I've got a phone line on the blue pair, and VDSL on the brown pair. Leaving the orange and green pairs available.

But when I try and connect a router in one room, and a laptop in the other room, I don't get a connection and also the phone line goes off. VDSL stays on.

Why is it even touching the blue pair? I thought Ethernet only used the green and orange pairs. Or have I got something wired incorrectly? I haven't seen any indication that I have.

  • \$\begingroup\$ Also, both the laptop and the router knock off the phone line. Leaving one connected or both.. it doesn't matter. \$\endgroup\$ – user2476549 Jun 12 '13 at 0:54
  • \$\begingroup\$ You could only piggyback phone line on 10Base-T on CAT-5 cable. 100Base-T and 1000BASE-T use all pairs. \$\endgroup\$ – Chetan Bhargava Jun 12 '13 at 0:54
  • \$\begingroup\$ How come I'm reading that 100Base-T only uses 2 pairs though? Apologies in advance, I'm new to electrical engineering. \$\endgroup\$ – user2476549 Jun 12 '13 at 1:05

Many ethernet devices short out the unused pairs. So if you want to use those pairs for something else you need to make sure you don't connect them to the ethernet device.

Also regarding your comment on the other answer you don't want to have any wires branching off the pairs you are using for ethernet. Even if that branch doesn't go anywhere it will still do horrible things to the signal integrity.

Finally note that while 10BASE-T and 100BASE-TX only use two pairs 1000BASE-TX uses all four pairs.


I would check your connections to ensure you have left the green and orange pairs open for use with the Ethernet. Remember that the orange pair is separated by the blue pair, the wires are not side by side in the connector. Also, I would not include the extra two twisted pairs in the RJ45 connectors (blue and brown) you plug into the router. That way, the router has no way of trying to put data on those lines.

Both 10Base-T and 100Base-T use the same wiring topology, although 100Base-T needs the higher quality cable due to the increased transmission speeds. This should be the green pair (RJ45 pins 1 and 2) and the orange pair (RJ45 pins 3 and 6). The other two twisted pairs can be used for other data transmission, as long as the signal does not produce interference with the Ethernet.

  • 2
    \$\begingroup\$ The suggestion of leaving the unwanted wires out of the connectors is a good one. Also, "as long as the signal does not produce interference with the Ethernet" is an important caveat, DSL is wideband designed to travel long distance so will put a fair bit of noise down the cable which the ethernet devices will not be expecting. \$\endgroup\$ – John U Jun 12 '13 at 8:03
  • \$\begingroup\$ There's a very important detail that I left out, and that's that the brown pair that carries VDSL is isolated to one room. All of the other pairs are connected together at a central point in the house. All pairs in room A, B, and C are connected together, except brown. Brown pair in room A is isolated, whereas the brown pairs in room B and C are connected together. I'm experimenting between point B and C.. which is basically one long ethernet cord, with the green, orange and brown pairs available. The only pair being used is the blue, for phone. I'll try taking out those two pairs and report. \$\endgroup\$ – user2476549 Jun 13 '13 at 4:06

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