Protocols and performance models for high speed, dual-bus, fiber-optic local area networks

Abstract

At high data transmission rates, the packet transmission time in a local area network (LAN) could become comparable to, or less than the medium propagation delay. Therefore, in high speed LANs, the ratio of the channel propagation delay to the packet transmission time, or the normalized propagation delay, a, could approach or even exceed 1. The performance of existing LAN schemes, for example, LANs employing the CSMA/CD protocol, degrades rapidly as a approaches 1. In a high speed environment therefore, LAN medium access protocols should be capable of yielding satisfactory performance over a wider range of a values. In this thesis, we propose two new LAN medium access protocols, named Z-Net and X-Net, as suitable candidates for operation at high speeds. The network architecture is based on two, unidirectional fiber-optic channels. A distinct feature of the architecture is the use of active taps to minimize the waste of channel bandwidth due to collisions. When collisions occur, one transmission continues to completion, while others are aborted. The proposed medium access protocols behave as random-access schemes at light load. As load increases, the behaviour is similar to that of a controlled-access scheme with implicit token-passing. Because of this hybrid nature of the access protocols, they possess the advantages of zero medium access delay at light load and bounded delay at all loads. The protocols are completely distributed, with all stations executing the same access protocol. Thus, the vulnerability of the network to a single station failure is minimized. Further, for the execution of the protocols, stations do not require a knowledge of other station locations in the network. This reduces maintenance effort, when there are frequent changes in station locations. The performance of the Z-Net and X-Net are evaluated using approximate analytic models. These results are then validated against the results obtained from simulation models. The channel utilization values of the analytic and simulation models are in very close agreement. Performance results show that both protocols achieve high channel utilization (with X-Net exhibiting superior performance) even when the packet transmission time is low compared to the channel propagation delay. Therefore, the proposed schemes are suitable for operating at high channel data rates. The bounded delay property makes them suitable for supporting real-time traffic.