What is ATM - Asynchronous Transfer Mode?

ATM divides information in to blocks called cells which are fixed in size. Each cell has 5 bytes header and 48 bytes information field. Information field carries used data and the header contains information relevant to the functioning of ATM. ATM cell switching is performed in dedicated harware that's why ATM is a faster data transfer technology.

ATM requires high bandwith, high speed medium for normal operation such as optical fiber networks. In North America the digital hierarcy for optical networks is SONET (Synchronous Optical Network) and international is SDH. SONET optical hierarcy is divided into blocks of 51.84 Mbps. Each of these blocks roughly corrosponds to the T3 line rate. The standard 51.84 Mbps is called STS-1 (Syncronous Transport Signal Level - 1) when referring to an electrical signal and OC-1 (Optical Carrier Level - 1) when referring to an optical carrier. Within SONET, an OC-n line is exactly n times of OC-1 i.e. 51.84 Mbps OC-1 to 2488.32 Mbps OC-48.

ATM seems similar to packet based networks but there are differences :

- Cells are fixed in length but packets might vary. Fixed length cells allow switching done in hardware whereas variable length packets mean that switching has to be done in software. This makes cell switching faster.

- Cell based networks are usually less efficient than packet based networks. For example, in frame relay network, all payloads are transmitted as a single frame. This makes it more efficient as compared to the fragmentation done by ATM. We can say that, ATM system has an overhead of 10 per cent compared to 1 per cent of frame relay. But despite the handicap in efficiency, ATM is faster than existing packet switched networks such as X.25 and frame relay.

ATM uses a multiplexing scheme to transmit several stream of cells together. There are basically 2 types of multiplexing :

- FDM (Frequency Division Multiplexing) : The bandwith of the common channel is divided between the user channels with each user having complete and exclusive possession of an assigned portion of bandwith.

- TDM (Time Division Multiplexing) : The bandwith if the sahred medium is divided between the users on a temporary basis with each getting the entire bandwith for a given period of time.

ATM like other modern networks, uses TDM.