Asynchronous Transfer Mode

ATM (Asynchronous Transfer Mode) is connection oriented network. It was designed in the early 1990s.ATM was more successful than OSI and it is now widely used within telephone system for moving IP Packets.

Since ATM networks are connection oriented, sending data requires first sending a packet to set up the connection. Connections are often called virtual circuits, in analogy with the physical circuits used within telephone system. Most ATM networks also support permanent virtual circuits, which are permanent connections between two hosts. Each connection, temporary or permanent has a unique connection identifier. Once connection has been established, either side can begin transmit data.

Cells: ATM is to transmit all information in small, fixed size packets known as cells. The cells are 53 bytes long of which 5 bytes are header and 48 bytes are payload. Part of the header is the connection identifier so the sending and receiving host and all the intermediate routers can tell which cells belong to which connection.

This information allows each router to know how to route each incoming cell.

The most common speeds for ATM networks are 155 Mbps and 622 Mbps.

ATM has its own reference model, different from the OSI model and TCP/IP model. It consists of three layers: the physical, ATM, and ATM adaptation layers.

The physical layer deals with the physical medium such as voltages, bit timing and various other issues. ATM has been designed such a way that transmission medium is independent

ATM layer deals with cells and cell transport. It defines the layout of a cell and tells what the header fields mean. A layer above the ATM layer has been defined to allow users to send packets larger than a cell. ATM interface segments these packets,

Transmit cells individually and reassembles at the other end. This layer is the AAL (ATM Adaptation Layer).

ATM Model is three dimensional model. The USER Plane deals with data transport, flow control, error connection and other user functions. The CONTROL Plane is concerned with connection management. The physical layer and AAL layer are each divided into two sublayers one at the bottom that does the work and a convergence sublayer on top that provides the proper interface to the layer above it. The PMD (Physical Medium Dependent) is a sublayer which interfaces to the actual cable. It moves the bits on and off handles the bit timing. The other sublayer of the physical layer is the TC (Transmission Convergence) sublayer.When cells are transmitted the TC layer sends them as a string of bits to the PMD Layer. AAL Layer

Is split into a SAR (Segmentation And Reassembly) sublayer and a CS (Convergence Sublayer)