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I've seen numerous articles about straight-through and crossover cables and why they are needed. With conventional straight-through cabling, one device transmits on pair 2 (pins 3 and 6) while the other transmits on pair 3 (pins 1 and 2).

But here's the thing they never clarify:

Which device, the computer or the switch, transmits on pair 2 and receives on pair 3? (And vice versa.)

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    \$\begingroup\$ That is for 10 and 100 Base T only. On 1000 Base T all four pairs are used in both directions. \$\endgroup\$ Jun 22, 2012 at 19:47
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    \$\begingroup\$ Just a note, in case you are wondering what crossover cable is. Google "crossover cables obsolete" and you will undestand that the asymmetry issue is long gone since every PHY chip made during last 10-15 years has an automatic switch to detect which "side" it belongs to. \$\endgroup\$
    – user924
    Jun 22, 2012 at 23:37
  • \$\begingroup\$ I'm trying to build a 10BASE-T PHY myself, and it would be helpful to (A) follow the convention and (B) not have to worry about auto-MDIX in my implementation before I even get it to work. \$\endgroup\$
    – user381521
    Jun 23, 2012 at 16:24

2 Answers 2

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According to this diagram from the Wikipedia page on crossovers, a network card (NIC) will transmit on 1 and 2, so a hub/router/switch will transmit on 3 and 6.

Crossover cable wiring

The ethernet twisted pair page states:

A 10BASE-T or 100BASE-TX node such as a PC, with a connector called medium dependent interfaces (MDI), transmits on pin 1 and 2 and receives on pin 3 and 6 to a network device using a "straight-through" cable.

It hasn't actually mattered in a long time though - auto-MDIX has been around for a decade or so.

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    \$\begingroup\$ A few years ago students of mine were working on an Ethernet interface in an FPGA. They did not implement auto-MDIX. To their supprise their circuit worked OK on one hub, but failed on some ports of another hub. That hub had some of its ports wired the oter way round, relying on auto detection to correct this. \$\endgroup\$ Jun 22, 2012 at 17:22
  • \$\begingroup\$ Sorry if I'm missing it, but where does it specify which device transmits on which pair? I'm trying to build my own bit-bang implementation of Ethernet, and auto-MDIX is certainly not something I can implement on my own or something I can rely on on the upstream switch. \$\endgroup\$
    – user381521
    Jun 22, 2012 at 17:56
  • \$\begingroup\$ the picture shows two NICe which is short for network interface card, which is the PC end of the link \$\endgroup\$ Jun 22, 2012 at 18:18
  • \$\begingroup\$ @Wouter - Ouch! \$\endgroup\$
    – MikeJ-UK
    Jun 22, 2012 at 18:51
  • \$\begingroup\$ Oh, I just learned the difference between MDI and MDIX and have it figured out now. Thanks! \$\endgroup\$
    – user381521
    Jun 23, 2012 at 16:21
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On older 100 Mb/s networks, it doesn't matter. What matters is that the receive pins of one side are connected to transmit pins of other side. Basically one side of the cable will have the "straight-through" pin-out and the other will have "cross-over" pin-out. The cross-over pin-out makes sure that one side's transmit pins connect to other side's receive pins and you can connect the "cross-over" side.

So if the switch is connected directly to the "cross-over" side of the cable or the computer only depends on the way you actually plug in the cable. The "cross-over" side can be connected so that it is near the switch or so that it is near the computer.

Also you'd use "cross-over" cables to connect two computers together. For a computer and a switch, a "straight-through" would be used.

Do note that mot of newer network equipment and all 1 Gb/s equipment automatically negotiates which pins do what.

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  • \$\begingroup\$ There's something wrong with my answer? \$\endgroup\$
    – AndrejaKo
    Jun 22, 2012 at 9:56
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    \$\begingroup\$ Not sure what you mean by "the way you plug in the cable" - I can't see it matters? And, as far as I know, it does matter on older networks (10 and 100Mbps) as the hardware doesn't support Auto-MDIX to figure out which pairs to use. \$\endgroup\$ Jun 22, 2012 at 9:56
  • \$\begingroup\$ @Martin Thompson I meant that it doesn't matter which side of the cross-over cable goes to which end, as it's asked in the question. \$\endgroup\$
    – AndrejaKo
    Jun 22, 2012 at 9:57
  • \$\begingroup\$ Ahh. I read the question asking "which device (PC or hub) usees which pair to transmit" \$\endgroup\$ Jun 22, 2012 at 10:00

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