The Care and Feeding of ISDN4BSD
2.3.2 D-Channel Layer 3	2.3 The D-Channel	2.4 The B-Channel

2.3.3 Interlayer Communication

Every layer in the ISDN protocol stack has to communicate with the other layers with which it is in contact: layer 1 interacts with layer 2 which in turn interacts with layer 1 and layer 3 and finally layer 3 which interacts with layer 2 and something which sits above it.

The way how this communication is done is also specified in the above referenced ITU Recommendations and it is done by so called ``primitives'' which separate the layers logically from each other in a well defined manner.