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Demarcation point on a serial link


Can someone confirm if this is where the demarcation point is located on a serial link?

Router connects to csu/dsu via ethernet.

csu/dsu connects to the pop via serial cable.

demar is a RJ45 female connection located in the telecommunications closet? It's after the csu/dsu isnt it?

Thanks for all help.

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Re: Demarcation point on a serial link

If telco owns what you call csu/dsu (that actually is either a router or a bridge because it has ethernet), the demarc is the ethernet connecteor in there.

If you own said above device, whatever it is, the demarc is the RJ45 (actually an RJ48) you mentioned.

New Member

Re: Demarcation point on a serial link


Thanks for your reply.


Re: Demarcation point on a serial link


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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