Tony:
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.
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Thanks
Victor
