Asynchronous transfer mode (ATM) is a high-bandwidth switching technology developed by the ITU Telecommunication Standards Sector (ITU-TSS). ATM can be layered on other physical layer technologies, such as Fiber Distributed Data Interface (FDDI) & SONET.

ATM is based on fixed-length, 53-byte cell, where as other technologies employ frames that very in length to accommodate different amounts of data. Because ATM cells are uniform in length, switching mechanisms can operate with a high level of efficiency. This high efficiency results in high data transfer rates. Some ATM systems can operate at an incredible rate of 622Mbps; a typical working speed for an ATM is around 155Mbps. The unit of transmission for ATM is called a cell. All cells are 53 bytes long & consist of a 5-byte header & 48 bytes of data. The 48-byte data size was selected by the standards committee as a compromise to suit both audio & data-transmission needs. Audio information, for instance, must be delivered with little latency (delay) to maintain a smooth flow of sound. Audio engineers therefore preffered a small cell so that cells would be more readily available when needed. For data however, large cell reduce the over head required to deliver a byte of information asynchronous delivery is another distinguishing features of ATM. Asynchronous refers to the characteristic of ATM in which transmission time shorts don’t occur periodically but are granted at irregular intersals. ATM uses a techniques called label multiplexing, which allocates time slots and demand. Traffic that is time-critical such, as voice or video can be given priority over data traffic that can be delayed slightly with no ill effect.

Device communication of ATM network establishing a virtual path which is identified by a virtual circuit can be established, which are in turn associated with virtual circuit identifier (VSIS). The VPI and VCI together make up a three-byte field included in cell header.


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