The Care and Feeding of ISDN4BSD
3.2.1 isdnd	3.2 The Userland Components	3.2.3 isdntel

3.2.2 isdntrace

This program is able to produce full ISDN traces of all activities on passive cards for all protocol layers for the D-channel and hex dump traces for B-channel data transfers. It might be configured with two passive cards and a special cable to convert your machine into an S0-bus analyzer.