Segmentation and Reassembly refers to the process used to divide and reformat data so as to allow it to be transported as the 48 byte payload in multiple ATM cells, followed by its reconstitution at the destination of the ATM Permanent virtual circuit through which it has been carried.

The process is analogous to the fragmentation of IP packets on reaching an interface with a Maximum Transmit Unit (MTU) size less than the packet size and the subsequent reassembly of the original packet once the fragments have reached the original packet's destination.

The process is conceptually simple: incoming data to be transmitted is chopped up into segments that fit into 48 byte chunks carried as ATM cell payloads. At the far end, these chunks are fitted back together to reconstitute the original data.

The details of the actual process vary according to the type of data being handled. There are several different schemes, referred to as ATM Adaptation Layers (AAL). The schemes are:

• AAL0 - Raw cells with no special format
• AAL1
• AAL2
• AAL3/4
• AAL5 - Used for most data traffic