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?
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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.
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