[PaulB]

A caveat (as I've stated before) - my knowledge of these things is, as I've previously stated, woeful ('O' Level (mid 1970s))

Obviously we've all heard "two stations at once" on the RT meaning that for any given frequency, only one station can be transmitting whilst many can be receiving. There have also been comments from Lee (and perhaps others) regarding how Pilot Aware spaces transmissions on its P3i frequency to avoid collisions. I assume that this is a balance between signal strength (ie range) and time taken to transmit in this case?

So..... does the same apply to ADS-B?

What about all these other 2-way systems that rely on wireless transmission? How do they avoid collisions? I assume that (for example) mobile phones use more than one frequency, but there many be many individual two way calls connected by one base station. Does each have a separate frequency or is there something more complex at pay here? What about all the things connected to my router here? Laptops, phones, TVs, Sonos, iPads desktop PCs. How do they have 2-way comms without collisions? do similar concepts apply to bluetooth, which I assume is wi-fi but on a different frequency?

[Paul_Sengupta]

I assume that (for example) mobile phones use more than one frequency, but there many be many individual two way calls connected by one base station. Does each have a separate frequency or is there something more complex at pay here?

Right...

For the old 1G systems, you had a separate frequency for each call on any particular base station. You had a set of allocated frequencies, and nearby base stations used different frequencies. There was a re-use pattern so that the same frequencies could be re-used outside the area where they would interfere with each other.

For 2G, GSM, you digitise the speech and can then divide it in time. You have the frequency use similar to 1G but then you also have timeslots, so you have eight timeslots per frequency, one frequency can support eight calls. As 2G progressed, you had further compression ("halfrate") meaning you could have sixteen calls per frequency.

For 3G, everyone is on the same frequency - sort of. There are various bands and carriers you can be on but within these, all mobiles use the same frequency. How does this work, you ask? Good question. They use code division. Power levels of phones are set so that they should all be picked up at the base station within a small range. They are allocated a code, which you can think of as a mask. All the signals are down in the noise, and of themselves are unrecognisable. Imagine putting a piece of paper with squares cut out in a certain pattern over a seemingly random mosaic. In this way you can take out the data for that one device. Each device with have a separate code/mask. 4G is similar to 3G in this respect.

All of these have separate uplink and downlink frequencies so transmit and receive are on separate frequencies.

Glad you asked?

[PaulB]

Indeed yes..... Thank you. Now, for the home wifi and bluetooth?

[rikur_]

WiFi all transmit on the same frequency (for a given access point) ..... they listen for no one else transmitting, then shout. If they don't get an acknowledgement, they assume that they clashed with another device (e.g. you might not be able to hear far away devices transmitting), wait a random time - then try again. Unlike wired ethernet, they can't explicitly detect a collision with another station transmitting at the same time.

[caveat]Some more sophisticated WiFi access solutions combine multiple channels[/caveat]