In addition to Bev's correct answer, I would like to offer a bit more clarity, since I used to troubleshoot facilities extensively with TELCO.
The service entrance in the basement of your building, where all the TELCO lines come in and are terminated, is the D-marc.
Let's say your office is on the 3rd floor of the building and you order a T1 line from TELCO. What they will do is extend the D-marc to your phone closet and call it an "extended D-marc."
That T1 circuit will be terminated in an RJ-48 jack, otherwise known as a Smart Jack or Network Interface Unit. It is typically loopable from the TELCO's Central Office for testing purposes. Take note that the plug and receptacles are RJ-45. When you say RJ-45, you are referring to the plugs and pinouts. RJ-48 refers to the mart Jack itself.
Anyway, for TELCO, that Smart Jack is their handoff to you -- the extended D-marc -- and they will test to that and be responsible up to that point. Unless, of course, you bought the CSU/DSU from them, but that is a rarity.
Finally, your CSU/DSU will plug into the Smart Jack. Or if the router comes with an onboard CSU/DSU, like a WIC-T1 card, you will plug the router into the Smart Jack.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...