This chapter describes and explains the main features of a broadband asynchronous transfer mode (ATM) network, from the network point of view. It does not describe in detail the various techniques for switching ATM cells, particularly as many of the switching architectures are proprietary.

Although this book is mainly concerned with the network aspects of ATM, some regard must be paid to the applications that can make use of a broadband communications infrastructure. It cannot be emphasised too strongly that the customer will pay for services only if these services support the required application and are seen to be priced at a level that makes their use worthwhile.

There are several performance measures that are specific for ATM, such as cell transfer delay and delay variation, cell-loss probability and cell-insertion rate. Cell-delay variation and cell loss arise from queuing and buffer overflow, respectively. They occur because of the characteristics and statistical variation of ATM traffic; they are therefore important measures of performance in ATM traffic engineering. ATM-layer phenomena depend on both ATM-layer and physical-layer performance. Physical-layer effects are not a subject of ATM traffic engineering per se. However, because random bit errors can cause cell loss (for example), it is important to be able to distinguish the reasons for cell losses when this phenomenon is measured on a real system. Thus, it is instructive to address the causes and effects of network performance in layers. The physical-layer performance is independent of the performance of the ATM layer; however, the latter is affected by the former. The performance of both layers will affect the performance of the AAL. Aspects of all three layers have an effect on the quality of service experienced by the user. This interdependence of cause and effect between layers is illustrated in Figure 5.1.

Interworking functions may be implemented in the broadband network, in the customer's equipment, in other types of networks, or in some combination of these. The three aspects of interworking that have to be considered are network interworking, service interworking and numbering and addressing.

Network structures form the basis of a routeing plan. Different customer groups can be linked by different types of route, choosing between combinations of hierarchies, mesh or star structures, direct, alternative and transit routes. The average and maximum numbers of transmission links between any pair of customers are important parameters in the trade-off between optimum technical performance and economic efficiency.

This chapter discusses intelligence in the network. Network management is the act of controlling, supervising and maintaining communication networks with the ultimate aim of maximising the profit for the network operator, while providing the best possible services and quality of service for the customer.

This Appendix briefly outlines the scope of current CCITT recommendations dealing with B-ISDN.