VPI (8/12 bits)
Protocol: ATM ATM PCI (Cell Layer)
Field: virtual path identifier
Length: 8 / 12 bits
Contents: cell routing information / hop-to-hop.
By means of VPI values the ATM cell is routed to the next ATM cross connect. The VPI length depends on the used ATM interface protocol: 8 bits with UNI, 12 bits with NNI.
Various VPIs are reserved for meta signalling and OAM channels which are used for internal network control data exchange.