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I am setting up ethernet in a friends house with cat 5 cabling. I do not do this professionally but I have made like 20 patch cables and installed about 20 wall sockets before in my own house.

My friend's house has two existing RJ45 wall sockets with an (old) existing cabling between them. I made two patch cables using the T-568-B wiring system and tested them both using a Goobay CAT-Network-Tester TP III. The tester goes through the pins and the other end lights the leds in the same order from 1 to 8.

When I connect all three segments together: - patch cable 1 - wall socket 1 - existing cable inside the walls - wall socket 2 - patch cable 2 and set up the transmitter in one end of patch cable 1 and the terminator at the end of patch cable 2, the tester shows only lights 2, 3, 4, 6, 7, 8.

This seems to indicate, that only green and brown pairs are used. However, cat 5 is supposed to work with green and orange.

So how come this connection works?

update: went to this friends house and checked the speed, the speed is 45Mbps download and 32 Mbps upload which is the same as in other parts of the house. So I am assuming that it is not working on one pair only. The physical layer must somehow detect that non-standard wiring is used and adapts to that

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It may depend on your definition of "works". Ethernet is an AC coupled communications link, so a wire that is mis-connected may still have enough capacitance to the other end of the link to enable data to come across.

Modern PHY (basically the cable modem) devices have pretty sophisticated techniques to compensate for cable variance. In some cases you may find that the actual speed of the link has dropped below standard rates because of that during the equalization and training phases of link establishment. That's why you also have more expensive testers that actually check the cabling the way an Ethernet PHY would use it.

For example the low cost DC type checkers wouldn't be able to tell that the internal wall cable connections were wired to RJ-61 standards, which would be problematic for Ethernet, but would pass continuity/short circuit tests. That's not a put-down, they're generally all most people will ever need, but then there are systems where a 50% drop in data rates would be unacceptable (data centers, medical imaging, etc.) and that's where you need the higher end stuff.

Best advice I can give is take the wall sockets off and inspect the wiring there, you may find a poor punchdown connection, RJ-61 wiring, or something else.

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