Institute for Systems Research Technical Reports
Permanent URI for this collectionhttp://hdl.handle.net/1903/4376
This archive contains a collection of reports generated by the faculty and students of the Institute for Systems Research (ISR), a permanent, interdisciplinary research unit in the A. James Clark School of Engineering at the University of Maryland. ISR-based projects are conducted through partnerships with industry and government, bringing together faculty and students from multiple academic departments and colleges across the university.
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Item Error Control for Multicasting in Satellite and Hybrid Communication Networks(2001) Friedman, Daniel E.; ISR; CSHCNA problem inherent in ARQ multicasting over a broadcast channel is thata retransmission typically benefits only a minority of destinationswhile all others wait unproductively. This results in poorthroughput to each receiving station in the network, with thethroughput diminishing as the number of receivers grows.If point-to-point links between the transmitter and each receiver werealso available, then conceivably retransmissions could be sent over suchsecondary links. This would reduce the frequency of retransmissionsinterrupting the flow of new packets on the broadcast link. That is,a hybrid satellite-terrestrial network architecture would allowgreater throughput for multicasting than a pure-satellite network.
This work examines ARQ multicasting in such a network, and confirms byanalysis and simulation that, within limits, such a throughput advantagecan be realized. A detailed discussion of implementation aspects forpoint-to-point and point-to-multipoint ARQ protocols in bothpure-satellite and hybrid networks is presented as well. This work alsoconsiders partitioning a fixed amount of bandwidth to maximize throughput,possibly subject to a cost constraint, and the effect of a "poorlistener" upon performance in both pure-satellite and hybrid 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 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 A Scheme to Improve Throughput for ARQ-Protected Satellite Communication(1997) Friedman, Daniel E.; Ephremides, Anthony; ISR; CSHCNAutomatic-repeat-request (ARQ) error control is often employed to assure high fidelity information transmission. However, ARQ error control can provide poor throughput for satellite multicasting. The throughput in such communication may be improved by the combination of a terrestrial network parallel to the satellite network and a judiciously modified ARQ protocol. In particular, retransmitted ARQ frames can be sent terrestrially in such a hybrid network, allowing higher throughput than in a pure- satellite network. This work presents analytic results to establish the potential for improving the throughput of satellite multicast communication employing ARQ error control by the adoption of such a hybrid network architecture.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 Experiments in Hybrid Networking with the Advanced Communications Technology Satellite(1996) Friedman, Daniel E.; Gupta, Sonjai K.; Zhang, C.; Ephremides, Anthony; ISR; CSHCNThis paper describes experiments conducted over ACTS that were motivated by the commercial potential of low-cost receive-only satellite terminals operating in a hybrid network environment. 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.Item Review of the VSAT ACTS Experiments at the Center for Satellite & Hybrid Communication Networks(1996) Friedman, Daniel E.; Gupta, Sonjai K.; Zhang, C.; Ephremides, Anthony; ISR; CSHCNThis paper describes experiments conducted over ACTS and the associated TI 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. 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 Error Control for Satellite and Hybrid Communication Networks(1995) Friedman, Daniel E.; Ephremides, A.; ISR; CSHCNBoth forward-error correction (FEC) and automatic-repeat-request (ARQ) error control schemes are used for assuring the accuracy of information transferred through imperfect channels. In satellite systems in which propagation times are typically large, ARQ error control can result in poor throughput to the destination. Also, an ARQ protocol for satellite multicast communication must be carefully crafted to assure good throughput to all destinations regardless of which stations require retransmissions.Supplementing a satellite link with a parallel terrestrial link may allow mitigating some problems of using ARQ in satellite communication systems. ARQ acknowledgments, and possibly retransmissions as well, can be sent terrestrially in such a hybrid network, and so avoid the large satellite propagation delay. The satellite transmission capability of a receiving station which communicates with the transmitter exclusively by terrestrial means can be eliminated and the system cost correspondingly reduced. Further, multicasting with a hybrid network may allow retransmissions to be conducted without interrupting the flow of new information to all destinations, so throughput need not drastically suffer if retransmissions are required.
The degree to which throughput can be improved by adopting a hybrid network is not clear. A hybrid network's effect on the fidelity of information delivered to the destination(s) is also not clear. An experiment is presented for investigating such error control issues of hybrid networking.
Item Neural Networks for Tactile Perception.(1987) Pati, Y.C.; Friedman, Daniel E.; Krishnaprasad, Perinkulam S.; Yao, C.T.; Peckar, M.C.; Yang, R.; Marrian, C.R.K.; ISRIntegrated tactile sensors appear to be essential for dextrous control of multifingered robotic hands. Such sensors would feature (1) compliant contact surfaces, (2) high resolution surface stress transduction, (3) local signal conditioning, and (4) local computation to recover contact surface stress. The last-mentioned item pertains to the basic inverse problem of tactile perception and the real time solution of this inverse problem is our primary concern. We think that good solutions to this problem (i.e., algorithms + implementations) will be needed for realizing dextrous hand control via tactile serving. In this paper we describe a processor chip designed to solve the mathematical inversion problem utilizing neural network principles. Simulations indicate that this chip can function in the presence of large amounts of electrical noise. In addition the effect of processing induced variability in sensor response can also be minimized using the maximum entropy estimate method described below. The tactile sensor design we refer to is the one reported in [1]. This particular design is based on piezo- resistive transduction via an array of diffuse resistors in silicon. Surface load on a compliant layer is transformed into resistance changes proportional to biaxial strains. Initial testing of the sensor has yielded repeatable, linear characteristics. The signal conditioning chip which acts as an interface between the sensor array and subsequent processor chips has also been fabricated. The neural network chip described in this paper has been simulated at the system level. The simulation results for this network based on a particular linear elastic model (described in section 2) of the compliant contact layer. We consider in the simulations some of the errors introduced by process variability in VLSI implementation. The simulations carried out using SIMNON a general purpose nonlinear simulation package developed at Lund Institute of Technology, Sweden (kindly provided us by Professor Astrom), are described in section 4.