Cellular network


How does my phone know which signals to catch from the millions of waves that transfer information in the air? Do they read all the information but process only the ones which are encrypted for me? Is the number if waves to different devices practically unlimited?

In: 2

Cellular networks are cellular, this means that you build ut up multiple cells. The radio waves that are used are nationally limited in range so you can reuse it one cell over, you can get the idea from [https://en.wikipedia.org/wiki/Cellular_network#/media/File:Frequency_reuse.svg](https://en.wikipedia.org/wiki/Cellular_network#/media/File:Frequency_reuse.svg) the reuse of frequencies and small cells is why there can be lot of cellphones used at one

So your phone could only receive signals from cell towers nearby. The signal is divided up into changes with a limited frequency range. The phone will filter out stuff that is not on that channel and only look at data on the channel it uses. You can compare it to how you tune a radio.


So the phone only looks at the data sent on the channel it uses, There will be data sent there that is for other phones too. I am not exactly sure how the radio signal is encoded on this level but it will have some structure.

So you have something like a receiver ID and length of the packet. So the pone can look at the ID and if it is not for them they can just ignore the length of the packet. So there is no need to look at all data sent in that channel just the one that is for you.

Technically phones uses CDMA where you decode the data intended for you from a stream with data to multiple phones but the general idea is still it is only a small part of the radio spectrum you listen to. The packet description above is more what WiFi use CDMA is just harder to understand and is not required to get for how you limit what you need to listen of at before that stage.

First the cell phone will be contacting the local cell tower and negotiating a frequency to communicate on. This tunes the cell phone to a particular channel so it can ignore all the other stuff going on, like how when you listen to the radio you don’t also hear other channels.

But even within that same frequency there are multiple cell phones all communicating. They aren’t communicating at the same *time* though, there is a strategy called “time-division multiple access” or “TDMA”. Everyone on the channel rapidly takes turns communicating so the phone only really needs to pay attention to its designated time slot.

In concept the cell phone could try to listen to other signals, but there are some major problems with this. One is that the phone only has a limited amount of power in its battery, and listening in on everything would use up power for no reason and be inefficient. The other major problem is practical: It couldn’t understand what was being communicated anyway. Cell phone traffic is encrypted and cannot be decoded in a reasonable amount of time without the key (and certainly not with a cell phone’s processing or battery power).

That is one of the things a SIM card provides, it has the encryption key for communicating burned directly into it (called the “Ki”). So while the cell towers are technically broadcasting such that everyone could hear what is being transmitted, nobody except its intended recipients can understand what is being said.