Design and perfomance evaluation of bus access schemes for salan-an experimental local area network

Abstract

Local Area Networks (LANs) spanning a limited geographic area have emerged of late as a field of wide applicability. One of the prime factors governing the design of a LAN is the Medium Access Control (MAC) scheme which arbitrates the access to the transmission medium by the connected data equipment. This work is concerned primarily with the design and performance evaluation of two MAC schemes developed for the experimental network SALAN (School of Automation Local Area Network). These schemes have been designed to incorporate the feature of the multiple priority function. Such a feature is necessary in a LAN that provides service to diverse applications with different time constraints. Voice-data integration is representative of such an application. The first MAC scheme implements multiple priority function by the method of staggered round initiations. However, the prioritization provided by this scheme becomes less effective under heavy network loading conditions. The second one is a more sophisticated version of the first one in that it overcomes the drawbacks of the first one and attains the perfection of the ideal priority discipline. However, the latter scheme entails additional network overhead and increased complexity of implementation. The performance of the proposed MAC schemes has been studied by simulation on a DEC 1090 system using SIMULA. Data traffic environments as well as mixed voice-data environments have been considered in the simulation study. In order to obtain hands-on experience in local area networking, a prototype of SALAN has been implemented using microprocessor-based communication nodes. To make the initial hardware design simple, SALAN at present does not feature the multiple priority function. SALAN has been used to interconnect the Intel Series III microcomputer development system and the Texas Instruments model 990/4 microcomputer system with a view to realising distributed file transfer between these systems. Further, a packet voice communication system also has been implemented on SALAN. As part of the future expansion of SALAN, the incorporation of priority scheduling is envisaged by a suitable modification of the communication nodes. In view of this fact, a methodology for the implementation of the second prioritised MAC scheme proposed is discussed in this thesis.