Each ATM cell contains an identification field in its header that includes a virtual path identifier( VPI) and virtual channel identifier( VCI). This identification is used to associate each cell with a single connection.
Between two reference points of a virtual path, only VPIs are processed by the ATM network nodes; the VCIs remain unprocessed. At the physical layer, each multiplex layer can support multiple virtual paths. In addition, each transmission medium can in turn contain several physical layers.