Tapping modem transmissions

[Matthew J Ghio <mg5n+@andrew.cmu.edu>]

It's not too technically difficult to tap modem connections at low
speed, I played around with it a few years back, and I was able to add a
third modem onto a 300 or 1200 baud connection and view what was being
transmitted.  Using the modem as a dumb interpreter of the signal works
well to analyze the raw data being transmitted at low speeds, but when
you use v.42 and LAPM, things get a bit more complicated.  You need
special hardware/software to interpret and decode the compressed &
checksummed data packets.