ATM Cell Format

An ATM cell or packet as specified by T1S1 sub-committee is 53 bytes.5 bytes comprise the header, and 48 bytes are payload. The header and payload are specified as follows:

   <------------- 5 bytes ---------------->|<---------- 48 bytes --------->|
   | VCI Label | control | header checksum | optional adaptation | payload |
   |  24 bits  | 8 bits  |   8 bits        |   layer 4 bytes     |44 or 48 |

The 48 bytes of payload may optionally contain a 4 byte ATM adaptation layer and 44 bytes of actual data, or all 48 bytes may be data, based on a bit in the control field of the header. This enables fragmentation and reassembly of cells into larger packets at the source and destination respectively. (Since the header definition may still be in flux, it is possible that presense or absence of an adaptation layer information may not be explicitly indicated with a bit in the header, but rather implicitly derived from the VCI label ). The control field may also contain a bit to specify whether this is a flow control cell or an ordinary cell, an advisory bit to indicate whether this cell is dropable in the face of congestion in the network or not, etc.

The ETSI definition of an ATM cell is similar, 53 bytes cell size, 5 byte header, 48 bytes data. However the difference is in number of bits for the VCI field, number of bits in the header checksum, and semantics and number of some of the control bits.