Work life is rapidly developing owing to digitalization and new work practices. As a consequence, intensive training is needed for working people to acquire new skills and knowledge. Such training might require larger efforts of weeks or months of full time work in order for professionals to get started on new roles at work. For this type of education, the boot camp format may be suitable. It is characterized by a focus on essentials in contents, scheduled work time, and social interaction and support amongst the course participants. This paper presents learning from boot camp training of conscript soldiers and it extends the experiences to training of working professionals. The novelty of the endeavor for the university is that the courses are given on a remote site by instructors without expert knowledge in the field; the university provides curated contents, formal examination and support to the instructors. This setup works well and has been in production for four years with minimal adjustments. It builds on good communication with the instructors, on teaching material structured for independent study, and on peer support among the students. We contrast the format with the popular massive open online courses and discuss how the boot camp format scales in terms of number of courses and course participants with respect to resources, primarily the time of the responsible teachers. The conclusion is that boot camp training should be considered for professional education and that it might not be more demanding for the teachers than any other course format.

Ever more powerful mobile devices are nowadays capable of collectively carrying out reasonably demanding computational tasks without offloading the processing to an edge server or a distant cloud-computing service. In this work, we explore such distributed computing and study how it is affected by the mobility as well as the number of nodes that collaborate. We choose distributed counting of a number of nodes in an enclosed area as application. Such application is useful for estimating attendance at events and measuring occupancy for facility management, as needed for monitoring of crowdedness with respect to safety and evacuation, climate control and comfort. For this application, we are interested in determining the time until all nodes know the correct number of nodes in the space where they reside. Our study shows the effect of mobility on the distributed process. We find that the process of collecting data opportunistically from a closed set of nodes is well described by empirical laws that we derive. We discuss the results and suggest further work needed to understand opportunistic computation and to develop it as a new model of computation among collectives of mobile nodes.

Accurate prediction of user mobility allows the efficient use of resources in our ubiquitously connected environment. In this work we study the predictability of the users' next location, considering a campus scenario with highly mobile users. We utilize Markov predictors, and estimate the theoretical predictability limits. Based on the mobility traces of nearly 7400 wireless network users, we estimate that the maximum predictability of the users is on average 82%, and we find that the best Markov predictor is accurate 67% of the time. In addition, we show that moderate performance gains can be achieved by leveraging multi-location prediction.

This paper presents an analysis of a large trace of user associations in a university wireless network, which includes around one thousand access points over five campuses. The trace is obtained from RADIUS authentication logs and its merit is in its recency, scale and duration. We propose a methodology for extracting association statistics from these logs, and look at visiting time distributions and processes of user arrivals to access points. We find that a large fraction of the network - around half of all access points - experiences time-varying Poisson arrival process, and association distributions can be modeled by two-stage hyper-exponential distributions at most of the access point. While network associations in campus wireless networks have been extensively studied in the literature, our study reveals changing patterns in user arrival processes and association durations, which seem to be characteristic for networks of predominantly mobile users, and allows the use of tractable network occupancy models.

A major challenge in mobile wireless devices for opportunistic networks is to decrease the energy consumption. The decrease should not come at a cost of reduced application throughput (i.e., goodput). This work evaluates the potential performance gains for mobile nodes that adopt duty cycling in an opportunistic context. The paper presents an analytical framework for evaluating goodput and energy consumption of nodes based on a probabilistic estimation of effective contact durations, and it validates this framework on a mobility scenario. The study shows that both goodput and energy consumption depend strongly on the distribution of listening durations, and that goodput is independent of the contact rate among nodes. This work also includes extensive trace-driven simulations and demonstrates that duty cycling considerably improves the performance of opportunistic networks by decreasing the energy consumption without significantly affecting the goodput.

Opportunistic networking (a.k.a. device-to-device communication) is considered a feasible means for offloading mobile data traffic. Since mobile nodes are battery-powered, opportunistic networks must be expected to satisfy the user demand without greatly affecting battery lifetime. To address this requirement, this work introduces progressive selfishness, an adaptive and scalable energy-aware algorithm for opportunistic networks used in the context of mobile data offloading. The paper evaluates the performance of progressive selfishness in terms of both application throughput and energy consumption via extensive trace-driven simulations of realistic pedestrian behavior, and demonstrates that the proposed algorithm is robust against the distributions of node density and initial content availability. The results show that in certain scenarios progressive selfishness achieves up to 85% energy savings during opportunistic downloads while sacrificing less than 1% in application throughput. Furthermore, the study demonstrates that in terms of total energy consumption (by both cellular and opportunistic downloads) in dense environments the performance of progressive selfishness is comparable to downloading contents directly from a mobile network.

Direct device-to-device communication based on the `store-carry-forward' paradigm is considered a feasible means for offloading mobile data traffic. Employing device-to-device communication however should satisfy user demand without greatly affecting battery lifetime. In order to provide satisfactory application throughput it is often assumed that nodes behave altruistic, and are willing to share contents for an infinite amount of time. In the context of energy savings, this assumption is overly optimistic. This work evaluates the performance of a publish/subscribe opportunistic content dissemination application that uses duty cycling for energy saving and allows nodes to behave in a selfish manner. Two types of selfishness are introduced: strict and mild. The paper presents the impact of selfishness on both application throughput and energy consumption via extensive trace-driven simulations, and demonstrates that introducing strictly selfish behavior on top of duty cycling leads to great decrease in energy consumption (up to 90% in certain cases) without causing significant loss in application throughput. Moreover, when the duration of the mild selfishness interval is chosen appropriately, mild selfishness can lead to even further decrease in energy consumption while at the same time increasing the application throughput.

In opportunistic networking, characterizing contact patterns between mobile users is essential for assessing feasibility and performance of opportunistic applications. There has been significant efforts in deriving this characterization, based on observations and trace analyses; however, most of the findings arise from studying contact opportunities at large spatial and temporal scales. Moreover, the user population is considered to be constant: no users can join or leave the system. Yet, there are many examples of scenarios which do not fully adhere to the previous assumption and cannot be accurately described at large scales. Urban environments, such as smaller city districts, are characterized by highly dynamic user populations. We believe that scenarios with varying population requires further investigation. In this paper, we present a novel modeling approach to study operation of opportunistic applications in scenarios where the population size is subjected to frequent changes, that is, it exhibits churn. We also propose an application for estimating the size of a mobile crowd, which we then use to validate our model in four scenarios: a city area, subway station, a conference and a scenario with a synthetic mobility model. We show that the model provides good representations of the investigated scenarios.

We derive a modular fluid-flow network congestion control model based on a law of fundamental nature in networks: the conservation of information. Network elements such as queues, users, and transmission channels and network performance indicators like sending/acknowledgment rates and delays are mathematically modeled by applying this law locally. Our contributions are twofold. First, we introduce a modular metamodel that is sufficiently generic to represent any network topology. The proposed model is composed of building blocks that implement mechanisms ignored by the existing ones, which can be recovered from exact reduction or approximation of this new model. Second, we provide a novel classification of previously proposed models in the literature and show that they are often not capable of capturing the transient behavior of the network precisely. Numerical results obtained from packet-level simulations demonstrate the accuracy of the proposed model.

Many mobility models currently used for evaluating wireless communication systems have weak resemblance to reality and there is a lack of understanding on which characteristics of human mobility affect system performance. In particular, most current mobility models assume a free flow of nodes and do not consider how mobility is affected by interactions with other persons and with the physical environment. They also assume a closed system, not considering the effect of node arrival and departure. The structure of space in which the mobility occurs is either not considered at all, or only in a limited way. In this work, we address human pedestrian mobility for evaluation of wireless communication to determine which of the aforementioned aspects need to be captured and to what level of detail. We focus on opportunistic communication in the form of ad-hoc and delay-tolerant networks. For the evaluation, we use mobility models from the field of transportation and urban planning that are used for designing and dimensioning public spaces for comfort and safety of pedestrians in rush hour and emergency evacuation. The models capture micro-mobility of pedestrians better than most mobility models used in mobile networking since the application domain requires realistic representation of node interactions with the physical environment and with other nodes. Our results show that the free flow assumption used in most models does not have a significant performance impact. We also conclude that performance is not very sensitive to accurate estimation of the probability distributions of mobility parameters such as speed and arrival process. Our results, however, suggest that it is important to capture the scenario and space in which mobility occurs since these may affect performance significantly.

Epidemic content dissemination has been proposed as an approach to mitigate frequent link disruptions and support content-centric information dissemination in opportunistic networks. Stochastic modeling is a common method to evaluate performance of epidemic dissemination schemes. The models introduce assumptions which, on one hand make them analytically tractable, while on the other, ignore attested characteristics of human mobility. In this paper, we investigate the fitness and limitations of an analytical stochastic model for content dissemination by comparison with experimental results obtained from real mobility traces. Our finding is that a homogeneous analytic model is unable to capture the performance of content dissemination with respect to content delivery delays.

A major challenge for wireless networks is to minimize the energy consumption in the mobile devices. This work evaluates potential performance gains of a dual-radio architecture in which a low power radio is used to wake up the primary high power radio. Our targeted domain is opportunistic communication directly between devices. We show that it can significantly reduce the energy consumption at a price of slightly decreased goodput. We also examine the effect of the MAC protocol on the performance of the dual-radio system and we point out that in dense scenarios MAC layer protocols, such as of 802.15.4, do not perform well. We observe that information on device density of an environment can be used to address this shortcoming.

A modular fluid-flow model for network congestion analysis and control is proposed. The model is derived from an information conservation law stating that the information is either in transit, lost or received. Mathematical models of network elements such as queues, users, and transmission channels, and network description variables, including sending/ acknowledgement rates and delays, are inferred from this law and obtained by applying this principle locally. The modularity of the devised model makes it sufficiently generic to describe any network topology, and appealing for building simulators. Previous models in the literature are often not capable of capturing the transient behavior of the network precisely, making the resulting analysis inaccurate in practice. Those models can be recovered from exact reduction or approximation of this new model. An important aspect of this particular modeling approach is the introduction of new tight building blocks that implement mechanisms ignored by the existing ones, notably at the queue and user levels. Comparisons with packet-level simulations corroborate the proposed model.

The fast spreading of smart mobile devices has changed the way people create and share multimedia contents. A recently proposed idea is to distribute contents opportunistically when devices are within each other's communication range. In this work we have seized this idea and designed and implemented a real-time music-sharing application that allows people to share a sound system for collective listening. The system utilizes a content-centric opportunistic networking middleware. The application provides an interface for users to play audio files stored in their own mobile devices through a shared jukebox that is within direct communication range. The jukebox plays the audio file that is first in queue in its playlist by streaming it from the publishing user device. All devices synchronize with the jukebox so that the users can observe the contents of the playlist on their displays in real-time. This paper presents the idea and motivation for such a system as well as its design and evaluation.

In this paper we examine content distribution in opportunistic networks. We design and evaluate strategies by which a node fetches and shares contents on behalf of other nodes, even when the contents are not of its own interest. We propose four relaying/caching options for improving the use of network resources: relay request on demand, hop-limit, greedy relay request, and weighted relay request with allocated caching positions. We also compare our strategies with a strategy from the literature. The proposed strategies are implemented in the OMNeT++ simulator, and evaluated on mobility traces with different churn levels. The results show that the performance of opportunistic caching is strongly dependent on the level of churn in the network. However, we find a strategy that outperforms the rest in all of the explored network settings.

Motivated by applications in industrial communication networks, in this paper we consider the trade-off between relationship anonymity and communication overhead in anonymity networks. We consider two anonymity networks; Crowds that provides unbounded communication delay and Minstrels, proposed in this paper, that provides bounded communication delay. While Crowds hides the sender's identity only, Minstrels aims to hide the receiver's identity as well. However, to achieve bounded message delay it has to expose the sender's identity to a greater extent than Crowds. We derive exact and approximate analytical expressions for the relationship anonymity for these systems. While Minstrels achieves close to optimal anonymity under certain conditions, our results show that, contrary to expectations, increased overhead does not always improve anonymity.

We present a demonstration scenario to validate a middleware architecture that we designed and implemented for distributing content between mobile users in an opportunistic manner. The peer-to-peer networking architecture allows content dissemination between mobile devices without relying on any infrastructure support. We developed a mobile application utilizing the services provided by the implemented middleware. The application facilitates opportunistic content distribution in both one-to-one and one-to-many dissemination modes.

Rapid growth of mobile wireless industry has generated numerous challenges for service operators. The provisioning of increasing demand in capacity and higher quality of service can be met with redimensioning of networks which makes a priori network planning difficult. We propose a network performance monitoring system to which all mobile devices contribute by voluntarily submitting periodical reports on the availability and quality of network access that they experience at their current geographical locations. Network performance measures such as signal strength, coverage, call block/drop statistics and operator service capabilities are reported to a server with a network status information database as experienced by mobile devices. The proposed database will indicate areas with lack of coverage, insufficient capacity, and high interference, which may serve as cues for operators to upgrade/retune their infrastructure equipment and for regulators to take action against operators that do not fulfill commitments made to their subscribers.

The demands for ubiquitous connectivity and high data rates have led to a proliferation of mobile devices such as smartphones, media tablets and netbooks. These devices, equipped with powerful communication and storage resources, are carried by humans during their daily activities and, thus, possess a high potential for opportunistic networking. In this networking mode, encounters between users define the “topology” of the network and can be exploited for content sharing. A large portion of user encounters occur in densely populated public urban areas such as city squares, subway stations, shopping malls, etc. In this paper, we propose an analytic model to study content dissemination characteristics inside smaller areas where the mobility of nodes does not affect their connectivity. Subsequently, we argue that this model is suitable for modeling larger areas as interconnections of these basic blocks.

In this paper, we study the performance of the proportional fair scheduler, which has been proposed for some of the emerging radio access systems. It maximizes the spectral efficiency of the systems, which is a strong incentive for network providers to use it. Our goal is to investigate how the proportional fair scheduling affects the performance of streaming users. Focus is on the throughput-delay trade-offs associated with proportional fair scheduling in CDMA/HDR systems. Special attention is devoted to defining appropriate performance measures and creating a realistic simulation environment. Our results indicate that the opportunistic scheduling may face difficulties in providing the user-level performance in cases where streaming flows constitute a significant share of the traffic load.

This paper investigates a class of mobile wireless sensor networks that are not connected most of the times. The characteristics of these networks is inherited from both delay tolerate networks (DTN) and wireless sensor networks. First, delay-tolerant wireless sensor networks (DTWSN) are introduced. Then, three main problems in the design space of these networks are discussed: Routing, data gathering, and neighbor discovery. An approach is proposed for deployment of DTWSNs based on the traditional opportunistic broadcast in delay tolerant networks with on-off periods. The delay and the throughput of the routing scheme were investigated in the DTN literature. However, the energy consumption was not studied thoroughly, which is focused here. Neighbor discovery in a sparse network could be a major source of energy consumption. Therefore, energy per contact measure is evaluated analytically based on the distribution of physical link duration. The results for 2D constant velocity model and random waypoint model are reported and the average PLD is suggested as an appropriate choice of beacon interval.

One of the intrinsic problems posed by mobility in wireless networks is the discovery of other nodes in the vicinity of a node to establish new links with them as well as detecting a link breakage. A widely used solution for this problem is to use different variations of beaconing. Here, we study the effect of the beacon settings on the link reliability and probability of link establishment. A poorly adjusted rate of beacon transmissions may lead to vast resource usage (power and bandwidth) on one side, or may lead to poor throughput on the other side. We use a general model without assuming a particular mobility model. The model is instantiated for periodic and exponential beaconing and it is then applied to compare two-way beaconing with one-way beaconing. Then, it is used to analyze the effect of connection setup time. Moreover, it is analytically proven that widely used periodic beaconing is not only the simplest beaconing scheme but also the optimal one in terms of beacon hit efficiency. Finally, the model is applied to a case study in delay-tolerant networks to study the effect of beacon interval on the capacity and the energy consumption of the network. Finally, a practical thumb of rule for beacon rate adjustment is suggested to bypass heavy computations.

One of the central problems in wireless video transmission is the choice of source and channel coding rates to allocate the available transmission rate optimally. In this paper, we present a structural distortion model for video streaming over time-varying fading channels. Based on the model, we study the end-to-end distortion for various bit-rate allocation strategies and channel conditions. We show that the robustness to channel variations is crucial for the streaming performance when frequent bit-rate adaptations are not feasible. It is achieved at the expense of higher source distortion in the encoder. Our findings are illustrated on a practical problem of distortion-optimal selection of transport formats in an adaptive modulation and coding (AMC) scheme used in HSDPA.

We present extensions to the IEEE 802.11 WLAN architecture to facilitate the deployment and operation of large-scale networks. A new distribution system (DS) is designed to work across IP networks removing the need for an Ethernet backbone. APs connected to different IP networks can join the DS to form a single WLAN, with link-layer mobility support for users. Two entities, portal and registry, are added to support self-configuration of access points, transport across the IP backbone and link layer mobility for stations. These extensions are transparent to the stations; they do not require any modification to work with the new architecture. Our DS design allows operators to share access points.

The use of licensed spectrum for wireless communication is driven by the need to control interference between different operators. However, with this mode of regulation, spectrum utilization is far from efficient and the growth of wireless networks is hindered by the shortage of free frequency bands and the vast investments for the acquisition of a license. In view of this situation, we present an alternative evolution path for the unobstructed growth of wireless networks and the efficient use of spectrum. The proposed architecture is based on the use of unlicensed spectrum and the open access of users to all public networks without prior contracts with operators. We highlight and discuss the inherent technical challenges that must be tackled before the proposed solution can be realized. Special attention is paid to the inherent need for alternative interference mitigation strategies.

The development of mobile ad hoc systems have considerably emphasized the need for a better understanding of the factors that influence the systemspsila performance, i.e., mobility patterns, radio propagation, traffic characteristics and their interrelations. Depending on the context where the system is being used-whether at work, at home, or in some means of transportation-there will be various applications that provide benefit of using the system. These different applications in turn generate traffic with very heterogeneous characteristics. In addition, the radio propagation depends on the surrounding environment and the density of communicating nodes in the system and in other systems that compete for radio resources; hence the available data rates in the system are hard to ascertain. The uncertainty regarding both the traffic and the available data rates make it hard to evaluate the performance of the system. This uncertainty is due to the lack of deployed systems. We hence believe that research in mobile ad hoc networks has to couple theory and practice in a continuous feedback loop with experimentations running full-fledged applications. We show how this methodology has proved to be beneficial in order to overcome challenges and to determine main issues for future research in the context of our PodNet project. It has also allowed us to revisit assumptions and scepticism surrounding the feasibility of mobile ad hoc networks. PodNet is an architecture, instantiated in a prototype, that is dedicated to cooperative content distribution. It enables exchange of podcasts (for any kind of multimedia content) amongst mobile devices in a peer-to-peer fashion using IEEE 802.11 in ad hoc mode. PodNet has clear advantages over traditional content distribution approaches (e.g., newspapers, 3G) in terms of public availability, practicability, capacity, resilience to failures, and jamming.

The eight papers in this special issue focus on delay and disruption tolerant wireless communication. The papers cover routing and network coding for spare mobile ad hoc networks, cross-layer design for sensor networks, satellite communication, and DTN architectural issues.

When the forwarding paths in an IP network change due to a link failure or a link weight modification, the forwarding tables in the routers may need to be updated. Each of these updates may cause transient loops if they are not performed in an appropriate order. In this paper, we propose an order to update the forwarding tables that avoids transient loops for non-urgent changes. The order is obtained by studying the changes in the forwarding tables, therefore it can be used in networks running any routing protocols, and for any type of forwarding path changes. After presenting the order, we prove that it is correct, and present an efficient algorithm to compute the order. Thereafter, we present several algorithms for performing forwarding table updates in accordance with the order. We also discuss how the update algorithms can be applied to both networks with centralized control and decentralized routing protocols. Finally, we study the update algorithms' performance on several network topologies and with varying parameter settings and for several types of forwarding path changes.

Ad-hoc wireless peer-to-peer methods are an important and viable alternative to the more traditional infrastructure based ones for distributing contents among a population of users. The PodNet project is a ongoing effort to create a broadcasting system for delay-tolerant content, in which peer nodes exchange contents opportunistically when within range, and their mobility acts as a complementary mechanism for the dissemination. This paper introduces the design of a Pod-Net node. In particular, the special case of a gateway, which bridges the infrastructure world of the Internet and the ad-hoc domain, is considered.

Network processor systems provide the performance of ASICs combined with the programmability of general-purpose processors. One of the main challenges in designing these systems is the memory subsystem used when forwarding and queueing packets. In this work, we study the queueing behavior and packet delays in a network processor system which works as a router. We introduce a system model and a simulation tool based on the model. Using the simulation tool, both best-effort and diffserv IPv4 forwarding were modeled and tested using real-world and synthetically generated packet traces. The results on queueing behavior have been used to dimension various queues, and can be used as guidelines for designing memory subsystems and queueing disciplines. In particular, a system with small queue sizes has been proposed. The results on packet delays also show that our diffserv setup provides good service differentiation for best-effort and priority packets. Finally, the study reveals that the choice of traces has a large impact on the results when evaluating router and switch architectures.

Applications that require low loss probabilities in today's Internet have to employ some end-to-end error-recovery mechanism. For interactive applications with strict delay constraints, the delay introduced by the applied schemes has to be low as well. In this paper we compare two schemes proposed for error recovery for real-time audio applications: media-dependent forward error correction (MD-FEC) and multiple description coding (MDC). We conclude that MDC always performs better than MD-FEC, and that the stationary loss probability plays a key role in the choice of the optimal parameters for these schemes. Combining the analytical results with the loss characteristics of measured traces of VoIP calls we conclude that in the current Internet these schemes give considerable gains for streams with a high code rate only, and for these streams MDC can decrease the average distortion significantly better than MD-FEC.

In this paper we propose an analytical model of a resilient end-node multicast streaming architecture based on multiple minimum-depth-trees that employs path diversity and forward error correction for improved resilience to node churns and packet losses. We study the performance of the architecture in the presence of packet losses and dynamic node behavior. We show that for a given redundancy the probability that an arbitrary node possesses a packet is high as long as the loss probability in the network is below a certain threshold. After reaching the threshold the packet possession probability suddenly drops; the rate decrease gets faster as the number of nodes in the overlay grows. The value of the threshold depends on the ratio of redundancy and on the number of the distribution trees. We study the overlay structure in the presence of node dynamics and conclude that stability can be achieved only if the root node serves a large number of nodes simultaneously

This paper investigates the possibility to differentiate services by using endpoint traffic controls, while sharing a single best-effort network The starting point is a scheme based on a combination of probe-based admission control in the end-systems for streaming traffic and error correction to isolate different flows. There are inherent limitations in pure host-based control mechanisms and we therefore investigate the improvement that can be achieved when explicit congestion notification is used in routers. In particular we investigate the sensitivity to different active queue management mechanisms and parameter settings. Simulations show that a high ECN marking rate helps to differentiate between flows, but it is important to make sure that the chosen AQM works well for the chosen parameters. This study also shows that a main benefit of ECN is to enable the extension of the scheme to wireless environments

This paper presents a distributed admission control for the distributed coordination function of IEEE 802.11 wireless LANs that limits the risk of congestion collapse due to a high arrival rate of flows. This control scheme requires no modification to the current distributed coordination function; it works by performing a short, non-disturbing probe that estimates the MAC service time. The flow is admitted if the estimate is below a threshold. We show how the threshold may be adjusted dynamically to maintain an average packet loss rate below a configurable limit. We show via extensive simulations that the admission control avoids congestion due to flow arrivals and that it maintains the loss probability below the given threshold regardless of the offered load or number of stations. Our simulations also analyze the main drawback of our scheme: a reduction in the link utilization. The admission control efficiently protects cells from overload and it may offer soft QoS guarantees to multimedia flows without the need for scheduling or polling mechanisms in IEEE 802.11

In this paper, we study the performance of the proportional fair scheduler, which has been proposed for the emerging 3.5G radio access systems. It maximizes the spectral efficiency of the systems, which is a strong incentive for network providers to use it. Our goal is to investigate whether the proportional fair scheduler also provides benefits to the streaming users. Focus is on the throughput-delay trade-offs associated with video streaming over HSDPA. Special attention is devoted to defining appropriate performance measures and creating a realistic simulation environment. Our results indicate that the opportunistic scheduling faces major difficulties in providing the user-level performance in cases where streaming flows constitute a significant share of the traffic load.

One of the central problems in video transmission over lossy channels is the choice of source and channel coding rates to allocate the available transmission rate optimally. In this paper, we present a structural distortion model for video streaming over time-varying fading channels. Based on this model we study the average video distortion for various bit-rate allocation strategies and channel conditions. We argue that sensitivity to channel variations should be one of the selection criteria when choosing an optimal bit-rate allocation. We demonstrate that a simple bit-rate allocation strategy, which aims to minimize current distortion, is not necessarily optimal in terms of time-average distortion. System robustness is crucial for the streaming performance when frequent allocation updates are not feasible. It is achieved at the expense of higher source distortion in the encoder.

TCP is a reliable transport protocol that has been tuned to perform well in networks where packet losses occur mostly because of congestion. However, wireless networks are different: TCP responds both to congestion-based and error-based losses by invoking a congestion control algorithm and reducing the sending rate, resulting in degraded end-to-end performance for wireless systems. We investigate a new end-to- end approach for improving TCP performance over lossy links by using adaptive, end-to-end forward error correction (FEC) for recovering losses and consequently avoiding the TCP back-off behaviour. Of course there is a clear trade-off between the capacity consumed by FEC and the gain achieved in the overall throughput. An adaptive algorithm is needed to calculate the optimum ratio of redundancy given the state of the connection. The sender uses feedback information from the receiver to dynamically tune the FEC parameters. Through simulations we evaluate the performance of TCP with end-to-end FEC in mixed wired and wireless networks. The simulation results show in different scenarios that the throughput can be significantly improved by adding end-to-end FEC to TCP. However, compared to other improved TCP variants such as Westwood+ the performance is not improved, hence a direct modification of TCP congestion control appears to be more efficient than adding end-to-end FEC.

We propose a load-balancing scheme for overlapping wireless LAN cells. Agents running in each access point broadcast periodically the local load level via the Ethernet backbone and determines whether the access point is overloaded, balanced or under-loaded by comparing it with the received reports. The load metric is the access point throughput. Overloaded access points force the handoff of some stations to balance the load. Only the under-loaded access points accept the roaming stations in minimizing the number of handoffs. We show via experimental evaluation that our balancing scheme increases the total wireless network throughput and decreases the cell delay.

We analyze the link-layer handoff process in wireless LANs based on the IEEE 802.11b standard and suggests how to reduce its duration. Firstly, we divide the process into three phases: detection, search and execution. Our performance measurements indicate that the detection and search phases are the main contributors to the handoff time. We show that the link-layer detection time can be reduced to three consecutive lost frames. We also show that the search time can be reduced at least by 20% using active scanning with the two timers that control its duration set to 1 ms and 10.24 ms. Several simulations illustrate the achieved reduction in handoff time.

This paper proposes a scheme for quality of service differentiation for single-service networks that is based on the use of two separate forms of traffic control at the transport layer: Streams are controlled by means of probe-based admission control and elastic flows are controlled by TCP. The controls allow separation of traffic into two distinct service classes. The stream class is designed to provide a consistent quality for interactive audiovisual communication, as favored by human perception. It is responsive to load variations as an aggregate through blocking of sessions, while TCP is responsive on the flow level. Streams can be isolated against disturbances from probes and TCP flows by means of error-control coding. We show that the two traffic controls can coexist without starvation, and the proposed scheme might thus provide a first step towards differentiated services end-to-end.

The throughput per access point has been used as the main figure of merit in performance characterization of wireless LANs. The usefulness of throughput as a performance measure is limited because low throughput can be produced by either low offered traffic or by radio conditions that limit the data rates. We propose the multi-rate performance index as an additional figure of merit to characterize the performance of wireless LAN cells. It represents which bit rates are used in the cell, and therefore it identifies cells whose throughput is limited by radio conditions rather than offered traffic. We include several measurements in real cells to illustrate its utility in performance analysis.

In this paper the performance of optical fast frequency-hopping with forward error correction is evaluated. Beat noise is the main performance limitation for a low-complexity implementation using optical filtering of incoherent light. Taking the beat noise into account, we show how to improve the performance with channel coding and soft decoding optimized for this channel.

This paper proposes a controlled load service that provides a network state with bounded and well known worst-case behavior. The service is primarily developed for real time applications. The full system for achieving quality of service to the application consists of an admission control combined with forward-error correction. The admission control is used to limit the packet-loss probability to a known value; the error-control coding (i.e., FEC) is then used to raise the quality above the level enforced by the admission control. The basic idea for the admission control is that a host must probe the path to the receiver before sending actual data. It accepts the session if the probe is received with no or at most a moderate amount of loss. The performance evaluation shows clearly that the proposed scheme avoids network congestion and high packet losses even over short time scales.

There has been a notable interest in the organization of routing information to enable fast lookup of IP addresses. The interest is primarily motivated by the goal of building multigigabit routers for the Internet, without having to rely on multilayer switching techniques. We address this problem by using an LC-trie, a trie structure with combined path and level compression. This data structure enables us to build efficient, compact, and easily searchable implementations of an IP-routing table. The structure can store both unicast and multicast addresses with the same average search times. The search depth increases as /spl Theta/(log log n) with the number of entries in the table for a large class of distributions, and it is independent of the length of the addresses. A node in the trie can be coded with four bytes. Only the size of the base vector, which contains the search strings, grows linearly with the length of the addresses when extended from 4 to 16 bytes, as mandated by the shift from IP version 4 to IP version 6. We present the basic structure as well as an adaptive version that roughly doubles the number of lookups/s. More general classifications of packets that are needed for link sharing, quality-of-service provisioning, and multicast and multipath routing are also discussed. Our experimental results compare favorably with those reported previously in the research literature.

Traffic dispersion, which means spreading traffic from a source over multiple disjoint paths through a network, has gained interest for the handling of bursty multimedia traffic. The strategy has been shown to improve network performance and to increase security and tolerance to faults. Our study focuses on the implementation of the technique in an ATM network, and discusses the protocol functions and signalling procedures necessary to support traffic dispersion. These include connection establishment and release for multiple paths, and the dispersion of cells at the sender with resequencing at the receiver. There are also simulation results included to show how dispersion affects the delay a cell suffers through a network. Lastly, we give an example of pseudocode for implementing resequencing and decoding of dispersed cells in a receiving AAL.

Packet video is investigated from a systems point of view. The most important issues relating to its transmission are identified and studied in the context of a layered network architecture model, leading to a better understanding of the interactions between network and signal handling. The functions at a particular layer can thereby be made less dependent on network implementation and signal format. In the layered network model, the higher layers provide format conversion, hierarchical source coding, error recovery, resynchronization, cost/quality arbitration, session setup and tear-down, packetization, and multiplexing. Provisions from the network layers pertain mainly to real-time transmission. Special consideration is given to hierarchical source coding, error recovery, statistical behavior, and timing aspects. Simulation results indicating practical solutions to some of the issues raised are presented for a hierarchical packet-video subband coding system.>

A novel coding scheme has been developed which is based on multidimensional subband coding. The digital video signal is filtered and sub-sampled in all three dimensions (temporally, horizontally and vertically) to yield the subbands, from which the input signal can be losslessly reconstructed in the absence of coding loss. The subbands can be more efficiently coded than the input signal in terms of compression and quality, because the restricted information in each band allows well-tailored encoding. The computational complexity of this coding scheme compares favorably to DCT with interframe prediction and inter/intra frame DPCM. The scheme has an architectural structure suitable for parallel implementation, and it yields high compression with sustained good quality.>

Models and results are presented that assess the performance of statistical multiplexing of independent video sources. Presented results indicate that the probability of buffering (or delaying) video data beyond an acceptable limit drops dramatically as the number of multiplexed sources increases beyond one. This demonstrates that statistical or asynchronous time-division multiplexing (TDM) can efficiently absorb temporal variations of the bit rate of individual sources without the significant variations in reception quality exhibited by multimode videocoders for synchronous TDM or circuit-switched transmission. Two source models are presented. The first model is an autoregressive continuous-state, discrete-time Markov process, which was used to generate source data in simulation experiments. The second model is a discrete-state, continuous-time Markov process that was used in deriving a fluid-flow queuing analysis. The presented study shows that both models generated consistent numerical results in terms of queuing performance.>