 transfer mode; (c) packet mode 41/4 Asynchronous Transfer Mode are not identified by their position in relation to a fixed time 8-bit CRC field for header error control (HEC). This field also reference but by means of address information in the header defin- supports the mechanism for cell delineation, i.e. the identifica- ing a 'virtual channel'. The technique is asynchronous in the sense tion of the start of each cell within a serial bit stream. that the cells carrying a particular address (i.e. within a particular virtual channel) may appear at irregular intervals within the cell- The payload may be user information (voice, data, images, etc in stream. The technique is connection oriented in that a 'virtual digital form), signalling, or O&M messages (operations & mainten- circuit' is established at call set-up time and this will associate the ance). virtual channels used on a series of network links to form the end to The payload is transferred transparently from end point to end end connection. point across the network. ATM offers a flexible transfer capability common to a broad range of services with widely varying traffic patterns and can be employed 41.3.2 on various transmission media operating at widely varying rates. ATM multiplexing (Figure 41.3) ATM adaptation functions are provided to accommodate the data formats and operating characteristics of specific services. 41.3.2.1 Multiplexing mechanism Information from a particular user is assembled into cells as it becomes available. Cells from a number of such sources will be 41.3.1 The ATM cell (Figure 41.2) placed in a queue in their order of appearance and the cells on the queue output will be placed in order in contiguous slots on the The cell consists of a header (label of 5 bytes) and a payload transmission link. If the aggregate information rate from the set of (information field of 48 bytes). sources being multiplexed exceeds the link capacity at a particular The header includes: time, then the queue fill will become greater, increasing the delay