Communicating with Virtual Paths and Virtual Channels

Clients of ATM (Asynchronous Transfer Mode) networks communicate by creating connections to one another, then exchanging data over these connections [1] [2] [4] [6] [7] [10]. The ATM standard provides for two types of connections: Virtual Path (VP) and Virtual Channel (VC). With a VP connection, clients set the VP identifier (VPI) field of the ATM header (Figure 1) when sending cells. The network then uses the VPI for routing, possibly remapping this field at every switching node within the network, until the cells reach their destinations. With VC connections, clients set the VC identifier (VCI) and the VPI when sending cells and the network uses both the VCI and VPI for routing. For VP connections, the VCI is preserved by the network—whatever value the sending client places in this field is delivered to the destination client(s) and is available for use by the client, for example, as a multiplexing field. With VC connections, the VPI is not necessarily preserved by the network and may have to be set to a particular value (such as zero). Therefore, VC connections do not allow the client to use the ATM header for multiplexing.