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I've been having some issues with my DSL line and when AT&T did a line test they found a lot of noise.

The technician came out and tested the line up to the house and said it was clean. This means that the wire in my house is probably bad or has some interference.

I did all the wiring in my house since we built the house, but I'm not sure how to test the cable for noise.

Is there a tool to test this with? Or how do I determine if there is interference? Should I just run a new cable?

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migrated from serverfault.com Jul 26 '11 at 23:10

This question came from our site for professional system and network administrators.

    
Running a new cable may not help much unless you did something silly. Assuming you used the right type of cable (don't put indoor cable outside in the weather), then the problem is almost always with the termination. Use the correct type of cable, don't bend it beyond the bend radius. Don't use staples to attach it to a wall. Don't run it directly next to power wiring. –  Zoredache Jul 26 '11 at 22:44
    
A common mistake I see is UTP run too close to flourescent light fixtures; this goes along with what @Zoredache says about power wiring. Also, large electric motors (refrigerators, A/C units, etc). –  Adrien Jul 26 '11 at 23:06
    
Unless you did something stupid like run the cable within inches of a fluorescent light, I'd be surprised if the cable itself is the source of noise. More likely a bad termination is at fault. (One possible problem is getting the pairs crossed in terminating: The wires need to be matched to the pins according to the standard -- simply getting connectivity from one end to the other is not sufficient. In particular, pins 3 and 6 are a pair, and if one simply wires pair/pair/pair/pair left to right two pairs will end up with their wires crossed.) –  Daniel R Hicks Jul 27 '11 at 0:09

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up vote 4 down vote accepted

UTP cabling is usually tested with a cable tester that basically uses time domain reflectometry. Testers with this ability are somewhat expensive, so they are usually out of the budget of a home user.

A tester might also transmit signal on one conductor, and expect the signal to be returned on another conductor (requires a terminator which builds a loop). The electronics will compare the transmitted signal against the returned signal.

Simple testers basically just test the resistance.

Almost all good testers require either a terminator on one end, or something that builds a loop out of the various conductors.

This wikpedia article on Copper cable certification includes lots of details about various tests that may be performed.

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Also just figured out that most routers include a Signal to Noise Ratio measurement which is a really good indicator of line noise. Just plugged in the router at the box and back in the wire closet and compared the two SNR to figure out the line noise issue. –  Bryant Oct 4 '11 at 17:18

This probably belongs on SuperUser (I've voted for it to move), but many common cable tester tools will provide good information on the quality of the line, with regard to faults, signal-to-noise, crosstalk, resistance, etc.

That said, a good cable tester would probably be a bit of an over-spend for a home project like this. Maybe just re-do the ends and punch downs (the most common points of failure), and see if that helps first.

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Although OP mentions his house, this certainly has application in the business environment as well. Not all of us have the luxury of a Facilities department that handles pulling cable for us. (And sometimes, when we do have a Facilities department, we don't want them anywhere near our cables anyway...) –  Adrien Jul 26 '11 at 23:09

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