And to throw another wrinkle out there a T-1 is a T-1 only differece is the signaling mode. The only change we made testing a T-1 was if it were D4/AMI we'd change it to ESF/B8ZS other wise you're not testing the whole T-1. AT&T had built in monitors on the lines, across the network it was always ESF at the end point it could be converted to D4. I have tried to explain to many a T-1 "tech" when they say why do I see 5 errors here and only 2 here. The reason is because you're not looking at the full bit stream when testing or monitoring D4. Usually they'd just get a glazed look and walk away.

Retired phone dude