Hi,
http://www.protocols.com/pbook/atm.htm
VPI - Virtual Path Identifier.
VCI - Virtual Channel Identifier. The VPI and the virtual channel identifier (VCI) together form the routing field, also called VPCI, which associates each cell with a particular channel or circuit. The VCI is a single-channel identifier; the VPI allows grouping of VCs with different VCIs and allows the group to be switched together as an entity. However, the VPIs and VCIs have significance only on the local link; the contents of the routing field will generally change as the cell traverses from link to link. These fields, in UNI, can support up to 16 million desk (user) to network sessions.
http://www.tekelec.com/ss7/protocols/atm10.asp
