This paper focuses on network delays as they apply to voice traffic. First the nature of the delay problem is discussed and this is followed by a review of enhanced circuit, packet, and hybrid switching techniques: these include fast circuit switching (FCS), virtual circuit switching (VCS), buffered speech interpolation (SI), packetized virtual circuit (PVC), cut-through switching (CTS), composite packets, and various frame management strategies for hybrid switching. In particular, the concept of introducing delay to resolve contention in SI is emphasized, and when applied to both voice talkspurts and data messages, forms a basis for a relatively new approach to network design called transparent message switching (TMS). This approach and its potential performance advantages are reviewed in terms of packet structure, multiplexing scheme, network topology, and network protocols. The paper then deals more specifically with the impact of variable delays on voice traffic. In this regard the importance of generating and preserving appropriate length speech talkspurts in order to mitigate the effects of variable network delay is emphasized. The results indicate that a desirable length of talkspurt "hangover" of about 200 ms will accomplish this without unduly affecting speech activity, and that, under these circumstances, the perceptable threshold of variable talkspurt delay can be as high as about 200 ms average. As such, the results provide a useful guideline for integrated services system designers. Finally, suggestions are made for further studies on performance analysis and subjective evaluation of advanced integrated services systems.

Delay Related Issues in Integrated Voice and Data Networks

An adaptive hierarchical routing protocol based on the extension of the new Arpanet scheme is proposed and its simulated performance is presented. The protocol can adapt to rapidly changing environments and works for arbitrarily large networks. A number of existing schemes as well as the proposed scheme are simulated under many different environments and clustering structures. The proposed protocol is found to be superior to the other protocols tested in many different types of network traffic and topological configurations. The results indicate that intercluster links must be reliable, because (1) the failure of these links can significantly degrade the routing performance, even though the protocol does not degrade as badly as the existing scheme and (2) hierarchical routing protocols usually prefer small clusters, which means that there will be many intercluster links. The tradeoff between two conflicting performance criteria, response speed and communication overhead, is shown. >

An adaptive hierarchical routing protocol

This paper describes the data communications activity at the National Physical Laboratory (NPL) during the period 1965-1975. The key feature of the NPL work was the development of the principle of packet switching, which was first proposed in a data communications context by D. W. Davies of the NPL in 1965. The report focuses on the construction of the NPL Data Communications Network, which first became operational in 1970. This network served both as a model for a possible U.K. national network and as a practical local area network (LAN) for the NPL site. The report describes the impact of the NPL work on other early networks, such as ARPANET and the British Experimental Packet-Switched Service (EPSS), and on data communications in general.

Data Communications at the National Physical Laboratory (1965-1975)

This is an expanded version of an earlier report, adding, new results. The first part introduces a simple analytic model showing that dynamic routing may increase network throughput or lower delay when trunks are moderately loaded, but will reduce throughput or increase delay at heavy trunk loads. The second part of the paper presents simulation results characterizing: 1) the operation of a network "protected" by end-to-end flow control, 2) the operation of an "unprotected" network, 3) the transition between these two modes, 4) the effect of changing the update interval for the routing algorithms, 5) the effect of slow local loops, and 6) the effect of increasing the number of alternate paths available for dynamic routing. Overall, the results show that dynamic routing improves network performance only over a small parameter range, and should dynamic routing be used at all, it must be implemented with great care.

Dynamic Routing and Flow Control

Data network simulation Experiments at the national physical laboratory 1968–1976

A design study of a hierarchically connected packet-switching network using simulation techniques

A Simulation Study of Routing and Flow Control Problems in a Hierarchically Connected Packet Switching Network.

The paper examines in general terms the requirements of a congestion avoidance system applied to packet switched networks. Two specific systems are then discussed, namely the system based on allocation of packet buffers in source nodes and, as an alternative, the isarithmic flow control scheme. It is seen that both systems are effective in single-level networks. The reasons for requiring a hierarchical topology are given, followed by an account of a simulation study of flow control, routing and congestion avoidance in such a context. It is shown that the behaviour of such a network in these respects is very different from that of a single—level network.

http://link.springer.com/content/pdf/10.1007%2F978-94-009-9062-3_46.pdf

Gregory R. A. Gomberg

Papers

A design study of a hierarchically connected packet-switching network using simulation techniques

A Simulation Study of Routing and Flow Control Problems in a Hierarchically Connected Packet Switching Network.

Congestion avoidance in hierarchical packet switched networks