A circuit-switched channel in Integrated Services Digital Network (ISDN) technologies that carries signaling information between the customer premises termination and the central office (CO) of the telecommunications service provider or telco. The letter «D» stands for «data» or «delta».
The D channel is used to signal the telco CO when connections need to be created or terminated. The D channel forms the “D” part of a 2B+D Basic Rate Interface ISDN (BRI-ISDN) line and carries signaling information at a rate of 16 Kbps. On a 23B+D Primary Rate Interface ISDN (PRI-ISDN) line, the D channel carries signaling information at the faster rate of 64 Kbps.
D channel communication uses a completely separate out-of-band communication network called the Signaling System 7 (SS7) network, as shown in the illustration. This telco network is dedicated solely to servicing system functions that are overhead as far as voice or data communication is concerned.
The SS7 network on which D channel communication takes place makes possible the low latency of dial-up ISDN connections, which are typically 1 or 2 seconds (compared to a latency of 15 to 30 seconds for analog phone connections).
In ISDN voice communication, D channels are also used to activate special calling features such as line call forwarding and caller ID.