (HWSNs). In particular, we formulate the key management problem as a multi‐objective optimization problem, in which the cost of the sensor network, and the security and survivability metrics of the sensor network are taken into account. To solve the multi‐objective optimization model, we develop a genetic algorithm (GA)‐based approach that can efficiently obtain near‐optimal solutions. We show the performance of our scheme through extensive numerical results. With a small amount of powerful sensor nodes in HWSN, we can balance the cost of the sensor network and the resilience of the sensor network with the required security constraint in different hostile environment. Copyright © 2008 John Wiley & Sons, Ltd.

In this paper, we discuss our preliminary work on signaling overload control for cellular services which requires adaptability and scalability as well as guaranteed level of service. We propose a set of controls that can provide different grades of service to different service classes effectively. The controls are resource sharing schemes proposed consisting of a token rate control combined with priority scheduling and an awareness of the transport network state. Simulation results are given illustrating the behavior of new signaling overload controls and comparative performance with the other existing controls in the literature.

In this paper, we present a comprehensive study on the design of secure and survivable wireless sensor networks (WSNs). Our goal is to develop a framework that provides both security and survivability features that are crucial to applications in WSNs, which are vulnerable to physical and network based security attacks, accidents, and failures. To achieve such a goal, we first examine the security requirements and survivability requirements. We then propose an architecture for security and survivability in WSNs with heterogeneous sensor nodes. To understand the interactions between survivability and security, we also design and analyze a key management scheme. The experiment results show that 1) a good design can improve both security and survivability of WSNs; and 2) in some situation, there is a trade-off between security and survivability.

In this paper, we propose a paradigm for signaling congestion control based on providing quality of service (QoS) to signaling traffic in wireless cellular networks (WCNs). An adaptive multi-class token bucket with an overflow buffer combined with priority scheduling is used to provide different grades of service to classes of signaling messages. Engineering specifically for WCNs, the signaling message congestion control is tied to the state of the traffic transport network. Simulation results are given illustrating the behavior of the new signaling overload control scheme and comparing it with existing schemes in the literature. The performance study shows that the proposed scheme can control congestion and improve performance.

In this paper we consider the problem of provisioning spare capacity in two-layer backbone networks using shared backup path protection. First, two spare capacity allocation (SCA) optimization problems are formulated as integer linear programming (ILP) models for the cases of protection at the top layer against failures at the bottom layer. The first model captures failure propagation using overlay information between two layers for backup paths to meet diversity requirements. The second model improves bandwidth efficiency by moving spare capacity sharing from the top layer to the bottom layer. This exposes a tradeoff between bandwidth efficiency and extra cross-layer operation. Next, the SCA model for common pool protection is developed to allow spare capacity sharing between two layers. Our previous SCA heuristic technique, successive survivable routing (SSR) is extended for these optimization problems. Numerical results for a variety of networks indicate that the common pool protection is attractive to enhance bandwidth efficiency without loss of survivability and that the SSR heuristic quickly results in near optimal solutions

This paper presents an investment strategy to reduce the risk associated with failures in wavelength division multiplexing (WDM) optical networks. The investment strategy determines how to allocate a fixed budget for implementing survivability techniques in different parts of the network such that the expected loss of traffic (ELT) is minimized. Two survivability schemes are considered in this paper: dedicated link protection and dedicated path protection. Two analytical techniques for evaluating network unavailability and ELT are presented in this paper: a fault tree analysis and an event tree. Based on the event tree approach, we propose a novel mixed integer linear programming (MILP) formulation for the investment strategy problem. Numerical results illustrating the investment strategy for both link and path protection are presented and discussed

In this paper, we propose an adaptive capacity allocation scheme at the call level for a blocked call cleared loss system. The scheme aims to maintain the connection blocking probability around a specified target value by dynamically adjusting the allocated capacity. Based on a fluid flow model of the loss system, Lyapunov Stability theory is used to derive an adaptive capacity adjustment scheme which guarantees overall system stability around the target call blocking probability. Numerical results are given which show that the Lyapunov control based scheme is robust to load variations and performs better than existing schemes in the literature.

This paper presents the use of adaptive bandwidth control (ABC) for a quantitative packet loss rate guarantee to aggregate traffic in packet switched networks. ABC starts with some initial amount of bandwidth allocated to a queue and adjusts it over time based on online measurements of system states to ensure that the allocated bandwidth is just enough to attain the specified loss requirement. Consequently, no a priori detailed traffic information is required, making ABC more suitable for efficient aggregate quality of service (QoS) provisioning. We propose an ABC algorithm called augmented Fuzzy (A-Fuzzy) control, whereby fuzzy logic control is used to keep an average queue length at an appropriate target value, and the measured packet loss rate is used to augment the standard control to achieve better performance. An extensive simulation study based on both theoretical traffic models and real traffic traces under a wide range of system configurations demonstrates that the A-Fuzzy control itself is highly robust, yields high bandwidth utilization, and is indeed a viable alternative and improvement to static bandwidth allocation (SBA) and existing adaptive bandwidth allocation schemes. Additionally, we develop a simple and efficient measurement-based admission control procedure which limits the amount of input traffic in order to maintain the performance of the A-Fuzzy control at an acceptable level.

The design of survivable mesh based communication networks has received considerable attention in recent years. One task is to route backup paths and allocate spare capacity in the network to guarantee seamless communications services survivable to a set of failure scenarios. This is a complex multi-constraint optimization problem, called the spare capacity allocation (SCA) problem. This paper unravels the SCA problem structure using a matrix-based model, and develops a fast and efficient approximation algorithm, termed successive survivable routing (SSR). First, per-flow spare capacity sharing is captured by a spare provision matrix (SPM) method. The SPM matrix has a dimension the number of failure scenarios by the number of links. It is used by each demand to route the backup path and share spare capacity with other backup paths. Next, based on a special link metric calculated from SPM, SSR iteratively routes/updates backup paths in order to minimize the cost of total spare capacity. A backup path can be further updated as long as it is not carrying any traffic. Furthermore, the SPM method and SSR algorithm are generalized from protecting all single link failures to any arbitrary link failures such as those generated by Shared Risk Link Groups or all single node failures. Numerical results comparing several SCA algorithms show that SSR has the best trade-off between solution optimality and computation speed.

Information assurance techniques employed in wired networks have limited direct applicability in wireless networks because of the unique aspects of wireless networks (e.g., user mobility, wireless communication channel, power conservation, limited computational power in mobile nodes, security at the link layer, etc.). The interaction between the components of information assurance, namely availability and security in a wireless network environment poses new challenges. In this article, we present a framework for understanding survivability and security in wireless network and also discuss the issues related to the interaction between survivability and security in hybrid wireless access networks.

In this paper we develop a performance modeling technique for analyzing the time varying performance of mobile ad hoc networks. Our approach is a novel hybrid of discrete event simulation and numerical method based queueing analysis. Network queues are modeled using fluid-flow based differential equation models which are solved using numerical methods, while node mobility is modeled using an adjacency matrix topology representation whose values are determined via discrete event simulation techniques. Numerical results are given illustrating the approach.

This paper proposes a novel energy saving scheme, termed the Gossip-based sleep protocol (GSP). With GSP, each node randomly goes to sleep for some time with gossip sleep probability p. When the value of p is small enough, the network stays connected. GSP does not require a wireless node to maintain the states of other nodes. It requires few operations and scales to large networks. Two versions of GSP, one for synchronous networks and one for asynchronous networks are proposed, and the advantages of the GSP approach through both simulations and analysis is showed in this paper.

The paper proposes an adaptive bandwidth control algorithm that efficiently provides an aggregate loss guarantee to resolve the problem of inefficient bandwidth allocation due to incomplete, inaccurate traffic descriptors supplied by users. Because the control attempts to allocate only just enough bandwidth to meet the QoS requirement, the amount of bandwidth saving compared to static allocation can be substantial. Another distinct advantage of our control algorithm is that no a priori information on the traffic characteristics of the aggregate is required. From the simulation study, the proposed control can maintain the packet loss QoS while attaining very high utilization, and is robust against different system configurations and controller parameters.

With the deployment of MPLS over a core IP backbone, it is possible for a service provider to build virtual private networks (VPNs) supporting various classes of services with QoS guarantees. Efficiently mapping the logical layout of multiple VPNs over a service provider network is an important and challenging traffic engineering procedure. The use of sink-tree (multipoint-to-point) routing paths in an MPLS network makes the VPN design problem different from traditional design approaches where a full-mesh of point-to-point paths is often the choice. The clear benefits of using sink-tree paths are the reduction in the number of label switch paths and the bandwidth saving due to the larger granularity of bandwidth aggregation within the network. The design of multiple VPNs over MPLS, using sink-tree routing, is formulated as a mixed integer programming problem to find simultaneously VPN logical topologies and their dimensions to carry multi-service, multi-hour VPN traffic from various customers. Such a problem formulation yields an NP-hard complexity. We propose a heuristic aiming to scale the VPN design problem by choosing a small-but-good candidate set of feasible sink-tree paths to solve the optimization problem. Numerical results are given showing the advantages of the proposed approach.

This paper addresses the spare capacity allocation (SCA) problem considering any single node failure in mesh networks. The SCA node failure problem aims at finding backup routes and providing sufficient spare capacity to protect traffic when any single node fails in a communication network. Here, we introduce our novel matrix formulation of the arc-flow SCA node failure model. In this model, working paths are given before pre-planned backup paths are routed and reserved. Because backup paths can not be guaranteed if general shortest path routing of working paths is used, we give a graph algorithm to find the working path which has at least one node-disjoint backup path. We extend our recent approximation algorithm, successive survivable routing (SSR), to solve the above SCA model. Numerical comparison shows that SSR has the best trade-off between solution optimality and computation speed.

Spare capacity allocation (SCA) is an important part of a fault tolerant network design. In the spare capacity allocation problem one seeks to determine where to place spare capacity in the network and how much spare capacity must be allocated to guarantee seamless communications services survivable to a set of failure scenarios (e.g., any single link failure). Formulated as a multi-commodity flow integer programming problem, SCA is known to be NP-hard. We provide a two-pronged attack to approximate the optimal SCA solution: unravel the SCA structure and find an effective algorithm. First, a literature review on the SCA problem and its algorithms is provided. Second, a integer programming model for SCA is provided. Third, a simulated annealing algorithm using the above INP model is introduced. Next, the structure of SCA is modeled by a matrix method. The per-flow based backup path information are aggregated into a square matrix, called the spare provision matrix (SPM). The size of the SPM is the number of links. Using the SPM as the state information, a new adaptive algorithm is then developed to approximate the optimal SCA solution termed successive survivable routing (SSR). SSR routes link-disjoint backup paths for each traffic flow one at a time. Each flow keeps updating its backup path according to the current network state as long as the backup path is not carrying any traffic. In this way, SSR can be implemented by shortest path algorithms using advertised state information with complexity of O( Link/sup 2/). The analysis also shows that SSR is using a necessary condition of the optimal solution. The numerical results show that SSR has near optimal spare capacity allocation with substantial advantages in computation speed.

This paper presents a framework for providing survivability to group communications where part of the underlying traffic layer infrastructure is connection-oriented. The framework is multi-layered to express the virtual overlays inherent to networked systems, describes survivability issues unique to group communications, and outlines tradeoffs between restoration techniques that can be extended from circuit-switched communications. The conclusion is disjoint dedicated backup route sets to provide survivability for connection-oriented group communications is preferable but more research needs to be done on coordination between layers and scalable techniques within the survivability framework presented.

Research and development on the survivability of networks has largely focused on public switched telecommunications networks and high speed data networks with little attention on the survivability of wireless access networks supporting cellular and PCS communications. This paper provides an overview of the survivability issues in PCS networks with emphasis on the unique difficulties presented by user mobility and the wireless channel environment. A multi-layer framework for the study of PCS network survivability is proposed. Metrics for quantifying network survivability are identified at each layer. It is shown that user mobility significantly worsens the network performance after even small failures, as disconnected users move among adjacent cells and attempt to reconnect to the network. Thus survivability strategies must be designed to contend with spatial as well as temporal network behavior. Possible survivability strategies and restoration techniques for each layer in the framework are also discussed.

In this paper we propose a priority scheme for reconnection of virtual circuits (VCs) in ATM networks that have been disrupted by a failure. ATM networks offer several service categories each designed to handle applications with specific traffic characteristics. A failure typically results in a large number of disrupted connections in each category, all of which must be restored simultaneously. A critical issue in the restoration is the order in which the service categories are processed, and the order of processing the connections and routing within each category. The proposed priority scheme aims at minimizing the impact of a failure on the network.

Due to the increasing reliance of society on the timely and reliable transfer of large quantities of information (such as voice, data, and video) across high speed communication networks, it is becoming important for a network to offer survivability, or at least graceful degradation, in the event of network failure. In this paper we aim to offer a solution in the selection of the K-best disjoint paths through a network by using graph theoretic techniques. The basic approach is to map an arbitrary network graph into a trellis graph which allows the application of computationally efficient methods to find disjoint paths. Use of the knowledge of the K-best disjoint paths for improving the survivability of ATM networks at the virtual path and virtual circuit level is discussed.

We consider the combined control problem of connection admission, flow rate, and bandwidth allocation (capacity, service-rate) under nonstationary conditions. A fluid flow model in state variable form describes the time varying mean behaviour of available bit rate (ABR) traffic, which competes with guaranteed traffic for network resources. Using nonlinear control we derive an integrated control strategy, for the finite buffer and finite server case, that is insensitive to any propagation delay. We also derive bounds on mix between ABR and guaranteed traffic to ensure that control strategy is effective, and select control design variables which can be used to influence the delivered QoS. The performance of proposed scheme is evaluated using analysis and simulation. Simulation results show that it achieves effective server and buffer utilisation (as predicted by analysis) and by appropriate choice of control design variables achieves prescribed bounded delays (for guaranteed traffic only) and zero cell loss (even for ABR traffic demands exceeding server capacity, which if not controlled would cause losses). Hence proposed scheme delivers guaranteed QoS to user and avoids retransmissions, thus increasing network utilisation.

We present a comparative analysis of bandwidth access control schemes under nonstationary traffic conditions for heterogeneous circuit-mode traffic offered to a B-ISDN network link. A unified model for analysing the behavior of several bandwidth access control schemes is developed using a Markov process model with acceptance functions. Numerical techniques are used to solve the associated Chapman-Kolmogorov equations and determine the nonstationary behavior. Performance results are given for several common bandwidth access control schemes, namely: complete sharing, complete partitioning, partial sharing, classical trunk reservation, and the probabilistic reservation policy. It is shown that the use of the average arrival rate to estimate the average connection blocking rate may be an underestimate for a system with a time varying arrival process. Also, it is found that a nonstationarity in one traffic class arrival process can impact other stationary traffic classes and the degree of variability depends on the particular access control scheme.

Evaluation of the behavior of queues with nonstationary arrival processes is of importance in several applications including communication networks. However, the analysis of nonstationary queues is in general computationally complex and seldom produces closed form expressions. Thus approximation methods may be more appropriate. The pointwise stationary fluid flow approximation (PSFFA) for determining the mean queue length of nonstationary queues is presented. The PSFFA combines steady state queueing results with a simple fluid flow model to develop a single nonlinear differential equation model of the queue. Numerical integration techniques are used to solve the PSFFA model and the method is illustrated by several examples. The power of this approach is that it can handle very general queueing systems.

Current standards reserve one bit in the ATM cell header to indicate loss priority. When congestion occurs at a queue lower priority cells can be discarded in order to insure a smaller cell loss rate for higher priority cells. Strategies for determining which cells to discard are termed space priority buffer management schemes. In this paper two adjustable space priority schemes are studied where a cell upon arriving to a full buffer can pushout a cell of opposite priority depending upon an adjustable parameter. A queueing analysis of an ATM switching node is conducted to compare the adjustable pushout schemes with other common space priority mechanisms.>

The authors present the results of a study to determine the effects of link failures on the performance of a network in terms of the occurrence of congestion due to traffic restoration after a failure. The network studied is a virtual circuit based packet switched wide area network. A generic queueing framework is developed to study the effect of failures and the subsequent traffic restoration on network performance. In general, the congestion resulting after a failure is a transient phenomenon. Hence, a numerical methods based nonstationary queueing analysis is conducted in order to quantify the effects of failures in terms of the transient behavior of queue lengths and packet loss probabilities. A bounding relationship is developed whereby a network node can determine whether or not congestion will occur as the result of traffic restoration after a failure.>

The results of a study to determine the effects of link failures on network performance are presented. The network studied is a virtual-circuit-based packet-switched wide area network. A generic queuing framework is developed to study the effect of failures, and the subsequent traffic restoration, on network performance. In general, the congestion resulting after a failure is a transient phenomenon. Hence, a numerical-method-based nonstationary queuing analysis is conducted in order to quantify the effects of failures in terms of the transient behavior of queue lengths and packet loss probabilities. A bounding relationship is developed whereby a network node can determine whether or not congestion will occur as the result of traffic restoration after a failure.>

The behavior of window flow control schemes that adapt to changing network conditions is studied. A dynamic window scheme, which adjusts the window size based on explicit network congestion indicators, is proposed. The throughput performance of the adaptive policy under both steady-state and nonstationary conditions is studied. The superiority of this scheme over some previously proposed adaptive window schemes based on implicit congestion detection is demonstrated.>

The problem of virtual circuit routing in data communication networks under nonstationary conditions is considered. A state variable modeling approach is adopted to describe the dynamic behavior of the possible paths for the virtual circuit through the network. The state model is developed by representing each link in a path as a set of nonlinear differential equations describing the dynamics of the virtual circuit and the network traffic in terms of time-varying mean quantities. An optimal control problem is formulated to determine the virtual circuit routing path which minimizes the number of packets in the network. The solution of the optimal control problem by Hamilton-Jacobi arguments is discussed leading to a threshold routing policy which can be implemented in a decentralized fashion. The performance of the routing algorithm is studied under both steady-state and nonstationary conditions, and its superiority over the performance of previous schemes in a dynamic load environment is demonstrated.>

Methods for the simulation of the nonstationary behavior of computer networks are discussed. Steady-state performance measures, typically computed over time histories, are modified to be applicable to the nonstationary case by redefining the measures as ensemble statistics. The application of well-known ensemble simulation techniques to determining the modified performance metrics from finite ensembles generated by independent replications is given. The simulation methods are illustrated with a comprehensive tutorial example and with a performance study which gives results for a shared buffer switch.>

Numerical techniques for modeling computer networks under nonstationary conditions are discussed, and two distinct approaches are presented. The first approach uses a queuing theory formulation to develop differential equation models which describe the behavior of the network by time-varying probability distributions. In the second approach, a nonlinear differential equation model is developed for representing the dynamics of the network in terms of time-varying mean quantities. This approach allows multiple classes of traffic to be modeled and establishes a framework for the use of optimal control techniques in the design of network control strategies. Numerical techniques for determining the queue behavior as a function of time for both approaches are discussed and their computational advantages are contrasted with simulation.>

Three distinct approaches for modeling computer networks under nonstationary conditions are presented. The first approach uses a queuing-theoretic formulation to develop numerical techniques for determining the nonstationary queue dynamics and for estimating the settling time for the queue. In the second approach, discrete-event simulation techniques are used for the evaluation of network performance during nonstationary periods. The third approach develops a nonlinear state model for representing the dynamics of the packet queues at the various transmission links of the network and establishes a framework for formulating optimal control problems for designing routing and flow control strategies that ensure optimal network performance under both transient and steady-state conditions.>

A simple analytical approximation to the optimal buffer-management policy valid in the moderate to heavy load ranges is presented. The throughput performance of the present adaptive policies under both steady-state and transient conditions is studied. The superiority of this scheme over the performance resulting from existing buffer-management schemes is demonstrated.>