Asynchronous Transfer Mode (ATM)
Asynchronous Transfer Mode (ATM) is a cell-switching and multiplexing technology for carrying voice, data, and video over one network. The ATM layer uses short units with a fixed length, called cells; IP packets may have variable lengths. An appropriate ATM Adaptation Layer (AAL) segments higher-layer data and reassembles it at the destination. Different streams can therefore share one cell interface while retaining different traffic profiles.
The term asynchronous describes how cells use a shared link as traffic arrives. Active connections interleave their cells in one stream, and a source sends a cell when it has data, without a fixed slot assigned in advance as in synchronous systems such as . On-demand allocation uses the link efficiently for variable, bursty computer traffic.
Why fixed-length cells matter
Fixed-length cells simplify hardware processing inside a switch. A switching element knows how long each unit occupies the path, so it can schedule successive cells and multiplex many connections without first parsing the length of every packet. The fixed format also makes the position of the next cell predictable in the stream.
ATM and the B-ISDN concept
During the 1980s and 1990s, B-ISDN (Broadband Integrated Services Digital Network) was developed as a concept for a common transport network supporting different services. ATM was selected as its transfer technology, combining cell switching with mechanisms for service-quality parameters. The ITU-T I.321 reference model separated user, control, and management planes, together with the physical layer, the ATM layer, and ATM Adaptation Layers (AAL). The ATM layer remained independent of the physical medium, while the AAL supplied service-dependent functions.
- Standardization: The first specifications were developed by the CCITT (now ) and ANSI.
- ATM Forum: In 1991, equipment manufacturers and service providers formed the ATM Forum to accelerate specifications and prepare interoperability profiles.
- Scope: ATM specifications covered operation in Local Area Networks (LANs), Metropolitan Area Networks (MANs), and Wide Area Networks (WANs).
B-ISDN was not deployed at the intended scale. ATM was nevertheless used for many years in telephone and internet-provider cores, as well as in some mobile-network backhaul systems. The B-ISDN model described functions and reference points, so it did not prescribe one physical device implementation.
The 53-byte ATM cell
The basic ATM transfer unit is a cell with a fixed length of 53 bytes. Higher-layer data is divided into fragments carried in the payloads of successive cells. The header is sent before the 48-byte information field, and the physical layer may insert unassigned cells to match the cell stream to the capacity of the medium.
Cell format and payload-size choice
An ATM cell consists of:
- 5-Byte Header: Contains addressing and control information.
- 48-Byte Payload: Carries the actual user data.
The header format depends on the interface. At the UNI, four header bits form GFC and have local significance, while the routing field has an 8-bit VPI and a 16-bit VCI. At the NNI, GFC is absent and the corresponding four bits extend VPI to 12 bits. VCI remains 16 bits. PTI has 3 bits, CLP has 1 bit, and HEC has 8 bits.
Why 48+5?
The 48-byte payload was a compromise adopted during ATM standardization. Representatives in North America argued for 64 bytes to reduce header overhead for data, while European operators proposed 32 bytes to limit the time needed to fill a cell with voice samples. The selected payload is 48 bytes. The five-byte header carries identifiers and control fields.
Key Header Fields
- VPI/VCI (Virtual Path/Channel Identifier): These fields identify the virtual path and channel to which the cell belongs at the current link. VPI groups channels into one path, while VCI distinguishes channels within that path. ATM is connection-oriented, so a switch uses the VPI/VCI values to select the next port and the translated identifiers for the .
- PT (Payload Type, PTI): Three bits indicate the cell content. For user data they also signal congestion and the end of an AAL5 frame; other values identify management cells.
- CLP (Cell Loss Priority):A single bit that indicates whether the cell can be discarded if the network encounters congestion. A value of '1' means the cell is lower priority and can be dropped first.
- HEC (Header Error Control): The last header byte, calculated from the preceding four bytes. The HEC code can correct a single-bit header error and detect multi-bit errors; a cell with an uncorrectable error may be discarded.