Performance Optimisation of Multiple Access Protocols for Multiservice VSAT Networks.

VSAT networks have been very successful as private networks offering a variety of data communication services. Recent advances in telecommunications technology combined with the relaxation of regulations and the globalisation of computer communications have created a growing demand for VSAT networks offering a much broader range of services, like telephony and multimedia applications. We address some of the technical issues that need to be resolved so that these services can be added to existing VSAT networks, without significant increases in the system's complexity and cost. We have focused on the development of adaptive, dynamic protocols that will result in the most efficient allocation of the space segment, one of the most expensive commodities in the service provision. We have shown that in order to achieve the best possible performance in a multiservice VSAT network, it is important to adopt a multiaccess protocol that suits a particular traffic mix. A proposed protocol, based on Selective Reject ALOHA, performs better than conventional random access protocols when traffic consists of variable length data messages. By making this protocol adaptive to changing traffic loads we can increase the system efficiency. For traffic consisting of voice and data services, it is necessary to use dynamic reservation for voice connections and long file transmission, while shorter data messages can be send using contention protocols. Results show that significant throughput improvements are possible if voice compression techniques are used to produce bit rates below 8 Kbits/s. The penalty paid is some degradation in the speech quality, but usually this is not very significant. The development of cheap, low-bit-rate vocoders, enabling a large number of voice users to share the limited bandwidth, and the use of Voice Activity Detection to take advantage of idle intervals in conversations, could further improve the efficiency of the channel utilisation. There is also a growing need for compatibility of VSATs with the developing terrestrial Integrated Broadband Communication Network (IBCN) and to extend VSAT coverage to areas such as network interconnection and provision of Asynchronous Transfer Mode (ATM)-based services. We highlight some of the key issues and provide some preliminary suggestions of the new role that VSATs could play in the ATM era