ATM technology

Asynchronous Transfer Mode (ATM) is a cell-switching, connection-oriented technology.

In ATM networks, end stations attach to the network using dedicated full duplex connections. The ATM networks are constructed using switches, and switches are interconnected using dedicated physical connections. Before any data transfers can begin, end-to-end connections must be established. Multiple connections can and do exist on a single physical interface. Sending stations transmit data by segmenting Protocol Data Units (PDUs) into 53-byte cells. Payload stays in the form of cells during network transport. Receiving stations reassemble cells into PDUs. The connections are identified using a virtual path identifier (VPI) and a virtual channel identifier (VCI). The VPI field occupies one byte in the ATM cell five-byte header; whereas, the VCI field occupies two bytes in the ATM cell five-byte header. Basically, a VPI:VCI pair identifies the source of the ATM cell. The function of the ATM switch is to recognize the source of the cell, determine the next hop, and output the cell to a port. The VPI:VCI changes on a hop-by-hop basis. Thus, VPI:VCI values are not universal. Each virtual circuit is described as a concatenation of VPI:VCI values across the network.