The D channel carries the ISDN Network Services between the user and the network. It maintains the user's relationship with the network.

This includes:

• the requests and responses used when you make or receive a call
• call progress messages
• messages informing you that the called party has closed the call
• error messages telling you why a call has not been established for you

The D channel operates at 16 000 bits per second in a BRI and at 64 000 bits per second in a PRI.