The phrase VPI meant to determine the identity of the virtual path.
VPI a number is quite logic, that is, to traverse from an IMA is required. Between the two switching ATM the VPI for the isolation of transport-related LT used. For a full description of the work, it should be said that for each port of DSLAM should be a VPI/VCI considered, it is the use of the Cross Connection between the ports to switch higher). Number VPI/VCI specific to a joint is and can be for the bandwidth guarantee ( CBR ) or non-warranty ( UBR ) considered.