Segmentation and reassembly

Segmentation and reassembly (SAR) is the process used to fragment and reassemble variable length packets into fixed length cells so as to allow them to be transported across Asynchronous Transfer Mode (ATM) networks or other cell based infrastructures.

Since ATM's payload is only 48 bytes, nearly every packet from any other protocol has to be processed in this way.

The process is conceptually simple: an incoming packet from another protocol to be transmitted across the ATM network 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 packet.

There are several different schemes, referred to as ATM adaptation layers (AALs).