What Is The Concept Of Cells In An ATM Network? What Type Of Connection Does An ATM Use?

The basic data unit in an ATM network is known as the cell. a cell in only 53 bytes long with five bytes allocated to the header and 48 bytes carrying the payload (user data may be less than 48 bytes). Most of the header is occupied by VPI (virtual path identifier) and VCI (virtual circuit identifier) that define the virtual connection through which a cell should travel from an end point to a switch or from a switch to another switch.

There are two types of connections used by ATM and these are PVC and SVC.  A permanent virtual circuit or PVC connection is established between two end points by network provider. The VPIs and VCIs are defined for the permanent connections, and values are entered for the tables of each switch. In a switched virtual circuit connection or SVC, each time an end point wants to make a connection with another end point, a new virtual circuit must be established then.

An ATM can not do the job by itself, but needs network layer addresses and the services of another protocol such as IP. The signalling mechanism of this other protocol makes a connection request using network layer addresses of the two end points. The actual mechanism depends on network layer protocol.