Channel Identifier(CID) is a channel number( CN) with which a transmission channel is identified. The designation is used in many networks and can refer to physical channels, logical channels or virtual channels.
In the frame relay frame, it is the Data Link Channel Identifier( DLCI) data field that provides the channel identifier, and in Distributed Queue Dual Bus( DQDB), it is the VCI data field for the virtual channel identifier. In UMTS, the channel identifier is exchanged between the base station and the radio network controller. And even with time division multiplexing, there are channel identification numbers with which each data packet is provided.