It is a CSU/DSU of some sort. I see the things quite a
bit, still in
service - there are still lots of 56K lines still in use, but one by one
they are being decommissioned.
Someone asked...
A CSU (Channel Service Unit) is essentially a modem for high speed lines,
like 56K, T1, and T3 circuits. They are generally far more complex than
your normal Sportster!
William Donzelli
william(a)ans.net