|The Care and Feeding of ISDN4BSD|
|2.2.1 Frame format on||2 What is ISDN||2.3.1 D-Channel Layer 2|
2.3 The D-Channel
To transmit signaling information in an orderly, error corrected way, some sort of protocol is needed. And where protocols come into play, protocol layers are not far away.
The D-channel uses three layers for communication and because of this you need three different protocols and protocol specifications implemented to transfer signaling information from an ISDN device to an exchange.