Alternatives for providing highly reliable access to X . 25 networks

Public and private packet-switching networks (PSNs) are being developed or planned throughout the world-for example in Canada (DATAPAC), the U.S. (TELENET), France (TRANSPAC), the U.K. (EPSS), Japan (DDX), Spain, Netherlands and others. 7 Such an explosive development of data communications services is one of the means for attaining the near-perfect availability required from future distributed information processing systems. 9 The reasons for this requirement are the increasing size, complexity and geographic dispersion of information processing systems, and the increasing real-time dependence of users on these critical, distributed systems for the timely and effective operation of their businesses. As a result, there is a sustained incentive towards improving the reliability and availability of PSNsl1 on the one hand and of information processing systems on the other. Both improvements use similar techniques, namely components of high reliability and redundancy of components. However, the reliability of the access path between the information system and the PSN is in some cases the weakest link in the overall reliability of distributed teleprocessing systems. lO Using the same reliability techniques for the access path (namely redundant, reliable physical circuit configurations) is not sufficient as the access path connects two dissimilar logical entities, and the implementation of the access protocol governing their inter-communication has to be considered. This paper is concerned with the reliability of access from synchronous packet-mode Data Terminal Equipments (DTEs) through multiple physical circuits to X.25 packetswitching networks offering virtual circuit services. The techniques discussed in this paper are also generally appli-