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 Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by American National Standards Institute (ANSI) and ITU-T (formerly CCITT) for digital transmission of multiple types of traffic. ATM was developed to meet the needs of ...

(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. Thus, it is an essential process for any ATM node. It is usually handled by a dedicated chip, called the SAR. 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. The process is analogous to the fragmentation of IP packets on reaching an interface with a

maximum transmission unit In computer networking, the maximum transmission unit (MTU) is the size of the largest protocol data unit (PDU) that can be communicated in a single network layer transaction. The MTU relates to, but is not identical to the maximum frame size tha ...

(MTU) less than the packet size and the subsequent reassembly of the original packet once the fragments have reached the original packet's destination. Since different types of data are encapsulated in different ways, the details of the segmentation 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 – Constant bitrate, circuit emulation ( T1, E1, etc.) * AAL2 – Variable bitrate synchronous traffic, eous traffic, e.g.

Frame Relay Frame Relay is a standardized wide area network (WAN) technology that specifies the physical and data link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated Se ...

transport * AAL5 – Used for most data traffic, such as IP

See also