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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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That is for 10 and 100 Base T only. On 1000 Base T all four pairs are used in both directions. – Brian Carlton Jun 22 '12 at 19:47
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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. – Rocket Surgeon Jun 22 '12 at 23:37
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. – user381521 Jun 23 '12 at 16:24

2 Answers

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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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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. – Wouter van Ooijen Jun 22 '12 at 17:22
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. – user381521 Jun 22 '12 at 17:56
the picture shows two NICe which is short for network interface card, which is the PC end of the link – Martin Thompson Jun 22 '12 at 18:18
@Wouter - Ouch! – MikeJ-UK Jun 22 '12 at 18:51
Oh, I just learned the difference between MDI and MDIX and have it figured out now. Thanks! – user381521 Jun 23 '12 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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There's something wrong with my answer? – AndrejaKo Jun 22 '12 at 9:56
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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. – Martin Thompson Jun 22 '12 at 9:56
@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. – AndrejaKo Jun 22 '12 at 9:57
Ahh. I read the question asking "which device (PC or hub) usees which pair to transmit" – Martin Thompson Jun 22 '12 at 10:00

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