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International seminar on computer networks and performance evaluation
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International Seminar on Computer Networks and Performance Evaluation The International Seminar on Computer Networking and Performance Evaluation took place on September 18-20, 1985, in Tokyo, Japan. This seminar, which was organized by T. Hasegawa (Kyoto University, Japan), H. Kobayashi (Science Institute of IBM Japan/the Working Group on Computer System Modeling and Performance Evaluation (WG7.3) of the International Federation of Information Processing Societies (IFIP), Japan), D. Potier (INRIA, France), and G. Pujolle (Universit~ Paris V I / INRIA, France), was planned to contribute to the development of the information systems by furnishing an opportunity to present and exchange ideas of the researchers around the world. It was sponsored by the following organizations: IBM Japan, INRIA, Kyoto University, IFIP WG7.3 and the International Teletraffic Congress (ITC). The seminar contained four main topics: Local Area Networks, Satellite Communications, Queueing Networks and Systems, and Computer Networks. One hundred thirty-three people from 16 countries attended the seminar. We provide below a report on the thirty-six presentations including four invited lectures delivered at this seminar.
1. Local Area Networks I
1.1. Optimization of Total System According to T. Saito (University of Tokyo, Japan), the final objective of performance evaluation of a computer is the optimization of the total system composed of the computer and controlled
North-Holland Computer Networks and ISDN Systems 11 (1986) 59-73
objects. The design consideration for the total optimization depends on the individual controlled objects, he explained. In an invited lecture entitled "Evaluation of Traffic Carrying Capability and Control of Complex Time Space Switches", Saito presented a survey of results on the optimization of complex time-space networks in the digital circuit switching networks for telephone system. He concluded that the network design should be optimized from the viewpoint of a control computer as well as the network traffic characteristics.
1.2. Process Assignment In a lecture entitled "Process Assignment on Distributed System with Communication Contentions", S. Shimojo, H. Miyahara and K. Takashima (Osaka University, Japan) considered a process allocation problem in a distributed system. The lecturer (S. Shimojo) explained to the audience that processes assigned to several processors are processed in parallel through communicating with each other via local memories when they are assigned to the same processor, and via communication network when assigned to several processors. He presented a queueing model and proposed a heuristic algorithm to find an optimal process assignment which minimizes the average job execution time. Finally, the speaker gave several numerical examples to demonstrate the applicability of their algorithm.
1.3. Hybrid Access Methods (The author, S.I. Samoylenko (USSR Academy of Sciences, USSR) was absent from the seminar so that the next is abstracted from his paper entitled "Hybrid Access Methods in Local-Area Networks".)
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In his paper, he discussed access methods that combine random access at low traffic loads with time-assignment access to handle heavy traffic. His approach yields high medium utilization and features low implementation complexity, he stated in his paper. His proposed method is compared in terms of medium utilization to carrier-sense multiple access with collision detection and some other techniques. He said in conclusion that it performs better than all other known methods. Moreover, he claimed that his procedure is simple as it can be implemented using readily calculable network characteristics (maximum end-to-end propagation time and maximum propagation time between a pair of adjacent nodes).
2. Satellite Communication I 2.1. Data Link Error Recovery In an invited lecture entitled "Performance of Data Link Error Recovery Schemes for Satellite Channels", G. Pujolle (Universit6 Paris VI, France) investigated the main data link error recovery schemes for satellite channels. The speaker first introduced several data link control protocols and their error recovery schemes which have been satisfactorily implemented in terrestrial links. However, satellite links differ from terrestrial links as the propagation delay involved is very large and the capacity reaches several Mbps, he pointed out to the audience. Pujolle presented a performance comparison of the main data link error recovery schemes in two examples: 64 Kbps and 2 Mbps satellite links. 2.2. Two-Node ALOHA Network P. Nain (INRIA, France), in a lecture entitled "Analysis of a Two-Node ALOHA-Network with Infinite Capacity Buffers", considered a two-node ALOHA-network with infinite capacity buffers. The lecturer assumed in his study that time is slotted and at the beginning of each time-slot, a station sends a packet, if any, to a central station with a constant probability. He also assumed that, when two transmissions occur in the same timeslot, both messages have to be retransmitted in a later time-slot. For geometric arrivals of packets, he obtained a close-form generating function for the stationary joint queue length distribution with
the aid of the theory of boundary value problems for regular functions, and derived ergodic conditions, he explained to the audience. Finally, the lecturer concluded by giving exact numerical resuits for the mean packet response time at each station. 2.3. ARQ Protocols According to G. Fayolle (INRIA, France) and R. Thomas (CNET P A A / T I M , France), most of the models for the analysis of the performance of ARQ protocols (Automatic Repeat Request) have assumed that the transmission error rate is taken to be time invariant. However, recent physical experiments have shown that, in many real situations, the error rate is indeed time-correlated, the lecturer (R. Thomas) observed. He further explained that it is the case for example on satellite channels, where an important measure of erroneous transmission is the random fading on the link due to the eventual existence of hygrometeores between the satellite and the receiving station. In a lecture entitled "Efficiency and Stability of Four ARQ Protocols in the Case of Fluctuating Error Rates", the speaker dealt with a model of "mass transport" over a satellite circuit, for which the successive error rates form a stochastic process. He first gave a mathematical model and introduced a matrix representation. Next he derived the efficiency of the protocols and showed asymptotic results in the case of the saturated sources. He further obtained the load of the system and ergodicity conditions from the analysis of sample path equations which are not standard. Finally, the speaker concluded with some numerical examples.
3. Queueing Network I 3.1. OSI Reference Model In an invited lecture entitled "CommunicationSystem Models Embedded in the OSI-Reference Model, A Survey", M. Reiser (IBM Ztirich Research Laboratory, Switzerland) provided analytical methods for major model classes in the framework of seven layers of the Open-Systems Interconnect Reference Model (OSI). The lecturer
ConferenceReports showed examples such as to cover, in parallel, the most important solution methodologies. On the Data-Link Layer (Layer 2), randomaccess systems, polled systems, and point-to-point links with error recovery were treated by the speaker. He presented flow balance, Markov chain, and virtual service time/sequence analyses as solution methods for layer 2. The Network Layer (Layer 3) leads to queueing network models, the speaker observed. He introduced the message-independence assumption and open queueing networks for layer 3. Next he presented end-to-end protocols for the Transport Layer (Layer 4). He focused on the window-flow control as an important class of such protocols. Multichain closed queueing networks were introduced and their solution methods were discussed with emphasis on reducing the exponential complexity of standard algorithms in exact and approximate methods. He explained that, to further reduce mathematical complexity, Layer 4 protocols are mapped onto tandem queues which may be treated analytically, numerically or approximately using "Norton's theorem". Orthogonal to the layer structure of network architectures are nodal services such as operating systems and buffer management, the speaker told the audience. The problem of buffer partitioning to avoid performance degradation and even deadlock was amenable to a solution using flow balance, he observed. However, in the general network context, limited buffer systems did not find a satisfactory solution, he said. Layers 5-7 (Session, Presentation, Application) constitute largely a virgin territory, he said in conclusion. 3.2. Universal Maximum Entropy Algorithm In his lecture "A Universal Maximum Entropy Algorithm for the Analysis of General Closed Networks", D.D. Kouvatsos (University of Bradford, England) gave a hybrid product form solution, based on the principle of Maximum Entropy, for general FCFS closed queueing networks with single servers and multiple job classes at equilibrium. The solution is maximally noncommittal with regard to missing information and is efficiently implemented by a universal algorithm using one-dimensional iterative techniques, the speaker explained to the audience. Finally, Kouvatsos demonstrated the numerical accuracy of the pro-
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posed algorithm by some illustrative examples and made comparisons with other methods. 3.3. Canonical Approximation E. Pinsky and Y. Yemini (Columbia University, U.S.A.) proposed an approximation technique for the performance analysis of distributed systems. Their method provided a closed-form approximation to global performance measures, whose precision increases, often exponentially, with the size of the system, the speaker (E. Pinsky) told the audience. He remarked that the complexity of computing these approximations is typically independent of the problem size, and the method is easily applicable to a wide variety of distributed systems. Finally, the speaker illustrated the method by analyzing some models of multihop packet radio networks, multiprocessor interconnection networks and distributed systems. The title of this lecture was "The Canonical Approximation in Performance Analysis". 3.4. A Discrete Queueing System G. Pujolle, J.P. Claude and D. Seret (Universit6 Paris VI, France) observed that most systems to be evaluated are not governed by continuous distribution functions as in the classical queueing theory but by events which occur at deterministic intervals of time. In a lecture entitled "A Discrete Queueing System with a Product Form Solution", the speaker (G. Pujolle) discussed a class of queueing networks where the time is discretized into slots. He assumed that each event (customer arrival or departure, silence) has a given probability of occurrence in a slot, and that the probability may be fixed or may depend on the state of the queue. The lecturer analyzed a single server queue and a queueing network with a product form solution when queues are in tandem. Finally, he gave an example to evaluate the average time spent in a packet switching network, and compared the results between the discrete time and continuous time queueing systems. 4. Local Area Networks II
4.1. Orwell Slotted Ring R.M. Falconer and J.L. Adams (British Telecorn Research Laboratories, U.K.) explained the
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limitations of established LAN protocols for handling multiple services such as speech, data, and low bit-rate video. In a lecture entitled "Carrying Integrated Services on the Orwell Slotted Ring", Falconer introduced a new slotted ring protocol (Orwell) developed as an Integrated Services LAN. A novel load control mechanism was used to bound the ring access delays to 2 ms or less, and simulation results showed that a 34 M b i t / s Orwell ring LAN has a throughput of up to 50 Mbit/s, the speaker told the audience. He remarked that, in the Orwell ring, no central control is provided for slot allocation; all control functions are fully distributed.
4.2. Fiber Optic Configurations In a lecture entitled "Fiber Optic Configurations for Local Area Networks", F.A. Tobagi, M.M. Nassehi, and M.E. Marhic (Stanford University, U.S.A.) discussed a number of fiber optic configurations for a new class of demand assignment multiple access local area networks requiring a physical ordering among stations. The lecturer (M.M. Nassehi) explained that, in such networks, the data transmission and linear-ordering functions may be distinguished and be provided by separate data and control subnetworks. The configurations proposed for the data subnetwork are based on the linear, star, and tree topologies, he said. He further explained that the control subnetwork must always have a linear unidirectional bus structure to provide linear-ordering functions. Due to the reciprocity and excess loss of optical couplers, the number of stations that can be accommodated on a linear fiber optic bus is severely limited, the speaker observed. He proposed two techniques to overcome the limitation. Finally, for each of the data and control subnetwork configurations, the speaker presented the computation of the maximum number of stations as a function of the power margin for both reciprocal and nonreciprocal couplers.
4.3. Comparison of Connection Oriented and Connectionless Communication architectures with a multi-layer protocol structure are extremely difficult to evaluate due to the model complexity, O. Gihr and P.J. Kuehn (University of Stuttgart, F.R.G.)
observed. In a lecture entitled "Comparison of Communication Services with Connection-Oriented and Connectionless Data Transmission", the speaker (O. Gihr) provided a hierarchical decomposition and aggregation technique which is oriented at the ISO Basic Reference Model. In particular, the lecturer focused on the comparison between connection-oriented and connectionless data transmissions quantitatively. The modelling and performance evaluation technique was exemplified for a local area network. He presented numerical results to show the influence of the various system and protocol parameters on throughput and delay. The generality of the approach allows the application to wide area networks as well, the speaker remarked in conclusion.
5. Queueing System I 5.1. Resequence Delay According to Y.C. Luke Lien (IBM Watson Research Center, U.S.A.), the resquence delay is an important performance measure in a multiple server service center when a first-in-first-out (FIFO) order is required. He observed that the delay happens either because of variable service requirement or because of heterogeneous servers. In his lecture entitled "Evaluation of the Resequence Delay in a Poisson Queueing System with Two Heterogeneous Servers", an analysis of the resequence delay for an M / M / 2 system with 'high capacity server first' scheduling discipline was presented. In conclusion, he told the audience that the solution technique is simple and the result is exact.
5.2. Scan-type TDM and Polling Systems H. Takagi and M. Murata (IBM Japan Science Institute, Japan), in a lecture entitled "Queueing Analysis of Scan-Type T D M and Polling Systems", studied scan-type service which is defined as the order of services for N users given by 1 ~ 2 ... ~ N - I ~ N - - - > N - - - > N - I ~ . . . ~2~1 1 ~ 2 ~ .... The speaker (H. Takagi) presented exact queueing analysis for scan-type T D M and (exhaustive and gated service) polling systems. He explained that the mean delay values in serving requests for scan systems are computed for
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each station which show discrimination among stations due to their relative positions. They were also compared to the corresponding values for cyclic service systems with extra return times from station N to station 1. In this case, scan systems provide better performance if cyclic systems have only nominal return times, the lecturer explained to the audience. Finally, he compared exhaustive and gated service scan" polling systems.
5.3. Multiqueue Systems with Finite Capacity P. Tran-Gia and T. Raith (University of Stuttgart, F.R.G.) observed that the class of multiqueue systems with cyclic service has a broad spectrum of applications, e.g., in modelling approaches for switching systems, token ring local area networks, etc.. In a lecture entitled "Multiqueue Systems with Finite Capacity and Nonexhaustive Cyclic Service", the speaker (P. Tran-Gia) presented an approximate analysis method for such a class of models, whereby the realistic assumption of a finite queue capacity was taken into account. He told the audience that the analysis is based on an imbedded Markov chain approach in conjunction with a two-moment approximation for the cycle time. Finally the speaker showed numerical results in order to illustrate the accuracy of the calculation method over a wide range of system parameters.
6. Queueing Network II
6.1. Fixed Point Approximation In a lecture entitled "Fixed Point Approximation in Models of Networks of Queues", R.J. Walstra (Hewlett-Packard Laboratories, U.S.A.) presented a framework for approximately analyzing networks of queues based on network decomposition. He explained that, in the approximate analysis by decomposition, performance measures of a network of queues are obtained by solving a system of simultaneous nonlinear equations. Typically, the solution is obtained by fixed point iteration, he observed. Walstra investigated a wide variety of existing methods of approximate analysis, and told that his approach of decomposition and fixed point iteration provides a uniform framework for these methods. His aim was to
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create a framework that can assist in devising and formally justifying new methods of approximate solution, he said in conclusion.
6.2. Tandem Exponential Queues In a lecture entitled "Tandem Exponential Queues with Finite Buffers", A. Brandwajn and Y.L. Lily Jow (Amdahl Corporation, U.S.A.) dealt with open networks of tandem exponential queues with finite buffers where blocking of service occurs at service completion if the following queue is at capacity. As the speaker (A. Brandwajn) explained, they relied on exact equivalence of state probabilities. To evaluate the conditional probabilities introduced through the equivalence, they used an explicitly defined approximation in which some variables are ignored in the condition state description. The lecturer observed that it results in an iterative approach with computational complexity of order near linear in the number of servers. He further explained that the method produces performance measures for individual servers and also an approximation for joint state probabilities for pairs of neighboring servers. The accuracy of their approach appears to be generally good in numerical examples, the speaker concluded.
6.3. Markovian Solver D. Potier (INRIA, France) and M. Veran (Bull Sems, France) observed that several important computer architecture features such as synchronization, blocking and priorities are difficult to analyze within the framework of product-form networks. Available methods are approximate and, in most cases, the quality of the results cannot be assessed, the speaker (D. Potier) explained. He pointed out to the audience, that Markovian analysis has a strong potential for this class of problems, but its practical application requires that specific tools be available: the state-space generation and steady-state computation must be performed in an automatic and efficient way. In the lecture entitled "The Markovian Solver of QNAP2 and Examples", the lecturer introduced the Markovian solver of modelling package QNAP2, which performs the above-mentioned functions for a large class of models. He presented design principles and application stipulations of
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this solver. He showed several examples which illustrate its application to the analysis of complex models. Finally, he highlighted future work and extensions.
7. Computer Networks
7.1. Modelling of New Services In an incited lecture entitled "Modeling of New Services in Computer and Communication Networks", P.J. Kuehn (University of Stuttgart, F.R.G.) stated that the introduction of digital networks and new services for communication, information processing and storage poses a large number of new problems for traffic engineering and network planning. Basis of the performance evaluation is the modelling of new services and of new network functions, he pointed out to the audience. For this purpose, he first characterized service attributes and network capabilities related to the protocol architecture and to standardization work. Finally, he discussed a set of traffic models which refer to workload characterization, protocols and switching networks.
7.2. Testing Protocol Implementation According to O. Rafiq (Bull Transac, France), in the distributed architecture for testing protocol implementations, a process which is called the responder system, is required to act as a user of the service provided by the protocol entity under test. The design of such a system poses a number of problems such as the influence of its functions on execution and the value of testing, he told the audience. In a lecture entitled "A Good Approach for Testing Protocol Implementations", the lecturer presented features and contributions of the so-called astride responder which is used by Bull to test the OSI protocol implementations. Finally, Rafiq told that it is simple, is small and provides an efficient testing methodology.
portant issue, they said, in designing computer networks with window control. The previous work on wait systems of multiple logical channel (LC) networks under stationary traffic condition showed that the optimal allocation can be achieved by equalizing the input regulation degree on each LC, they observed. However, the window allocation problem under time-varying traffic condition is unsettled, the speaker (I. Akiyoshi) told the audience. In a lecture entitled "Dynamic Window Control in Wait System of Computer Network", the lecturer presented a window control scheme which changes window sizes according to traffic fluctuation so as to keep the balance of regulation. He gave simulation results and compared with the static window control scheme. Their scheme prevents unfairness among LC's and produces a delay improvement under time-varying traffic condition, the speaker concluded.
7.4. Effect of System Parameter Changes S.G. Chang (Hanyang University, Korea) and D.V. Tcha (Korea Advanced Institute of Science and Technology, Korea), in a lecture entitled "Analyzing the Effect on the Message Delay of Some System Parameter Changes in Communication Networks", discussed the following question: how the implicit function formula of Kuhn-Tucker conditions can be exploited in analyzing the effect on the message delay of the changes in link capacities and traffic requirements in store-and-forward communication networks. Based on some properties of the involved delay function, the speaker (D.W. Tcha) first analyzed the model formulated as single-commodity convex-cost network flows, and next extended to the multicommodity flow model. Finally, he provided a numerical example. The results might be usefully exploited in designing an adaptive quasi-static routing scheme, the lecturer said in conclusion.
8. Satellite Communication II
7.3. Dynamic Window Control
8.1. Hybrid Go-Back-N ARQ
I. Akiyoshi (Kobe University of Commerce, Japan), H. Suzuki, H. Nakanishi, H. Sanada and Y. Tezuka (Osaka University, Japan) dealt with the window allocation problem, which is an im-
In a lecture entitled "Hybrid Go-Back-N ARQ with Extended Code Blocks for Error Control of Satellite Communications", P.S. Yu and D.T. Tang (IBM Watson Research Center, U.S.A.) presented
Conference Reports a new hybrid Go-Back-N ARQ scheme with extended code block. The lecturer (P.S. Yu) explained that their scheme may be regarded as a generalization of previous hybrid Go-Back-N ARQ schemes with half-rate codes proposed by Lin and Yu. In their generalization, a complete code block consists of a data block and an extended code block of the same length, the speaker said. He observed that both were augmented by parity bits for error detection. Normally, "full length" data blocks are transmitted, and, during error recovery, the transmitter will then transmit the extended code block, he told the audience. He said that the proposed scheme achieves high throughput efficiency over a wide range of channel error rate. The speaker further explained that it is particularly attractive for communication systems where the round trip propagation delay is large and the bit rate is high. Finally, he presented the derivation of a lower bound on throughput efficiency.
8.2. New Access Scheme for Mixed Traffic In a lecturer entitled " F O D A - T D M A : A New Satellite Access Scheme For Mixed Traffic. Presentation and First Simulation Results", R. Beltrame, A.B. Bonito, N. Celandroni and E. Ferro (CNUSE Institute, National Research Council, Italy) presented a Fifo Order based Demand Assignment (FODA) access scheme which is designed to handle packetized data and voice traffic in a multiple access satellite broadcast channel of Mbits band. The channel is shared by as many as 64 simultaneously active stations in a range of 255 addressable stations, the lecturer (N. Celandroni) told the audience. A sophisticated traffic environment was assumed, including different types of service requirements and an arbitrary load distribution among the stations. They also presented the results of the 2 M b i t / s simulation tests carried out in an already existing hardware environment.
8. 3. Bulk Data Transfer Protocol F. Ivanova and I. Valet (INRIA, France) first explained the N A D I R project, which is intended to conduct an experimental research program to study and experiment with computer communication through satellite systems such as TELECOM1 in France. Next they discussed the design and implementation of specific protocols which are
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adapted to satellite transmission characteristics. These protocols have been defined to optimize the transfer of data with respect to the main characteristics of satellite links (i.e., high throughput, long propagation delay and specific bit error rate), they told the audience. In a lecture entitled "Performance Measurements of N A D I R Bulk Data Transfer Protocol on Satellite Links", they studied the implementation and measurements of the point-to-point bulk data transfer by using a satellite simulator called ANIS. They showed its performance for high throughput (900 Kbit/s) and for one, two and three successive satellite hops. According to the speaker (F. Ivanova) the results showed that the N A D I R protocol is able to perform well under various transfer conditions, and its efficiency remains good for high data rate in conjunction with high bit error rate and an increase in the propagation delay. She concluded that their protocol is used as a basis to develop specific applications taking advantage of the satellite link characteristics.
9. Multi-Access Communication
9.1. Finite Population Multiple-Access Channels X.M. Chang (Nankai University, China) and F.K. Hwang ( A T & T Bell Laboratories, U.S.A.) considered the k-channel version of the Capetanakis model for multiple-access protocols with finite population. They assumed in their study that n users share a channel which is able to successfully transmit up to k - 1 messages simultaneously. It was also assumed that, if k or more users transmit simultaneously, then a collision occurs and those users have to retransmit. In a lecture entitled " T h e Minimax Number of Calls for Finite Population Multiple-Access Channels", they studied the case where d of n users are active, i.e., have a message to send, and the rest inactive. A protocol is a sequence of calls that schedules the transmissions of all active users where a call requests the active users (there may be none) in a specified subset of users to transmit, the speaker (F.K. Hwang) explained. He presented the evaluation of an algorithm by the minimax numbers of required calls for small d and large d. Finally, the speaker claimed that their results apply to the case when at most d active
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users exist, and that many of their results carry over to the Hayes-Gudjohnsen model.
9.2. Thrashing Phenomena In a lecture entitled "An Analytical Method of Thrashing Phenomena of Retransmit Model in Computer Networks", Y. Onozato, K. Katayama (University of Electro-Communications, Japan) and S. Noguchi (Tohoku University, Japan) investigated the thrashing phenomena of retransmit model, which has been discovered in computer networks. The speaker (Y. Onozato) introduced a queueing model of retransmit model and analyzed it in line with the catastrophe theory. He explained that the global behavior of the model is determined by the fold catastrophe. Finally, he demonstrated the usefulness of their analysis by giving several numerical examples and simulation results.
9.3. Recurrence Equation Conflict resolution algorithms ( C R A ) for broadcast communications have become more and more popular since the work of Capetanakis as well as Tsybakov and Mikhailov (CTM algorithm), W. Szpankowski (Purdue University, U.S.A.) observed. In a lecture entitled "Analysis of a Recurrence Equation Arising in Stack-Type Algorithms for Collision-Detecting Channels", he considered a class of CTM algorithms for which a common recurrence equation for the expected length of the conflict resolution interval (CRI) may be found. Szpankowski presented a closed form expression through an analysis of the equation, and then gave an asymptotic approximation. Finally, he applied his approximation to compute maximum throughput of the analyzed CRA algorithms.
processing time spent clearing the unsuccessful attempts, an often ignored factor, he said. Sze further explained that his model also includes subsequent reattempts by failed attempts. He presented the derivation of an approximation, necessary and sufficient conditions for its solution, and conditions for uniqueness in a special case. Finally, Sze provided an example which shows the decrease in the throughput near the system capacity, and how analyzing the total offered load overestimates the stability of the system.
10.2. Transient Queueing Analysis H.R. van As (IBM Ziirich Research Laboratory, Switzerland) observed that, as a result of dynamic sharing of network resources, packet switching networks are very sensitive to overload situations. Therefore, efficient congestion control mechanisms are of vital importance, he told the audience. In a lecture "Transient Queueing Analysis of a Two-Level Global Congestion Control Mechanism", the lecturer presented a global congestion control mechanism based on either a brusque cut off or a smooth throttling technique, and demonstrated the inherent transient effects. The lecturer showed that the underlying Markovian queueing models take into account the delay until the control becomes effective, which is caused by the geographic distances. He further explained that the transient queueing analysis is based on the numerical evaluation of simultaneous first-order differential equations which describe the system behavior. For this purpose, the fourth-order Runge-Kutta procedure was used, he said. In particular, he analyzed the system reaction to a rectangular overload peak. Finally, he presented the numerical results for the mean system occupancy and the loss probabilities as a function of time, which indicated the explanatory value of transient queueing analysis.
10. Queueing System II 10.3. Fork-Join Queue 10.1. Overload Analysis According to D.Y. Sze (Bell Communications Research Inc., U.S.A.), for a stochastic system near capacity, some attempts can fail due to system timeouts or customer impatience. In a lecture entitled "A Queueing Model for Overload Analysis", Sze built a queueing model that includes
In a lecture entitled "Simple Computable Bounds and Approximations for the Fork-Join Queue", F. Baccelli (INRIA, France), A.M. Makowski (University of Maryland, U.S.A.) and A. Shwartz (Technion, Israel) considered a queueing system, known as the Fork-Join queue, with a basic performance measure defined as the delay
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between the Fork and Join dates. The speaker (P. Nain on behalf of the authors) explained that simple bounds are derived for some of the statistics of this quantity under standard renewal assumptions. These bounds are obtained, in both transient and steady-state regimes, by stochastically comparing the original system to two other queueing systems with a structure simpler than the original system, yet with identical stability characteristics, the speaker told the audience. He further explained that, in steady-state, the computation reduces to standard G I / G I / 1 calculations and the obtained approximations thus constitute a first sizing-up of system performance. Finally, the binding methodology shown in this presentation is of independent interest to study various other queueing system, the speaker observed.
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carried out by the interface connecting the computers to the common bus. Usually, in more conventional systems, this kind of interface merely controls communication aspects such as modulation, demodulation, bus access, data formatting, etc., she observed. She further explained that her idea is to assign to the interface also the responsibility for the control of information transfer between tasks situated in different computers. Besides increasing the overall execution speed, the solution frees each computer from most of the intertask communication burden, she said. Additional, the task synchronization becomes transparent to the programmer, causing substantial savings in specific design efforts, the speaker said in conclusion.
11.3. A Comparison of Distributed and Centralized Systems 11. Distributed Systems
11.1. Circulating Queue Algorithm In a lecture entitled "Performance Analysis of a Circulating Queue Algorithm for Distributed Database Updating", C. Nogues (Empesa Nacional Hidroelectica del Ribagoszana, Spain) and R. Puigjaner (Universitad Politecnica de Barcelona, Spain) presented a new method for multiple-copy file updating. The speaker (C. Nogues) explained that the transaction queue circulates through a virtual ring linking all sites. The queue is updated at each site and copied into the execution site queue, she said. Finally, the speaker presented a performance study by analytical and simulation models.
11.2. Intertask Communication F. Rosemberg (Weizmann Institute of Science, Israel) presented a novel design for the implementation of the Ada intertask communication mechanism in common bus systems. In a lecture entitled "An Ada Oriented Protocol for Intertask Communication in Real Time Common Bus Systems", she first explained a hierarchical layered partition, appropriate to the systems. Then, she introduced her design decisions concerning the bus access mechanism. Finally, she proposed an Ada-oriented protocol, servicing the intertask communication. She explained that most of its execution is
In a lecture entitled "A Comparison of Distributed and Centralized Processing Systems", F.A. Tobagi and H. Kanakia (Stanford University, U.S.A.) discussed the relative performance of distributed processing and centralized processing of an algorithm. The speaker (H. Kanakia) explained that the algorithm is abstracted as consisting of a number of tasks with precedence constraints among them. The comparison was based on the average execution time of a request to execute a given algorithm. The main assumption about the processing system used is that the processing capacity of a centralized system is the sum of capacities of processors in a distributed system, the speaker told the audience. He presented a range of parameters of algorithms for which a centralized system outperforms a distributed system, both executing the same algorithm. Finally, the lecturer remarked that it is not true for all algorithms by giving examples.
11.4. Formal Specification by SDL W. Fischer, K.P. Sauer and W. Denzel (University of Stuttgart, F.R.G.) observed that, for some basic communications mechanisms within distributed systems, analytical approaches are available for calculating throughput, transfer times, waiting times and some other quantities, but for a large class of problems such methods do not exist or are not applicable due to the systems' complexity. The
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lecturer (W. Fischer) told the audience that system simulation is a common way to overcome this problem at least to a certain degree. In a lecture entitled "A Simulation Technique for Distributed Systems Based on a Formal Specification by SDL", the speaker described the simulation technique, which works with a formal description of the system of processes to be simulated. He further explained that their approach uses directly a formal specification by the "Specification and Description Language" (SDL) recommended by CCITT.
The Proceedings of this conference have been published by Elsevier Science Publishers (North-Holland) under the title Computer Networking and Performance Analysis and edited by T. Hasegawa, H. Takagi and Y. Takahashi. 1986. xii + 514 pages. ISBN: 0-444-87960-9. US$63.00 / Df1.170.00.
H. Takagi M. Murata IBM Japan Science Institute
Ninth Data Communications Symposium The Ninth Data Communications Symposium was held at Whistler Mountain, British Columbia, Canada from September 10-13, 1985. The General Chairman of the symposium was William P. Lidinsky; the sponsors were the ACM/SIGCOMM, the IEEE Communications Society and the IEEE Computer Society. The symposium continued its tradition of 18 years ago as a meeting of practitioners in the field of data communications. In an attempt to solicit participants from industry working on the frontier of data communications, seven panels were presented: space satellite earth stations, DARPA interdomain addressing, packet switching network operational experience, message handling and directory services, remote procedure calls versus problem oriented protocols, ISDN, and end user applications. The lectures reflected a mix of theory and practice. The seven sessions covered transport protocol case studies, theoretical foundations of protocols, communication system performance, packet switching network management, prototype networks, naming and addressing and routing, and application level issues. The technical program was put together by Bart W. Stuck, the Program Chairman. Two keynote addresses were presented: Ed Sussenguth, a lead architect for IBM's Systems Net-
work Architecture (SNA), examined trends in data communications. Shen Lin, a key designer for AT&T Electronic Tandem Network (ETN) private line networks, surveyed network design methodologies. Two tutorials were also presented: Peggy Karp covered message handling protocols (e.g., X.400), and Stuart Wecker examined voice/data PBXs. We feature below a description of the technical presentations, as well as brief information on the panel discussions.
Session 1: Transport Protocol Case Studies Development of a TCP / IP R.K. Brandriff, CA. Lynch and M.H. Needleman (University of California, Berkeley, U.S.A.) described the design and implementation decisions that have been made in developing software to support the DARPA TCP/IP protocols for the IBM OS/370 environment at the University of California Division of Library Automation. They explained that the implementation is designed to support over 100 concurrent TCP connections, all of which are managed by a single program, which acts as a specialized sub-operating system. In their presentation, the lecturers also discussed problems
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International Seminar on Computer Networks and Performance Evaluation The International Seminar on Computer Networking and Performance Evaluation took place on September 18-20, 1985, in Tokyo, Japan. This seminar, which was organized by T. Hasegawa (Kyoto University, Japan), H. Kobayashi (Science Institute of IBM Japan/the Working Group on Computer System Modeling and Performance Evaluation (WG7.3) of the International Federation of Information Processing Societies (IFIP), Japan), D. Potier (INRIA, France), and G. Pujolle (Universit~ Paris V I / INRIA, France), was planned to contribute to the development of the information systems by furnishing an opportunity to present and exchange ideas of the researchers around the world. It was sponsored by the following organizations: IBM Japan, INRIA, Kyoto University, IFIP WG7.3 and the International Teletraffic Congress (ITC). The seminar contained four main topics: Local Area Networks, Satellite Communications, Queueing Networks and Systems, and Computer Networks. One hundred thirty-three people from 16 countries attended the seminar. We provide below a report on the thirty-six presentations including four invited lectures delivered at this seminar.
1. Local Area Networks I
1.1. Optimization of Total System According to T. Saito (University of Tokyo, Japan), the final objective of performance evaluation of a computer is the optimization of the total system composed of the computer and controlled
North-Holland Computer Networks and ISDN Systems 11 (1986) 59-73
objects. The design consideration for the total optimization depends on the individual controlled objects, he explained. In an invited lecture entitled "Evaluation of Traffic Carrying Capability and Control of Complex Time Space Switches", Saito presented a survey of results on the optimization of complex time-space networks in the digital circuit switching networks for telephone system. He concluded that the network design should be optimized from the viewpoint of a control computer as well as the network traffic characteristics.
1.2. Process Assignment In a lecture entitled "Process Assignment on Distributed System with Communication Contentions", S. Shimojo, H. Miyahara and K. Takashima (Osaka University, Japan) considered a process allocation problem in a distributed system. The lecturer (S. Shimojo) explained to the audience that processes assigned to several processors are processed in parallel through communicating with each other via local memories when they are assigned to the same processor, and via communication network when assigned to several processors. He presented a queueing model and proposed a heuristic algorithm to find an optimal process assignment which minimizes the average job execution time. Finally, the speaker gave several numerical examples to demonstrate the applicability of their algorithm.
1.3. Hybrid Access Methods (The author, S.I. Samoylenko (USSR Academy of Sciences, USSR) was absent from the seminar so that the next is abstracted from his paper entitled "Hybrid Access Methods in Local-Area Networks".)
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In his paper, he discussed access methods that combine random access at low traffic loads with time-assignment access to handle heavy traffic. His approach yields high medium utilization and features low implementation complexity, he stated in his paper. His proposed method is compared in terms of medium utilization to carrier-sense multiple access with collision detection and some other techniques. He said in conclusion that it performs better than all other known methods. Moreover, he claimed that his procedure is simple as it can be implemented using readily calculable network characteristics (maximum end-to-end propagation time and maximum propagation time between a pair of adjacent nodes).
2. Satellite Communication I 2.1. Data Link Error Recovery In an invited lecture entitled "Performance of Data Link Error Recovery Schemes for Satellite Channels", G. Pujolle (Universit6 Paris VI, France) investigated the main data link error recovery schemes for satellite channels. The speaker first introduced several data link control protocols and their error recovery schemes which have been satisfactorily implemented in terrestrial links. However, satellite links differ from terrestrial links as the propagation delay involved is very large and the capacity reaches several Mbps, he pointed out to the audience. Pujolle presented a performance comparison of the main data link error recovery schemes in two examples: 64 Kbps and 2 Mbps satellite links. 2.2. Two-Node ALOHA Network P. Nain (INRIA, France), in a lecture entitled "Analysis of a Two-Node ALOHA-Network with Infinite Capacity Buffers", considered a two-node ALOHA-network with infinite capacity buffers. The lecturer assumed in his study that time is slotted and at the beginning of each time-slot, a station sends a packet, if any, to a central station with a constant probability. He also assumed that, when two transmissions occur in the same timeslot, both messages have to be retransmitted in a later time-slot. For geometric arrivals of packets, he obtained a close-form generating function for the stationary joint queue length distribution with
the aid of the theory of boundary value problems for regular functions, and derived ergodic conditions, he explained to the audience. Finally, the lecturer concluded by giving exact numerical resuits for the mean packet response time at each station. 2.3. ARQ Protocols According to G. Fayolle (INRIA, France) and R. Thomas (CNET P A A / T I M , France), most of the models for the analysis of the performance of ARQ protocols (Automatic Repeat Request) have assumed that the transmission error rate is taken to be time invariant. However, recent physical experiments have shown that, in many real situations, the error rate is indeed time-correlated, the lecturer (R. Thomas) observed. He further explained that it is the case for example on satellite channels, where an important measure of erroneous transmission is the random fading on the link due to the eventual existence of hygrometeores between the satellite and the receiving station. In a lecture entitled "Efficiency and Stability of Four ARQ Protocols in the Case of Fluctuating Error Rates", the speaker dealt with a model of "mass transport" over a satellite circuit, for which the successive error rates form a stochastic process. He first gave a mathematical model and introduced a matrix representation. Next he derived the efficiency of the protocols and showed asymptotic results in the case of the saturated sources. He further obtained the load of the system and ergodicity conditions from the analysis of sample path equations which are not standard. Finally, the speaker concluded with some numerical examples.
3. Queueing Network I 3.1. OSI Reference Model In an invited lecture entitled "CommunicationSystem Models Embedded in the OSI-Reference Model, A Survey", M. Reiser (IBM Ztirich Research Laboratory, Switzerland) provided analytical methods for major model classes in the framework of seven layers of the Open-Systems Interconnect Reference Model (OSI). The lecturer
ConferenceReports showed examples such as to cover, in parallel, the most important solution methodologies. On the Data-Link Layer (Layer 2), randomaccess systems, polled systems, and point-to-point links with error recovery were treated by the speaker. He presented flow balance, Markov chain, and virtual service time/sequence analyses as solution methods for layer 2. The Network Layer (Layer 3) leads to queueing network models, the speaker observed. He introduced the message-independence assumption and open queueing networks for layer 3. Next he presented end-to-end protocols for the Transport Layer (Layer 4). He focused on the window-flow control as an important class of such protocols. Multichain closed queueing networks were introduced and their solution methods were discussed with emphasis on reducing the exponential complexity of standard algorithms in exact and approximate methods. He explained that, to further reduce mathematical complexity, Layer 4 protocols are mapped onto tandem queues which may be treated analytically, numerically or approximately using "Norton's theorem". Orthogonal to the layer structure of network architectures are nodal services such as operating systems and buffer management, the speaker told the audience. The problem of buffer partitioning to avoid performance degradation and even deadlock was amenable to a solution using flow balance, he observed. However, in the general network context, limited buffer systems did not find a satisfactory solution, he said. Layers 5-7 (Session, Presentation, Application) constitute largely a virgin territory, he said in conclusion. 3.2. Universal Maximum Entropy Algorithm In his lecture "A Universal Maximum Entropy Algorithm for the Analysis of General Closed Networks", D.D. Kouvatsos (University of Bradford, England) gave a hybrid product form solution, based on the principle of Maximum Entropy, for general FCFS closed queueing networks with single servers and multiple job classes at equilibrium. The solution is maximally noncommittal with regard to missing information and is efficiently implemented by a universal algorithm using one-dimensional iterative techniques, the speaker explained to the audience. Finally, Kouvatsos demonstrated the numerical accuracy of the pro-
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posed algorithm by some illustrative examples and made comparisons with other methods. 3.3. Canonical Approximation E. Pinsky and Y. Yemini (Columbia University, U.S.A.) proposed an approximation technique for the performance analysis of distributed systems. Their method provided a closed-form approximation to global performance measures, whose precision increases, often exponentially, with the size of the system, the speaker (E. Pinsky) told the audience. He remarked that the complexity of computing these approximations is typically independent of the problem size, and the method is easily applicable to a wide variety of distributed systems. Finally, the speaker illustrated the method by analyzing some models of multihop packet radio networks, multiprocessor interconnection networks and distributed systems. The title of this lecture was "The Canonical Approximation in Performance Analysis". 3.4. A Discrete Queueing System G. Pujolle, J.P. Claude and D. Seret (Universit6 Paris VI, France) observed that most systems to be evaluated are not governed by continuous distribution functions as in the classical queueing theory but by events which occur at deterministic intervals of time. In a lecture entitled "A Discrete Queueing System with a Product Form Solution", the speaker (G. Pujolle) discussed a class of queueing networks where the time is discretized into slots. He assumed that each event (customer arrival or departure, silence) has a given probability of occurrence in a slot, and that the probability may be fixed or may depend on the state of the queue. The lecturer analyzed a single server queue and a queueing network with a product form solution when queues are in tandem. Finally, he gave an example to evaluate the average time spent in a packet switching network, and compared the results between the discrete time and continuous time queueing systems. 4. Local Area Networks II
4.1. Orwell Slotted Ring R.M. Falconer and J.L. Adams (British Telecorn Research Laboratories, U.K.) explained the
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limitations of established LAN protocols for handling multiple services such as speech, data, and low bit-rate video. In a lecture entitled "Carrying Integrated Services on the Orwell Slotted Ring", Falconer introduced a new slotted ring protocol (Orwell) developed as an Integrated Services LAN. A novel load control mechanism was used to bound the ring access delays to 2 ms or less, and simulation results showed that a 34 M b i t / s Orwell ring LAN has a throughput of up to 50 Mbit/s, the speaker told the audience. He remarked that, in the Orwell ring, no central control is provided for slot allocation; all control functions are fully distributed.
4.2. Fiber Optic Configurations In a lecture entitled "Fiber Optic Configurations for Local Area Networks", F.A. Tobagi, M.M. Nassehi, and M.E. Marhic (Stanford University, U.S.A.) discussed a number of fiber optic configurations for a new class of demand assignment multiple access local area networks requiring a physical ordering among stations. The lecturer (M.M. Nassehi) explained that, in such networks, the data transmission and linear-ordering functions may be distinguished and be provided by separate data and control subnetworks. The configurations proposed for the data subnetwork are based on the linear, star, and tree topologies, he said. He further explained that the control subnetwork must always have a linear unidirectional bus structure to provide linear-ordering functions. Due to the reciprocity and excess loss of optical couplers, the number of stations that can be accommodated on a linear fiber optic bus is severely limited, the speaker observed. He proposed two techniques to overcome the limitation. Finally, for each of the data and control subnetwork configurations, the speaker presented the computation of the maximum number of stations as a function of the power margin for both reciprocal and nonreciprocal couplers.
4.3. Comparison of Connection Oriented and Connectionless Communication architectures with a multi-layer protocol structure are extremely difficult to evaluate due to the model complexity, O. Gihr and P.J. Kuehn (University of Stuttgart, F.R.G.)
observed. In a lecture entitled "Comparison of Communication Services with Connection-Oriented and Connectionless Data Transmission", the speaker (O. Gihr) provided a hierarchical decomposition and aggregation technique which is oriented at the ISO Basic Reference Model. In particular, the lecturer focused on the comparison between connection-oriented and connectionless data transmissions quantitatively. The modelling and performance evaluation technique was exemplified for a local area network. He presented numerical results to show the influence of the various system and protocol parameters on throughput and delay. The generality of the approach allows the application to wide area networks as well, the speaker remarked in conclusion.
5. Queueing System I 5.1. Resequence Delay According to Y.C. Luke Lien (IBM Watson Research Center, U.S.A.), the resquence delay is an important performance measure in a multiple server service center when a first-in-first-out (FIFO) order is required. He observed that the delay happens either because of variable service requirement or because of heterogeneous servers. In his lecture entitled "Evaluation of the Resequence Delay in a Poisson Queueing System with Two Heterogeneous Servers", an analysis of the resequence delay for an M / M / 2 system with 'high capacity server first' scheduling discipline was presented. In conclusion, he told the audience that the solution technique is simple and the result is exact.
5.2. Scan-type TDM and Polling Systems H. Takagi and M. Murata (IBM Japan Science Institute, Japan), in a lecture entitled "Queueing Analysis of Scan-Type T D M and Polling Systems", studied scan-type service which is defined as the order of services for N users given by 1 ~ 2 ... ~ N - I ~ N - - - > N - - - > N - I ~ . . . ~2~1 1 ~ 2 ~ .... The speaker (H. Takagi) presented exact queueing analysis for scan-type T D M and (exhaustive and gated service) polling systems. He explained that the mean delay values in serving requests for scan systems are computed for
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each station which show discrimination among stations due to their relative positions. They were also compared to the corresponding values for cyclic service systems with extra return times from station N to station 1. In this case, scan systems provide better performance if cyclic systems have only nominal return times, the lecturer explained to the audience. Finally, he compared exhaustive and gated service scan" polling systems.
5.3. Multiqueue Systems with Finite Capacity P. Tran-Gia and T. Raith (University of Stuttgart, F.R.G.) observed that the class of multiqueue systems with cyclic service has a broad spectrum of applications, e.g., in modelling approaches for switching systems, token ring local area networks, etc.. In a lecture entitled "Multiqueue Systems with Finite Capacity and Nonexhaustive Cyclic Service", the speaker (P. Tran-Gia) presented an approximate analysis method for such a class of models, whereby the realistic assumption of a finite queue capacity was taken into account. He told the audience that the analysis is based on an imbedded Markov chain approach in conjunction with a two-moment approximation for the cycle time. Finally the speaker showed numerical results in order to illustrate the accuracy of the calculation method over a wide range of system parameters.
6. Queueing Network II
6.1. Fixed Point Approximation In a lecture entitled "Fixed Point Approximation in Models of Networks of Queues", R.J. Walstra (Hewlett-Packard Laboratories, U.S.A.) presented a framework for approximately analyzing networks of queues based on network decomposition. He explained that, in the approximate analysis by decomposition, performance measures of a network of queues are obtained by solving a system of simultaneous nonlinear equations. Typically, the solution is obtained by fixed point iteration, he observed. Walstra investigated a wide variety of existing methods of approximate analysis, and told that his approach of decomposition and fixed point iteration provides a uniform framework for these methods. His aim was to
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create a framework that can assist in devising and formally justifying new methods of approximate solution, he said in conclusion.
6.2. Tandem Exponential Queues In a lecture entitled "Tandem Exponential Queues with Finite Buffers", A. Brandwajn and Y.L. Lily Jow (Amdahl Corporation, U.S.A.) dealt with open networks of tandem exponential queues with finite buffers where blocking of service occurs at service completion if the following queue is at capacity. As the speaker (A. Brandwajn) explained, they relied on exact equivalence of state probabilities. To evaluate the conditional probabilities introduced through the equivalence, they used an explicitly defined approximation in which some variables are ignored in the condition state description. The lecturer observed that it results in an iterative approach with computational complexity of order near linear in the number of servers. He further explained that the method produces performance measures for individual servers and also an approximation for joint state probabilities for pairs of neighboring servers. The accuracy of their approach appears to be generally good in numerical examples, the speaker concluded.
6.3. Markovian Solver D. Potier (INRIA, France) and M. Veran (Bull Sems, France) observed that several important computer architecture features such as synchronization, blocking and priorities are difficult to analyze within the framework of product-form networks. Available methods are approximate and, in most cases, the quality of the results cannot be assessed, the speaker (D. Potier) explained. He pointed out to the audience, that Markovian analysis has a strong potential for this class of problems, but its practical application requires that specific tools be available: the state-space generation and steady-state computation must be performed in an automatic and efficient way. In the lecture entitled "The Markovian Solver of QNAP2 and Examples", the lecturer introduced the Markovian solver of modelling package QNAP2, which performs the above-mentioned functions for a large class of models. He presented design principles and application stipulations of
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this solver. He showed several examples which illustrate its application to the analysis of complex models. Finally, he highlighted future work and extensions.
7. Computer Networks
7.1. Modelling of New Services In an incited lecture entitled "Modeling of New Services in Computer and Communication Networks", P.J. Kuehn (University of Stuttgart, F.R.G.) stated that the introduction of digital networks and new services for communication, information processing and storage poses a large number of new problems for traffic engineering and network planning. Basis of the performance evaluation is the modelling of new services and of new network functions, he pointed out to the audience. For this purpose, he first characterized service attributes and network capabilities related to the protocol architecture and to standardization work. Finally, he discussed a set of traffic models which refer to workload characterization, protocols and switching networks.
7.2. Testing Protocol Implementation According to O. Rafiq (Bull Transac, France), in the distributed architecture for testing protocol implementations, a process which is called the responder system, is required to act as a user of the service provided by the protocol entity under test. The design of such a system poses a number of problems such as the influence of its functions on execution and the value of testing, he told the audience. In a lecture entitled "A Good Approach for Testing Protocol Implementations", the lecturer presented features and contributions of the so-called astride responder which is used by Bull to test the OSI protocol implementations. Finally, Rafiq told that it is simple, is small and provides an efficient testing methodology.
portant issue, they said, in designing computer networks with window control. The previous work on wait systems of multiple logical channel (LC) networks under stationary traffic condition showed that the optimal allocation can be achieved by equalizing the input regulation degree on each LC, they observed. However, the window allocation problem under time-varying traffic condition is unsettled, the speaker (I. Akiyoshi) told the audience. In a lecture entitled "Dynamic Window Control in Wait System of Computer Network", the lecturer presented a window control scheme which changes window sizes according to traffic fluctuation so as to keep the balance of regulation. He gave simulation results and compared with the static window control scheme. Their scheme prevents unfairness among LC's and produces a delay improvement under time-varying traffic condition, the speaker concluded.
7.4. Effect of System Parameter Changes S.G. Chang (Hanyang University, Korea) and D.V. Tcha (Korea Advanced Institute of Science and Technology, Korea), in a lecture entitled "Analyzing the Effect on the Message Delay of Some System Parameter Changes in Communication Networks", discussed the following question: how the implicit function formula of Kuhn-Tucker conditions can be exploited in analyzing the effect on the message delay of the changes in link capacities and traffic requirements in store-and-forward communication networks. Based on some properties of the involved delay function, the speaker (D.W. Tcha) first analyzed the model formulated as single-commodity convex-cost network flows, and next extended to the multicommodity flow model. Finally, he provided a numerical example. The results might be usefully exploited in designing an adaptive quasi-static routing scheme, the lecturer said in conclusion.
8. Satellite Communication II
7.3. Dynamic Window Control
8.1. Hybrid Go-Back-N ARQ
I. Akiyoshi (Kobe University of Commerce, Japan), H. Suzuki, H. Nakanishi, H. Sanada and Y. Tezuka (Osaka University, Japan) dealt with the window allocation problem, which is an im-
In a lecture entitled "Hybrid Go-Back-N ARQ with Extended Code Blocks for Error Control of Satellite Communications", P.S. Yu and D.T. Tang (IBM Watson Research Center, U.S.A.) presented
Conference Reports a new hybrid Go-Back-N ARQ scheme with extended code block. The lecturer (P.S. Yu) explained that their scheme may be regarded as a generalization of previous hybrid Go-Back-N ARQ schemes with half-rate codes proposed by Lin and Yu. In their generalization, a complete code block consists of a data block and an extended code block of the same length, the speaker said. He observed that both were augmented by parity bits for error detection. Normally, "full length" data blocks are transmitted, and, during error recovery, the transmitter will then transmit the extended code block, he told the audience. He said that the proposed scheme achieves high throughput efficiency over a wide range of channel error rate. The speaker further explained that it is particularly attractive for communication systems where the round trip propagation delay is large and the bit rate is high. Finally, he presented the derivation of a lower bound on throughput efficiency.
8.2. New Access Scheme for Mixed Traffic In a lecturer entitled " F O D A - T D M A : A New Satellite Access Scheme For Mixed Traffic. Presentation and First Simulation Results", R. Beltrame, A.B. Bonito, N. Celandroni and E. Ferro (CNUSE Institute, National Research Council, Italy) presented a Fifo Order based Demand Assignment (FODA) access scheme which is designed to handle packetized data and voice traffic in a multiple access satellite broadcast channel of Mbits band. The channel is shared by as many as 64 simultaneously active stations in a range of 255 addressable stations, the lecturer (N. Celandroni) told the audience. A sophisticated traffic environment was assumed, including different types of service requirements and an arbitrary load distribution among the stations. They also presented the results of the 2 M b i t / s simulation tests carried out in an already existing hardware environment.
8. 3. Bulk Data Transfer Protocol F. Ivanova and I. Valet (INRIA, France) first explained the N A D I R project, which is intended to conduct an experimental research program to study and experiment with computer communication through satellite systems such as TELECOM1 in France. Next they discussed the design and implementation of specific protocols which are
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adapted to satellite transmission characteristics. These protocols have been defined to optimize the transfer of data with respect to the main characteristics of satellite links (i.e., high throughput, long propagation delay and specific bit error rate), they told the audience. In a lecture entitled "Performance Measurements of N A D I R Bulk Data Transfer Protocol on Satellite Links", they studied the implementation and measurements of the point-to-point bulk data transfer by using a satellite simulator called ANIS. They showed its performance for high throughput (900 Kbit/s) and for one, two and three successive satellite hops. According to the speaker (F. Ivanova) the results showed that the N A D I R protocol is able to perform well under various transfer conditions, and its efficiency remains good for high data rate in conjunction with high bit error rate and an increase in the propagation delay. She concluded that their protocol is used as a basis to develop specific applications taking advantage of the satellite link characteristics.
9. Multi-Access Communication
9.1. Finite Population Multiple-Access Channels X.M. Chang (Nankai University, China) and F.K. Hwang ( A T & T Bell Laboratories, U.S.A.) considered the k-channel version of the Capetanakis model for multiple-access protocols with finite population. They assumed in their study that n users share a channel which is able to successfully transmit up to k - 1 messages simultaneously. It was also assumed that, if k or more users transmit simultaneously, then a collision occurs and those users have to retransmit. In a lecture entitled " T h e Minimax Number of Calls for Finite Population Multiple-Access Channels", they studied the case where d of n users are active, i.e., have a message to send, and the rest inactive. A protocol is a sequence of calls that schedules the transmissions of all active users where a call requests the active users (there may be none) in a specified subset of users to transmit, the speaker (F.K. Hwang) explained. He presented the evaluation of an algorithm by the minimax numbers of required calls for small d and large d. Finally, the speaker claimed that their results apply to the case when at most d active
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users exist, and that many of their results carry over to the Hayes-Gudjohnsen model.
9.2. Thrashing Phenomena In a lecture entitled "An Analytical Method of Thrashing Phenomena of Retransmit Model in Computer Networks", Y. Onozato, K. Katayama (University of Electro-Communications, Japan) and S. Noguchi (Tohoku University, Japan) investigated the thrashing phenomena of retransmit model, which has been discovered in computer networks. The speaker (Y. Onozato) introduced a queueing model of retransmit model and analyzed it in line with the catastrophe theory. He explained that the global behavior of the model is determined by the fold catastrophe. Finally, he demonstrated the usefulness of their analysis by giving several numerical examples and simulation results.
9.3. Recurrence Equation Conflict resolution algorithms ( C R A ) for broadcast communications have become more and more popular since the work of Capetanakis as well as Tsybakov and Mikhailov (CTM algorithm), W. Szpankowski (Purdue University, U.S.A.) observed. In a lecture entitled "Analysis of a Recurrence Equation Arising in Stack-Type Algorithms for Collision-Detecting Channels", he considered a class of CTM algorithms for which a common recurrence equation for the expected length of the conflict resolution interval (CRI) may be found. Szpankowski presented a closed form expression through an analysis of the equation, and then gave an asymptotic approximation. Finally, he applied his approximation to compute maximum throughput of the analyzed CRA algorithms.
processing time spent clearing the unsuccessful attempts, an often ignored factor, he said. Sze further explained that his model also includes subsequent reattempts by failed attempts. He presented the derivation of an approximation, necessary and sufficient conditions for its solution, and conditions for uniqueness in a special case. Finally, Sze provided an example which shows the decrease in the throughput near the system capacity, and how analyzing the total offered load overestimates the stability of the system.
10.2. Transient Queueing Analysis H.R. van As (IBM Ziirich Research Laboratory, Switzerland) observed that, as a result of dynamic sharing of network resources, packet switching networks are very sensitive to overload situations. Therefore, efficient congestion control mechanisms are of vital importance, he told the audience. In a lecture "Transient Queueing Analysis of a Two-Level Global Congestion Control Mechanism", the lecturer presented a global congestion control mechanism based on either a brusque cut off or a smooth throttling technique, and demonstrated the inherent transient effects. The lecturer showed that the underlying Markovian queueing models take into account the delay until the control becomes effective, which is caused by the geographic distances. He further explained that the transient queueing analysis is based on the numerical evaluation of simultaneous first-order differential equations which describe the system behavior. For this purpose, the fourth-order Runge-Kutta procedure was used, he said. In particular, he analyzed the system reaction to a rectangular overload peak. Finally, he presented the numerical results for the mean system occupancy and the loss probabilities as a function of time, which indicated the explanatory value of transient queueing analysis.
10. Queueing System II 10.3. Fork-Join Queue 10.1. Overload Analysis According to D.Y. Sze (Bell Communications Research Inc., U.S.A.), for a stochastic system near capacity, some attempts can fail due to system timeouts or customer impatience. In a lecture entitled "A Queueing Model for Overload Analysis", Sze built a queueing model that includes
In a lecture entitled "Simple Computable Bounds and Approximations for the Fork-Join Queue", F. Baccelli (INRIA, France), A.M. Makowski (University of Maryland, U.S.A.) and A. Shwartz (Technion, Israel) considered a queueing system, known as the Fork-Join queue, with a basic performance measure defined as the delay
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between the Fork and Join dates. The speaker (P. Nain on behalf of the authors) explained that simple bounds are derived for some of the statistics of this quantity under standard renewal assumptions. These bounds are obtained, in both transient and steady-state regimes, by stochastically comparing the original system to two other queueing systems with a structure simpler than the original system, yet with identical stability characteristics, the speaker told the audience. He further explained that, in steady-state, the computation reduces to standard G I / G I / 1 calculations and the obtained approximations thus constitute a first sizing-up of system performance. Finally, the binding methodology shown in this presentation is of independent interest to study various other queueing system, the speaker observed.
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carried out by the interface connecting the computers to the common bus. Usually, in more conventional systems, this kind of interface merely controls communication aspects such as modulation, demodulation, bus access, data formatting, etc., she observed. She further explained that her idea is to assign to the interface also the responsibility for the control of information transfer between tasks situated in different computers. Besides increasing the overall execution speed, the solution frees each computer from most of the intertask communication burden, she said. Additional, the task synchronization becomes transparent to the programmer, causing substantial savings in specific design efforts, the speaker said in conclusion.
11.3. A Comparison of Distributed and Centralized Systems 11. Distributed Systems
11.1. Circulating Queue Algorithm In a lecture entitled "Performance Analysis of a Circulating Queue Algorithm for Distributed Database Updating", C. Nogues (Empesa Nacional Hidroelectica del Ribagoszana, Spain) and R. Puigjaner (Universitad Politecnica de Barcelona, Spain) presented a new method for multiple-copy file updating. The speaker (C. Nogues) explained that the transaction queue circulates through a virtual ring linking all sites. The queue is updated at each site and copied into the execution site queue, she said. Finally, the speaker presented a performance study by analytical and simulation models.
11.2. Intertask Communication F. Rosemberg (Weizmann Institute of Science, Israel) presented a novel design for the implementation of the Ada intertask communication mechanism in common bus systems. In a lecture entitled "An Ada Oriented Protocol for Intertask Communication in Real Time Common Bus Systems", she first explained a hierarchical layered partition, appropriate to the systems. Then, she introduced her design decisions concerning the bus access mechanism. Finally, she proposed an Ada-oriented protocol, servicing the intertask communication. She explained that most of its execution is
In a lecture entitled "A Comparison of Distributed and Centralized Processing Systems", F.A. Tobagi and H. Kanakia (Stanford University, U.S.A.) discussed the relative performance of distributed processing and centralized processing of an algorithm. The speaker (H. Kanakia) explained that the algorithm is abstracted as consisting of a number of tasks with precedence constraints among them. The comparison was based on the average execution time of a request to execute a given algorithm. The main assumption about the processing system used is that the processing capacity of a centralized system is the sum of capacities of processors in a distributed system, the speaker told the audience. He presented a range of parameters of algorithms for which a centralized system outperforms a distributed system, both executing the same algorithm. Finally, the lecturer remarked that it is not true for all algorithms by giving examples.
11.4. Formal Specification by SDL W. Fischer, K.P. Sauer and W. Denzel (University of Stuttgart, F.R.G.) observed that, for some basic communications mechanisms within distributed systems, analytical approaches are available for calculating throughput, transfer times, waiting times and some other quantities, but for a large class of problems such methods do not exist or are not applicable due to the systems' complexity. The
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lecturer (W. Fischer) told the audience that system simulation is a common way to overcome this problem at least to a certain degree. In a lecture entitled "A Simulation Technique for Distributed Systems Based on a Formal Specification by SDL", the speaker described the simulation technique, which works with a formal description of the system of processes to be simulated. He further explained that their approach uses directly a formal specification by the "Specification and Description Language" (SDL) recommended by CCITT.
The Proceedings of this conference have been published by Elsevier Science Publishers (North-Holland) under the title Computer Networking and Performance Analysis and edited by T. Hasegawa, H. Takagi and Y. Takahashi. 1986. xii + 514 pages. ISBN: 0-444-87960-9. US$63.00 / Df1.170.00.
H. Takagi M. Murata IBM Japan Science Institute
Ninth Data Communications Symposium The Ninth Data Communications Symposium was held at Whistler Mountain, British Columbia, Canada from September 10-13, 1985. The General Chairman of the symposium was William P. Lidinsky; the sponsors were the ACM/SIGCOMM, the IEEE Communications Society and the IEEE Computer Society. The symposium continued its tradition of 18 years ago as a meeting of practitioners in the field of data communications. In an attempt to solicit participants from industry working on the frontier of data communications, seven panels were presented: space satellite earth stations, DARPA interdomain addressing, packet switching network operational experience, message handling and directory services, remote procedure calls versus problem oriented protocols, ISDN, and end user applications. The lectures reflected a mix of theory and practice. The seven sessions covered transport protocol case studies, theoretical foundations of protocols, communication system performance, packet switching network management, prototype networks, naming and addressing and routing, and application level issues. The technical program was put together by Bart W. Stuck, the Program Chairman. Two keynote addresses were presented: Ed Sussenguth, a lead architect for IBM's Systems Net-
work Architecture (SNA), examined trends in data communications. Shen Lin, a key designer for AT&T Electronic Tandem Network (ETN) private line networks, surveyed network design methodologies. Two tutorials were also presented: Peggy Karp covered message handling protocols (e.g., X.400), and Stuart Wecker examined voice/data PBXs. We feature below a description of the technical presentations, as well as brief information on the panel discussions.
Session 1: Transport Protocol Case Studies Development of a TCP / IP R.K. Brandriff, CA. Lynch and M.H. Needleman (University of California, Berkeley, U.S.A.) described the design and implementation decisions that have been made in developing software to support the DARPA TCP/IP protocols for the IBM OS/370 environment at the University of California Division of Library Automation. They explained that the implementation is designed to support over 100 concurrent TCP connections, all of which are managed by a single program, which acts as a specialized sub-operating system. In their presentation, the lecturers also discussed problems