Real Time Communications in Local Area Networks

Multiple services integration is a basic concept on which the ISDN is based. Merging diierent services on the same network is going to be a problem as they impose diierent throughput and delay requirements. The support of these real time applications on LANs present considerable diiculties due to the distributed multiaccess nature of these networks. Developing real time applications on LANs requires both eecient network protocols and a real time operating system on the nodes. Most protocols studies have been focused on the access protocols, e.g. comparing the respective merits of contention and demand assignment. In this paper we propose an optimization of the higher levels protocols: a stream communication mode based on network resources reservation is required to avoid queuing delays. A new class of the ISO Transport Protocol (TP5) adapted to the real time transmission requirement is then presented. Our analysis, and the experiments on an Ethernet network, show that the local scheduling in the nodes, when running the network protocols, generates delays of an order of magnitude higher than the network access's. This derives from the limited average load on such LANs. An implementation of a preemption priority mechanism in the UNIX scheduler, ensuring computing resources reservation is then described. The experiments connrm the improvement of performances. Next task is to study and develop eecient protocols for real time applications, to solve the synchronization and the data encoding problem. 0 Real time communications in local area networks 1 Real time applications During the current decade, the extremely rapid evolution of digital transmission technologies and hence the increase in transmission rates up to hundreds of Mb/sec made possible the building of digital networks based either on circuit or packet switching techniques. An integration of telephony, video, data and many other communication services is expected in ISDN's which will provide end-to-end digital connectivity to support such wide range of services. However, contrarily to classical applications such as data transfer, the applications based on real time transmission of voice or video impose time constraints on transmission delays. In fact, the existing protocols and particularly the OSI protocols were designed to come up to the need of standard applications. The development of high speed and speciically real time applications demands the optimization of these protocols at both the transport and the application levels. A brief comparaison of multiaccess protocols is sketched in the next Section. In Section 3 we analyse the …