Page "Asynchronous Transfer Mode" Paragraph 16

ATM operates as a channel-based transport layer, using virtual circuits ( VCs ).

This is encompassed in the concept of the Virtual Paths ( VP ) and Virtual Channels.

Every ATM cell has an 8-or 12-bit Virtual Path Identifier ( VPI ) and 16-bit Virtual Channel Identifier ( VCI ) pair defined in its header.

Together, these identify the virtual circuit used by the connection.

The length of the VPI varies according to whether the cell is sent on the user-network interface ( on the edge of the network ), or if it is sent on the network-network interface ( inside the network ).