Stands for "Virtual Channel Identifier." The VCI, used in conjunction with the VPI (virtual path indicator), indicates where an ATM cell is to travel over a network. ATM, or asynchronous transfer mode, is a method that many ISPs (Internet Service Providers) use to transfer data to client computers. Because ATM sends packets over fixed channels, the data is easier to track than information sent over the standard TCP/IP protocol.
The VCI within each ATM cell defines the fixed channel on which the packet of information should be sent. It is a 16-bit field, compared to the VPI, which is only 8 bits. Since this numerical tag specifies the virtual channel that each packet belongs to, it prevents interference with other data being sent across the network.