Browsing by Author "Ephremides, Anthony"
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Item The Acts Experiments program at the Center for Satellite and Hybrid Communication Networks(1997) Friedman, Daniel E.; Gupta, Sonjai K.; Zhang, C.; Ephremides, Anthony; ISR; CSHCNThis paper describes experiments conducted over ACTS and the associated T1~VSAT terminal. The experiments were motivated by the commercial potential of low-cost receive-only satellite terminals that can operate in a hybrid network environment, and by the desire to demonstrate frame relay technology over satellite networks. The first experiment tested highly adaptive methods of satellite bandwidth allocation in an integrated voice- data service environment. The second involved comparison of FEC and ARQ methods of error control for satellite communication with emphasis on the advantage that a hybrid architecture provides, especially in the case of multicasts. Finally, the third experiment demonstrated hybrid access to databases through the use of Mosaic and compared the performance of internetworking protocols for interconnecting LANs via satellite. A custom unit termed Frame Relay Access Switch (FRACS) was developed by COMSAT Laboratories for these experiments; the preparation and conduct of these experiments involved a total of twenty people from the University of Maryland, the University of Colorado, and COMSAT Laboratories, from late 1992 through 1995.Item Admission Control Schemes for Spot-Beam Satellite Networks(1993) Ramseier, S.; Ephremides, Anthony; ISR; CSHCNIn this report, we consider communication networks with a satellite with multiple spot beams. We describe the structure and features of these networks, and we consider admission control schemes which optimize the network revenue if several services types with different revenues are present. We show that in some cases the blocking of some calls even if capacity is available can considerably increase the network revenue. We will point out, however, that complete sharing, i.e., accepting calls on a first- come, first-served basis, is optimum for systems with similar traffic types.Item Broadband Access via Satellite(1999) Hadjitheodosiou, Michael H.; Ephremides, Anthony; Friedman, Daniel E.; Ephremides, A.; ISR; CSHCNSatellites are well suited for broadband communications. In this paperwe consider the special features of satellite systems, some of thebroadband applications that are well-suited for satellites and someof the technologies which make possible broadband satellitecommunications, as well as the research programs that led to theirdevelopment. We describe how such technologies, and other factors,have contributed to the evolution of broadband satellite systems,and discuss some of the challenges in establishing such systems.We finish by offering some concluding remarks on the role ofsatellites for broadband access.The research and scientific content in this material hasbeen published in Computer Networks, vol. 31, pp. 353-378, 1999. Item Cell Size in Hybrid Wireless Systems(1999) ElBatt, Tamer A.; Ephremides, Anthony; Ephremides, Anthony; ISR; CSHCNIn this paper we study, analytically and via simulation, the impact ofreducing the cell size of a wireless system on the system capacity andhand-off failure rate. First, we focus on a pure cellular system. Wedevelop exact and approximate models for cellular systems in order toemphasize the blocking probability/forced termination probabilitytrade-off. This trade-off motivates the problem of optimizing the systemperformance with respect to the cell size. For hybrid systems, weinvestigate how jointly optimizing the number of cells per spot-beam, andthe bandwidth partitioning could improve the QoS parameters of interest.For both systems, the numerical solution is only feasible for smallnumber of cells. For large systems, a simulation study is presented.Item Combating Performance Degradation in Highly Mobile Networks Using Rate Control(1997) Stamatelos, D.; Ephremides, Anthony; ISR; CSHCNWe examine a simple technique for combating performance degradation by, adjusting the rate of the received signal. Using adaptive demodulation, the receiver can respond to deteriorating conditions trading BER for rate and vice-versa, without involving the transmitter. This way the receiver has more control on the tolerated distortion. The implementation of this technique does not require modifications of the hardware of neither the transmitter nor the receiver, and its application can be very practical. The performance degradation which occurs by receiving a degraded version reliably versus receiving a non-degraded version unreliably, depends on the service and can be very subjective, making this technique very attractive for voice and video communication. This technique, independently or jointly with slow power control, can be a useful tool in network control and resource allocation.- A copy of this report has been published in the proceedings of
The 1st Annual Advanced Telecommunications/Information Distribution Research Program Conference, January 21-22, 1997.Item Decomposition of a Multiple Access Network into a Number of Interconnected Subnetworks.(1989) Karatzaglu, Minas A.; Ephremides, Anthony; ISRA random access system with a finite number of buffered terminals is considered. The problem of how to decompose this system into a number of interconnected subsystems in order to maximize its throughput is studied. Two different interconnection topologies are considered, a directional ring topology and a fully connected network of subsystems. For each of these two topologies the problem of maximizing the throughput is reduced to a simple maximization problem. We apply the attained results to a slotted ALOHA and a CSMA/CD system. For the former a significant improvement in the throughput performance is reported, while simulation analysis shows an expected deterioration in the delay performance, for low intensity traffic.Item Delay Analysis of Interacting Queues with an Approximate Model.(1986) Ephremides, Anthony; Zhu, R.Z.; ISRAn approximate model of coupled Markov chains is proposed and analyzed for a slotted ALOHA system with a finite number of buffered nodea. This model differs from earlier ones in that it attempts to capture the interdependence between the nodes. The analytical results lead to a set of equations that, when solved numerically, yield the average packet delay. Comparison between computational and simulation results for a small number of nodes show excellent agreement for most throughput values, except for values near saturation. Numerical comparisons for a two-node system show that a non-symmetric loading of the system provides better delay-throughput performance than a symmetric one.Item The Design and Simulation of a Mobile Radio Network with Distributed Control.(1987) Baker, Craig; Ephremides, Anthony; Flynn, Julia A.; ISRA new architecture for mobile radio networks, called the linked cluster architecture, is described, and methods for implementing this architecture using distributed control techniques are presented. We illustrate how fully distributed control methods can be combined with hierarchical control to create a network that is robust with respect to both node loss and connectivity changes. Two distributed algorithms are presented that deal with the formation and linkage of clusters and the activation of the network links. To study the performance of our network structuring algorithms, a simulation model was developed. The use of Simula to construct software simulation tools is illustrated. Simulation results are shown for the example of a high frequency (HF) intratask force (ITF) communication network.Item A Design Concept for Reliable Mobile Radio Networks with Frequency Hopping Signaling.(1987) Ephremides, Anthony; Wieselthier, Jeffrey E.; Baker, Dennis J.; ISRThe design of a packet radio network must reflect the operational requirements and environmental contraints to which it is subject. In this paper, we outline those features that distinguish the High Frequency (HF) Intra Task Force (ITF) Network from other packet radio networks, and we present a design concept for this network that encompasses organizational structure, waveform design, and channel access. Network survivability is achieved through the use of distributed network control and frequency hopping spread spectrum signaling.Item A Design Concept for Reliable Mobile Radio Networks with Frequency Hopping Signaling.(1986) Ephremides, Anthony; Wieselthier, Jeffrey E.; Baker, Dennis J.; ISRThe design of a packet radio network must reflect the operational requirements and environmental constraints to which it is subject. In this paper, we outline those features that distinguish the High Frequency (HF) Intra Task Force (ITF) Network from other packet radio networks, and we present a design concept for this network that encompasses organizational structure, waveform design, and channel access. Network survivability is achieved through the use of distributed network control and frequency hopping spread spectrum signaling. We demonstrate how the execution of the fully distributed Linked Cluster Algorithm can enable a network to reconfigure itself when it is affected by connectivity changes such as those resulting from jamming. Additional resistance against jamming is provided by the use of frequency hopping, which leads naturally to the use of code division multiple access techniques that permit the simultaneous successful transmission by several users. Distributed algorithms that exploit CDMA properties have been developed to schedule contention-free transmissions for much of the channel access in this network. Contention-based channel access protocols can also be implemented in conjunction with the Linked Cluster network structure. The design concept presented in this paper provides a high degree of survivability , flexibility, to accommodate changing environmental conditions , user demands.Item Discrimination Against Partially Overlapping Interference - Its Effect on Throughput in Frequency-Hopped Multiple Access Channels.(1987) Wieselthier, J.E.; Ephremides, Anthony; ISRIn this paper we derive the probability of correct packet reception and the resulting channel throughput achievable in an asynchronous slow-frequency-hopped multiple user channel. Reed- Solomon coding is used to correct errors caused by other user interference in an otherwise noiseless channel. We analyze and evaluate an M-ary FSK signaling scheme, which permits the discrimination against interfering signals that are present for a sufficiently small fraction of the hop duration, and results in substantial increases in channel throughput over previous models.Item A Distributed Reservation Scheme for Spread Spectrum Multiple Access Channels.(1987) Wisseltheir, J.E.; Ephremides, Anthony; ISRA distributed reservation-based multiple access protocol is proposed, analyzed and evaluated. This scheme is based on the quasi-orthogonality property that can be achieved through the use of spread spectrum signaling, which permits the correct reception of a signal despite the simultaneous presence of other channel users. The effect on performance of the ability to tolerate varying levels of other-user interference is demonstrated. No acknowledgment or feedback information from the destination is required, nor is any form of coordination among the users necessary.Item A Distributed Reservation-Based CDMS Protocol that Does Not Require Feedback Information.(1987) Wieselthier, J.E.; Ephremides, Anthony; Tarr, J.A.B.; ISRIn some multi-user radio systems the receiver is not allowed to transmit feedback information to the senders. In other applications the transmitters cannot receive feedback information sent by the receiver. For these cases a multiple access protocol is needed that maintains satisfactory performance in the absence of feedback. In this paper, such a protocol is introduced and an exact analysis of it is provided. The protocol, which uses an unconventional reservation mechanism, exploits the capability of interference rejection that Code Division Multiple Access (CDMA) provides.Item Dynamic Server Allocation to Parallel Queues with Randomly Varying Connectivity(1992) Tassiulas, L.; Ephremides, Anthony; ISR; CSHCNConsider N parallel queues competing for the attention of a single server. At each time slot each queue may be connected to the server or not depending on the value of a binary random variable, the connectivity variable. The server is allocated to one of the connected queues at each slot; the allocation decision is based on the connectivity information and on the lengths of the connected queues only. At the end of each slot, service may be completed with a given fixed probability. Such a queueing model is appropriate for some communication networks with changing topology (radio networks with mobile users, or networks with variable links such as meteor-burst communication channels). In the case of infinite buffers, necessary and sufficient conditions are obtained for stabilizability of the system in terms of the different system parameters. The allocation policy that serves the longest connected queue stabilizes the system when the stabilizability conditions hold. The same policy minimizes the delay for the special case of symmetric queue (i.e., queues with equal arrival, service, and connectivity statistics) is provided. In a system with a single buffer per queue, an allocation policy is obtained that maximizes the throughput and minimizes the delay when the arrival and service statistics of different queues are identical.Item Efficient Simulation of DEDS by Means of Standard Clock Techniques: Queueing and Integrated Radio Network Examples(1993) Wieselthier, Jeffrey E.; Barnhart, Craig M.; Ephremides, Anthony; ISR; CSHCNSimulation is an important tool in the study of communication networks manufacturing systems, and other complicated man-made systems. Many systems of this type are examples of discrete- event dynamic systems (DEDS) for which analytical models have not yet been developed; thus simulation is the primary method for their performance evaluation and control. Although in many cases simulation can provide a good estimate of system performance, it can also be extremely time consuming, and therefore expensive. This is especially true when it is necessary to evaluate system performance for a large number of values of one or more parameters or for different control policies.Item Energy-Efficient Routing for Connection-Oriented Traffic in Wireless Ad-hoc Networks(2001) Michail, Anastassios; Ephremides, Anthony; ISR; CSHCNWe address the problem of routing connection-oriented traffic in wireless ad-hoc networks with energy efficiency. We outlinethe trade-offs that arise by the flexibility of wireless nodes totransmit at different power levels and define a framework forformulating the problem of session routing from the perspective ofenergy expenditure.A set of heuristics are developed for determiningend-to-end unicast paths with sufficient bandwidth and transceiverresources, in which nodes use local information in order to selecttheir transmission power and bandwidth allocation. We propose a setofmetrics that associate each link transmission with a cost andconsiderboth the cases of plentiful and limited bandwidth resources, thelatter jointly with a set of channel allocation algorithms.
Performance is captured by call blocking probability and averageconsumed energy. A detailed simulation model has been developed andused to evaluate the algorithms for a variety of networks.
Item Enhanced Throughput for Satellite Multicasting(1999) Friedman, Daniel E.; Ephremides, Anthony; Ephremides, A.; ISR; CSHCNFaithful information delivery in satellite multicasting requires appropriate error control. If multicast automatic-repeat-request (ARQ)is employed, a retransmission does not benefit receivers which do notrequire it, and consequently the throughput suffers greatly as thenumber of receivers increases. This performance degradation might bealleviated substantially by conducting retransmissions through terrestrialpaths from the transmitter to each receiver instead of through themulticast satellite link. By sending a retransmission directly to thereceiver(s) which requires it, higher throughput can be provided in sucha hybrid network than in a pure-satellite network. In this work,we examine the throughput improvement provided by the hybrid network.The research and scientific content in this material hasbeen accepted for presentation at the International Mobile SatelliteConference, Ottawa, June 16-18, 1999. Item Enhancement of Cellular Service via the Use of Satellite Capacity(1995) Ayyagari, Deepak; Ephremides, Anthony; ISR; CSHCNIn mobile communications, new services are being launched that provide extended geographical coverage of cellular service via the satellite. The additional satellite capacity can be shared among all the cells, thereby augmenting the dedicated cellular capacity available within the cells. This additional shared capacity has the inherent potential for increasing efficiency, as the extra channels can be assigned wherever they are needed rather than pre-allocating them. We considered the performance advantage the satellite offers in off-loading congestion within the cellular covered area. We propose a model for the hybrid network based on the recently proposed Multiple Service Multiple Resource concept. This model has been used in performing a detailed analysis that demonstrates the marked improvement in performance obtained by augmenting cellular capacity via the satellite and quantifies the gains achieved therein.Item Ergodicity of M-dimensional Random Walks and Random Access Systems.(1989) Karatzaglu, Minas A.; Ephremides, Anthony; ISRWe apply a set of recent results by Malysev and Mensikov, concerning necessary or sufficient conditions for ergodicity of constrained M-dimensional random walks to the problem of stability of M coupled queueing systems that describe a system of M buffered terminals accessing a common channel by means of the slotted ALOHA protocol. We obtain a necessary and sufficient condition for the stability of such a system. Although the condition does not yield a descriptive characterization of the stability region, it allows a reduction of the stability problem of a M-user system to the determination of the steady state distribution of a (M-1)user system. The plausibility of a recent conjecture concerning the stability of this system is also discussed.Item An Exact Analysis and Performance Evaluation of Framed Aloha with Capture.(1987) Wieselthier, J.E.; Ephremides, Anthony; Michaels, Larry A.; ISRA combinatorial technique is introduced that can be applied to the analysis of a variety of framed, contention-based protocols of multiple access, with or without dynamic frame-length control. Furthermore, we consider a general capture model that includes the case of perfect capture, as well as capture models that depend on the number of packets involved in a collision. We demonstrate the application of our technique to several of these cases.