Log Nr. 214 INNOVATIVE NETWORKING CONCEPTS TESTED ON THE ADVANCED COMMUNICATIONS TECHNOLOGY SATELLITE Daniel Friedman, Sonjai Gupta, Chuanguo Zhang, and Anthony Ephremides Center for Satellite and Hybrid Communication Networks Institute for Systems Research University of Maryland College Park, MD 20742 (301) 405-7900 Submitted September 29, 1994 for the NASA-CCDS Conference Albuquerque, NM January 8-12, 1995 Address correspondence to: Daniel Friedman
INNOVATIVE NETWORKING CONCEPTS TESTED ON THE ADVANCED COMMUNICATIONS TECHNOLOGY SATELLITE Daniel Friedman, Sonjai Gupta, Chuanguo Zhang, and Anthony Ephremides Center for Satellite and Hybrid Communication Networks Institute for Systems Research University of Maryland College Park, MD 20742 (301) 405-7900 Abstract This paper describes a program of experiments conducted by the CSHCN using ACTS. This program of three experiments commenced in 1992 and utilized the ACTS T1-VSAT terminal. The experiments were motivated by the commercial driver of low-cost receive-only satellite terminals that can operate 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 X.25 and frame relay communication protocols for interconnecting LANs via satellite. The preparation and conduct of these experiments involved sixteen people from the University of Maryland, the University of Colorado, and COMSAT Laboratories. INTRODUCTION The Center for Satellite and Hybrid Communication Networks (CSHCN) at the University of Maryland was founded to explore and develop the possibilities of hybrid networks. Hybrid networking in this context refers to the connection of satellite and terrestrial communication links in parallel. Hybrid networks are well-suited for many commercial communication applications, such as file transfer and remote database access, in which there are asymmetric bandwidth requirements. That is, there is a need to communicate much information in one direction between two points, but there is much less information to send in the opposite direction. While a two-way satellite channel may be used for such applications, using a one-way satellite channel for the bulk information transfer and a parallel terrestrial channel for control purposes may be a less expensive system. In such a hybrid network the satellite terminal need not have transmit capability, and so a less expensive receive-only terminal suffices. In addition to a cost savings, some improvement in net information transfer rate may also be achieved.
The CSHCN proposed in 1992 a series of experiments in hybrid LAN interconnection. The experiments described below were based on an architecture whereby an end user communicated simultaneously via two links, one satellite and one terrestrial. Flow control and link management messages, application commands, and transport services could be routed via either the satellite or the terrestrial link in a manner designed to exploit the inherent advantages of each for optimal end-to-end performance. The terrestrial link used was either the public telephone network or Internet. These experiments were conducted on NASA s Advanced Communications Technology Satellite (ACTS) in the late summer and early fall of 1994. One experiment examined dynamic bandwidth allocation. A second experiment investigated error-control schemes for use in a hybrid network. Finally, a third experiment considered using a hybrid architecture for remote multimedia database access and also compared the performance of some networking protocols in LAN interconnection. Frame Relay was used as the layer-2 protocol over the satellite link, using a prototype Frame Relay Access Switch (FRACS) developed for the CSHCN by COMSAT Laboratories. Although frame relay has been used over terrestrial networks, this was the first instance of its use over a satellite link. One of the experiments described below compared the performance of frame relay with that of the more traditional X.25 protocol. All the experiments were based on a two-node satellite network configuration as in Figure 1. One node was at the University of Maryland at College Park, and the other was at the University of Colorado at Boulder. The experiment equipment configuration in Maryland consisted of two Sun workstations connected through a High Speed Serial Interface to the FRACS, and the FRACS was in turn connected to the ACTS T1-VSAT (Very Small Aperture Terminal). The FRACS-to-VSAT connection consisted of a T1 connection for traffic, and an RS-232 connection for control messages (such as bandwidth allocation requests). The workstations ran CSHCN-developed software to implement the bandwidth allocation algorithm, the error-control schemes, and the multimedia database server. SunLink Frame Relay software was used for a frame relay connection between the workstations and the FRACS. A similar arrangement was used at the University of Colorado, except for an important difference. The Colorado T1-VSAT was not collocated with the FRACS but instead was on the premises of the National Telecommunications and Information Administration (NTIA), also in Boulder. An optical link was used for the T1 traffic connection between University of Colorado and NTIA sites, and a modem link was used for the FRACS control messages. The experiments were motivated by the commercial driver of the availability of low cost of receive-only satellite terminals, as mentioned earlier. However, the scope of the experiments was expanded to include many more facets of hybrid network operation. They were designed to support investigations into the network management challenges that will confront at all levels future commercial and military network service providers as they try to meet market demands and mission requirements for multimedia network services.
ACTS T1 T1 FRACS VSAT LAN VSAT FRACS Univ. of Maryland Univ. of Colorado PSTN or Internet FIGURE 1. Configuration for ACTS Experiments EXPERIMENT 1 The on-board processing capability of the ACTS satellite was explored in the first experiment. The purpose of the experiment was to find the best possible way to rapidly adapt the link bandwidth to fluctuating traffic levels, both for data traffic and for mixed-media traffic such as voice and data. The FRACS had within it the capability to determine the amount of bandwidth required at any given time by observing the rate of the incoming data traffic. If the amount of allocated bandwidth differed from the required amount, the FRACS could then dynamically allocate or de-allocate multiple circuits (each a 64 kbit/s channel) as necessary via signaling to the ACTS satellite system. For data traffic, we proposed and tested a sophisticated threshold-based algorithm to compare with the FRACS s crude rate-based algorithm. The thresholds were specified as percentages of the data queue size. Whenever the queue size went above the threshold, an additional 64 kbit/s channel was requested from the ACTS system. Slightly below each such threshold was a "release" threshold, and whenever the queue size went below the release threshold, a channel was released. Tests were done to find the optimal difference between pairs of request and release thresholds, and also between the thresholds of a pair. The threshold idea was motivated by theoretical analysis
that has shown its optimality (Lin and Kumar 1984, Walrand 1984, Rubinovitch 1985, Viniotis and Ephremides 1988). The bandwidth allocation algorithm for mixed media traffic was even more sophisticated. This algorithm was designed not only to minimize the data delays and queue overflows, but also to minimize the voice call blocking frequency. The algorithm used two-dimensional "threshold" structures, known as switch functions. The switch functions consisted of different thresholds on the data queue size, for different amounts of total available bandwidth. Whenever one of the switch functions was crossed, the algorithm allocated a greater portion of the available bandwidth for data transmission. Voice calls were blocked whenever there were more voice calls than the bandwidth allowed for voice traffic. Voice call compression was another way to accommodate extra voice calls. However, care was taken not to exceed the specified call blocking frequency, by requesting additional bandwidth whenever too many voice calls were being blocked. The theoretical work on the algorithm is not yet complete, but following previous work in this area, our algorithm seems to be a logical extension. Extensive testing of this algorithm was done beforehand to determine proper thresholds for accommodating or rejecting arriving voice call requests. The cost functions used were based on average data delay, the voice call blocking frequency, the number of overflowed data packets, and the average bandwidth. The ACTS satellite has a significant delay associated with its allocating bandwidth. This delay is not fixed and varies with the overall load on the satellite network. Furthermore, the delay is sometimes many seconds. To simulate a system in which the delay is both fixed and smaller, a scheme was devised and tested for pre-allocating several channels but using a variable number of them, in accordance with our bandwidth allocation algorithms. EXPERIMENT 2 The focus of the second experiment was to explore ARQ operation in a hybrid network. The notion motivating this work was that it might be possible to increase net information throughput and reduce queueing delays in a satellite ARQ system by sending the acknowledgments terrestrially instead of by satellite, thus avoiding the satellite propagation delay for the acknowledgments. Throughput increase might also be achieved by retransmitting packets (as may be necessary) terrestrially instead of by satellite. Calculations showed that judiciously using the hybrid configuration can indeed increase the throughput, sometimes significantly. Both go-back-n and selective-repeat ARQ protocols were tested in satellite-only and hybrid configurations. A continuous source of traffic was used in the experiment to simulate a file transfer-type of communication application. The Internet and a 14.4 kbit/s telephone modem connection were employed as the terrestrial link for hybrid operation. In addition to throughput and delays, the fidelity of the communication was considered. That is, the information delivered to the destination was compared with the original information at the source station, and a residual error rate computed. Some simple FEC codes were also tested to compare net throughput and residual error rate of FEC and ARQ operation.
The nature of errors produced by the satellite channel was of particular concern in developing this experiment. Since this experiment depends on the errors produced in the satellite channel, the ability to exercise some control over the production of such errors was desired. Furthermore, the ACTS system is designed to produce very few errors in the information it conveys. Hence software was developed to artificially insert errors. It had originally been expected that burst errors would be seen when using the ACTS Ka-band channel, particularly during operation in severe weather. Accordingly, Reed-Solomon codes had been included in the group of FEC codes to test, and a precise model was sought to describe the burst errors. Later, the preliminary results of another ACTS experimenter were learned. These results indicate that the errors developed in the [uncoded] ACTS channel are most likely produced according to an independently and identically distributed model; such a model of error production was used in the experiment software. A series of point-to-two point ARQ protocol tests was also conducted, as the start of an inquiry into point-to-multipoint ARQ schemes for satellite and hybrid networks. In a purely-satellite broadcast system employing ARQ, each destination station must have a relatively expensive transmit and receive satellite terminal in order to receive information and send ARQ acknowledgments. Furthermore, if a single destination station requires a packet retransmission, then the origination station must send the packet over the satellite link. This retransmission interrupts the stream of new packets for all destination stations. Only one destination station benefits from the retransmission; the others are forced to wait unproductively during this time. This delay may possibly be circumvented, and the throughput for the system correspondingly increased, by sending ARQ acknowledgments and retransmitted packets terrestrially. Again, a receive-only satellite terminal is adequate for the destination stations if a hybrid configuration is used, and a significant cost savings may be obtained in addition to the aforementioned throughput improvement. EXPERIMENT 3 The third experiment consisted of two parts. The first part was multimedia database remote access through a hybrid network; the second was internetworking protocol performance comparison in a LAN-LAN interconnection environment. The first part of this experiment utilized a hybrid network, comprising a T1 satellite link and a modem terrestrial link, to interconnect a database client/server pair. The access of multimedia database information was an ideal use of this asymmetric configuration, taking advantage of the shorter delay of the terrestrial link, and the higher bandwidth, broadcast and dynamic interconnection capabilities of the satellite link. In particular, all short interactive access requests were routed through the terrestrial link, and bulk rate database information was routed through the satellite link. This experiment provided and demonstrated an advanced structure for the proliferation of multimedia network information services in the infrastructure of the information super-highway. The second part of the experiment compared two prominent internetworking protocols, namely X.25 and frame relay, in a LAN-LAN interconnection environment. A T1 satellite link was used to interconnect Ethernet LANs at the University of Maryland and the University of Colorado.
Two Sun SPARC workstations, one at each of the two sites, were installed with gateway protocols, X.25 and frame relay, to serve as internetworking gateways. The parameters of interest were the throughput and the average packet delay. The LAN traffic was composed of two major components, background traffic and foreground traffic. The background traffic was emulated with a new traffic model, which is the so-called self-similar traffic model (Leland, Taqqu, Willinger, and Wilson 1993 and 1994). This model was proposed by Bellcore, and is considered more realistic than traditional Poisson-related traffic models. The foreground traffic was comosed of different kinds of network application processes, such as File Transfer Protocol, database access and Telnet, representing bulk data transfer and interactive data transfer. In this portion of the experiment, the competition between the applications and the background traffic for Ethernet access and gateway buffering was studied. RESULTS Measurements and results of all the above experiments were collected during the conduct of the experiments and will promptly be disseminated. Acknowledgements This work was supported by the NASA Center for Satellite and Hybrid Communication Networks at the University of Maryland. References Leland, W. E., M. S. Taqqu, W. Willinger, and D. V. Wilson (1993) "On the Self-Similar Nature of Ethernet Traffic," Proc. ACM Sigcomm 93, pp. 183-193. Leland, W. E., M. S. Taqqu, W. Willinger, and D. V. Wilson (1994) "On the Self-Similar Nature of Ethernet Traffic (Extended Version)," IEEE/ACM Trans. Networking, 2:1-15. Lin, W. and P. R. Kumar (1984) "Optimal Control of a Queueing System with Two Heterogeneous Servers," IEEE Trans. Automatic Conrol, AC-29 (8):696-703. Rubinovitch, M. (1985) "The Slow Server Problem: A Queue with Stalling," J. Appl. Prob., 22:879-892. Viniotis, I., and A. Ephremides (1988) "On the Optimal Dynamic Switching of Voice and Data in Communication Networks," Proc. Computer Networking Symposium, pp. 8-16. Walrand, J. (1984) "A Note on Optimal Control of a Queueing System with Two Heterogeneous Servers," Systems and Control Letters 4:131-134.