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A disadvantage of the analogue radio1 used for communication is that it does not receive when it transmits. Everybody is trained to wait until the frequency is quiet before starting to talk, but it's still possible that two stations will start talking at the same time, in which case everybody else will hear an unintelligible mess but the two talking will not be aware of it. The station who talks longer won't be aware at all and if they talk for similar time, neither may be aware.

What I would like to know is how humans deal with the problem and whether there is any specific guideline for resolving these conflicts.
It seems like if the controller just says "calling centre, say again" the two may just both start talking past each other again.

1 Just like any other shared frequency radio, but digital systems seem to have more options to handle it, especially for the relatively low bandwidth requirement of voice communication.

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    $\begingroup$ in networking this situation is called carrier sense multiple access and the (unofficial protocol used would be aloha) $\endgroup$
    – ratchet freak
    Mar 18, 2014 at 17:42
  • $\begingroup$ Lol, I just posted my answer which linked to the same thing -- I promise I didn't copy your link! :) $\endgroup$
    – kevin42
    Mar 18, 2014 at 17:44
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    $\begingroup$ The normal way this is handled is that immediately after the conflicted transmission ends someone jumps on freq and transmits "blocked!". $\endgroup$
    – casey
    Mar 18, 2014 at 18:03
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    $\begingroup$ @JustinKiang which will be blocked as well continuing the loop ... $\endgroup$
    – ratchet freak
    Mar 18, 2014 at 19:09
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    $\begingroup$ @JanHudec "Blocked" is actually standard phraseology - other than that the AIM has a few words on radio technique but nothing specific about what to do if your transmission was blocked - it's largely an area where common sense and courtesy seems to suffice :-) $\endgroup$
    – voretaq7
    Mar 18, 2014 at 19:58

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Well in regard to "How are these situations resolved?" you've already answered your own question in a way: the intended recipient responds with something like "Blocked" or "Calling center, say again." (If they are expecting to hear from a specific aircraft they'll often respond with "N12345, say again.", and if the frequency congestion is really bad controllers may add "All other aircraft standby").
As kevin42 pointed out this is very similar to what digital CSMA/CD systems do - they detect a collision ("Blocked"), wait a while, and try their transmission again.

As a practical matter, two-way voice communication over the radio is possible, but difficult: for example a discrete "uplink" and "downlink" frequency as you have with some radio systems would be impractical in the sort of mesh topology that you have with aviation radio (you could implement it for a single air-to-ground pair trivially, but aviation radios are also used for air-to-air communication, and it's important that pilots hear what's going on as this enables them to build a mental picture of the airspace, as well as ensure that they're not blocking other pilot's transmissions).

Regarding analog-vs-digital, the traditional AM radio used in aircraft has one major advantage over its digital counterparts: when two digital signals overlap the result is garbage -- you can't recover any of the information. When two analog AM signals overlap you get heterodyning (the two signals mix, and your receiver pipes that mix into your ears).
The resulting mixed signal sounds AWFUL, there's an ear-bleeding squeal from the out-of-phase carriers and the voice modulations combine to a garbled mess much of the time, but with experience you can usually pick out one of the communications (particularly if you're close to one of the transmitters, where it will overpower the more distant signal).

The ability to step on a transmission as described above this can be useful: Instructions have been issued over a "stuck mic" before to get other aircraft off the frequency.

In a similar vein, if you have an emergency you probably aren't going to listen before you talk - you're going to key the mic with your mayday and even if the controller misses half of what you're saying under the heterodyne noise they'll know there's an emergency when they hear the other half of your transmission and tell everyone else to standby while they talk to you.

In both of those situations a digital system implementing CSMA/CD would detect that the frequency was in use and not transmit (and if were a digital system and transmitted anyway both digital signals would be garbled and unusable).

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    $\begingroup$ A digital system would need higher bandwidth to have enough time for the retransmits or some form of managed access. It would also be more expensive and less reliable and there is a big advantage to simplicity here. I was mainly interested about what aviators do when transmissions overlap, not really thinking that anything should be fixed (multiple people talking on a conference call does not cause any squeal, but is not any more intelligible anyway). $\endgroup$
    – Jan Hudec
    Mar 18, 2014 at 20:01
  • $\begingroup$ @JanHudec Honestly I think the big failing (more than the heterodyning from overlapping transmissions) is what you pointed out in your question: you can't hear the frequency when you're talking (which could at least theoretically be solved with some changes to the radio design 2 antennas - one RX and one TX. Not sure why nobody's built an aviation radio like that, except possibly our hatred for draggy antenna poles :) $\endgroup$
    – voretaq7
    Mar 18, 2014 at 20:10
  • $\begingroup$ Two antennas don't really solve anything. The transmitter would still feed a lot of energy to the receiver making it impossible to pick out and amplify the much weaker other signal. I asked about details over here. Unlikely to get it working on an airplane, but it could be doable on the ground, so at least the controller would know when their instructions are being blocked. $\endgroup$
    – Jan Hudec
    Mar 18, 2014 at 20:18
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    $\begingroup$ the second to last paragraph is why you repeat mayday a few times $\endgroup$
    – ratchet freak
    Mar 18, 2014 at 20:47
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Most of the time there will be an identifiable fragment of one transmission that will allow the controller to say, "Aircraft at flight level three three zero say again." After that interchange is completed, it is followed by, "Other aircraft calling Aloha center say again."

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It isn't really an analog vs. digital problem. Even WiFi connections have the same problem, and they rely on built in collision avoidance techniques to make it work. But it turns out that what humans do is pretty similar to the collision avoidance techniques used in wireless communications.

For example, two people try to contact the tower at the same time and step on each other's transmission. So the tower doesn't respond to either. Both people try again after some amount of time. As long as there is just a tiny amount of difference in the amount of time they wait, one of the retransmissions will come first, and the other person will wait.

In uncontrolled airspace, when using the advisory frequency things aren't as simple, but in practice at least one of the people will stop talking first and realize they were colliding with the other and repeat their transmission. The bottom line is that I don't think this is a problem, except when the airspace is very busy, and in that case a different technology is unlikely to make much of a difference.

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