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I have a potential application where the vast majority of the traffic will be in one direction, and, concurrently, I have a potential alternate use for a pair of wires in the cable that would facilitate this. So, it's got me wondering if any network equipment will negotiate gigabit in one direction and 100mb in the other direction. The idea is that one direction will have 2 pairs and the other direction will only have 1. I assume that this is impossible because the timing of handshakes (and whatever else goes on in the ethernet protocol) will be completely stirred. I'm not sure whether to call this partial duplex or mismatched duplex or.... Any thoughts?

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    There have been plenty of protocols that support asymmetric transfer/receive rates but standard Ethernet is not among them. You might get a better result out of something like VDSL2 bridges or optics that allow you to use a single pair to establish a connection. These present as Ethernet to either side but are actually encapsulated over a negotiated/variable speed transport. – rnxrx Sep 19 '17 at 17:06
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin Feb 19 '18 at 18:35
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No. Negotiation means that both ends agree to the speed, and duplex.

The cabling standard requires four pairs for a single connection, and you cannot mix applications within the same four-pair cable sheath. Also, 1000BASE-T (1 Gbps) uses all four pairs for both send and receive.

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  • Aha! 1000BASE-T (garden variety?) does indeed use the wires for bi-directional communication. I must have been looking at the documentation for 1000BASE-TX. So, the question is indeed moot for economy networking gear. Not to mention that it does likely break the 'standard' to use otherwise unused pairs for alternate uses although folks have used the extra pairs in 100mb for phone lines in homes since the advent, I think. – chmedly Sep 18 '17 at 23:14
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    Do you mean 100BASE-TX, because 1 Gbps ethernet on UTP is 1000BASE-T? 10BASE-T and 100BASE-TX use two pairs, but 1000BASE-T requires four pairs. – Ron Maupin Sep 19 '17 at 0:44
  • 1000BASE-TX was defined by TIA but very few devices have ever been made, due to the required and then-rare category 6 cabling. – Zac67 Sep 19 '17 at 6:29

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