Asynchronous Transfer Mode and its Virtual Connections

Asynchronous Transfer Mode (ATM) is derived from standards developed by the ITU-T that were based on BISDN (Broadband ISDN) technology.

ATM is a connection-oriented service in which transmitted data is organized into fixed-length cells. Upper-layer protocols and user data such as an IP packet are segmented into 48-byte protocol data units (PDUs). These PDUs are prepended with a 5-byte ATM header, and the resulting 53-byte cells are input into an ATM switch and multiplexed together. These cells then contend for vacant slots in the outgoing ATM cellstream.

Each ATM cell header contains a virtual path identifier (VPI) and a virtual channel identifier (VCI), which together define the ATM virtual circuit the cell needs to follow on its path toward its destination. The arrival rate, or delay, of one particular cell stream is not periodic. Therefore, the cell transfer is referred to as Asynchronous Transfer Mode, in contrast to synchronous transfer, such as TDM transport, which uses fixed time periods for frame transmission and reception.

Each ATM cell contains information that identifies the virtual connection to which it belongs. That identification has two parts : a virtual channel identifier and a virtual path identifier. Both the VPI and VCI are used at the ATM layer. The virtual channels and the virtual paths are contained within the physical transmission path, as shown in the picture below.

The virtual channel is a unidirectional communication capability for the transport of ATM cells. To originate or terminate a virtual channel link, a VCI is either assigned or removed. Virtual channel links are concatenated to form a virtual channel connection (VCC), which is an end-to-end path at the ATM layer.

A virtual path is a group of virtual channel links, all of which have the same endpoint. To originate or terminate a virtual path link, the VPI is either assigned or removed. Virtual path links are concatenated to form a virtual path connection (VPC).

Reference :
“Advanced MPLS Design and Implementation”, Vivek Alwayn, Cisco Press, 2002.