d multiplexing within bility of large-scale integrated digital technology, offer the possi- B-ISDN nodes. bility of an integrated approach to handling all types of traffic Asynchronous Transfer Mode (ATM), the name used by SG source, including a range of new services, some, such as video- XVIII to describe the chosen solution, owes much to the early ATD based services, requiring much broader channel capacities. proposals but some features were also adopted from 'Fast Packet It is against the background of this Broadband Integrated Services Switching' (FPS), a technique developed in the US aimed at reduc- Network (B-ISDN) that the technique known as ATM is being ing the end to end delays inherent in X.25 packet switching. FPS defined. still used variable length packets but had much reduced protocol complexity to permit a high speed hardware implementation. ATM employs fixed length cells having 48 information bytes and 41.2 History 5 header bytes which contain address information and other func- tions. Circuit switched techniques such as those used for voice communi- cations have been used in combination with STDM (synchronous time division multiplex) techniques. Here, the bandwidth on a link ATM basic principles is divided into channels consisting of (typically) eight bits. The identity of a particular channel is defined by its relationship to a Asynchronous Transfer Mode is a specific packet oriented mechan- fixed frame reference. In contrast, in packet switching a block of ism for the transfer of digital information based on the use of 'cells' data is associated with a 'header' which both defines its length and which are of constant length, having a payload field and a 'header'. contains an address defining its identity. The cells are transmitted contiguously on a transmission link and its 2 its 3 its 4 its 6 ts n-4 its n-3 ts n-2 its n-1 its n its 1 Fixed frame reference Payload Payload header T trailer Figure 41.1 ATM compared with STDM and packet mode: (a) synchronous time division multiplex; (b) asynchronous