The D-channel is a key element in ISDN operation. The user and the network use this channel to convey information to each other regarding service requests.
Carrying the signaling on the D-channel removes signaling from the other channels, freeing them for other services. In addition, the other channels (i.e., Bs and Hs) are “clear” because in-band signaling information is not required.
Because the D-channel is used for signaling, and signaling in the ISDN is packet-oriented, the D-channel by design is a packet-mode-only channel. In other words, all traffic that passes between the customer’s location and the local exchange on the D-channel must be in packet format. Any excess capacity on the D-channel can be used to pass user packet information in X.25 formats. This excess capacity provides the customer with another bearer channel.
The ISDN standards describe the three levels of protocols the D-channel carries and uses for signaling between a user and the network.
The Physical Layer defines the electrical and mechanical characteristics of the ISDN interface.
The Data Link Layer is a bit-oriented protocol and performs the traditional Data Link Layer function of ensuring that the messages from the Network Layer are sent and received correctly over the physical link.
The third layer, the Message Layer, defines the specific messages to be used for requesting and accepting services and transferring user data.