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If I have a full duplex relay, can half-duplex devices effectively communicate with this relay? Also, can interference cancellation be applied to the relays or devices in this case? To be more specific: I am considering two half-duplex D2D (device-to-device communication) devices which would communicate using a full duplex relay (say special devices equipped with full duplex capabilities). If otherwise (i.e they should all be full duplexes), I guess the number of transmitting full duplex devices must be equal to full duplex relays.

It appears a simple question but googling it have not helped me.

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    Please be more specific. What devices and models do you mean?
    – Ron Maupin ♦
    Commented Sep 25, 2018 at 23:38

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If you're asking about ethernet duplex, then the full- or half-duplex nature of a given link is negotiated when the link comes up, or alternatively set in configuration. (See autonegotiation at Wikipedia).

All full-duplex ethernet links are capable of working in half-duplex, and will do so when connected to a half-duplex device. (Presuming correct autonegotiation and configuration.) This is the normal case for many millions of devices, and is no different from speed abilities: the faster devices negotiates its speed downwards to match the slower device.

If you have a situation where two devices F and H (full- and half-duplex) are connected to a full-duplex switch, like this:

    ===+===+===
       |   |
       F   H

The normal situation is that F will be connected by full-duplex, H will be connected by half-duplex and the switch will store and forward frames as appropriate.

No extra frame loss will occur because of this situation, only the usual possibilities of noise etc, and is likely to have the error rate of the lesser capable link.

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  • This is a beautiful very well organized answer! +1
    – Abdulhameed
    Commented Sep 26, 2018 at 7:53
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They can. A lot of security cameras work as half dulpex sending pictures to a server which is in full duplex. You will have packet loss but it will work

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  • Your answer is well appreciated. Are there any underlying assumptions for them to work effectively
    – Abdulhameed
    Commented Sep 26, 2018 at 0:37
  • @Mary Hi are you sure about that? Why would there be packet loss because of this?
    – jonathanjo
    Commented Sep 26, 2018 at 7:39
  • Basically, cause nature of half duplex you will not running in a collision free domain anymore but a CSMA/CD. So the moment they match to send frames at the same time, cause they think the "medium" is quiet enough for them to do it, there you will have your packet loss. I am not saying it is going to cause a packet loss because of that, I am saying there will be packet loss in an environment like that
    – BloodyMery
    Commented Sep 26, 2018 at 9:58
  • Technically, those are collisions and not packet loss. Once the medium is clear the device will retransmit the frame, and the packet will eventually get there. So increased latency sure, but not necessarily packet loss.
    – boomi
    Commented Sep 26, 2018 at 11:36
  • @mary ah yes of course, just the normal potential for collisions dropping frames on any half-duplex link.
    – jonathanjo
    Commented Sep 26, 2018 at 12:40
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Wanted to add this because it was not mentioned here yet:

If the duplex mode is fixed/forced to full-duplex for the full-duplex device and the other device is using auto-negotiation or is fixed/forced to half-duplex, then they can communicate with each other, but not efficiently due to a Duplex mismatch.

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  • You should consider replacing not efficiently with very poorly...
    – Zac67 ♦
    Commented Apr 19, 2021 at 6:42

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