This paper studies the statistical characterization of ground-to-air (G2A) and reconfigurable intelligent surface (RIS)-assisted air-to-ground (A2G) communications in RIS-assisted AAV networks under the impact of channel aging. A comprehensive channel model is presented, which incorporates the time-varying fading, three-dimensional (3D) mobility, Doppler shifts, and the effects of channel aging on array antenna structures. We provide analytical expressions for the G2A signal-to-noise ratio (SNR) probability density function (PDF) and the corresponding cumulative distribution function (CDF), demonstrating that the G2A SNR follows a mixture of noncentral$\chi ^{2}$distributions. The A2G communication is characterized under RIS arbitrary phase-shift configurations, showing that the A2G SNR can be represented as the product of two correlated noncentral$\chi ^{2}$random variables (RVs). Additionally, we present the PDF and the CDF of the product of two independently distributed noncentral$\chi ^{2}$RVs, which accurately characterize the A2G SNR’s distribution. Our paper confirms the effectiveness of RIS-assisted communication in mitigating channel aging effects within the coherence time. Finally, we propose an adaptive spectral efficiency method that ensures consistent system performance and satisfactory outage levels when the AAV and the ground user equipment are in motion.

As a widely adopted protocol for anomaly detection in attributed networks, reconstruction error prioritizes comprehensive feature extraction to detect anomalies over interrogating the differential representation between normal and abnormal nodes. Intuitively, in attributed networks, normal nodes and their neighbors often exhibit similarities, whereas abnormal nodes demonstrate behaviors distinct from their neighbors. Hence, normal nodes can be accurately represented through their neighbors and effectively reconstructed. As opposed to normal nodes, abnormal nodes represented by their neighbors may be erroneously reconstructed as normal, resulting in increased reconstruction error. Leveraging from this observation, we propose a novel anomaly detection protocol called Neighbor-aware Mask-Filling Anomaly Detection (NMFAD) for attributed networks, aiming to maximize the variability between original and reconstructed features of abnormal nodes filled with information from their neighbors. Specifically, we utilize random-mask on nodes and integrate them into the backbone Graph Neural Networks (GNNs) to map nodes into a latent space. Subsequently, we fill the masked nodes with embeddings from their neighbors and smooth the abnormal nodes closer to the distribution of normal nodes. This optimization improves the likelihood of the decoder to reconstructing abnormal nodes as normal, thereby maximizing the reconstruction error of abnormal nodes. Experimental results demonstrate that, compared to the existing models, NMFAD exhibits superior performance.in attributed networks.

This paper studies the statistical characterization of ground-to-air (G2A) and reconfigurable intelligent surface (RIS)-assisted air-to-ground (A2G) communications with unmanned aerial vehicles (UAVs) in terrestrial and non-terrestrial networks under the impact of channel aging. We first model the G2A and A2G signal-to-noise ratios (SNRs) as non-central complex Gaussian quadratic random variables (RVs) and derive their exact probability density functions, offering a unique characterization for the A2G SNR as the product of two scaled non-central chi-square RVs. Moreover, we also find that, for a large number of RIS elements, the RIS-assisted A2G channel can be characterized as a single Rician fading channel. Our results reveal the presence of channel hardening in A2G communication under low UAV speeds, where we derive the maximum target spectral efficiency (SE) for a system to maintain a consistent required outage level. Meanwhile, high UAV speeds, exceeding 50 m/s, lead to a significant performance degradation, which cannot be mitigated by increasing the number of RIS elements.

A cyber–physical system (CPS) is a multidimensional complex system integrating computing, communication, and control technologies. Because of their key functionality within the system, CPS requires large robustness and security to ensure its reliable operation. Due to its importance in supporting overall system security, anomaly detection (AD) is likely to continue to play an important role in the CPS security. Moreover, unsupervised AD models based on deep learning have shown better performances in rule training, adaptive update, detection efficiency, and accuracy. Due to the nature of the CPS systems, CPS data is more likely to exhibit implicit correlative relationship among data, which would be vital to exploit for CPS security provisions in more complex data environments. In view of this observation, we propose the data-correlation-aware unsupervised deep learning model for AD in CPS, which uses an undigraph structure to store samples and implicit correlation among samples. We design a dual-autoencoder to train both original features and implicit correlation features among data, and we construct an estimation network using Gaussian mixture model (GMM) to evaluate the probability distribution of samples to complete the anomaly analysis. Experimental results compared the performance with relevant AD models based on deep learning which did not use data-correlation analysis. The results showed that, under some representative application scenarios of CPS considered, data-correlation-aware unsupervised deep-learning model achieved superior results in parameter sensitivity, ablation, relationship between correlation degree and detection performance, visualization, and detection effects.

In this paper, we propose and investigate an aerial reconfigurable intelligent surface (aerial-RIS)-aided wireless communication system. Specifically, considering practical composite fading channels, we characterize the air-to-ground (A2G) links by Namkagami-m small-scale fading and inverse-Gamma large-scale shadowing. To investigate the delay-limited performance of the proposed system, we derive a tight approximate closed-form expression for the end-to-end outage probability (OP). Next, considering a mobile environment, where performance analysis is intractable, we rely on machine learning-based performance prediction to evaluate the performance of the mobile aerial-RIS-aided system. Specifically, taking into account the three-dimensional (3D) spatial movement of the aerial-RIS, we build a deep neural network (DNN) to accurately predict the OP. We show that: (i) fading and shadowing conditions have strong impact on the OP, (ii) as the number of reflecting elements increases, aerial-RIS achieves higher energy efficiency (EE), and (iii) the aerial-RIS-aided system outperforms conventional relaying systems.

Intelligent reflecting surface (IRS) is being considered as a prospective candidate for next generation wireless communication due to its ability to significantly improve coverage and spectral efficiency by controlling the propagation environment. One of the ways IRS increases spectral efficiency is by adjusting phase shifts to perform passive beamforming. In this letter, we integrate the concept of IRS aided communication to the domain of multi-direction beamforming, whereby multiple receive antennas are selected to convey more information bits than existing spatial modulation (SM) techniques at any specific time. To complement this system, we also propose a successive signal detection (SSD) technique at the receiver. Numerical results show that the proposed design is able to improve the average successful bits transmitted (ASBT) by the system, which outperforms other state-of-the-art methods proposed in literature.

In this paper, we study the statistical characterization and modeling of distributed multi-reconfigurable intelligent surface (RIS)-aided wireless systems. Specifically, we consider a practical system model where the RISs with different geometric sizes are distributively deployed, and wireless channels associated to different RISs are assumed to be independent but not identically distributed (i.n.i.d.). We propose two purpose-oriented multi-RIS-aided schemes, namely, the exhaustive RIS-aided (ERA) and opportunistic RIS-aided (ORA) schemes. A mathematical framework, which relies on the method of moments, is proposed to statistically characterize the end-to-end (e2e) channels of these schemes. It is shown that either a Gamma distribution or a Log-Normal distribution can be used to approximate the distribution of the magnitude of the e2e channel coefficients in both schemes. With these findings, we evaluate the performance of the two schemes in terms of outage probability (OP) and ergodic capacity (EC), where tight approximate closed-form expressions for the OP and EC are derived. Representative results show that the ERA scheme outperforms the ORA scheme in terms of OP and EC. In addition, under i.n.i.d. fading channels, the reflecting element settings and location settings of RISs have a significant impact on the system performance of both the ERA or ORA schemes.

We propose an innovative framework to improve communication reliability of IoT sensory traffic, when the network is subject to congestion. The proposed architecture is based on information dispersion and relies on the fact that congestion occurs in space-and time-differing parts of the network. Our approach uses encoding and spreading of data flows across various parts of the network, while ensuring that the original data could be recovered at the destination with sufficient accuracy, even if only parts of the encoded data survive the congestion losses. The encoding is based on spatial use of erasure coding, while the recovery is based on Dictionary-based Compressed Sensing. We term this approach Congestion-Tolerant Framework. Our work is specifically targeted at the paradigm of IoT that promises to provide wide-area sensory connectivity to devices on a massive scale, interconnecting billions of such devices. Our framework is applicable to both, real-time and non-real-time IoT networking paradigms.

In this paper, we consider a distributed MIMO system with a set of geographically distributed nodes forming a multi-antenna source S, and a set of geographically distributed full duplex (FD) nodes forming a multi-antenna destination D. S and D each forms a transmit and a receive cluster, respectively. D uses FD mode to jam the signal from S at eavesdropper E, incurring some residual self-interference. We assume that neither S nor D has knowledge of their instantaneous channel state information to E. Furthermore, to simplify the system design, especially for the mobile case, we assume as well that S has no knowledge of the ICSI of its channel to D. Hence, adopting ergodic secrecy rate (ESR) as the performance metric, we are interested in evaluating the ESR averaged over the nodes' locations, i.e., cluster averaged ESR. To accomplish this, we obtain a lower bound (LB) and an upper bound of the ESR. As part of our work, we also obtain bounds on the cluster averaged ergodic rate between the two clusters. We demonstrate by simulations that the LB can act as a good approximation of the ESR for practical purposes.

We propose and study the cooperative multi-input multi-output (MIMO) architecture to enable and improve the secrecy transmissions between clusters of mobile devices in the presence of an eavesdropper with certain location constraint. The cooperative MIMO system in this paper (referred to as reconfigurable distributed MIMO) is formed by temporarily activating clusters of nearby trusted mobile devices, with each cluster being centrally coordinated by its corresponding cluster head. We assume that the transmitters apply a practical eigendirection precoding scheme to transmit the confidential signal and artificial noise, while the eavesdropper can be located in multiple possible locations in the proximity. We first obtain the expression of the secrecy rate, where the required average mutual information between the transmitters and the eavesdropper is characterized by closed-form approximations. The proposed approximations are especially useful in the secrecy rate maximization, where the original non-convex problem can be solved by the successive convex approximations. Numerical results show that the secrecy rate can be significantly improved by leveraging the location constraint of the eavesdropper, compared to the existing result. We also demonstrate that the secrecy rate can be further improved by increasing the cluster size.

Radio frequency identification (RFID) is a technology that incorporates the use of electromagnetic fields to uniquely identity objects. Among different types of RFID tags, passive tags have some salient features such as light weight, low cost, small size, etc. However, the downside of passive RFID systems is very limited reading range due to lacking their own energy sources (passive RFID tags communicate solely by backscattering the reader's power). A novel concept of passive RFID tag-to-tag (T2T) communication has been recently proposed, via which passive tags in proximity (at centimeter level) can directly communicate with each other with the existence of an external energy source. Utilizing this concept, we proposed a Network of Tags (NeTa) that passive tags can connect with each other through multiple hops, using a the novel concept of turbo backscattering operation. This significantly enhances the scalability of such a T2T network. However, to implement the proposed NeTa architecture, one of the main issues is the inter-tag interference, which brings challenge to the routing protocol design. In our previous work [1], we introduced protocol design for both tag-to-reader routing and tag-to-tag routing, considering basic hardware capability of tags, i.e., tags cannot measure the strength of received signals. In this paper, we extend upon the results in [1] and focus on tag-to-tag routing for two distinct types of tags with different hardware capabilities - tags can measure and attenuate the received signal before backscattering. These functions can greatly reduce the inter-tag interference and therefore enhance the network throughput. The protocol design is based on solutions of two mixed integer nonlinear programming (MINLP) problems, respectively. The performances of the proposed protocols are analyzed and the impacts of several network factors (e.g., tag density, the transmit power of the reader, etc.) are investigated.

We propose a new distributed Multi-Input Multi-Output (MIMO) architecture for mobile networks, which we refer to as reconfigurable distributed MIMO (RD-MIMO), where the communicating mobile nodes temporarily recruit adjacent nodes to operate as distributed antenna arrays. To best serve the communicating nodes, the node clusters are continuously reconfigured due to the node mobility and varying channel conditions. The frequency of reconfiguration depends on the required system performance, exhibiting a tradeoff between performance and complexity. We propose a practical node selection scheme, which activates only a small subset of transmitters. We evaluate the asymptotic performance of the scheme as a function of the number of recruited nodes, demonstrating that there is an optimal number of such nodes. Compared with the system that blindly activates all available transmitting nodes, our results show that the proposed RD-MIMO architecture with node selection achieves superior performance, especially as evident at low SNR. Furthermore, assuming the Brownian motion model, an analytical expression for the reconfiguration time to select a new cluster of transmitting nodes is obtained. Numerical results show that the obtained expression serves as a good estimation for the order of magnitude of the cluster reconfiguration time for other mobility patterns, such as the random walk model.

We study the dynamic energy optimization problem in data centers. We formulate and solve the following offline problem: given a set of jobs to process, where the jobs are characterized by arrival instances, required processing time, and completion deadlines, and given the energy requirements of switching servers ON or OFF, in which time-slot which server has to be assigned to which job, and in which time-slot which server has to be switched ON or OFF, so that the total energy is optimal for some time horizon. We formulate the offline problem as a binary integer program that can be considered as a new version of generalized assignment problem which includes new constraints stemming from deadline characteristics of jobs and the activation energy of servers. We propose an online algorithm that solves the problem heuristically, and we compare it to random assignment solution.

In this paper, we consider MIMO beamforming in the presence of Rayleigh product channels. Based on a derived largest eigenvalue distribution, the key performance metrics of the beamforming system are obtained, assuming perfect channel knowledge at the transmitter and receiver. Using the closed-form expressions, we gain insights into the behavior of MIMO beamforming systems in scenarios of practical interest.

Given n source nodes and k relay nodes, we model the optimal relay topology problem allowing for simultaneous optimization of the relay node locations and traffic through the network, so that the overall number of packet retransmissions is minimized. We argue that state-of-the-art models and algorithms for relay placement in wireless networks do not reflect salient characteristics of the optimal relays topology and lead to suboptimal solutions. We do not constrain the position of relays to a finite set of discrete points, as the latter may not be feasible in practical networks. In this case, we show that just listing a set of feasible sites for the relays is already at least APX-hard. Exploiting convexity in a special case of the network communication cost function, we give an optimal algorithm for the relay placement problem. However, the algorithm is exponential on the number of nodes in the network. We suggest a practical heuristic algorithm for relay placement: RePlace. We compare RePlace numerically to the optimal algorithm and show that RePlace achieves the optimal or almost optimal solutions. We implement RePlace in the full network stack simulator JiST/SWANS. The relay topologies generated by RePlace eliminate overhead communication cost almost entirely.

Quality of Experience (QoE) measures a user's satisfaction with a service delivery. However QoE is a very subjective measure and is context dependent, making it difficult for a service provider to estimate and optimize user's QoE. In this paper, we look at how the provider can maximize QoE by optimizing wireless bandwidth allocation, especially for mobile cloud applications. The multi-stimuli version of the “IQX” hypothesis is used to model the QoE of a user, and this model is used in formulation of a nonlinear optimization problem, which is solved using NSGA-II. Simulations using realistic parameters based on 802.11n demonstrate a reduction in the required bandwidth by as much as 33% (i.e., more users can be accommodated by the system), while maintaining the same level of QoE. Our evolutionary-algorithm-based approach is able to discover the optimal bandwidth allocation. The problem of equalizing user QoE is explored and a tradeoff between QoE and fairness is studied, while being characterized using a Pareto front.

Broadcast is a fundamental operation in networks, especially in reconfigurable wireless ad hoc networks. For example, some form of broadcasting is used by all on-demand mobile networks routing protocols, when there is uncertainty as to the location of the destination node, or for service discovery. In this work, we present a new approach to efficient broadcast in networks with dynamic topologies, and we introduce the time sequence scheme (TSS), a new online local broadcasting algorithm for such networking environments. TSS ranks by priority, in a distributed way, candidate broadcasting nodes so that the overall number of re-broadcasts in the network is minimized. We evaluate TSS, showing that its performance comes remarkably close to the corresponding theoretical performance bounds, even in the presence of packet loss due, for example, to MAC-layer collisions. Furthermore, we compare our algorithm with a number of recently proposed schemes considering their performance in various realistic network mobility scenarios. We demonstrate that TSS performance is robust in the context of mobility induced topology reconfigurations-including temporal network partitioning-during propagation of the broadcast message.

Improvement of traffic safety by cooperative vehicular applications is one of the most promising benefits of vehicular ad hoc networks (VANETs). However, to properly develop such applications, the influence of different driving parameters on the event of vehicle collision must be assessed at an early design stage. In this paper, we derive a stochastic model for the number of accidents in a platoon of vehicles equipped with a warning collision notification system, which is able to inform all the vehicles about an emergency event. In fact, the assumption of communications being used is key to simplify the derivation of a stochastic model. The model enables the computation of the average number of collisions that occur in the platoon, the probabilities of the different ways in which the collisions may take place, as well as other statistics of interest. Although an exponential distribution has been used for the traffic density, it is also valid for different probability distributions for traffic densities, as well as for other significant parameters of the model. Moreover, the actual communication system employed is independent of the model since it is abstracted by a message delay variable, which allows it to be used to evaluate different communication technologies. We validate the proposed model with Monte Carlo simulations. With this model, one can quickly evaluate numerically the influence of different model parameters (vehicle density, velocities, decelerations, and delays) on the collision process and draw conclusions that shed relevant guidelines for the design of vehicular communication systems, as well as chain collision avoidance applications. Illustrative examples of application are provided, although a systematic characterization and evaluation of different scenarios is left as future work.

Broadcast is a fundamental operation in networks, especially in wireless Mobile Ad Hoc NETworks (MANET). For example, some form of broadcasting is used by all on-demand MANET routing protocols, when there is uncertainty as to the location of the destination node, or for service discovery. Being such a basic operation of the networking protocols, the importance of efficient broadcasting has long been recognized by the networking community. Numerous papers proposed increasingly more efficient implementation of broadcasting, while other studies presented bounds on broadcast performance. In this work, we present a new approach to efficient broadcast in networks with dynamic topologies, such as MANET, and we introduce a new broadcasting algorithm for such networking environments. We evaluate our algorithm, showing that its performance comes remarkably close to the corresponding theoretical performance bounds. Furthermore, we compare the performance of the proposed algorithm with other recently proposed schemes, especially in the context of various mobility settings.

Cooperative Networking is a new technology which exploits the massive deployment of nodes in wireless sensor networks. Cooperative Networking synergistically integrates Networking with cluster-based Cooperative Communications to improve reliability and enhance network performance. In this paper, we consider the effect of link-level feedback and retransmissions on the performance of wireless sensor networks using Cooperative Networking, and we present scenarios where link-level retransmission offers a significant improvement in network throughput. Generally, Cooperative Networking with link-level retransmission provides higher throughput when the network node density is low (i.e., sparse networks) or in environments with adverse conditions such as high probability of transmission loss and low connectivity among the nodes.

In cooperative networks, transmitting and receiving nodes recruit neighboring nodes to assist in communication. We model a cooperative transmission link in wireless networks as a transmitter cluster and a receiver cluster. We then propose a cooperative communication protocol for establishment of these clusters and for cooperative transmission of data. We derive the upper bound of the capacity of the protocol, and we analyze the end-to-end robustness of the protocol to data-packet loss, along with the tradeoff between energy consumption and error rate. The analysis results are used to compare the energy savings and the end-to-end robustness of our protocol with two non-cooperative schemes, as well as to another cooperative protocol published in the technical literature. The comparison results show that, when nodes are positioned on a grid, there is a reduction in the probability of packet delivery failure by two orders of magnitude for the values of parameters considered. Up to 80% in energy savings can be achieved for a grid topology, while for random node placement our cooperative protocol can save up to 40% in energy consumption relative to the other protocols. The reduction in error rate and the energy savings translate into increased lifetime of cooperative sensor networks.

In this paper, we propose a simple and practical opportunistic routing algorithm, and we analyze its performance along a multi-hop wireless network path, while considering link-level interference among the network nodes. Through our analysis, we show that our algorithm results in significant improvement in throughput, especially for short-haul paths. The proposed algorithm can be easily integrated into most routing protocols with only minor modifications. Consequently, the algorithm provides a practical and effective approach for implementation of opportunistic routing in wireless networks.

Gossiping, a probabilistic algorithm for network-wide broadcasting, can significantly reduce the number of transmissions in one-to-all message spread. Based on the Continuum Percolation Theory, we develop a novel analytical model of gossiping for wireless ad hoc networks. This model provides an intuitive way for setting the gossiping probability, so that a gossip achieves high degree of network coverage with limited number of transmissions. We also propose and study a distributed gossiping scheme, in which nodes set their gossiping probabilities according to locally acquired information. Both gossiping schemes are demonstrated to be scalable; i.e., for fixed network area, the expected number of gossip transmissions does not depend on the number of nodes in the network. The analytical results are verified via simulations of large ad hoc networks.

We study the problem of predicting the residual link lifetime (RLL) in MANETs, where the nodes are able to measure the relative distances between them (e.g., by using the UWB technology). We propose a mobile-projected trajectory (MPT) algorithm, whose input is periodically sampled, noisy range measurements between the two nodes of a link. It estimates a projected trajectory, which is then used to compute the predicted RLL. An enhancement technique, which we call incremental sampling, is proposed where the estimated trajectory is further refined to improve the accuracy of the RLL prediction. We have evaluated the performance of the MPT algorithm with two different mobility models and for different parameters, and have shown that MPT yields robust performance; i.e., the main strength of the MPT algorithm lies in its capability to accurately predict the RLL with limited range input data. For example, after a measurement-acquisition time equal to 25% of the link lifetime, the algorithm yields 90% prediction accuracy; 80% accuracy is achieved after 20% of the link lifetime. After only 15% of link lifetime, the algorithm still achieves 60% prediction accuracy.

One of the main challenges in wireless networks is improving the network throughput and capacity. As commodity wireless network cards support the usage of orthogonal channels, allowing multiple simultaneous communications on these channels will improve the performance of wireless networks. Previous MAC protocols either were expensive to implement as they needed more than one packet radio per node or required synchronization or caused network partitioning. We propose a new MAC protocol that uses hash functions to distribute the control overhead over all the channels. In addition, it uses busy tones to reserve the channels and solve the hidden terminal and the channel rendezvous problems in multi channels. We compared our protocol with another multi-channel MAC protocol that uses more than one packet radio per node (DCA) and with 802.11. Our protocol produces comparable results with DCA and performs much better than 802.11. The results show that our protocol utilizes the multiple channels available to increase the capacity of the network. The proposed protocol is expected to improve the usage of ad hoc networks as it does not need synchronization like previous protocols.

Abstract Unlike a terrestrial network, an underwater sensor network can have significant height which makes it a three‐dimensional network. There are many important sensor network design problems where the physical dimensionality of the network plays a significant role. One such problem is determining how to deploy minimum number of sensor nodes so that all points inside the network is within the sensing range of at least one sensor and all sensor nodes can communicate with each other, possibly over a multi‐hop path. The solution to this problem depends on the ratio of the communication range and the sensing range of each sensor. Under sphere‐based communication and sensing model, placing a node at the center of each virtual cell created by truncated octahedron‐based tessellation solves this problem when this ratio is greater than 1.7889. However, for smaller values of this ratio, the solution depends on how much communication redundancy the network needs. We provide solutions for both limited and full communication redundancy requirements. Copyright © 2008 John Wiley & Sons, Ltd.

Data replication is, indeed, a widely used redundancy scheme for improving data accessibility in distributed systems at the cost of increased storage space and communication overhead. In this paper, we study the performance of data replication under mobile ad hoc networking environments in the presence of frequent network partitions by analytical modeling and by statistical analysis of simulation results. In particular, we examine the statistics of network partitions for a number of mobility models, and we propose distribution models to approximate the size of the network partitions. We then establish the relation between the network partitioning pattern and the effectiveness of the data replication scheme, which could be used to dynamically adjust the degree of replication depending on the current network operational conditions, while optimizing the trade-off between storage and data accessibility.

In an intermittent-connectivity network, there rarely exists a connected path between a source node and its destination. These networks arise frequently when each node has a limited transmission range, such as a communication network between separated villages or a surveillance network with a large geographical span. One method of addressing the low connectivity of the network uses redundancy. A node generates and stores data; upon reaching the communication range of another node, it replicates the data to it. Multiple copies of the packet decrease the time to offload the data to the destination, but increase the energy and storage used in the system. In this paper, we quantify the resource-delay trade-off and the throughput capacity for intermittent-connectivity networks with Quality of Service restrictions such as limited communication bandwidth. Many routing protocols have been proposed for these intermittent-connectivity networks. Using the Shared Wireless Infostation Model as an example strategy, we mathematically represent the intermittent-connectivity network and adjust the model to include a Quality of Service constraint. By completely defining a mathematical model, we allow network designers control over system performance through the adjustment of allocated resources such as communication bandwidth, fraction of time a node spends in sleep mode, or required reliability of packet offloading.

We extend the results of O. Arpacioglu and Z.J. Haas, (2004) on the capacity of single-user-detection based wireless networks, and we determine the implications of our results on the scalability of such networks. In particular, we consider a wireless network of N nodes that are equipped with isotropic antennas. The nodes are stationary or moving arbitrarily in a network domain of an arbitrary one, two, or three dimensional shape, as opposed to the two dimensional circular domain in O. Arpacioglu and Z.J. Haas, (2004). In this arbitrary dimensional setting, we derive bounds on the per node end-to-end throughput capacity and the maximum number of simultaneous transmissions whose SINRs exceed a given threshold. We derive these bounds with both the bounded propagation model in O. Arpacioglu and Z.J. Haas, (2004) and a large class of bounded propagation models, which we refer to as the general propagation model. Our results show that with the general propagation model, the maximum number of simultaneous transmissions, whose SINRs exceed the threshold has an upper bound that does not depend on N, and the per node end-to-end throughput capacity is O(1/N) for a large class of wireless networks. Moreover, we establish several required conditions for scalability. These conditions show that, for any propagation model, to achieve a desired per node end-to-end throughput as N grows, it is necessary to keep the average source-to-destination hop count bounded. Also, for the particular propagation model of O. Arpacioglu and Z.J. Haas, (2004), we show that the size of the network domain must grow with N at a rate that depends on the dimension of the network domain and the path loss exponent

Secure routing protocols for mobile ad hoc networks have been developed recently, yet, it has been unclear what are the properties they achieve, as a formal analysis of these protocols is mostly lacking. In this paper, we are concerned with this problem, how to specify and how to prove the correctness of a secure routing protocol. We provide a definition of what a protocol is expected to achieve independently of its functionality, as well as a communication and adversary models. This way, we enable formal reasoning on the correctness of secure routing protocols. We demonstrate this by analyzing two protocols from the literature.

Due to their low network connectivity, sparsely-connected networks can support delay-tolerant applications only. Thus, such communication environments are examples of Delay-Tolerant Networks (DTNs). While in some DTN connectivity is predictable (e.g., scheduled), in others, connectivity is created randomly. For instance, to save battery power, nodes in a sensor network may be programmed to follow a pre-arranged sleep patterns, or may enter the sleep state opportunistically. In a sparsely-connected network, source-to-destination route discovery cannot be implemented, since at any particular time, there rarely exists a connected path between the source node and the destination node. Consequently, the conventional approaches to implement network routing do not work and new methods are needed. Flooding (as implemented, for example, using epidemic routing) could be used to spread information through the network. With multiple copies of each packet in the network, the time to offload the data to the destination can be significantly reduced, but at the expense of increased energy and storage utilization. However, in some DTNs, network resources may be limited, for instance due to physical constraints of the network devices. In this paper, we consider the implications of the limited network resources, such as limited communication bandwidth, on the performance and the capacity of a DTN.In particular, leveraging from our previous work on DTN, we use the Shared Wireless Infostation Model (SWIM) to derive a strategy to mathematically represent a DTN, which allows us to evaluate the network throughput capacity, while including a variety of restrictions on network resources. Using this mathematical model derived here, will allow the network designers to adjust the allocated network resources, as to trade off system performance and system resources, while achieving the required network capacity.

Because of connectivity richness in mobile ad hoc networks, multiple paths often exist between a source and a destination. Since many applications require uninterrupted connectivity of a session, the ability to find long-living paths can be very useful. In this paper, we propose three path-selection algorithms and evaluate their performance in a homogeneous network based on two criteria: 1) the selected path is the most likely to meet a target residual path lifetime requirement, and 2) the selected path has the longest residual path lifetime among all the available paths. We also introduce two performance metrics to compare the proposed algorithms among themselves and with a baseline random-selection algorithm. All three algorithms demonstrate comparable performance in satisfying the first criterion, although the first algorithm performs consistently better than the other two for both criteria. As the path-set size increases, the proposed algorithms yield greater performance gain over the baseline algorithm. Furthermore, we show that these algorithms perform better in a higher mobility environment than when the mobility is low.

Whereas energy efficiency in wireless sensor network is of critical importance, idle listening has been recognized as a main source of wasted power. Many studies have proposed various approaches to scheduling the active and sleeping periods of sensor nodes, as to reduce the power consumption of idle listening. While noticing the fact that there is no universally accepted approach, one which can meet the diversity of different application, in this paper, we propose a scheduling scheme for active and sleeping periods that is based on the packet arrival pattern. More particularly, we propose an arrival model which is targeted at application characterized by bursty arrival. The bursty arrival times are assumed to be distributed exponentially with different rates for the packet arrival intra-burst and inter-burst. Based on this packet arrival model, we introduce a Bursty Arrival Dependent Sleeping Scheduling (BASS) scheme, in which each node dynamically and independently adjusts its wakeup rate. Through analysis and simulation, we evaluate the impact of the proposed scheme on the duty cycle and on the delay of the MAC layer. We show that as the bursty arrival rate decreases, the ON/OFF duty cycle decreases linearly and the MAC-layer delay is minimally affected. Our results suggest that the BASS scheme provides a superior solution for sensor network with bursty arrival. Comparing BASS scheme with S-MAC, results demonstrate 45%—70% gains in the BASS case in term of energy efficiency, without degrading performance. BASS scheme also obtains much better performance (e.g. 10 times better in the range of parameters in this paper) with the same amount of power consumption.

Although most wireless terrestrial networks are based on two-dimensional (2D) design, in reality, such networks operate in three-dimensions (3D). Since most often the size (i.e., the length and the width) of such terrestrial networks is significantly larger than the differences in the third dimension (i.e., the height) of the nodes, the 2D assumption is somewhat justified and usually it does not lead to major inaccuracies. However, in some environments, this is not the case; the underwater, atmospheric, or space communications being such apparent examples. In fact, recent interest in underwater acoustic ad hoc and sensor networks hints at the need to understand how to design networks in 3D. Unfortunately, the design of 3D networks is surprisingly more difficult than the design of 2D networks. For example, proofs of Kelvin's conjecture and Kepler's conjecture required centuries of research to achieve breakthroughs, whereas their

The personal environment service (PES) configures user's computing and communication environmental based on the user's personal organizer profile, which stores the user preference data. PES then coordinates the reconfiguration of the user's physical environment, such as appliances, through the use of short-range wireless communications.

In this paper, we propose a multi-state based hybrid location update scheme, which integrates the time-based and the movement-based methods. In the proposed scheme, a mobile terminal updates its location after n cell boundary crossing and a time interval of T[sec]. We derive an analytical solution for the performance of the hybrid scheme with exponential cell resident time and evaluate it numerically with time-varying random walk mobility model, which we model as a multi-state Markov chain. Furthermore, we also evaluate the scheme for arbitrary cell resident times by simulation. From the numerical analysis and the simulation results, we prove that the proposed scheme significantly outperforms the time-based and the movement-based methods, when implemented alone, more accurately adapting to the time-varying user mobility

We address the problem of secure and fault-tolerant communication in the presence of adversaries across a multihop wireless network with frequently changing topology. To effectively cope with arbitrary malicious disruption of data transmissions, we propose and evaluate the secure message transmission (SMT) protocol and its alternative, the secure single-path (SSP) protocol. Among the salient features of SMT and SSP is their ability to operate solely in an end-to-end manner and without restrictive assumptions on the network trust and security associations. As a result, the protocols are applicable to a wide range of network architectures. We demonstrate that highly reliable communication can be sustained with small delay and small delay variability, even when a substantial portion of the network nodes systematically or intermittently disrupt communication. SMT and SSP robustly detect transmission failures and continuously configure their operation to avoid and tolerate data loss, and to ensure the availability of communication. This is achieved at the expense of moderate transmission and routing overhead, which can be traded off for delay. Overall, the ability of the protocols to mitigate both malicious and benign faults allows fast and reliable data transport even in highly adverse network environments.

In this work, we analyze and evaluate the maximum achievable throughput of split-channel MAC schemes that are based on the RTS/CTS (ready-to-send/clear-to-send) dialogue and that rely on pure ALOHA or on p-persistent carrier sensing multiple access (CSMA) contention resolution techniques. Our results show that, when radio propagation delays are negligible and when the pure ALOHA mechanism is used, then for a network with relatively large number of nodes, the maximum achievable throughput of the split-channel MAC schemes is lower than that of the corresponding single-channel MAC schemes. When the split-channel MAC schemes employ the p-persistent CSMA mechanism, then they out-perform the corresponding single-channel schemes when the maximum end-to-end propagation delays are at least 25% of the transmission time of the control packets on the single shared channel.

In order to improve the throughput performance of medium access control (MAC) schemes in wireless communication networks, some researchers proposed to divide a single shared channel into several sub-channels: one as control sub-channel and the others as data sub-channels. In this paper, we analyze and evaluate the maximum achievable throughput of a class of generic multi-channel MAC schemes that are based on the RTS/CTS (ready-to-send/clear-to-send) dialogue and on ALOHA contention resolution. We study these multi-channel MAC schemes under two split-channel scenarios: the fixed-total-bandwidth scenario and the fixed-channel-bandwidth scenario. In the fixed-total-bandwidth scenario, we show that the throughput of the multi-channel MAC schemes is inferior to that of the corresponding single-channel MAC scheme, which sends the RTS/CTS packets and DATA packets on a single shared channel. For the fixed-channel-bandwidth scenario, where CDMA or similar techniques can be applied, we derive the optimal number of the data sub-channels that maximizes the throughput. The analytical framework that we derive in this paper can also be used to evaluate other contention resolution technique, when the average contention period is known.

Virtual backbone routing (VBR) is a scalable hybrid routing framework for ad hoc networks, which combines local proactive and global reactive routing components over a variable-sized zone hierarchy. The zone hierarchy is maintained through a novel distributed virtual backbone maintenance scheme, termed the distributed database coverage heuristic (DDCH), also presented in this paper. Borrowing from the design philosophy of the zone routing protocol, VBR limits the proactive link information exchange to the local routing zones only. Furthermore, the reactive component of VBR restricts the route queries to within the virtual backbone only, thus improving the overall routing efficiency. Our numerical results suggest that the cost of the hybrid VBR scheme can be a small fraction of that of either one of the purely proactive or purely reactive protocols, with or without route caching. Since the data routes do not necessarily pass through the virtual backbone nodes, traffic congestion is considerably reduced. Yet, the average length of the VBR routes tends to be close to optimal. Compared with the traditional one-hop hierarchical protocols, our results indicate that, for a network of moderate to large size, VBR with an optimal zone radius larger than one can significantly reduce the routing traffic. Furthermore, we demonstrate VBR's improved scalability through analysis and simulations

We analyze the asymptotic cost of discovering a route within a flat ad hoc network and we show that one can discover a route with cost that is proportional only to the area of the network and that is independent of the number of network nodes. Furthermore, we show that this is optimal and that bordercasting (a query propagation protocol where a node retransmits a query to a set of nodes at some hop-distance away) possesses this density-independence property. We present the design of bordercast and the associated maintenance protocols, and we evaluate their performance. In particular, we highlight that the aggregation of local information by boredercasting at each network node is a fundamental building block for the construction of scalable protocols in flat ad hoc networks.

Mobile ad hoc networks (MANETs) are self-organized networks that do not rely in their operation on wired infrastructure. As in any networking technology, security is an essential element in MANET as well for proliferation of this type of networks. But supporting secure communication in MANETs proved to be a significant challenge, mainly due to the fact that the set of nodes in the network can frequently and rapidly change and due to the lack of access to the wired infrastructure. In particular, the trust model and the authentication protocols, which were developed for wired and infrastructure-based networks, cannot be used in MANETs. In this paper, we address the problem of efficient authentication of mobile users distributed in geographically large networks and we propose a new authentication scheme for this case of MANETs. The proposed scheme exploits randomized groups to efficiently share authentication information among nodes that together implement the function of a distributive certification authority (CA). We evaluate the performance of the proposed randomized groups authentication scheme based on two metrics: the total cost of the scheme's operation and its reliability. As an example of the results and as a reference point, we show that the proposed scheme achieves successful authentication when the random group size is around 10 - 12 nodes, irrespectively of the number of mobile nodes, of the number of CAs, and of the mobility model of the nodes in the network.

In this paper, we study the problem of maximizing the throughput of UWB-based wireless ad-hoc networks by optimal allocation of radio resources, such as transmission rates and transmission powers. In particular, we determine the transmission rate of a link (which is transmission-power dependent) that maximizes the link's throughput. We show that if a link's transmission rate is adapted to this optimal transmission rate then, as the number of concurrently operating links increases, the maximal total network throughput approaches a constant and each link's throughput approaches zero. We conclude that for sufficiently large transmission power, the throughput of the single-link case exceeds the total network throughput of the multiple-links case. We then present an optimal transmission scheduling scheme that maximizes the network throughput. Based on this scheme, and for the case of small maximal attainable power and weak interference levels, the scheduling scheme allocates simultaneous transmissions of multiple links. However, for the case of large maximally attainable power and strong interference levels, the scheduling scheme assigns separate times for transmission on each link. The breakpoint of when it is preferable to use a separate time for any link, as opposed to simultaneous transmission over multiple links, is determined by the value of the maximal transmission power of the links. Finally, we argue that, in contrast with previous studies, the design of the optimal Medium MAC protocol is dependent on the choice of a routing scheme.

Discrete event simulators are important scientific tools and their efficient design and execution is the subject of much research. In this paper, we propose a new approach for constructing simulators that leverages virtual machines and combines advantages from the traditional systems-based and language-based simulator designs. We introduce JiST, a Java-based simulation system that executes discrete event simulations both efficiently and transparently by embedding simulation semantics directly into the Java execution model. The system provides standard benefits that the modern Java runtime affords. In addition, JiST is efficient, out-performing existing highly optimized simulation runtimes. As a case study, we illustrate the practicality of the JiST framework by applying it to the construction of SWANS, a scalable wireless ad hoc network simulator. We simulate million node wireless networks, which represents two orders of magnitude increase in scale over what existing simulators can achieve on equivalent hardware and at the same level of detail. Copyright © 2005 John Wiley & Sons, Ltd.

In this paper, we investigate the impacts of transmission concurrency on the throughput of the downstream channel in multihop cellular networks. In particular, this paper focuses on the tradeoff between the improvement of resource reuse and the increase in radio interference. We evaluate and compare the throughput for various concurrency types of multihop relaying. We also propose a hybrid control strategy, in which we advocate the use of both, the concurrent and the nonconcurrent transmissions. Our results show that the throughput can be increased by allowing transmission concurrency, but the concurrency does not always result in higher throughput, relative to the nonconcurrency case, because of the interference between concurrently transmitting links. In the random topology with shadowing, depending on the particular circumstances, either the concurrent or the nonconcurrent case can be the optimal transmission strategy. Thus, in order to achieve a significant improvement, a hybrid scheme should be employed. Additionally, if concurrency is applied, it should be used among the different downstream paths, rather than between the hops on the same downstream path.

We consider a wireless network of N nodes equipped with omnidirectional antennas, and we extend the capacity results of some previous works by finding bounds on the maximum achievable per-node end-to-end throughput, λe, while using a general network model and a bounded propagation model. Specifically, we show that when the network domain has a fixed area, λe is Θ(1/N) even when the mobility pattern of the nodes, the temporal variation of transmission powers, the source-destination pairs, and the possibly multi-path routes between them are optimally chosen. This result continues to hold even when the nodes are capable of maintaining multiple transmissions and/or receptions simultaneously, or when the communication bandwidth is partitioned into sub-channels of smaller bandwidth. We also address how λe depends on the other network parameters such as the area of the network domain, the path loss exponent, or the average number of hops between a source and a destination. Finally, we determine some required conditions to achieve a non-vanishing per-node end-to-end throughput as the number of nodes in the network grows large.

In this paper, we study the effect of multihop relaying on the throughput of the downstream channel in cellular networks. In particular, we compare the throughput of the multihop system with that of the conventional cellular system, demonstrating the achievable throughput improvement by the multihop relaying. We also propose a hybrid control strategy for the multihop relaying, in which we advocate the use of both, the direct transmission and the multihop relaying. Our study shows that most of the throughput gain can be obtained with the use of a two- and three-hop relaying scheme. Substantial throughput improvement could be additionally obtained by operating the concurrent relaying transmission in conjunction with the nonconcurrent transmission. We also argue here that the multihop relaying technology can be utilized for mitigating unfairness in quality-of-service (QoS), which comes about due to the location-dependent signal quality. Our results show that the multihop system can provide more even QoS over the cell area. The multihop cellular network architecture can also be utilized as a self-configuring network mechanism that efficiently accommodates variability of traffic distribution. We have studied the throughput improvement for the uniform, as well as for the nonuniform traffic distribution, and we conclude that the use of multihop relaying in cellular networks would be relatively robust to changes in the actual traffic distribution.

In this paper, we propose to concurrently search for a number of mobile users in a wireless cellular network based on the probabilistic information about the locations of mobile users. The concurrent search approach guarantees that all k mobile users will be located within k time slots. It is shown that even in the worst case when mobile users appear equally in all the cells of the network, the concurrent search approach is able to reduce the average paging cost by 25%. More importantly, this is achieved without an increase in the worst case paging delay or in the worst case paging cost. Depending on the total number of mobile users to be located, total number of cells in the network, and the probabilistic information about the locations of mobile users, the reduction of the average paging cost due to the usage of the concurrent search approach ranges from 25% to 88%. The case in which perfect probabilistic information is unavailable is also studied.

To effectively support communication in such a dynamic networking environment as the ad hoc networks, the routing framework has to be adaptable to the spatial and temporal changes in the characteristics of the network, such as traffic and mobility patterns. Multiscoping, as is provided through the concept of the Zone Routing Protocol (ZRP) for example, can serve as a basis for such an adaptive behavior. The Zone Routing framework implements hybrid routing by every network node proactively maintaining routing information about its local neighborhood called the routing zone, while reactively acquiring routes to destinations beyond the routing zone. In this paper, we propose the Independent Zone Routing (IZR) framework, an enhancement of the Zone Routing framework, which allows adaptive and distributed configuration for the optimal size of each node's routing zone, on the per-node basis. We demonstrate that the performance of IZR is significantly improved by its ability to automatically and dynamically tune the network routing operation, so as to flexibly and robustly support changes in the network characteristics and operational conditions. As a point of reference, through this form of adaptation, we show that the volume of routing control traffic overhead in the network can be reduced by an order of magnitude, under some set of parameter values. Furthermore, the adaptive nature of IZR enhances the scalability of these networks as well.

In this paper, we develop an analytical framework for evaluating multipath routing in mobile ad hoc networks. The instability of the topology (e.g., failure of links) in this type of network due to nodal mobility and changes in wireless propagation conditions makes transmission of time-sensitive information a challenging problem. To combat the inherent unreliability of these networks, we propose a routing scheme that uses multiple paths simultaneously by splitting the information between a multitude of paths, so as to increase the probability that the essential portion of the information is received at the destination without incurring excessive delay. Our scheme works by adding an overhead to each packet, which is calculated as a linear function of the original packet bits. The resulting packet (information and overhead) is fragmented into smaller blocks and distributed over the available paths. The probability of reconstructing the original information at the destination is derived in an analytical form and its behavior is studied for some special cases. It is shown that, under certain constraints, the packet dropping probability decreases as the number of used paths is increased.

In this paper, we extend the analysis of multipath routing presented in our previous work, so that the basic restrictions on the evaluation and optimization of that scheme can be dropped (e.g., disjoint paths and identical paths in terms of failure probability). In that work, we employed diversity coding in order to provide increased protection against frequent route failures by splitting data packets and distributing them over multiple disjoint paths. Motivated by the high increase in the packet delivery ratio, we study the increase we can achieve through the usage of multiple paths in the general case, where the paths are not necessarily independent and their failure probabilities vary. For this reason, a function that measures the probability of successful transmission is derived as a tight approximation of the evaluation function P/sub succ/. Given the failure probabilities of the available paths and their correlation, we are able to find in polynomial time the set of paths that maximizes the probability of reconstructing the original information at the destination.

An Erratum has been published for this article in Wireless Communications and Mobile Computing 2004; 4(4): 473. We extend the previously well-known results on the capacity of wireless networks and present the implications of our results on network scalability. In particular, we find bounds on the maximum achievable per-node end-to-end throughput, λe, and the maximum number of simultaneously successful wireless transmissions, Ntmax, under a more general network scenario than previously considered. Furthermore, in the derivation of our results, we make no restrictions on the mobility pattern of the nodes or on the number simultaneous transmissions and/or receptions that nodes are capable of maintaining. In our derivation, we analyze the effect of parameters such as the area of the network domain, A, the path loss exponent, γ, the processing gain, G, and the SINR threshold, β. Specifically, we prove the following results for a wireless network of N nodes that are equipped with omnidirectional antennas: (1) λe is Θ(1/N) under very general conditions. This result continues to hold even when the communication bandwidth is divided into sub-channels of smaller bandwidth. (2) Ntmax has an upper bound that does not depend on N, which is the simultaneous transmission capacity of the network domain, NtQ. For a circular network domain, NtQ is O(Amin{γ/2,1}) if γ≠2 and O(A/log(A)) if γ = 2. In addition, NtQ is O(γ2) and O(G/β). Moreover, lack of attenuation and lack of space are equivalent, where NtQ cannot exceed 1 + G/β. (3) As N → ∞ a desired per-node end-to-end throughput is not achievable, unless the average number of hops between a source and a destination does not grow indefinitely with N, A grows with N and N is O(Amin{γ/2,1}) if γ≠2 and O(A/log(A)) if γ = 2. Copyright © 2004 John Wiley & Sons, Ltd.

The IEEE 802.11 multiple access control protocol was modified for use in multi-channel, multi-hop ad hoc network, through the use of a new channel-status indicator. In particular, we have evaluated the improvement due to the multi-channel use. We report in this paper on the results of the throughput per node and the end-to-end delay for the modified IEEE 802.11 protocol for different network sizes. Using these results, we were able to propose a number of throughput scaling laws. Our simulation results show that the throughputs per node with multiple channels for the line and the grid ad hoc network topologies will increase by 47.89%, and by 1.39-163%, respectively, for networks with 16 to 64 nodes, as compared with that of single channel.

In this paper, the performance of a distributed mobility management scheme, the Randomized Database Group (RDG), for mobile ad hoc networks is presented. In this scheme, databases are used to store the location of the network nodes and to manage the mobility of nodes. When a mobile's location changes, a number of randomly selected databases are updated. When a mobile's location is needed, such as upon a call arrival, a number of randomly selected databases are queried. A number of different RDG query schemes are studied and their performance are compared. In particular, the optimum update-group size and the query-group size are found. We also present the probability of the first query being successful and the average query delay to find the mobile's location. Finally, we estimate the cost of implementing the RDG scheme as a function of different number of databases.

On-demand routing reduces the control overhead in mobile ad hoc networks, but it has the major drawback of introducing latency between route-request arrival and the determination of a valid route. This paper addresses the issue of minimizing the delay in on-demand routing protocols through optimizing the Time-to-Live (TTL) interval for route caching. An analytical framework is introduced to compute the expected routing delay when a source node or an intermediate node has a cached route with any given TTL value. Furthermore, numerical methods are proposed to determine the optimal TTL of a newly cached route. We present simulation results that support the validity of our analysis. Using the proposed analytical framework, we study how the routing delay is affected by route length, route-request frequency, and the frequency of topology variation. We show that the proposed optimal route-cache TTL strategy can significantly reduce the routing delay over systems that either does not use route-cache or keeps route-cache indefinitely long. We further show that the performance gain of optimizing the route-cache TTL increases with increasing traffic pattern localization.

A central challenge in ad hoc networks is the design of routing protocols that can adapt their behavior to frequent and rapid changes in the network. The performance of proactive and reactive routing protocols varies with network characteristics, and one protocol may outperform the other in different network conditions. The optimal routing strategy depends on the underlying network topology, rate of change, and traffic pattern, and varies dynamically. This paper introduces the Sharp Hybrid Adaptive Routing Protocol (SHARP), which automatically finds the balance point between proactive and reactive routing by adjusting the degree to which route information is propagated proactively versus the degree to which it needs to be discovered reactively. SHARP enables each node to use a different application-specific performance metric to control the adaptation of the routing layer. This paper describes application-specific protocols built on top of SHARP for minimizing packet overhead, bounding loss rate, and controlling jitter. Simulation studies show that the resulting protocols outperform the purely proactive and purely reactive protocols across a wide range of network characteristics.

In wireless ad hoc networks, capacity can be traded for delay. This tradeoff has been the subject of a number of studies, mainly concentrating on the two extremes: either minimizing the delay or maximizing the capacity. However, in between these extremes, there are schemes that allow instantiations of various degrees of this tradeoff. Infostations, which offer geographically intermittent coverage at high speeds, are one such an example. Indeed, through the use of the Infostation networking paradigm, the capacity of a mobile network can be increased at the expense of delay. We propose to further extend the Infostation concept by integrating it with the ad hoc networking technology. We refer to this networking model as the Shared Wireless Infostation Model (SWIM). SWIM allows additional improvement in the capacity-delay tradeoff through a moderate increase in the storage requirements. To demonstrate how SWIM can be applied to solve a practical problem, we use the example of a biological information acquisition system - radio-tagged whales - as nodes in an ad hoc network. We derive an analytical formula for the distribution of end-to-end delays and calculate the storage requirements. We further extend SWIM by allowing multi-tiered operation; which in our biological information acquisition system could be realized through seabirds acting as mobile data collection nodes.

To improve channel throughput and the fairness of random access channels, we propose a new backoff algorithm, namely, the sensing backoff algorithm (SBA). A novel feature of the SBA scheme is the sensing mechanism, in which every node modifies its backoff interval according to the results of the sensed channel activities. In particular, every active node sensing a successful transmission decreases its backoff interval by an additive factor of the transmission time of a packet. In order to find the optimum parameters for the SBA scheme, we have studied the optimum backoff intervals as a function of different numbers of active nodes (N) in a single transmission area with pure ALOHA-type channels. We find that the optimum backoff interval should be 4N times the packet transmission time when the random access channel operates under a pure ALOHA scheme. Based on this result, we have calculated numerically the optimum values of the parameters for SBA, which are independent of N. The SBA scheme operates close to the optimum backoff interval. Furthermore, its operation does not depend on a knowledge of N. The optimum backoff interval and the SBA scheme have been studied also by simulation. It is shown that the SBA scheme out-performs other backoff schemes, such as binary exponential backoff (BEB) and multiplicative increase linear decrease (MILD). As a point of reference, the SBA scheme offers a channel capacity of 0.19 when N is 10, while the MILD scheme can only offer 0.125. The performance gain is about 50%.

This paper presents a mobile tracking scheme that exploits the predictability of user mobility patterns in wireless PCS networks. In this scheme, a mobile's future location is predicted by the network, based on the information gathered from the mobile's recent report of location and velocity. When a call is made, the network pages the destination mobile around the predicted location. A mobile makes the same location prediction as the network does; it inspects its own location periodically and reports the new location when the distance between the predicted and the actual locations exceeds a threshold. To more realistically represent the various degrees of velocity correlation in time, a Gauss-Markov mobility model is used. For practical systems where the mobility pattern varies over time, we propose a dynamic Gauss-Markov parameter estimator that provides the mobility parameters to the prediction algorithm. Based on the Gauss-Markov model, we describe an analytical framework to evaluate the cost of mobility management for the proposed scheme. We also present an approximation method that reduces the computational complexity of the cost evaluation for multidimensional systems. We then compare the cost of predictive mobility management against that of the regular, nonpredictive distance-based scheme, for both the case with ideal Gauss-Markov mobility pattern and the case with time-varying mobility pattern.

In ad hoc networks, each node utilizes its limited resources to carry out the collective operation of the network. It is not always in the best interests of the network's nodes to demand the continuous participation of all nodes in the network operations. We propose an energy dependent participation (EDP) scheme, where a node periodically re-evaluates its participation in the network based on the residual energy in its battery. More importantly, a node gives special consideration to supporting the communication needs of its active network applications and preventing further network partitioning. EDP's localized partition checking algorithm is particularly well suited for the zone routing protocol, where the link-state information is proactively maintained within each node's local zone and routes to faraway nodes are reactively obtained via global queries. Through simulations, we evaluate the impact of our proposed scheme on battery life and network connectivity. Our results suggest that the EDP scheme can increase the usable lifetime of a battery-constraint ad hoc network by over 50%.

Many ad hoc routing protocols are based on some variant of flooding. Despite various optimizations, many routing messages are propagated unnecessarily. We propose a gossiping-based approach, where each node forwards a message with some probability, to reduce the overhead of the routing protocols. Gossiping exhibits bimodal behavior in sufficiently large networks: in some executions, the gossip dies out quickly and hardly any node gets the message; in the remaining executions, a substantial fraction of the nodes gets the message. The fraction of executions in which most nodes get the message depends on the gossiping probability and the topology of the network. In the networks we have considered, using gossiping probability between 0.6 and 0.8 suffices to ensure that almost every node gets the message in almost every execution. For large networks, this simple gossiping protocol uses up to 35% fewer messages than flooding, with improved performance. Gossiping can also be combined with various optimizations of flooding to yield further benefits. Simulations show that adding gossiping to AODV results in significant performance improvement, even in networks as small as 150 nodes. We expect that the improvement should be even more significant in larger networks.

Topological changes in mobile ad hoc networks frequently render routing paths unusable. Such recurrent path failures have detrimental effects on the network ability to support QoS-driven services. A promising technique for addressing this problem is to use multiple redundant paths between the source and the destination. However while multipath routing algorithms can tolerate network failures well their failure resilience only holds if the paths are selected judiciously. In particular the correlation between the failures of the paths in a redundant path set should be as small as possible. However selecting an optimal path set is an NP-complete problem. Heuristic solutions proposed in the literature are either too complex to be performed in real-time or too ineffective or both. This paper proposes a multipath routing algorithm called Disjoint Pathset Selection Protocol (DPSP) based on a novel heuristic that in nearly linear time on average picks a set of highly reliable paths. The convergence to a highly reliable path set is very fast and the protocol provides flexibility in path selection and routing algorithm. Furthermore DPSP is suitable for real-time execution with nearly no message exchange overhead and with minimal additional storage requirements. This paper presents evidence that multipath routing can mask a substantial number of failures in the network compared to single path routing protocols and that the selection of paths according to DPSP can be beneficial for mobile ad hoc networks since it dramatically reduces the rate of route discoveries.

In ad hoc networks, the hidden- and the exposed-terminal problems can severely reduce the network capacity on the MAC layer. To address these problems, the ready-to-send and clear-to-send (RTS/CTS) dialogue has been proposed in the literature. However, MAC schemes using only the RTS/CTS dialogue cannot completely solve the hidden and the exposed terminal problems, as pure "packet sensing" MAC schemes are not safe even in fully connected networks. We propose a new MAC protocol, termed the dual busy tone multiple access (DBTMA) scheme. The operation of the DBTMA protocol is based on the RTS packet and two narrow-bandwidth, out-of-band busy tones. With the use of the RTS packet and the receive busy tone, which is set up by the receiver, our scheme completely solves the hidden- and the exposed-terminal problems. The busy tone, which is set up by the transmitter, provides protection for the RTS packets, increasing the probability of successful RTS reception and, consequently, increasing the throughput. This paper outlines the operation rules of the DBTMA scheme and analyzes its performance. Simulation results are also provided to support the analytical results. It is concluded that the DBTMA protocol is superior to other schemes that rely on the RTS/CTS dialogue on a single channel or to those that rely on a single busy tone. As a point of reference, the DBTMA scheme out-performs FAMA-NCS by 20-40% in our simulations using the network topologies borrowed from the FAMA-NCS paper. In an ad hoc network with a large coverage area, DBTMA achieves performance gain of 140% over FAMA-NCS and performance gain of 20% over RI-BTMA.

In this paper, we study the impact of heterogeneous receivers on the throughput of multicast flow control and propose a new multicast flow control algorithm to optimally partition group members into multiple subgroups. Our main contributions are as follows. First, we cast the multicast flow control problem in the Internet as the list partition problem and then prove that the list partition problem is equivalent to the optimal paging problem in cellular networks. The result is not only interesting in itself but also essential to derive the first known analytical bounds for the throughput of multicast flow control. Furthermore, we propose an algorithm to solve not only the list partition problem but also the optimal paging problem and the problem of bulk data transfer using multiple multicast groups. The complexity of our algorithm is one order less than the best known algorithm designed only for the problem of bulk data transfer using multiple multicast groups in the literature. While earlier work uses simulations to justify the usage of multiple subgroups to deliver information to a large amount of receivers in heterogeneous networks, we provide the first analytical support.

We present some results regarding the paging cost gains and the cost-delay tradeoff that can be achieved by using sequential paging to locate mobile users in cellular networks. We describe tight bounds on the average paging cost and the paging delay and quantify the intuition that greater gains are achieved when the mobile user's location probabilities are concentrated in a small portion of the location area. We also examine the impact of errors in the location estimates on the paging cost.

Multicasting is an efficient means of one to many communication and is typically implemented by creating a multicasting tree. Because of the severe battery power and transmission bandwidth limitations in ad hoc networks, multicast routing can significantly improve the performance of this type of network. However, due to the frequent and hard-to-predict topological changes of ad hoc networks, maintenance of a multicasting tree to ensure its availability could be a difficult task. We borrow from the concept of alternate path routing, which has been studied for providing QoS routing, effective congestion control, security, and route failure protection, to propose a scheme in which a set of multicasting trees is continuously maintained. A tree is used until it fails at which time it is replaced by an alternative tree in the set, so that the time between failure of a tree and resumption of multicast routing is minimal. We introduce the scheme and present a number of heuristics to compute a set of alternate trees. The heuristics are then compared in terms of transmission cost, improvement in the average time between multicast failures and the probability of usefulness. Simulations show significant gains over a wide range of network operational conditions. In particular, we show that using alternate trees has the potential of improving mean time between interruption by 100-600% in a 50 node network (for most multicast group sizes) with small increase in the tree cost and the route discovery overhead.

We study the performance of route query control mechanisms for the zone routing protocol (ZRP) for ad hoc networks. The ZRP proactively maintains routing information for a local neighborhood (routing zone), while reactively acquiring routes to destinations beyond the routing zone. This hybrid routing approach can be more efficient than traditional routing schemes. However, without proper query control techniques, the ZRP cannot provide the expected reduction in the control traffic. Our proposed query control schemes exploit the structure of the routing zone to provide enhanced detection and prevention of overlapping queries. These techniques can be applied to single- or multiple-channel ad hoc networks to improve both the delay and control traffic performance of ZRP. Our query control mechanisms allow ZRP to provide routes to all accessible network nodes, with less control traffic than purely proactive link state or purely reactive route discovery, and with less delay than conventional flood searching.

In this paper, we present the implementation issues of a virtual backbone that supports the operations of the uniform quorum system (UQS) and the randomized database group (RDG) mobility management schemes in an ad hoc network. The virtual backbone comprises nodes that are dynamically selected to contain databases that store the location information of the network nodes. Together with the UQS and RDG schemes, the virtual backbone allows both dynamic database residence and dynamic database access, which provide high degree of location data availability and reliability. We introduce a distributed database coverage heuristic (DDCH), which is equivalent to the centralized greedy algorithm for virtual backbone generation, but only requires local information exchange and local computation. We show how DDCH can be employed to dynamically maintain the structure of the virtual backbone, along with database merging, as the network topology changes. We also provide a means to maintain connectivity among the virtual backbone nodes. We discuss optimization issues of DDCH through simulations. Simulation results suggest that the cost of ad hoc mobility management with a virtual backbone can be far below that of the conventional link-state routing.

Alternate path routing (APR) can provide load balancing and route failure protection by distributing traffic among a set of diverse paths. These benefits make APR appear to be an ideal candidate for the bandwidth limited and mobile ad-hoc networks. However, we find that APR's potential is not fully realized in ad-hoc networks because of route coupling resulting from the geographic proximity of candidate paths between common endpoints. In multiple channel networks, coupling occurs when paths share common intermediate nodes. The coupling problem is much more serious in single channel networks, where coupling also occurs where one path crosses the radio coverage area of another path, The network's inherent route coupling is further aggravated by the routing protocol, which may provide an incomplete view of current network connectivity. Through analysis and simulation, we demonstrate the impact of route coupling on APR's delay performance in ad-hoc networks. In multiple channel environments, APR is able to provide a 20% reduction in end-to-end delay for bursty data streams. Though these gains are appreciable, they are about half what we would expect from APR with independently operating routes. Route coupling is so severe in single channel networks that APR provides only negligible improvements in quality of service.

Neighbor discovery protocols are based on the receipt of an acknowledgement in response to a node advertisement. Traditionally, the advertisement and acknowledgement are sent in opposite directions across the same link. As a result, these protocols are only able to discover bidirectional links. The exclusion of unidirectional links can reduce the network's capacity and also lead to network partitioning. These concerns are particularly relevant to ad hoc networks, where bandwidth and transmission power are limited resources and links are not necessarily bidirectional. To support the discovery and subsequent use of unidirectional links, we propose the multi-hop broadcast and unicast relay of link layer acknowledgements. Through simulation of an ad hoc network with reduced transmission power support, we demonstrate that the proposed multi-hop feedback techniques are able to discover nearly all of the unidirectional links, improving network connectivity by 30% and reducing the length of shortest hop paths by 25%.

Previous work in ad hoc network routing protocol development has demonstrated how global route discovery can be performed more efficiently by leveraging the known topology of each node's local surrounding area (routing zone). In this paper, we demonstrate how routing zones can also be applied to the maintenance of these discovered routes. In particular, knowledge of routing zone connectivity can be used to automatically bypass link failures and provide "shortcuts" across sub-optimal segments of the route. These mechanisms contribute to a reduction in network load and end-to-end delay, while improving the stability of active connections.

A distributed mobility management scheme using randomized database groups (RDG) is proposed and analyzed for ad-hoc networks. In the proposed scheme, location databases are stored in the network nodes, comprising a virtual backbone within the flat network architecture. Upon location update or call arrival, a mobile's location information is written to or read from, respectively, a group of randomly chosen databases. Compared with a centralized scheme (such as the home location register) with fixed associations, this scheme is more suitable for ad-hoc networks, where the connectivity of the nodes with the rest of the network can be intermittent and sporadic, and the databases are relatively unstable. The expected cost due to call loss and location updates using this scheme is analyzed in the presence of database disconnections. Based on the expected cost, we present the numerical determination and approximation of the optimal total location database number, the optimal database access group size, and the optimal location update frequency, under different network stability, traffic, and mobility conditions. Numerical results show that the RDG scheme provides an robust and efficient approach to ad-hoc mobility management.

This paper presents a mobile tracking scheme that exploits the predictability of user mobility patterns in wireless PCS networks. Instead of the constant velocity fluid-flow or the random-walk mobility model, a more realistic Gauss-Markov model is introduced, where a mobile's velocity is correlated in time to a various degree. Based on the Gauss-Markov model, a mobile's future location is predicted by the network based on the information gathered from the mobile's last report of location and velocity. When a call is made, the network pages the destination mobile at and around the predicted location of the mobile and in the order of descending probability until the mobile is found. A mobile shares the same prediction information with the network and reports its new location whenever it reaches some threshold distance away from the predicted location. We describe an analytical framework to evaluate the cost of mobility management for the proposed predictive distance-based scheme. We then compare this cost against that of the regular, non-predictive distance-based scheme, which is obtained through simulations. Performance advantage of the proposed scheme is demonstrated under various mobility and call patterns, update cost, page cost, and frequencies of mobile location inspections.

The performance of the dynamic packet reservation multiple access (DPRMA) protocol for medium access control (MAC) is investigated in this work. This MAC protocol is designed for use in a wireless ATM system. A primary feature of DPRMA is the centralized controller, a base station in a cellular system, whose responsibility is to allocate resources on the uplink channel. The base station does this in an intelligent fashion based on the resource demands submitted by each active mobile and the Quality of Service (QoS) requirements of each type of user. Each user is responsible for providing updated information about its immediate bandwidth needs. The ability of the mobile to dynamically change its bandwidth reservation request is another primary feature of the system. The system investigated in this work contains voice, video conferencing, and data traffic.

The dual busy tone multiple access (DBTMA) scheme was designed for distributed multi-hop networks. The protocol uses the RTS/CTS dialogue to reserve the shared channel. In multi-hop networks, some nodes in the range of the transmitter and/or the range of the receiver might not be able to hear a successful RTS/CTS message exchange. This may lead to access collisions and data destruction. Hence we use two narrow bandwidth busy tones, to notify neighbor nodes of the on-going use of the channel. In this paper, we analyze the capacity of the DBTMA protocol. The effects of various parameters on the network utilization in multi-hop networks are also discussed. We compare, through analytical results and simulation means, the performance of the DBTMA protocol with schemes that solely use the RTS/CTS dialogue to prevent collisions and we show that DBTMA provides superior performance to such schemes.

In this paper, we investigate the diversity properties of routes acquired through traditional route query floods. The on-demand nature of query-based (reactive) routing is appropriate for networks with dynamic network topologies (such as ad-hoc networks). An intrinsic feature of route query protocols is the support for multiple route replies. While it is widely assumed that the set of discovered routes is fairly diverse, an examination of the query propagation behavior reveals that these routes typically have many nodes/links in common. Without sufficient diversity, the routing protocol is limited in its ability to promptly recover from (or even prevent) interrupted service caused by route failures. Based on our understanding of query propagation behavior, we propose a novel mechanism that uses information from terminated query threads to "inject" diversity into the collection of route replies. We study the impact of our diversity injection scheme on the demanding ad-hoc networking environment. Our results demonstrate how diversity injection can leverage a small investment of temporary cache to reduce both the rate of route failures and the bandwidth consumed by overhead query traffic.

The zone routing protocol (ZRP) is a hybrid routing protocol that proactively maintains routes within a local region of the network (which we refer to as the routing zone). Knowledge of this routing zone topology is leveraged by the ZRP to improve the efficiency of a reactive route query/reply mechanism. The ZRP can be configured for a particular network through adjustment of a single parameter, the routing zone radius. We address the issue of configuring the ZRP to provide the best performance for a particular network at any time. Previous work has demonstrated that an optimally configured ZRP operates at least as efficiently as traditional reactive flood-search or proactive distance vector/link state routing protocols (and in many cases, much more efficiently). Adaptation of the ZIP to changing network conditions requires both an understanding of how the ZRP reacts to changes in network behavior and a mechanism to allow individual nodes to identify these changes given only limited knowledge of the network behavior. We demonstrate the effects of relative node velocity, node density, network span, and user data activity on the performance of the ZRP. We then introduce two different schemes ("min searching" and "traffic adaptive") that allow individual nodes to identify and appropriately react to changes in network configuration, based only on information derived from the amount of received ZRP traffic. Through test-bed simulation, we demonstrate that these radius estimation techniques can allow the ZRP to operate within 2% of the control traffic resulting from perfect radius estimation.

A distributed mobility management scheme using a class of uniform quorum systems (UQS) is proposed for ad hoc networks. In the proposed scheme, location databases are stored in the network nodes themselves, which form a self-organizing virtual backbone within the flat network structure. The databases are dynamically organized into quorums, every two of which intersect at a constant number of databases. Upon location update or call arrival, a mobile's location information is written to or read from all the databases of a quorum, chosen in a nondeterministic manner. Compared with a conventional scheme [such as the use of home location register (HLR)] with fixed associations, this scheme is more suitable for ad hoc networks, where the connectivity of the nodes with the rest of the network can be intermittent and sporadic and the databases are relatively unstable. We introduce UQS, where the size of the quorum intersection is a design parameter that can be tuned to adapt to the traffic and mobility patterns of the network nodes. We propose the construction of UQS through the balanced incomplete block designs. The average cost, due to call loss and location updates using such systems, is analyzed in the presence of database disconnections. Based on the average cost, we investigate the tradeoff between the system reliability and the cost of location updates in the UQS scheme. The problem of optimizing the quorum size under different network traffic and mobility patterns is treated numerically. A dynamic and distributed HLR scheme, as a limiting case of the UQS, is also analyzed and shown to be suboptimal in general. It is also shown that partitioning of the network is sometimes necessary to reduce the cost of mobility management.

The performance of dynamic packet reservation multiple access (DPRMA), which was introduced in previous publications, is investigated. In particular, we show here how the DPRMA scheme integrates three different traffic types: voice, video, and data, through a single access control mechanism that permits users to specify their immediate bandwidth requirements. The primary feature of the DPRMA is the dynamic matching of the traffic source generation rates with the assigned fraction of the channel capacity. This is accomplished by a multiple-threshold-driven control algorithm with hysteresis that regulates the actual amount of channel capacity assigned to users. We discuss how the multiple thresholds are selected for the three traffic types and demonstrate the improvement of the DPRMA mechanism in increased throughput and reduced number of collisions.

In this paper, the outage probability of the "multiply-detected macrodiversity (MDM)" scheme is studied. The previously introduced MDM scheme is a non-selection based, postdetection combining scheme, which employs a maximum likelihood decision algorithm on signals received at a number of base-stations. Previously published performance results indicate that the MDM scheme considerably outperforms other conventional forms of macrodiversity throughout a large portion of the cell area. In this paper, we analyze and simulate the outage performance of the MDM scheme and compare those with the (S/I)-diversity scheme. As a point of reference, if the outage probability is defined as BER above 0.0001, the outage is eliminated at least 45% of the time as compared with (S/I)-diversity, for propagation attenuation exponent of 4 and shadowing standard deviation of 4 dB.

In this paper, we investigate the performance of a novel routing protocol, the zone routing protocol (ZRP), that was introduced in Haas (1997). The protocol is targeted at a special class of ad-hoc networks, which we refer to as the reconfigurable wireless networks (RWNs). RWNs are distinguished from other ad-hoc networks by their increased node mobility, larger number of nodes, and wider network span. We demonstrate that ZRP significantly reduces the delay and the amount of routing overhead by providing each node with continuous updates of its local neighborhood (routing zone) topology only. The structure of the routing zone is exploited to efficiently acquire routes on demand for destinations that lie beyond a node's routing zone. By adjusting a single parameter-the size of the routing zone-the ZRP can adapt to a variety of network operational conditions.

This paper studies the database failure recovery procedure for cellular phone networks as part of the EIA/TIA IS-41 standard. We show that, under some cost assumptions, a periodic update interval should be chosen to be approximately equal to the call interarrival time, with more frequent updates for more unreliable system. We also show that the cost of an optimized system is relatively small and stable, if the system is even moderately reliable. Finally, if the system is at least moderately reliable, the effects of call origination rate and the rate at which location areas are crossed are rather small, assuming that the periodic update interval was chosen as stated above. Thus in such cases, optimization of the size of the location area can be made independent of the optimization of the periodic update process.

In this paper, we study the performance of route query control mechanisms for the recently proposed Zone Routing Protocol (ZRP) for ad-hoc networks. The ZRP proactively maintains routing information for a local neighborhood (routing zone), while reactively acquiring routes to destinations beyond the routing zone. This hybrid routing approach has the potential to be more efficient in the generation of control traffic than traditional routing schemes. However, without proper query control techniques, the ZRP can actually produce more traffic than standard flooding protocols.Our proposed query control schemes exploit the structure of the routing zone to provide enhanced detection (Query Detection (QD1/QD2)), termination (Loop-back Termination (LT), Early Termination (ET)) and prevention (Selective Bordercasting (SBC)) of overlapping queries. We demonstrate how certain combinations of these techniques can be applied to single channel or multiple channel ad-hoc networks to improve both the delay and control traffic performance of the ZRP. Our query control mechanisms allow the ZRP to provide routes to all accessible network nodes with only a fraction of the control traffic generated by purely proactive distance vector and purely reactive flooding schemes, and with a response time as low as 10% of a flooding route query delay.

In this paper, we advocate the use, propose the design, and describe the implementation of an asymmetric transport-layer protocol for mobile systems. In an asymmetrically designed protocol, peer functions are implemented through algorithms and procedures that are of substantially different complexity, with the lower complexity procedures used on the mobile device. We have devised and implemented an example of such a communication protocol, termed by us Mobile-TCP that provides the TCP functionality. Although Mobile-TCP is based on the indirect interaction model, the semantics of the TCP protocol are preserved.

The stochastic nature of wireless communication that manifests itself by the fading processes leads the designers of wireless networks to over-engineer their designs. This random behavior can, however, be reduced by diversity methods. For example, Rayleigh fading can be mitigated by microdiversity and the effects of shadow fading can be reduced by macrodiversity techniques. In this paper, we advocate the use of a macrodiversity scheme based on signal-to-interference ratio, in combination with postdetection combining. This scheme has been termed multiply-detected macrodiversity (MDM). We discuss the sources of the improvement and demonstrate that the MDM scheme yields a considerable gain over the traditional macrodiversity scheme that is based on selection of the strongest signal path. Application of the MDM scheme in practical wireless networks results in reduction in the BER throughout over 50% of the system coverage.

Presents and investigates the performance of a secure access scheme to shared information. The primary target application is the electronic newspaper for mobile, wirelessly accessing users. In this application, a dynamically changing set of users is allowed to access the newspaper server. The authors based the solution on the locker key scheme, in which a user's access permission is granted by the server placing a universal encryption key in the user's buffer. The newspaper is then encrypted with the universal key and made public. Some of the salient features of the proposed scheme are: the newspaper is encrypted once and a single copy is stored in the server, the encryption is done off-line, considerably reducing the server congestion, and there is no need to redistribute the universal key upon its change. Furthermore, the authors show that, using some realistic parameter values, the scheme can reduce the access time two to three orders of magnitude over a scheme in which the encryption is performed in real-time on a request-by-request basis.

We investigate the capacity of cellular systems. In particular, we study how the reuse factor can be improved given the knowledge of the mobiles' locations; i.e., we evaluate the minimum number of channels required to support a cellular infrastructure with a given number of mobiles in each cell. We assume that the mobiles' locations are sampled from the uniform random distribution or are fixed on a uniform grid. Moreover, we show the effect of a number of parameters, such as the number of mobiles per cell, the minimum allowable signal-to-interference ratio, and limited knowledge of mobile location. The assumption of a single interferer, used in our study, is also justified.

We propose a time-frequency-code-slicing technique that allows multiple users with different speed requirements access to a communications resource in a manner that is cost effective over a wide range of access speeds. Conventional systems require low-speed users to either pay a cost premium (e.g., high-speed burst modems for TDMA) or end up wasting some of the spectrum (e.g., universal time slots). The advantages of our proposed scheme can be demonstrated with any two combinations (or all three) of time slicing, frequency slicing, and code slicing. For instance, with a time-frequency-slicing approach, users can be assigned different portions of the spectrum (e.g., on a slot-by-slot basis), allowing them access to only the needed fraction of the shared resource. With time-code slicing, users are dynamically granted portions of the code space, during specified time periods. Finally, in frequency-code slicing, a user may be assigned a number of codes in different frequency bands. These codes can be of constant or variable chip rates.

The author studies the performance of some extensions to the staggering switch, an almost-all-optical packet switch architecture based on two rearrangeably nonblocking stages interconnected by optical delay lines with different amounts of delay. In particular, the packet loss probability of the switch is investigated for bursty traffic, and a hardware modification that will make it possible to overcome the performance penalty of the time correlation is proposed. The switch growability patterns and an extension to the wavelength-division-multiplexing (WDM) case are discussed. A scheme that will allow optical packet synchronization for the synchronously operated switch is presented.>

The authors introduce the design of the optical distribution channel, which serves as a local-area network and facilitates high-speed interconnection among heterogeneous machines in a way that is economically justifiable. In particular, they describe a design of an almost-all-optical local-area network that is capable of providing a gigabit per second transmission rate directly to the user. The network design is based on the 802.6 topology, a field coding technique in which the header and the data fields are encoded at different rates, and the principle of almost-all-optical switching.>

The authors address the selection of a single packet size for broadband integrated packet networks (e.g., B-ISDN) that carry many traffic types, each with different performance requirements. To meet the requirements of different types of traffic in an integrated network (for example, synchronous stream traffic and delay sensitive bursty traffic, or two different types of synchronous traffic), the probability distribution of the end-to-end network delay must be determined. New insights and procedures for estimating such statistics for store-and-forward integrated packet networks that have a single packet size are presented. In particular, the authors address two problems: determining the required build-out delay to achieve a given packet loss probability for real-time stream traffic (e.g., voice), and determining a packet size that results in the required delay characteristics for delay sensitive traffic (e.g., data for parallel processing applications). The central limit theorem is used to approximate the probability distribution function for the end-to-end packet delay in a network of M/D/1 nodes.>

Issues in congestion control are discussed, and a novel congestion control scheme for high-speed networks is described. The scheme is based on periodic transmission of sample time-stamped packets through the network. Upon reception, the packet delays are calculated, averaged, and used to determine the state of the network. The information on the state of the network is then used to drive the network admission control. The major advantage of the proposed scheme over conventional congestion control techniques is that it copes with traffic surges that are shorter than the network round-trip delay. This is achieved by controlling traffic admission with a continuous estimate of the network state. The scheme is targeted towards networks that carry aggregated traffic, and can be applied to ATM-based networks.>

Providing a single user with a very high (gigabit per second) bandwidth is a challenge that the research community is trying to address these days. The paper describes a design of an 'almost-all' optical local-area network that is capable of providing Gbps directly to the user. The network design is based on the 802.6 topology, the 'field coding' technique (in which the header and the data fields are encoded in different rates), and the principle of 'almost all' optical switching. Several of the design considerations were: cost, integration, and flexibility. By providing all of these attributes, the optical distribution channel may be an example of future high-speed local area networks, in line with the FDDI-follow on effort.>

The horizontally oriented protocol structure (HOPS) is proposed. The horizontal structure, coupled with a small number of layers, eliminates unnecessary replication and processing of functions and lowers the unnecessary interprocess communication burden of the protocol execution-for example, unnecessary error detection for voice packets in integrated applications and unnecessary error detection in multiple layers for data applications are eliminated, the overhead associated with multi-peer-to-peer connection to multilayer architecture is substantially reduced, etc. The proposed architecture lends itself more naturally to full parallel implementation. Parallelism offers the potential of increased protocol processing rates, reduction in processing latency, and further reduction in processing overhead; and with the advances in VLSI, full parallel implementation becomes more and more feasible. The scheme also features selective functionality, which permits the protocol to easily adjust itself to the kind of performance demanded by the network-application requirements.>

Blazelan is an extended bus local-area network that provides high-throughput low-latency communication and can be used for distributed and parallel processing systems. The basic network design and operation are presented, and two techniques are introduced to increase the network capacity: loop concatenation and loop replication. It is shown that with the two techniques, the Blazelan capacity approaches that of the output queuing system.>

A packet-switching network with photonic data path, Blazenet, that provides low delay and has minimal memory requirements is described. It can be extended to support multicast and priority delivery. The Blazenet design is described, addressing the issues of packet-switching and traffic congestion. A detailed switching node design is presented. Extended features that can be incorporated into Blazenet's design-priority traffic, limiting packet lifetime, and broadcast and multicast-are described. Some issues of the higher layers that have direct implication on Blazenet's operation are presented.>