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.