On the Nonconcavity of Throughput in Certain Closed Queueing Networks

by K. E. Stecke

Analytic queueing network models are be- ing used to analyze various optimization pro- blems such as server allocation, design and capacity issues, optimal routing, and workload allocation. The mathematical pro- perties of the relevant performance measu- res, such as throughput, are important for op- timization purposes and for insight into system performance. We show that for clos- ed queueing networks of m arbitrarily con- nected single server queues with in custo- mers, throughput, as a function of a scaled, constrained workload, is not concave. In fact, the function appears to be strictly quasi- con- cave. There is a constraint on the total workload that must be allocated among the servers in the network. However, for closed networks of two single server queues, we pro- ve that our scaled throughput is concave when there are two customers in the network and strictly quasi-concave when there are more than two customers. The mathematical properties of both the scaled throughput and reciprocal throughput are demonstrated graphically for closed networks of two and three single server queues. To appear in: Per- formance Evaluation.

Marketing of Databases Produced in the United Kingdom

by L. Bibby and H. East

The paper considers the growth in the number of UK-produced online databases, which has been in parallel with the expansion of European host services, and discusses the approaches of commercial and nonprofit organisations to marketing. The findings of a survey of marketing practices and attitudes amongst UK producers of non- numerical databases are described. To appear in: Jour- nal of Information Science.

Report on the INS Experiment

by T. Takahashi

NTT has been undertaking thorough studies aimed at the establishment of a new comprehensive telecommunications network for the 21st century, called the Information Network System (INS). The INS is expected

to play an important role in the infrastructure of the advanced information society of the future. An INS Model System plan has been initiated as the first big step toward the realization of the INS. Its construction began in September 1982 in both the center and suburbs of Tokyo, and it become operational on September 28, 1984. Through the model, studies are being made on the technical feasibility of INS systems and equipment and on a variety of service requirements. This paper describes the INS Model System, in- cluding such aspects as system configura- tion, services provided, in'formation pro- viders, monitors and actual operation status. To appear in: Computer Networks and ISDN Systems.

On Program Development Effort and Productivity

by I. Vessey

Many claims are made in the literature con- cerning large productivity increases as a result of the introduction of various new programm- ing tools and techniques. Seldom do the researchers substantiate these claims, however. Further, many of the reported studies have been unsuccessful because much of the existing research is methodologically flawed and poorly ground- ed in theory. This paper investigates the methodological problems of previous studies and reports the results of a field study con- ducted to assess the feasibility of conducting controlled studies of the programming pro- cess. A field study of COBOL programs from three commercial organizations investigated the effects of programming style (i.e., struc- tured programming) and programmer skill on (i) the effort required to develop programs and (ii) programming productivity; program size was used as a control variable. The results of the study support the underlying concept: that use of disciplined approaches and well- defined variables leads to more readily inter- pretable and more conclusive results. To ap- pear in: Information Et Management.

Two Processor Scheduling with Limited Preemption

by E. L. Lloyd

We introduce deterministic task system scheduling with limited preemption--that is, a preempted task can resume execution only on the processor where it originally executed. Two classes of limited preemptive schedules are introduced, corresponding to whether or not unforced idle time is allowed in the

schedule. Our main result is to show that, on two processors, these two classes are equivalent with respect to optimal schedule lengths. We use this result to establish a hierarchy of optimal schedule lengths. To ap- pear in: Performance Evaluation.

Regenerative Stochastic Petri Nets

by P. J. Haas and G. S. Shedler

The stochastic Petri net(S P N) model is well suited to formal representation of concurren- cy, synchronization, and communication. We define the marking process of an SPN in terms of a general state space Markov chain which describes the net at successive transi- tion firing times. Using structural properties of the SPN and recurrence theory for generalized semi-Markov processes, we establish conditions which ensure that the marking process of an SPN is a regenerative process and that the expected time between regeneration points is finite. This leads to a theory of regenerative simulation in the SPN setting. To appear in: Performance Evalua- tion.

The Design of a Small Packet-Switching Node

by A. Patel and M. Purser

This paper discusses the design of a packet-switching node which handles virtual circuits. Particular reference is made to the switching node's capacity and loading. It is shown that the switch and its associated communications links can be dimensioned to handle offered traffic, using techniques bor- rowed from the telephone network. To ap- pear in: Computer Networks and ISDN Systems.

Bounds for the Optimal Decentralized Access Pro- tocol in a Local Area Net- work

by Z. Rosberg

Here, we consider a local area network under the set of decentralized access pro- tocols. Lower and upper bounds are obtained for the optimal long-run average waiting cost. These bounds are shown to be quite tight for a typical network. To appear in: Performance Evaluation.

183