1

I've got a bit of a tricky question (or more like curious problem),

I wonder why a network connection with wrong wiring actually worked:

We needed a new network cable from a consumer-grade router (100 Mbit/s network ports) to another room to connect a smart TV to the LAN.

An electrician did the required work, but obviously did only a short test with some laptop if the cable worked fine.
Anyway, the TV could not connect to the LAN, the electrician did not show up when we asked him to check his work again.

So we did some tests ourselves with some long (20 meter/65 feet) network cable and a continuity tester.

It turned out that he used the T568B standard for wiring, but he obviously overlooked the numbering note inside the RJ45 wall socket: Ordering of the wires in the connecting panel should have been 8,7 5,4 6,3 2,1, but he just connected the wires in the color order of T568B, so 8,7 6,5 4,3 2,1.
This leads to 6 being connected to 5, 5 to 4 and 4 to 6, all others were fine.

I also checked with the continuity tester that this was indeed the wiring (I used the long 20 meter cable to get back to the place where the router is located to test this).

Now, why does a network connection via some laptop work with such a wrong wiring anyway?
I tested it with my own laptop, it worked fine. From my understanding a 100 Mbit/s network needs wires 1,2,3 and 6, but in my case only 1,2 and 3 were wired correctly.

Is there a fallback in network chips to also work with such bad wiring?
And for additional fun: When I connected the TV with the 20 meter LAN cable to the RJ45 wall socket, the network connection worked fine.
When I used a short (1 meter) cable, it did not work. Both cables were fine, at least according to my continuity tester (I don't have a real network cable tester at home :).
And I also turned off WLAN on the router&laptop to really make sure it does not use this as a backup.

3
  • I suggest trying electronics.stackexchange.com
    – barlop
    Jul 27, 2014 at 11:19
  • Have been trying to look up which wires are used in simplex (half-duplex) mode, but can not find it. May be the connection only works in half-duplex, and only some hardware can handle it. Jul 27, 2014 at 11:42
  • You say he did this with the RJ45 wall socket, but how was this done on the other side? If it is "wrong" the same way on both ends, it could largely work as expected.
    – YLearn
    Jul 27, 2014 at 20:26

1 Answer 1

1

The effect you described might be caused by capacitive coupling between the open lead (6) and shield and/or the TX wires 1/2. The capacitance increases lineary by the length of your cable. This explains, why it works with the 20 meter cable but not with the 1 meter cable.

This is just a guess, as I experienced the same effect and was looking for an explanation. I was not yet able to find a confirmation of my hypothesis. Has anybody better facts?

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy