DSL Much Slower when plugged into certain phone wall jacks

Question

I have successfully confirmed that my DSL connection is 1/3 of my full speed when plugged into 2 different wall jacks in my apartment. My building manager linked those two wall jacks together, with port A coming from the D-MARK and port B connected to port A.

Port C is not connected to either of those ports, and is receiving the full bandwidth for my DSL connection. When I plug it into A and B, both of them show the same speed.

There are no phones, no modems, no devices of any type on these lines with the exception of the DSL modem.

Port C is in a very inconvenient location to run wires from. Calling my ISP only resulted in "you need to pay for a technician to come out." I know how to hook up and wire phone jacks, I just have no idea where the fault/problem would be here.

What are some troubleshooting techniques I can try to get the other ports working, or to find out where the true problem lies?

Answer 1

quite simply, the problem is probably line noise. Port C is less noisy since it has not been split. Noise means your connection is dropping down to a slower mode with better error correction.

Answer 2

Test from the DMARC to Jack A. You should have less than 15 dB of stress on the line (honestly though, if you have over 5 you have a potential problem depending on the dB loss between the CO and the DMARC itself). Depending on how A and B are punched down at the shared Jack, you could try removing the second jack and retesting. It is possible that the second cable is causing the problem. Be sure that the cable (if cat 3/5) is no more than 325 feet, as that could be part of the problem. You can try pulling the wires out of the jack and punch them down again. It is quite common that a jack has been punched down improperly which can lead to all sorts of connection issues (tip/ring shorts, grounding, loose connections, etc). It is also possible that the jacks are low quality and replacing them may increase your signal strength. You should also check the way they are punched down at the DMARC. If you have Jacks A and C getting a connection that are separate of each other in the apartment, then they are spliced together somehow at the DMARC and that could be causing the issue as well.