Considered is a network of N nodes which wirelessly communicate among themselves in peer-to-peer fashion. For such a network, we apply well known information-theoretic concepts, but applied to switching networks (rather than to physical links), to find an easily computed upper bound on the rate at which information may be carried for any number of nodes, any internodal connectivity pattern, and any hop-by- hop routing algorithm. Found is the maximum factor by which a given traffic matrix, corresponding the relative exogenous demand among each pair of nodes, may be scaled, subject to constraints that (1) each node transmits to only one intended receiver at a time, (2) each node receives from only one transmitter at a time, and (3) no node can transmit and receive simultaneously. Sample results are presented.

We consider a wireless network with n peer-to-peer nodes that are randomly distributed over some service area. Given the traffic and channel gain matrix, we use the flow deviation (FD) method to find an upper bound on the achievable capacity of the network, assuming no interference. In addition, we show that the achievable capacity without interference is lower- bounded by 2/3 of the upper bound in special cases and 1/3 of the upper bound in the worst case.

A multiple-input multiple-output (MIMO) downlink system in which data is transmitted to two users over a common wireless channel is considered. The channel is assumed to be fixed for all transmissions over the period of interest and the ratio of anticipated average arrival rates for the two users, also known as the relative traffic rate, is the system design parameter. A packet-based traffic model is considered where data for each user is queued at the transmit end. A queueing analog for this system leads to a coupled queueing system for which a simple policy is known to be throughput-optimal under Markovian assumptions. Since an exact expression for the performance is not available, as a measure of performance (in heavy traffic), a diffusion approximation is established. This diffusion process is a two-dimensional (2-D) Semimartingale Reflecting Brownian Motion (SRBM) living in the positive quadrant of 2-D space

A smart antenna's ability to simultaneously resolve more than one user on the same channel is exploited to help expedite the process of random access, especially for the reverse channel. However, use of a smart antenna requires frequent array adaptation, a difficult task in a random access environment. In papers H. Jin and Acampora (2005), we proposed and studied the SINR performance of a media access protocol intended for use with smart antennas. In this paper, we study the random access performance of the proposed protocol when a fast collision resolution algorithm is used. Found are the maximum achievable throughput and an upper bound on the expected delay. The impact of the number of antennas on the performance with both flat fading and frequency selective fading is studied. Topical results show significant improvement in throughput for systems with smart antennas. For example, when the fading is flat, the gain in maximum achievable throughput can be as large as 232% and 550% with four and eight antenna elements respectively. Even when severe frequency selective fading is present, there is a gain of 40% and 145% with four and eight antenna elements, respectively. We also show that dynamically adjusting the retransmission probability in the collision resolution process is not worthwhile since this extra complexity produces a gain of only about 6.2% in the best case

In this paper, we present a framework that improves the reliability and reduce the power consumption of a wireless ad hoc network through the deployment of supplementary nodes called agents. Agents are stationary nodes that have less stringent power constraints than the battery-operated mobile nodes. Placed in a mesh-like formation, agents enhance the connectivity of an ad hoc network by reducing the probability of having a partitioned system. We also show that the power consumption of user nodes is significantly decreased with a clever choice of the routing strategy, one that picks the routes that go through the agent nodes as often as possible, hence preventing the mobiles from relaying packets. In addition, the impact of a power control mode is analyzed. Even though such a mode already promises power efficiency, we demonstrate that the amount of power savings is further increased when the power control is used within the agent scheme. Finally, we present the traffic simulation results to assess the maximum throughput of no-agent and agent-supported networks under various routing strategies like bandwidth-conserving, power-conserving, etc. Some degradation in maximum throughput can be observed as a result of forcing routes to go through agent nodes

Smart antennas have the ability to cancel both cochannel interference (CCI) and intersymbol interference (ISI), and thus could be used in a cellular system to enhance coverage or increase capacity. A long standing problem is that of providing fast and efficient array adaptation in a random access packet-switching environment. This paper considers the media access control (MAC) design problem when smart antennas are used in a wireless data network and multipath fading is flat. The case of time dispersive fading will be considered in a companion paper. We propose a reservation-based MAC scheme in which the antenna weights will be computed from slot to slot to capture the actual interference and multipath fading. Performance of this scheme under the condition of flat Rayleigh fading is evaluated, and the training overhead is found, from which important design parameters for array training may be deduced. Bounds are also produced for the delay performance. Simulation results show that in an interference limited multicell environment, smart antennas can be used to significantly increase frequency reuse. If we require that the signal-to-interference-plus-noise (SINR) ratio must be at least 10 dB more than 90% of the time, then equipping each base station with eight antenna elements will allow two mobiles to simultaneously receive from the same base station. The number of training symbols required by the overhead in each transmission slot is between three and four times the number of antenna elements. We assume throughout that once adapted at the beginning of a packet, the antenna adapts throughout the packet to maintain the same SINR over the entire packet.

A smart antenna's ability to simultaneously resolve more than one user on the same channel is exploited to help expedite the process of random access, especially for reverse channel signalling. However, the use of a smart antenna requires frequent array adaptation, a difficult task in a random access environment. In our paper of 2003, we proposed and studied the SINR performance of a media access protocol intended for use with smart antennas. In this paper, we study the random access performance of the proposed protocol when a tree-based fast collision resolution algorithm is used. The maximum achievable throughput and an upper bound on the expected delay are found. The impact of the number of antennas on the performance with both flat fading and frequency selective fading is studied. Typical results show significant improvement in throughput for systems with smart antennas. We also show that dynamically adjusting the retransmission probability in the collision resolution process is not worthwhile since this extra complexity produces a gain of only about 6.2% in the best case.

The objective of this study is to evaluate effectiveness of multihopping technique applied to CDMA packet communication system. The system model is designed so that mobile stations that have no direct connectivity to any base station setup multi-hopped paths by way of neighbor mobile stations. Two policies are addressed in selecting relay stations based on battery usage, minimized usage on relay station and minimized usage on all stations along multihopped path. With these conditions, system characteristic is evaluated in terms of system capacity and coverage area extension. The result of computer simulation shows that multihopping technique does extend the coverage area with effectively equal performance to the conventional single hop system when the system load is relatively light.

As cellular networks evolve into 3G and beyond, they will be designed around IP infrastructure to enable cost benefits besides better integration with the Internet. Several proposals have been made to handle fast handoffs in an IP based cellular network. All these proposals focus on the problem of routing packets in a fast and efficient manner. However, handoffs can also be used to exploit macrodiversity and increase coverage and capacity as is done in current CDMA networks via soft handoffs. We show that soft handoffs are not suitable for IP cellular networks and propose a new method to exploit macrodiversity via fast hard handoffs. Our method does not suffer the disadvantages of soft handoffs in IP cellular networks. We show through simulations that we can achieve coverage gains approaching those obtained by soft handoffs with our scheme.

There is on-going convergence of IP and traditional telecommunication networks (both wired and wireless). To facilitate fast handoff and real-time location tracking, and reduce the location update overhead in IP based mobile networks, hierarchical IP mobility management schemes, i.e., Mobile IP for macro-mobility (inter mobile domains) and cellular-network-optimized schemes for micromobility within mobile domains, are needed. We apply the connection tree concept that was originally proposed for wireless ATM networks to IP micro-mobility management for mobile users with QoS requirements by using such new concepts as virtual multicast, virtual sender/receiver, null filter specification and dormant reservation. Our scheme enables fast and smooth handoff by localizing most of the location update related operation and pre-establishing connections to/from multiple base stations. It integrates seamlessly with such IP QoS mechanisms as RSVP, enables quick reservation restoration during handoff, and naturally supports soft handoff.

Provision of multimedia services to both stationary and non-stationary terminals is one of the key challenges in modern day telecommunications. A multi-tiered cellular radio system may provide adequate capacity needed to ensure reliable delivery of high quality broadband services to home, office, and mobile users. At the lowest tier, one may envisage picocellular base stations interconnected via short, highly focused free-space optical links, in a multihop mesh arrangement, thereby eliminating the need for new broadband access cabling to backhand traffic from the base stations. Ultimately, the traffic generated in these picocells must be relayed to an entry/exit port (end-office) of the core network. An intelligent routing/load-balancing strategy will help avoid hot spots on the mesh and increase network capacity. The performance metric would be the probability that a newly generated virtual connection must be blocked to maintain QoS guarantees, within this mesh network. A new dynamic routing/load-balancing algorithm is proposed to achieve high performance in terms of blocking probability in the presence of both uniform and nonuniform traffic distributions. Simulation results show that the algorithm dramatically helps in reducing congestion at hot spots in the network. For a toroidal grid configuration it is seen that the new dynamic routing to different end-offices achieves a 20% reduction in blocking probability when compared with a routing scheme wherein picocells have static affiliations with end-offices.

There is on-going convergence of IP and traditional telecommunication networks (both wired and wireless). To facilitate fast handoff and real-time location tracking, and reduce the location update overhead in IP based mobile networks, hierarchical IP mobility management schemes, i.e., Mobile IP for macro-mobility (inter mobile domains) and cellular-network-optimized schemes for micro-mobility within mobile domains, are needed. In this paper, we apply the connection tree concept that was originally proposed for wireless ATM networks to IP micro-mobility management for mobile users with QoS requirements by using such new concepts as virtual multicast, virtual sender/receiver, null filter spec and dormant reservation. Our scheme enables fast and smooth handoff by localizing most of the location update related operation and pre-establishing connections to/from multiple base stations. It integrates seamlessly with such IP QoS mechanisms as RSVP, enables quick reservation restoration during handoff, and naturally supports soft handoff.

The future wireless communication networks will be all-IP based and have frequency reuse factor one thanks to the advancements in smart antennas. In this paper, we study the admission control problems in such networks and use both analysis and simulations to compare four admission control algorithms in terms of the maximum traffic load they can accommodate such that the QoS targets can be met. It is found that the performance of a simple cell threshold based admission strategy is at least comparable with more complicated ones. To account for the impact of handoff on the system performance, we introduce a new concept called handoff density, which, together with traditional traffic density, determines the system performance. The results in this paper also show that in general handoff worsens the system performance.

Use of an adaptive antenna array or a space-time processor at each base station of a wireless network can substantially abate the effects of multipath fading and co-channel interference. Among the expected benefits are higher data rates, greater frequency reuse factors, and overall higher capacity systems as needed to enable wireless multimedia services. Media access control (MAC) protocols which facilitate the deployment of such a processor have been previously proposed and studied. These MAC protocols invoke the delivery of a pilot tone from each packet access unit in the network as needed, so that the array antenna at the associated base station may rapidly adjust its weighting coefficients, or its per branch equalization coefficients, thereby ensuring subsequent reliable communication between the access unit and the base station. This paper considers two modifications to these earlier protocols, both based upon the notion of piggybacking information requests on to the actual information messages and both intended to improve utilization efficiency and mean delay performance. Results show that a maximum link utilization efficiency of 97% is readily achieved with either modification, and that this maximum utilization efficiency is independent of the number of remote users in the network. Note that the utilization efficiency refers to the throughput at maximum loading, i.e., when the remotes always have queued requests. The modifications also help in achieving a considerable reduction in the average delay in low-load regimes: for typical system parameters, the average delay at low load is only about 10% of that produced by the original schemes.

Small cell sizes can lead to high rates of handoff in a broadband wireless environment. The virtual connection tree architecture has been proposed as an effective method to deal with this in a wireless ATM network. We review the virtual connection tree approach and we present and analyze a simple protocol designed to prevent cell loss and ensure cell sequence during handoff. Then we show how the connection tree concept can be modified to permit simple, efficient and loss-free handoffs in IP-based cellular networks. Finally, we determine the transient delay profile during a handoff and characterize overall performance of the approach.

The wireless local loop (WLL) is an emerging technology that allows rapid connection to the wired network from remote locations. A crucial issue for the WLL is the system design so that the number of subscribers can be maximized while providing the required quality of service. In this paper, we consider a broadband wireless local loop with fixed point users and we evaluate the subscriber capacity for a slotted CDMA scheme which accommodates integrated voice, data, and video traffic. The impact of the system parameters on the WLL performance is studied via simulation and provides a useful insight on how to efficiently design the network.

We consider a symmetric grid network consisting of N distinct nodes. The number or allowable calls (either the number of circuit switched calls or the maximum number of virtual connections such that QoS objectives are maintained) between any two nodes of the network is assumed to be a constant. We first determine this constant assuming that the network is fully loaded. Then, we find the maximum additional capacity needed on each link such that single link, and double link failures can be tolerated by rerouting calls around faded links. Results show that the maximum additional capacity needed to recover from any single link, double link, or single node failure, with no loss of connections (except for those connections terminating at a failed node) scales as 1//spl radic/(N). Thus, we conclude that rerouting, combined with an admission policy which blocks new call attempts such that a fraction of capacity proportional to 1//spl radic/(N) is reserved for failure recovery, provides totally failsafe operation in the presence of such failure events.

The basic mobile IP has a so called "triangle routing" problem, namely, all datagrams for a mobile host have to been routed first to its home agent and then forwarded by the home agent to the mobile host. Those paths are often significantly longer than optimal. Although the route optimization extensions in mobile IP can solve this problem, it requires the Internet hosts that communicate with a mobile host to be able to understand some mobility related messages. Unfortunately, that's not true for most Internet hosts. We propose a novel and simple virtual home agent based route optimization solution to this problem. It is based on two observations: a permanent home address is not really necessary for many Internet applications on a mobile host and a mobile host's mobility is highly localized during a certain period of time. By having a foreign network within a mobile host's localized footprint to be it's virtual home network, it can greatly alleviate the triangle routing problem because of the mobility locality nature, and, at the same time, enable the mobile host to communicate with any conventional Internet hosts that are not mobility aware.

A new medium-access protocol is proposed for sharing a high-speed radio channel among a number of small wireless packet-access units, some of which may be stationary and some of which may be within moving vehicles. Such a system could provide fixed-point pedestrian and remote users with wireless access to CPU and database resources of an underlying asynchronous transfer mode (ATM) wireline network, essentially extending the ATM bandwidth-upon-demand interface directly to the wireless units and enabling delivery of multimedia services (albeit at the lower peak rate afforded by the radio channel). A primary goal of the proposed medium-access protocol is the pre-delivery of a signal from each packet-access unit as needed to rapidly compute the weights needed by a base station's adaptive array processor or a space-time processor, thereby protecting the packet flow in each direction from the effects of both multipath propagation and adjacent channel interference arising in neighboring radio cells. An impairment-robust direct sequence spread-spectrum-based polling signal is invoked to stimulate a pilot tone from a given remote immediately prior to packet transfer in either direction, thereby permitting the base station to determine a good set of antenna element combining or power splitting weights to be used for that packet. Reasonable approximations are invoked to study the performance of the proposed protocol and the link utilization efficiency and average message delay are found. By proper choice of protocol parameters, a radio resource utilization efficiency of about 95% is readily achieved. The accuracy of the approximations is confirmed by extensive computer simulations.

Studies of the capacity of cellular systems, stated in terms of the admissible number of remote users, have generally been limited to voice telephony. We address the problem of comparing the interference-limited performance of CDMA and TDMA systems in a packet switched environment. The objective is to determine whether the capacity advantages claimed for circuit-switched CDMA still apply in a packet-switched environment, where the natural time diversity of bursty transmission may be a significant factor. Under a set of specific assumptions about the wireless environment (including path loss, shadow fading, multipath delay spread, co-channel interference, power control, coding), we evaluate the number of users which can be admitted to the system while maintaining some desired quality-of-service level. Four different classes of users with different characteristics and requirements are considered. The system capacity is found to significantly depend on the QoS objectives, which might be stated in terms of availability of some specified signal to interference level, packet loss rate, or mean tolerable delay. The main finding is that strict requirements imposed on the radio access level tend to favor CDMA, whereas if some form of packet recovery at the higher layers is allowed (implying a relaxed set of requirements on the radio interface), then a somewhat higher capacity may be achieved by TDMA.

A new type of broadband access system is proposed for providing high-quality, bandwidth-upon-demand telecommunication services to the home and office. Communication terminals attach to the network via short radio links, and users can roam freely within a house or building unencumbered by the availability of wired "telecommunications outlets". Basic service is extended through small, high-capacity radio cells; the base stations are interconnected via short, highly focused free-space optical links in a multihop mesh arrangement; and the need for new broadband access cabling is totally surmounted. A tiered arrangement of radio cells further extends service to both out-of-building pedestrian and vehicular users, and service is universally available. Benefits of the approach are described, and issues involving reliability, availability, capacity, and hand off are identified and addressed. Opportunities to refine the basic approach are suggested for further study.

In this paper, we study the benefits of wavelength translation in all-optical networks providing clear channel circuit-switching among users. We first establish approximate analytical models for a static-routing circuit-switched network with an arbitrary topology, both with and without wavelength translation. We then study the performance of the nonblocking centralized switch, the mesh-torus network and the ring network, using the analytical models and simulation results. It is shown that the analytical models match the simulation results very well in the case of the centralized switch and the mesh network. The results of our study also show that the benefits of wavelength translation are modest for the centralized switch and the ring network. On the other hand, the results show that wavelength translation can significantly improve the performance of a large mesh-torus network.

This paper describes a geographically distributed ATM switching architecture that exploits new possibilities offered by lightwave technology. Small modules that provide both an access and an ATM switching function are distributed over some extended geographical area and connected to a passive optical medium. Each module is equipped with some small number of electrooptic transmitters and receivers that provide access to the medium. Assignment of wavelengths to transmitter/receiver pairs create logical channels that are wavelength-multiplexed onto the medium, thus creating a logical connection diagram among the modules. Advantages of the lightwave-based distributed architecture are the huge bandwidth of the medium, a high degree of modularity to facilitate growth, high reliability, and the ability to provide "clear channels" among modules. On the other hand, if the externally offered flow of ATM cells among modules is too uniform, then each module may be capable of supporting only a fraction of the load that could be carried by any port of a fully connected centralized ATM switch with output queueing. Fortunately, under the much more realistic assumption of nonuniform traffic, the independence between the logical connection diagram and the physical topology of the medium can be exploited by reconfiguring the connection diagram to "match" the nonuniformity of the cell traffic. Simulation results show that, as the nonuniformity becomes larger, the reconfiguration technique allows the traffic-handling capability of a distributed switch to match that of a centralized switch.>

Presents a heuristic algorithm for effectively assigning a limited number of wavelengths among the access stations of a multihop network wherein the physical medium consists of optical fiber segments which interconnect wavelength-selective optical switches. Such a physical medium permits the limited number of wavelengths to be re-used among the various fiber links, thereby offering very high aggregate capacity. Although the optical connectivity among the access station can be altered by changing the states of the various optical switches, the resulting optical connectivity pattern is constrained by the limitation imposed at the physical level. The authors also study two routing schemes, used to route requests for virtual connections. The heuristic is tested on a realistic traffic model, and the call blocking performance of new requests for virtual connections is studied through extensive simulations and compared against the blocking performance of an ideal infinite capacity centralized switch (lowest possible call blocking caused exclusively by congestion on the finite capacity user input/output links, never by the switch fabric itself). Surprisingly, the authors find that, for a wide range of parameters, the blocking performance of the lightwave network is almost the same as that of the ideal centralized switch. From these results, they conclude that the heuristic algorithm is effective and the routing scheme is efficient.>

The authors present a heuristic algorithm for effectively assigning a limited number of wavelengths among the access stations of a multihop network wherein the physical medium consists of optical fiber segments which interconnect wavelength-selective optical switches. Such a physical medium permits the limited number of wavelengths to be re-used among the various fiber links, thereby offering very high aggregate capacity. Although the optical connectivity among the access station can be altered by changing the states of the various optical switches, the resulting optical connectivity pattern is constrained by the limitation imposed at the physical level. They also present and study two admission control schemes, used to admit or reject requests for virtual connections. The heuristic is tested on a realistic traffic model, and the call blocking performance of new requests for virtual connections is studied through extensive simulations and compared against the blocking performance of an ideal infinite capacity centralized switch (lowest possible call blocking caused exclusively by congestion on the finite capacity user input/output links, never by the switch fabric itself). Surprisingly, they find that, for a wide range of parameters, the blocking performance of the lightwave network is almost the same as that of the ideal centralized switch. From these results, they conclude that the heuristic algorithm is effective and the admission control scheme is efficient.>

An architecture is presented for a high-speed cellular radio access network based on ATM transport technology. Central to this approach is a new concept known as the virtual connection tree which avoids the need to involve the network call processor for every cell handoff attempt. Such an approach can readily support a very high rate of handoffs, thereby enabling use of physically small radio cells to provide very high system capacity, but may occasionally cause the volume of traffic to be handled by one cell site to exceed that cell site's capacity. A simple analytical methodology is developed which can be used for admission control, the purpose of which is to limit the number of in-progress calls such that two new quality of service metrics (overload probability and average time in overload) can be kept suitably low. Finally, a general framework is presented for overall system organization and signaling.>

Some of today's telecommunications networks have the ability to superimpose some form of logical connectivity, or virtual topology, on top of the underlying physical infrastructure. According to the degree of independence between the logical connectivity and the physical topology, the network can dynamically adapt its virtual topology to track changing traffic conditions, and cope with failure of network equipment. This is particularly true for lightwave networks, where a logical connection diagram is achieved by assignment of transmitting and receiving wavelengths to the network stations that tap into, and communicate over, an infrastructure of fiber glass. Use of tunable transmitters and/or receivers allow the logical connectivity to be optimized to prevailing traffic conditions. With rearrangeability having thus emerged as a powerful network attribute, this paper discusses the reconfiguration phase which is the transition between the current logical connection diagram and a target diagram. We consider here an approach where the network reaches some target connectivity graph through a sequence of intermediate connection diagrams, so that two successive diagrams differ by a single branch-exchange operation. This is an attempt at logically reconfiguring the network in a way that is minimally disruptive to the traffic. We propose and compare three polynomial-time algorithms that search for "short" sequences of branch-exchange operations, so as to minimize the overall reconfiguration time. For networks made of up to 40 stations, theoretical and simulation results show that, when a randomly selected diagram is to be changed to another randomly chosen diagram, the average number of branch-exchange operations required grows linearly with the size of the network.>

Multihop lightwave networks with hot potato routing offer the potential for significantly higher aggregate capacity as compared against minimum distance routing since each interconnecting link can be operated at a rate much higher than the electro-optic constraint forced by store-and-forward buffering. If the links can be operated at a speed sufficiently high as to offset the hot potato misrouting inefficiency, then a net capacity gain will be enjoyed. In this paper, we study such a network in which contention for an outbound link is resolved by assigning to each packet a priority based on its distance to destination and its age (number of misroutes previously suffered). An approximate analysis based on a reasonable set of assumptions is presented, queueing delay, packet loss performance, and network aggregate capacity (saturation throughout) are compared for distance-age prioritization, age-distance prioritization, and no prioritization. Results suggest that the lowest delay, lowest packet loss performance, and highest saturation throughput are offered by distance-age prioritization (priority given first to a packet closest to its destination and second to the oldest of multiple packets of the same distance to destination). The tails of the probability distribution for the number of hops taken by a representative packet are, however, shorter for age-distance routing. The capacity gain with link speed-up and hot potato routing can be quite significant compared to link speed-constrained store and forward buffering, and grows with both link speed-up factor and network size.>

An optical multihop network architecture that uses wavelength-based cell-switching and that consists of optically separated passive star couplers interconnected by 2*2 electronic asynchronous transfer mode (ATM) switches (network interface units) is presented. A fixed assignment TDMA-WDMA scheme provides a means for sharing the passive star couplers' bandwidth without cell collisions. Adaptive shortest path cell routing exploits the multiplicity of paths in the multihop environment to reduce queuing delay and provide load balancing across all WDMA channels. Analytic models of general network performance are developed and verified by means of computer simulation. The network performance for ShuffleNet configurations is quantitatively studied. The proposed architecture provides for unlimited scalability, both geographically and in the number of users, and is less costly and complex to implement than conventional multihop approaches.>

A practical approach to the analysis and operation of medium to large-size distributed asynchronous transfer mode (ATM) switches based on an original optical reconfigurable architecture is described. The optical nature of the switch provides enormous bandwidth with input/output port rates on the order of gigabits/second, and the distributed approach is highly modular, creating an easily growable switch. The interconnection pattern between input and output ports is dynamically reconfigurable; the switch can thus exploit nonuniformities in the offered traffic and optimize its connectivity accordingly. For a 24*24 switch, this leads to performance that matches that of a fully connected output buffered ideal centralized switch. Simulation results for a switch of size 81*81 seem to indicate that the traffic-handling capability scales well with the size of the switch when nonuniformity prevails in the offered traffic, a realistic assumption. Clustering techniques would permit the optimization and operation of much larger reconfigurable distributed ATM switches.>

The efficiency of protocols coordinating the data transmission between the transmitter and receivers in a network of stations connected using a passive star coupler, equipped with fixed transmitters and tunable receivers, and using wavelength-division multiplexing is discussed. Two reservation-based protocols with varying degrees of signaling complexity are proposed: the dynamic allocation scheme (DAS), which dynamically assigns slots on a packet-by-packet basis, and the hybrid time-division-multiplexing (TDM) scheme (HTDM), which combines the TDM and the DAS scheme and allows both preassigned and dynamic slot assignment. Analytical results are derived for the delay performance of the two schemes and compared with that of TDM. It is shown that the performance of DAS under ideal conditions is close to optimal, but its signaling costs are exorbitantly high. On the other hand, HTDM has lower signaling needs, but has higher delays when compared to DAS.>

The Center for Telecommunications Research supports research education, industrial collaboration, and technology transfer in the area of integrated telecommunications networks for multimedia information movement and management. Personnel include 23 faculty, 55 fully supported Ph.D. students, 20 unsupported Ph.D. students, 30 master's and bachelor's level students, eight research scientists and engineers, and six administrative workers. The CTR enjoys the active support and participation of 22 companies with whom it regularly interacts to share ideas, formulate policy and research plans, collaborate on specific projects, and transfer research innovations for subsequent commercial development. The research program has been organized into three cross-discipfinary focal projects

The authors present an analytical approach to the performance analysis of multihop lightwave networks with hot-potato routing and multiclass traffic. Explicit closed-form expressions are derived for the hop distribution and the probability of 'don't care'. The computational complexity is thus reduced as compared to previous approaches, and priorities are readily handled. Results for two different classes of traffic are given. The probability of a packet belonging to a particular class of traffic existing on a preferred path is calculated by finding the fixed point of a scale function. The throughput and network delay as well as queuing delay of a particular class are then calculated using the expressions derived for them.>

A network with N stations connected via a passive star coupler using wavelength division multiplexing (WDM) is considered. Each station is equipped with a transmitter, transmitting at a fixed frequency, and a tunable receiver, capable of receiving frequencies over the entire transmitting spectrum. Two reservation-based protocols are proposed for a local area network (LAN) environment: the dynamic allocation scheme (DAS), which dynamically assigns slots on a packet-by-packet basis, and the hybrid TDM scheme (HTDM) which is a combination of TDM and the DAS, allowing both preassigned slot and dynamic slot assignment. Analytical results are derived for the delay performance of the two schemes and these results are compared with that of the TDM. It is shown that the performance of DAS is close to optimal when the propagation delay is zero. However, it requires a large signaling bandwidth. On the other hand, HTDM has lower signaling needs, provides lower delays when compared to TDM (particularly under nonuniform traffic conditions), and is capable of handling bursty data traffic. Thus, HTDM offers a more practical solution for implementation for LANs.>

Centralized and distributed approaches to the design and implementation of a multi-gigabit per second asynchronous transfer mode (ATM) switch are discussed. The implementation constraints associated with a centralized switch are demonstrated and an alternative distributed optical network architecture is described. The distributed approach exploits unique opportunities presented through the use of a passive optical medium. A network based on these principles (TeraNet), has been implemented that offers user access rates of 1 Gbit/s. The distributed ATM switch fabric resides in small, geographically dispersed access stations interconnected by a shared, all-optical medium carrying many wavelength-multiplexed channels. Although the underlying multihop architecture for the ATM network produces a lower per-port throughput than that afforded by a large centralized ATM switch, significant implementation advantages exist. Possible techniques to yield a higher per-port throughput approaching that of a large centralized switch are also discussed.>

The authors propose a minimally disruptive approach that transitions the network through a sequence of branch exchange operations, so that only two links are disrupted at any given time. It is shown that the problem of finding the shortest sequence, so as to minimize the duration of the reconfiguration phase, is equivalent to the problem of finding a decomposition of an auxiliary graph into the largest number of vertex-disjoint cycles. The authors then propose and compare three different polynomial-time greedy algorithms, on the basis of performance and time complexity. Noticing that the length of a sequence increases at most linearly with the size of the network, the authors derived the average rate of growth from simulation results.>

The achievable aggregate capacity for a variant of the basic multihop approach in which minimum distance store-and-forward routing is replaced by a hot-potato routing algorithm is determined. With hot-potato routing, all packets simultaneously arriving at a given node and not intended for reception at that node are immediately placed onto the outbound links leaving that node; if two or more packets contend for the same outgoing link to achieve a minimum distance routing, then all but one will be misrouted to links which produce longer paths to the eventual destination. Attention is confined to the development of an analytical methodology for finding the probability distribution of the number of hops with hot potato routing for symmetric networks under uniform traffic load. Results show that the maximum throughput achievable with hot-potato routing can be as low as 25% of that for store-and-forward routing, and that the relative degradation increases as the number of nodes grows larger. This implies that the link speed up needed to produce a significant overall capacity advantage with hot potato should be at least a factor of 10.>

A straightforward approach is found for computing the total aggregate capacity of a multihop network with hot-potato routing, and for finding the probability distribution of the number of hops taken by a representative test packet. The rows and columns of the state transition matrix used by this approach each contain, at most, two nonzero elements. The approach is valid for any symmetric network connection diagram under conditions of uniform loading. Results obtained for recirculating perfect shuffle connection diagrams show that the achievable aggregate capacity degrades relative to the capacity maximizing store-and-forward routing as the number of network nodes increases, but even for networks containing several thousand nodes, it is no worse than 25% of that for store-and-forward. Increasing the link data speed by more than a factor of four will produce, overall, a higher aggregate capacity.>

A network is studied in which contention for an outbound link is resolved by assigning to each packet a priority based on its distance to destination and its age (number of misroutes previously suffered). Queueing delay, lost packet performance and network aggregate capacity (saturation throughput) are compared for distance-age prioritization, age-distance prioritization and no prioritization. Results suggest that the lower delay, lower lost packet performance, and high saturation throughputs are offered by distance prioritization. The tails of the probability distribution for the number of hops taken by a representative packet are, however, shorter for age-distance routing. Typical results show that, for a multihop shuffle network with 10240 nodes, the saturation throughput is about 40% higher for distance-age prioritization. For a smaller network with 384 nodes, the improvement falls to 25%.>

A metropolitan-area radio system that, from a centrally located base station, provides continuously 360 degrees coverage over a large service region is proposed. The base station blankets the service region with a raster of very narrow pencil beams that can be rapidly scanned to any position in synchronism with the switching sequences of a time-division-multiple-access (TDMA) assignment. By deploying multiple scanning spot beams, the allocated spectrum can be reused many times. A centralized network controller, executing an efficient TDMA assignment algorithm, dynamically allocates the resources of a small transceiver pool among the far larger number of beam positions in response to real-time requests for service. By varying the dwell time at each beam position in response to the traffic intensity of the position, highly efficient resource utilization is provided. The high antenna gain of the base station antenna provides adequate arain fade margin to permit operation in the uncongested portion of the ratio spectrum above 20 GHz. The system is particularly well suited to emerging, direct-to-end-user wideband digital service offerings.>

The optimization problem of rearrangeable multihop lightwave networks is considered. The authors formulate the flow and wavelength assignment problem, when minimizing the maximum flow in the network, as a mixed integer optimization problem subject to linear constraints. The problem is decomposed into two independent subproblems, the wavelength assignment (or connectivity problem) and the flow assignment (or routing problem). A simple heuristic provides a meaningful formulation to the connectivity problem, in a form similar to a transportation problem. An algorithm is then proposed which finds a heuristic initial logical connectivity diagram and the corresponding routing, and then iterates from that solution by applying branch-exchange operations to the connectivity diagram. The algorithm was tested on illustrative traffic matrices for an 8 node network with two transmitters and two receivers per node, and an improvement in achievable throughput over the Perfect Shuffle interconnection pattern was shown in all cases.>

The multihop architecture, which provides a way of tapping the capacity potential available in lightwave networks, is described. Within this architecture, each network node is equipped with some small number of transmitters and receivers, each of which can communicate on one wavelength. Transmitters and receivers are connected to an optical medium, which is physically configured in such a way that the entire spectrum of wavelengths in use is potentially accessible by each node. Then, an assignment of transmit and receive wavelengths to each node defines the logical connectivity among nodes. Furthermore, the use of optically agile transceivers (i.e. slowly tunable lasers or optical filters) permits the logical connectivity to be updated in response to changing traffic patterns and failure/recovery of nodal transmitters and receivers. The flow and wavelength assignment problem is studied. Solutions in which the traffic flows over a derived connectivity diagram such that the largest flow on any link is minimized are sought. Examples of the resulting connectivity diagram and traffic flow are presented for selected nonuniform point-to-point traffic matrices.>

The technical feasibility of a lightwave communication system capable of providing service to thousands of subscribers within cities or large suburban areas is examined. Each subscriber in this system has available upon demand an aggregate bit rate of up to 100 Mb/s. The aggregate bit rate of the entire network is on the order of 5 Tb/s. The network topology is that of a star-on-star wherein each subscriber line terminates on one of a multitude of remote concentrators. Each high-speed link from a remote concentrator to a centrally located time-multiplexing photonic switch consists of a single-mode fiber carrying several wavelength-division multiplexed (WDM) channels. Each channel operates in a time-division multiplexed (TDM) mode at a data rate of 2 Gb/s and is powered by a multimode laser. No regeneration is required at the central switch, and at the remote concentrators, each channel is terminated by a direct-detection receiver. The approach achieves high throughput by reusing the same channels, that collectively occupy a tiny fraction of the optical band, among all interconnecting links and, with the exception of large-dimensional switch arrays, employs readily available technology.>

Considers a vision system that uses the echo of a transmitted ultrasonic pulse as the basis for identifying objects. The return of a single pulse from an object generates a three-dimensional acoustical characteristic signature across the aperture of a receiving antenna which can be directly used for object identification. Thus, there is no need to reproduce an accurate visual representation to recognize which of a class of objects is present. Since the signal bandwidth is narrow relative to that of an optical imaging system, faster execution should be possible. The need to remove clutter which can arise in a conventional imaging system, caused by the compression of three dimensions into two, is totally avoided. The authors calculate a fundamental limit (in the Shannon sense) on the number of objects that can be distinguished by an ultrasonic system and show that, for modest signal-to-noise ratios and an object space comparable to the beamwidth, an enormous number of different objects can still, in principle, be resolved.>

A three-tiered digital switching hierarchy is considered in which traffic from remote users is grouped, concentrated into time-multiplexed lines, and then transmitted to a central location for interconnection by a central space division switch. While the configuration provides efficient sharing of both transmission and switching resources, the TDM (time-division multiplexing) assignment problem for a conventional time-space-time (T-S-T) switch can become quite formidable due to the various hierarchical constraints. A time-slot assignment strategy is proposed that circumvents this problem by temporarily storing the incoming traffic at the central switch. Unlike the traditional T-S-T approach, the in-bound and out-bound assignments are entirely decoupled, which greatly simplifies the assignment process and produces a fast circuit switching capability. Necessary and sufficient conditions governing the assignability of traffic to the three-tiered switching system are stated and proved, and a block diagram of the proposed switching architecture is presented.>

We propose and analyze a wide-band indoor communication system that uses radio as the transmission medium either on a stand-alone basis or to supplement a hard-wired network for those situations where complete portability is desired. One principal impairment to such a system is intersymbol interference caused by frequencyselective fading. A novel media-access scheme is proposed which permits the use of resource sharing, wherein a small pool of time slots is effectively shared among all users to provide added protection against channel impairments on an as-needed basis. Our results show that the use of resource sharing and diversity provide excellent protection against intersymbol interference caused by frequency-selective fading with negligible impact on throughput. Furthermore, resource sharing plus diversity can permit significantly higher data rates without large queueing delays. For example, a wireless network with a 10 Mbit/s data rate in a 10 MHz bandwidth using four antennas at the base station has a less than 10-4outage probability at a 10-4BER in buildings with less than 58 ns rms delay spread. A loading of 75 percent is permitted for a queueing delay of less than 20 packet transmission times all but 0.01 percent of the time.

A new, high-performance packet-switching architecture, called the Knockout Switch, is proposed. The Knockout Switch uses a fully interconnected switch fabric topology (i.e., each input has a direct path to every output) so that no switch blocking occurs where packets destined for one output interfere with (i.e., block or delay) packets going to different Outputs. It is only at each output of the switch that one encounters the unavoidable congestion caused by multiple packets simultaneously arriving on different inputs all destined for the same output. Taking advantage of the inevitability of lost packets in a packet-switching network, the Knockout Switch uses a novel concentrator design at each output to reduce the number of separate buffers needed to receive simultaneously arriving packets. Following the concentrator, a shared buffer architecture provides complete sharing of all buffer memory at each output and ensures that all packets are placed on the output line on a first-in first-out basis. The Knockout Switch architecture has low latency, and is self-routing and nonblocking. Moreover, its Simple interconnection topology allows for easy modular growth along with minimal disruption and easy repair for any fault. Possible applications include interconnects for multiprocessing systems, high-speed local and metropolitan area networks, and local or toll switches for integrated traffic loads.

We consider the traffic handling capability of a new switching architecture which generalizes upon the structure of a traditional time multiplex switching system. For the traditional approach, lowbandwidth end users are formed into groups, each of which shares a single high bandwidth time-division multiplexed (TDM) line into the central time multiplex switch. Each user synchronously generates packets of data in preassigned time slots at a rate consistent with its offered traffic, and the feeder for the TDM line serving a group of users merely time multiplexes the arriving packets prior to routing by the central switch; analogously, each output port of the switch feeds a demultiplexer which routes the packets to the appropriate user within its group. The generalized approach permits each user group to share some multitude of TDM lines interconnecting that group with the central switch, and the group multiplexers and demultiplexers are replaced by switches which route packets from users to TDM lines (and vice versa). For this structure, we derive a set of necessary and sufficient conditions on the user-touser offered traffic such that a valid, nonconflicting TDM assignment of packets-to-time slots exists. These conditions reveal that the constraints imposed by the three-tiered switching hierarchy do not limit the useable capacity of the switch. Consequently, with no loss of traffic bearing efficiency, it is possible to reduce the number of multiplexers used to serve the end-user population, achieve greater trunking efficiency since small user groups served by one TDM line are replaced by larger groups serving multiple lines, and modularly grow the system by adding TDM lines to each group commensurate with the traffic offered by that group. As a byproduct, it is shown that such a system designed to switch low-speed circuits of some particular data rate can, with no hardware change, switch circuits at lower rates (subrate switching). These conclusions have important ramifications for the design of terrestrial and satellite-based switching systems.

Resource sharing is a technique which can improve the circuit availability of digital satellites operating at frequencies above 10 GHz, without requiring large fade margins. In this paper, the resource sharing concept is generalized by fully exploiting the available clear-air carrier-to-noise ratio of the satellite link to achieve very high transmission capacity while maintaining low rain outage. During clear-air conditions, convolutional codes with a large channel signaling alphabet are employed to permit a high rate of information transfer. When the fade depth exceeds the built-in fade margin, the signaling alphabet is reduced and enough time slots are borrowed from a resource sharing reserved pool to maintain the data rate at the fade site. It is shown that this approach greatly diminishes the impact of rain attenuation on satellite communications. Effective utilized capacities exceeding 85 percent of that possible if it never rains are feasible, and the increase in capacity compared to a system not using resourcesharing protection is typically a factor of 3-10.

A useful technique is derived for upper bounding the bit error rate of convolutional coding-Viterbi decoding in the presence of modem implementation errors or cochannel interference. This method utilizes the state transition matrix to find the contribution of each error event. By slightly underestimating the distance of each error path, the randomness of the implementation errors or interference can be averaged on a bit-by-bit basis. This produces an effective reduction of signal-to-noise ratio, and the elements of the state transition matrix merely reflect this reduction. The resulting bound is both accurate and very simple to apply.

A satellite system with several independent spot beams is considered, wherein each spot beam can be scanned over a limited portion of some large service region. The spectral band is totally reused in each spot beam, thereby providing high system capacity. Such a system combines the high antenna gain advantages of a fixed beam system while providing access to the satellite from any point within the overall service region. Moreover, the transmission resources of the satellite can be dynamically allocated to match time-varying changes of the possibly nonuniform terrestrial traffic patterns. The limited-scan antenna forms multiple spot beams, each of which scans only in one dimension; by alternating polarizations among scan strips, high isolation between beams is guaranteed. We show that nearly equal volumes of traffic can be served by each limited scan beam by proper orientation of the scan regions, and complete interconnectivity among all points in all scan regions is achieved easily. A practical antenna design to implement this concept is presented.

We present block diagram designs for the baseband processing modules required to implement high-speed (600 Mbit/s) burst modems for use in TDMA satellite systems. Our main concern is to obtain designs which can be implemented with a minimum of high-speed (ECL) devices so as to reduce design and hardware costs. A major consideration in the modem operation is to accurately synchronize it to a reference frame-marker broadcast by a master earth station. Since frame mark recovery must be done at the full serial bit rate, the proposed design relies on the periodic nature of the frame-marker occurrences to enable utilization of short unique words and simple highSpeed electronics to provide extremely reliable marker recovery with virtually no false alarms (or missing markers), even on very noisy channels. Estimates of false alarm rates and the mean time to lock are given. Considerations of clock stability indicate that short-term (150 ms) instabilities must be on the order of a part in 107in order to maintain proper operation of the synchronization circuitry; such stability also minimizes guard time requirements between bursts from the several ground stations sharing a given transponder.

Viterbi decoding of binary convolutional codes on bandlimited channels exhibiting intersymbol interference is considered, and a maximum likelihood sequence estimator algorithm is derived. This algorithm might be applied either to increase the allowable data rate for a fixed power transmitter or to reduce the required power for a fixed data rate. Upper and lower bounds on the bit error rate performance of several codes are found for selected values of the ratio of information rate to channel bandwidth, and results are compared against both conventional equalization techniques and the Shannon capacity limit. Results indicate that this algorithm can provide the power saving associated with low rate (highly redundant) codes without suffering the noise enhancement of linear equalization techniques.

Analog means for implementing the Viterbi decoding algorithm at high data rates are presented. One approach employs sample-and-hold circuits and voltage adders to store and update the path metrics based upon maximum-likelihood decisions. Experimental results obtained from a breadboard realization of such a decoder are reported. An alternate approach employing tapped delay lines to store the analog channel waveform is also described. Analytical results pertaining to each implementation are presented.