My understanding is that wireless devices, such as a cell phone, must speak to a master device like a cell tower and the cell tower will then arbitrate who gets to talk and when. Maybe my understanding is wrong, but assuming it is right, how do the devices even request their turn and alert their presence without stepping all over the communications already there? In a video explaining the process, it used an example of one teacher and many students and the students must raise their hand and have the teacher select them to talk individually or else [s]he can't understand anything. But how do they raise their hands? In a wireless world, I imagine they'd all be shouting "Teacher call on Sam!" "Teacher, call on Jenny!" simultaneously and all while someone else is talking. So how does it arbitrate without confusion?
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2Depends on the protocol. For the one you have described - if there is an interference, the call will be cancelled and retried after some random time by each device. By the way, the problem is not limited to wireless communication. https://computer.howstuffworks.com/ethernet8.htm – Eugene Sh. Feb 08 '19 at 19:22
2 Answers
This depends on the protocol in use. But typically there's a teacher-student broadcast channel, whether that is a dedicated frequency band as in GSM, or some specific time-frequency slot, where it's desired that new students just shout "here I am, I'm CodeMonkey!".
After that phase the teacher says "OK, I heard CodeMonkey and Maria are new."
If you're not on the list but tried to shout your name, then you know you didn't get through, and will typically retry later on – often with something like "not going to try for a random number of announcement periods" (random backoff), to avoid making contention worse.
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In GSM, there is a separate channel for this, called "RACH", which uses random timing and retries to avoid collisions. Once the initial setup is done, the phone can move to a dedicated timeslot.
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