A Virtual Channel Identifier (VCI) is a unique identifier which indicates a particular virtual circuit on a network. It is a 16-bit field in the header of an ATM cell. The VCI, together with the Virtual Path Identifier (VPI) is used to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination.
ATM switches use the VPI/VCI fields to identify the Virtual Channel Link (VCL) of the next network that a cell needs to transit on its way to its final destination. The function of the VCI is similar to that of the data link connection identifier (DLCI) in frame relay and the Logical Channel Number & Logical Channel Group Number in X.25.
Cisco Systems "Multiband Switch and Service Configuration"