. ATM is described in more detail in Chapter 41, and SDH in Chapter Tanebaum, A.S. (1989) Computer Networks, Prentice Hall. Asynchronous Transfer Mode Mike Hillyard BSc (Eng) CEng MIEE BNR Europe Ltd Contents Overview and content 41/3 History 41/3 ATM: Basic principles 41/3 Standards for ATM 41/8 41.5 Quality of service performance requirements 41/18 41.6 References 41/20 This page intentionally left blank Overview and content 41/3 41.1 Overview and content 'ATD' (asynchronous time division), arguably lies somewhere between STDM and packet switching and is intended for the Current communications networks fall into discrete types depend- transport of data, video and voice services. (Figure 41.1) Versions ant on the types of traffic being handled. Voice networks have been of the technique emerged in Europe in the 1980s in which a bit- based on the use of circuit switching which offers guaranteed path streám was sub-divided into short, fixed-length slots (32 bytes availability and uniform delay during a connection. Network con- maximum) with each slot having an address field attached to it. The gestion results in the blocking of calls rather than delay. Data term 'cell' was used to describe the slot plus the address field and services often possess a very bursty traffic pattern and packet transmission links would carry a contiguous stream of fixed length networks, with their flexible mechanism for interleaving traffic cells. from different sources, offer an efficient solution. Variable delays, The work on ATD was a major influence when CCITT Study however, are introduced into the path as a result of queueing and Group XVIII commenced the definition of the target transfer mech- these, though acceptable for data, would not be tolerable for voice anism for B-ISDN in 1988. This transfer mechanism was to be and other constant bit-rate (CBR) services. standardised for use at the User/Network interface for B-ISDN and The increasing use of optical transmission and the general availa- to be used as the basis for switching an