Copyright © 1995-2012 Daniele Orlandi
The ISDN provides serveral functionalities to the host connected to it.
Through the D channel, the host may talk with the network in order to setup and manage callsi using the q.931/DSSS-1 and q.sig protocols.
Tough mostly unused due to the very limited bandwidth, the D channel supports transport of X.25 network traffic.
Point-to-point data transmission is available through the B channels. Transported data may be of various nature, goint from voice, video, and faxes to packet communication and encapsulated asynchronous TTY data.
Being very different in nature, I thought that each functionality should have had a different place in the infrastructure, instead of having everything in one place, in some way the approach I4L followed.