How do you troubleshoot a cat-5 drop?
December 14, 2009 7:40 AM   Subscribe

How do you troubleshoot a cat-5 drop?

Background: I recently moved into an awesome basement apartment where internet access is provided by a cat-5 drop from the second floor where the router connected to Fios resides.

Problem: The drop doesn't work. Using a cheap cable tester the landlord and I discovered that only two of the four pairs were working. I thought maybe the jacks were the problem so I repunched both jacks following the T568A standard. Briefly, three pairs were now working with only one pair failing. After moving the wire slightly none of the pairs worked. The other drops in the house have the same problem. All patch cables used have been tested and verified to work. The RJ-45 jacks were replaced when I repunched everything. There are no visible breaks in the wire.

Question: So, now what? He offered to pull a new cable through the walls but I'd like to save him the hassle if I can. Since other drops to other parts of the house have the same problem I'm thinking it isn't the cable itself. At this point, all I care about is getting the cable to pass using the handheld tester. My only idea right now is to test the jacks with a multimeter. How do the pros go about troubleshooting something like this?
posted by Loto to Computers & Internet (16 answers total) 2 users marked this as a favorite
Presumably you are using both solid wire and parts for solid wire. If not, there's you're problem. Jacks made for solid wire will not work right if you use stranded wire.

And you say that there's more than one drop that doesn't work? It's either wrong hardware for cable or a rodent. (probably)
posted by wierdo at 7:49 AM on December 14, 2009

You have a short in the cable somewhere.

The cheapest, easiest solution will be to pull a new cable.
posted by Thistledown at 8:03 AM on December 14, 2009

TDR. (time domain reflectometry).

It will pinpoint the fault to within inches.
posted by FauxScot at 8:20 AM on December 14, 2009

Some 3Com network cards have the capability to do TDR. That still means pulling a new cable, though. It might be cool to know within inches where the kink is, but you don't need to know really.
posted by odinsdream at 8:23 AM on December 14, 2009

Can he show you a map of the network topology? What is on the common path to the failing drops? Look there.
posted by TruncatedTiller at 8:55 AM on December 14, 2009

Since other drops to other parts of the house have the same problem

This suggests to me that the problem — probably a short — is in the patch panel where all these cables originate. Depending on the topology there might be a patchbay in a wiring closet on each floor where the "horizontal" runs start, and there's some sort of switch connecting to an interfloor backbone. (This is the standard topology in some office tower buildings and some big residential apartment buildings.)

If more than one drop is failing in an identical way I'd look for bad wiring in the punch block on the back of the patchbay. It might be that someone wired it incorrectly and shorted things together. That's about all that I can think would cause simultaneous failures in the same wires in multiple drops.

Of course, the alternative is that several horizontal runs have just failed independently but in a similar way, perhaps due to poor installation (bend radius too small, kinks, stapled down). Crushing Cat-5 under stapes meant for NM wiring will sometimes cause shorts.

Using a TDR would be the best, since it'll tell you where the short is; run it from a few different failed drops and see if it points you back to the patchbay, or if it sends you back to different places along the route. (To assist in locating the break, note the feet measurements on the cable runs both near the central patchbay and at the drop. That gives you an idea where the break is, or at least lets you know whether it's at the bay or somewhere along the route. Virtually all decent Cat-5/6 has distance measurements printed along it every so often.)
posted by Kadin2048 at 9:17 AM on December 14, 2009

To clarify:

This is a basement apartment in a single family house. That's why I'm looking at it rather than someone who knows what they are doing. There is no real patch panel, just a multi-jack face plate where each run originates with patch cables running to the router. I installed a new jack at this faceplate and also at the faceplate in my apartment.

The other drops I am referring to are to different jacks he has in the house. We tested the kitchen jack and it had the same problem as mine, two pairs failing.

I was hoping the cable itself wouldn't be bad but that seems likely at this point. According to the landlord, he had them run it during construction of the house (four years ago?) and punched the jacks himself. I'm wondering if the contractors stapled it all down, although the the coax cable for the TV seems fine (run to and from the same location).
posted by Loto at 10:20 AM on December 14, 2009

Run new cable. I'm not sure what time you think you'd be saving by knowing where the break is. You'd need to figure out where that break is in the wall, punch out the wall, cut the cable, solder or splice the mess (resulting in inferior cable quality), put it all back, and then repair & repaint the wall. Voltage reflection won't detect more than one break so if there is another break somewhere you'll need to do it again.

If the cable is not stapled, running new cable should be easy since you can use the existing cable to pull the new strand.
posted by chairface at 11:25 AM on December 14, 2009

The 100 Base TX ethernet standard actually only uses two pairs. Also, your keystone jacks list two possible pinouts. Use the T568B (sometimes just "B") pinout.
posted by Chuckles at 11:28 AM on December 14, 2009

Chuckles is right- use the B pinout. Whiteorange, orange, whitegreen, blue, whiteblue, green, whitebrown, brown. 100bt uses the orange and green pairs. (And it won't work right if the 2 signals aren't on pairs of wires, it uses the twists in the wires to cancel out interference.)

All the jacks should be wired the same; there is no crossing over that happens in the jacks depending on which end of the run the "source" is at.

If jiggling the wires causes the signal to fluctuate, I'd say they weren't punched down right, or the jacks are faulty. It's pretty hard to wreck cat5 cable. (Unless the cable was severely mistreated at some point in its life.)

Another way to test would be to crimp on some rj45 connectors to either end and connect the cable directly to the devices.

To test them with a multimeter, you would take one end of the wire, strip the insulation and twist each pair together. On the other end, check for continuity on that pair. If that holds, you could see if the resistance is the same across each pair. You could also cut a patch cable in half and make test wires out of it. Twist the wires on one end together, and check the continuity on either end. That way you don't have to tear the in-wall cable apart as much.

(Are your punch-downs solid? IE, did you use a punch down tool?)
posted by gjc at 2:20 PM on December 14, 2009

It may seem obvious, but just in case if it comes down to pulling new cables, make sure you remember that you already have a fishing line run to each location.

Just make sure you tape the new cable securely to the old one and away you go. There would be nothing worse than having the new cable come off halfway through pulling it.
posted by sah at 2:25 PM on December 14, 2009

Unless the ends are terminated differently it shouldn't matter whether I use B or A, correct?
posted by Loto at 2:58 PM on December 14, 2009

It shouldn't matter. Somehow though, I rewired a friends place three times, and it only actually worked when I accepted that B was the one I should have used from the beginning and redid a bunch of stuff..

I mean.. I really do agree that it shouldn't matter, and I don't believe in magic, but we are overlooking something..
posted by Chuckles at 5:28 PM on December 14, 2009

One thing. You mentioned "stapling the cables down."

Stapling is often carelessly done. It nicks the cable jacket and often the wire sheaths, causing shorts. Seen that lots of times.
posted by Thistledown at 8:11 PM on December 14, 2009

Unless the ends are terminated differently it shouldn't matter whether I use B or A, correct?

As long as both ends are A or both ends are B, the connection should work. B is standard. If one end is A and the other is B, it won't work. 100BASE-T doesn't care what color the pairs are, only that they're connected to the same pins on both ends.
posted by flabdablet at 12:22 AM on December 15, 2009

100BT uses pins 1 & 2, 3 & 6. So if you can figure out which pairs in the cable are broken, you can wire the jacks so that those pairs aren't used (wired to pins 4,5,7 & 8 in the connector).

The wiring — how you connect the pins in the connector to the pairs in the wire — doesn't really matter, so long as you wire everything "straight through" (same on both ends) and don't mix pairs. (I.e., pins 1 and 2 need to be the two halves of one pair, like white-orange and orange.)

As long as you have two good pairs in the wire, you can run 10/100BT. If you want gigabit, you'll need to pull new cable.
posted by Kadin2048 at 4:33 PM on December 15, 2009

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