Two simple models of queueing on an N \times N space-division packet switch are examined. The switch operates synchronously with fixed-length packets; during each time slot, packets may arrive on any inputs addressed to any outputs. Because packet arrivals to the switch are unscheduled, more than one packet may arrive for the same output during the same time slot, making queueing unavoidable. Mean queue lengths are always greater for queueing on inputs than for queueing on outputs, and the output queues saturate only as the utilization approaches unity. Input queues, on the other hand, saturate at a utilization that depends on N , but is approximately (2 -\sqrt{2}) = 0.586 when N is large. If output trunk utilization is the primary consideration, it is possible to slightly increase utilization of the output trunks-upto (1 - e^{-1}) = 0.632 as N \rightarrow \infty -by dropping interfering packets at the end of each time slot, rather than storing them in the input queues. This improvement is possible, however, only when the utilization of the input trunks exceeds a second critical threshold-approximately ln (1 +\sqrt{2}) = 0.881 for large N .

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Input Versus Output Queueing on a Space-Division Packet Switch

Most artificial intelligence research investigates intelligent behavior for a single agent--solving problems heuristically, understanding natural language, and so on. Distributed Artificial Intelligence (DAI) is concerned with coordinated intelligent behavior: intelligent agents coordinating their knowledge, skills, and plans to act or solve problems, working toward a single goal, or toward separate, individual goals that interact. DAI provides intellectual insights about organization, interaction, and problem solving among intelligent agents. This comprehensive collection of articles shows the breadth and depth of DAI research. The selected information is relevant to emerging DAI technologies as well as to practical problems in artificial intelligence, distributed computing systems, and human-computer interaction. 

"Readings in Distributed Artificial Intelligence" proposes a framework for understanding the problems and possibilities of DAI. It divides the study into three realms: the natural systems approach (emulating strategies and representations people use to coordinate their activities), the engineering/science perspective (building automated, coordinated problem solvers for specific applications), and a third, hybrid approach that is useful in analyzing and developing mixed collections of machines and human agents working together. 

The editors introduce the volume with an important survey of the motivations, research, and results of work in DAI. This historical and conceptual overview combines with chapter introductions to guide the reader through this fascinating field. A unique and extensive bibliography is also provided.

Readings in Distributed Artificial Intelligence

hat a difference 25 years makes! When this article was written the telecommunications industry was still a heavily regulated, non-competitive but highly integrated place; the Internet was the private preserve of congnoscenti at a handful of research universities; integrated circuits with a feature size of 2 micron were state-of-the-art; and the room-sized minicomputers of the day could send and receive maybe a thousand packets per second, on a good day. This article articulated an observation that was gaining recognition throughout the early 1980s: first, that the fragmentation of the communications infrastructure into several distinct application-specific networks was wasteful and unnecessary; and second, that by combining concepts from packet switching with the implementation practices of circuit switching (highly parallel, hardware switching systems), one could develop systems with unprecedented flexibility and the performance levels and reliability needed to support large-scale, ubiquitous deployment. By the mid to late 1980s, these ideas were being pursued most vigorously in the development of ATM standards and technology, which was widely expected to play a leading role in the next generation of the public telecommunications infrastructure. The article outlined some of the key elements of such an integrated network technology, capable of supporting voice, data, and video communication and having the flexibility to accommodate new services as the need arose. Integrated switching architectures, generalized connection signaling protocols, quality-of-service, multicast switching, and routing were all identified as key challenges to be addressed. Subsequent years have seen tremendous progress on all these fronts in both the ATM context and in the now all-important Internet context. The changes in packet switching technology over the last two decades are particularly striking. While advanced switching architectures were first developed in the ATM context, these concepts have been applied with striking success in Ethernet switches and IP routers in recent years. Emerging IP router products with aggregate capacities exceeding a terabit per second are becoming available now, and gigabit Ethernet switches are available for just hundreds of dollars per port. Progress on other fronts has been more disappointing. While practical and effective solutions for connection signaling, quality-of-service, congestion control, and multicast have been developed and demonstrated, there has been little progress toward widespread commercial deployment. We still operate separate networks for voice, data, and video, and while the Internet promises to play a growing role in voice and video communication, it cannot achieve that promise without major technical improvements. Fortunately, the potential value of such improvements is becoming more and more clear, as the Internet’s role in our evolving information society continues to develop. While challenges remain, there seems little doubt that the necessary changes can and will be made, and that the full realization of the vision outlined in this article 25 years ago is now within our grasp. Jonathan S. Turner

New directions in communications (or which way to the information age

A general class of hypercube structures is presented in this paper for interconnecting a network of microcomputers in parallel and distributed environments. The interconnection is based on a mixed radix number system and the technique results in a variety of hypercube structures for a given number of processors N, depending on the desired diameter of the network. A cost optimal realization is obtained through a process of discrete optimization. The performance of such a structure is compared to that of other existing hypercube structures such as Boolean n-cube and nearest neighbor mesh computers.

Generalized Hypercube and Hyperbus Structures for a Computer Network

The authors study the performance of four different approaches for providing the queuing necessary to smooth fluctuations in packet arrivals to a high-performance packet switch. They are (1) input queuing, where a separate buffer is provided at each input to the switch; (2) input smoothing, where a frame of b packets is stored at each of the input line to the switch and simultaneously launched into a switch fabric of size Nb*Nb; (3) output queuing, where packets are queued in a separate first-in first-out (FIFO) buffer located at each output of the switch; and (4) completely shared buffering, where all queuing is done at the outputs and all buffers are completely shared among all the output lines. Input queues saturate at an offered load that depends on the service policy and the number of inputs N, but is approximately 0.586 with FIFO buffers when N is large. Output queuing and completely shared buffering both achieve the optimal throughput-delay performance for any packet switch. However, compared to output queuing, completely shared buffering requires less buffer memory at the expense of an increase in switch fabric size. >

Queueing in high-performance packet switching

Concurrent processing depends on interconnection networks for communication among processors and memory modules. Various network topologies and switching strategies are covered here.

Tse-yun Feng

Papers

A Survey of Interconnection Networks