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I am wondering how does the devices on a wireless connection recognize each other between all other devices, for example A wifi signal from an access point at 2.4GHz wave on channel 2 would communicate at the frequencey of 2413MHz (I think.. but an example more than an accurate statement)

So if you have 5 different devices connecting to it how does it recognize each one? I'm not talking about the upper layer attributes like IP and Mac addresses but more on the actual physical layer, google is just giving me blanks and bad answers and wikipedia and other websites don't give me anything technical.

If the sender sends signals, say 010101010, with the first digits being the address all devices can ignore this except the device with the right address, but what if another devices sends a signal during that send time, they all are at the same frequency so there is no way of finding out who sent that.. so it could actually corrupt the data being transferred, it would be like a room of people just shouting out and all have the same voice? (I don't really know how this works)

BUT this is not the case in all current wireless connections, this is what i want to understand, how does this happen? How do they distinguish each other? If you have 10 devices sending information all at once and on the same frequency and channel then how does the receiver know who sent what?

My main aim is whether you can recognize multiple RFID tags at once at the same time by an RFID reader, for example if 20 people walked at once near an RFID reader it will recognize all of them.. is this possible? This is the main thing i want to find out, also i want to know a the general concept of how the standard wireless connections work and the differences between them, that would be cool, this caught too much of my attention.

Any refrence or help on this topic would be most appreciated! Almost a week and i still cant figure this out nor found any content on it.

Thank you in advance

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    IP and MAC addresses are not upper-layer attributes. Layer-1 is the radio waves. Layer-2, using MAC addresses, is how Wi-Fi devices distinguish each other.
    – Ron Maupin ♦
    Commented Jul 21, 2016 at 21:15
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    @Ron Maupin you would need to receive data to know the mac address, imagine all devices sending 0s and 1s all at the same time, how would it distinguish them? If you're a bartender and 10 people shouted out what they want but letter by letter of the word (w-a-t-e-r) how would you know who said what? I could shout'W' for water and instantly someome else shouts 'M' for milk and the 3rd instanrly after the 2nd shouts 'B' for beer then you'd register WMB....... which is wrong.. so how do you keep track is what i want to know.
    – bakz
    Commented Jul 21, 2016 at 23:19
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    "imagine all devices sending 0s and 1s all at the same time, how would it distinguish them?" You don't. When devices send at the same time, that is a collision, and the data are unreadable. Wi-Fi uses CSMA-CA to try to avoid collisions. You need to get a frame header in order to distinguish which device is sending, and you check that by MAC address.
    – Ron Maupin ♦
    Commented Jul 21, 2016 at 23:31
  • Thank you, makes sense. I don't really know much about networking on the physical layer, appreciated. +
    – bakz
    Commented Jul 22, 2016 at 13:50

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Based on your comment, I think I understand your confusion. The Wi-Fi medium (radio waves) is a shared medium, and only one device at a time can send, even your PC and WAP must take turns and not send at the same time, which is why advertised Wi-Fi speeds are just so much marketing hype. If multiple devices send at the same time, you have a collision, and a device gets garbled data, which it ignores. Wi-Fi uses CSMA-CA to try to avoid collisions.

The Wi-Fi protocol is set up to fairly share the airwaves with all other devices on the same frequency, even those which are not part of your network. Devices will even bow out and give time to other devices. If you and a neighbor are operating on the same frequency, each of your devices will give time to all the other devices on the same frequency. That is why your network will slow down your neighbor's network, and vice versa. Neither of you owns the public airwaves, and you must share them.

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    I see, the csma-cd protocol explains most of it, but the explanation said that the csma-cd protocol sends a bit of the frame at once which would still make collision of transmitted data, but the idea makes sense if each device gets to send a whole frame at once. This makes sense, I never knew it was that tedious of a job transmitting wireless data! Thanks a lot, the protocol opened many doors for me, i was able to get some results of what i need using it. I guess RFID has anti collision destections implemented as well :) if there is anything else that can help please add it.
    – bakz
    Commented Jul 22, 2016 at 13:40
  • CSMA-CD is what ethernet uses, but Wi-Fi uses CSMA-CA. Rather than the collision detection (CD) used by ethernet, Wi-Fi uses collision avoidance (CA).
    – Ron Maupin ♦
    Commented Jul 22, 2016 at 13:55
  • To be specific in Wi-Fi, CA implemented by RTS/CTS or CTS-to-self protocols. As this protocols is optional in Wi-Fi without it CSMA-CD used, and it work same as Ethernet CSMA-CD.
    – mmv-ru
    Commented Jul 22, 2016 at 16:13

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