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A little bit of backstory, just in case you want some context. In the last couple of weeks the network at least internally has slowed down. As far as I can tell the only things that have changed has been new emergency lighting has been installed and some servers have been moved from a different subnet.

Anyway onto the problem / question.

We have a NaviTEK-NT tester and below is an image of one of the network ports results. I have been told the ports should be Cat5e as that is what was requested when the points got installed.What exactly is happening in the image below? As Far as I can tell it's got 2 pairs going 20M, which is about the length of the cable between the network point I use and the patch cabinet. I then have 2 pairs going around 1M, which is maybe the rough length of the Ethernet cables combined (about 0.3/0.4m * 2).

In my head this indicates that maybe their is a break or something as I'd expect 4 pairs of 20M ~ cable length. However as far as the manual is concerned I would see a red line cutting the cable if there was a break.

TLDR: In the image below why are 2 pairs 20m and the other 2 are 1m in length. What does this indicate? It should be patched up as Cat5e though I wouldn't rule out it being just a cat5.

If I have missed anything let me know and I will try and find out anything else that could help. Thank you very much.

Network Test Results

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    My guess is that the pairs are split or only partially terminated on the patch panel.
    – Ron Trunk
    Commented Mar 20, 2019 at 13:14
  • That is a toy tester. It is simply testing the wiremap, but there are many other tests to pass the test suite, and you need a real cable tester to pass those. See this answer for the primary tests.
    – Ron Maupin ♦
    Commented Mar 20, 2019 at 13:54
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Commented Dec 14, 2019 at 21:45

2 Answers 2

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I offer another interpretation: 4 wire cable for 100Mbit/s.

This is basically the same as zac67's answer "Either it's a bad LSA contact, a bad panel insert or a damaged cable.", but probably the situation is intentional, not accidental.

It is compelling that it's exactly pairs 1/2 and 3/6 that are 20m long, while the other pairs are just shown with the length of the patch cable used to connect the tester to the cable.

EDIT: Actually, the above statement might be wrong.

The cable tester's graphical result is somewhat puzzling. It does show "S" (shielding) as a interrupted link, but the presentation of wire pairs 4&5 and 7&8 is shown as single strech of 1m cable.

If wire pairs 4&5 and 7&8 were discontiguous, i.e. present in the 1m patch cords, but "absent" (probably: "used elsewhere") in the ~20m of fixed-installation cable, they should be shown the same way as the shielding link: with a gap.

Then again, this might be a misinterpretation by the cable tester.

There were days when installed 8-wire cable was used to provide two 8P8C connectors (a.k.a. "RJ45"), but equipped with just 4 wires each (1,2,3,6), to "save some money", and of course because 100Mbit/s Ethernet was a big thing back then and because it ran fine on just two twisted pairs.

Saving money that way has a veeeeery long tail, as we now see. And then there were "Cat5" labelled patch cords too, with just two pairs inside. m-(

You may want to check if you happen to have found one of these old tripwires (pun intended).

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  • This seems to be the most applicable answer for our situation. I am wondering how I would go about testing and proving this is the case? At the moment I am going around on a wild goose chase and proving this is the case might enable other problems to be looked at. In an ideal world we'd replace these 100Mbit/s cables with something a bit more modern! Though I doubt that will happen :)
    – Eddy
    Commented Mar 27, 2019 at 8:43
  • In the wiring closet, I would expect the fixed-installation cables are terminated on a patch panel, which presents the 8P8C ("RJ45") connectors to the front. With some acrobacy, you might be able to glance at the panel's backside, and see how many wires are actually there, on each RJ45 outlet. If an incoming 8wire fixed installation cable serves to outlets with 4 wires each - then it becomes obvious. Other than that, you may want to disassemble a set of (pairs of?) cable points in the floor/wall, to check how they are wired.
    – Marc 'netztier' Luethi
    Commented Mar 27, 2019 at 11:31
  • @Eddy, Category-5e can run 1000Base-T (1 Gbps). If your cables only work with 100Base-TX (100 Mbps), then they have been incorrectly installed in the first place because 100Base-TX only requires the two pairs to work, but 1000Base-T requires all four pairs.
    – Ron Maupin ♦
    Commented Mar 27, 2019 at 12:53
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A cable tester of that type injects a pulse into the cable and analyzes the time it takes to reflect back. That tester can't really test/certify a cable deployment (you'd need attenuation, NEXT/FEXT, ...) but it does show a problem - the short lengths you've got indicate a "bump" in the cable that reflects enough energy to trigger the analyzer.

Most likely, the bump is caused by bad termination. Either it's a bad LSA contact, a bad panel insert or a damaged cable. An impedance change is also possible but that would reflect on all pairs. Another cause could be a split pair - double-check the color codes on the panel contacts.

Even with Cat-5e cabling (which isn't as delicate as Cat-6A by far) you need to have a deployment certified so you know you can rely on each connection.

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