Segmentation and Reassembly
Encyclopedia
Segmentation and Reassembly refers to 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
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 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.
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:
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
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 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.
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
ATM Adaptation Layers
The use of Asynchronous Transfer Mode technology and services creates the need for an adaptation layer in order to support information transfer protocols, which are not based on ATM...
(AAL). The schemes are:
- AAL0 - Raw cells with no special format
- AAL1 - Constant bitrate, circuit emulation (T1, E1, etc.)
- AAL2 - Variable bitrate synchronous traffic, e.g. voice data
- AAL3/4 - Variable bitrate asynchronous traffic, e.g. Frame Relay transport
- AAL5 - Used for most data traffic, such as IP
See also
- Packet segmentationPacket segmentationIn a data communications networks, packet segmentation is the process of dividing a data packet into smaller units for transmission over the network...
- Packet aggregationPacket aggregationIn a packet-based communications network, packet aggregation is the process of joining multiple packets together into a single transmission unit, in order to reduce the overhead associated with each transmission....