How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
As you probably noticed, phone calls are quite low quality. This didn’t use to be always the case. In the early days of telephony, when calls were manually connected by human operators they would use a patch cord to connect your phone directly to the phone of the person you’re calling, you would have a direct electrical link to an another phone.
Eventually, the demand for calls grew and it was no longer feasible to work that way. This was especially difficult for long distance calls, if there were a 100 people trying to call from Boston to New York you would need a 100 cables between the cities to carry the calls. So, phone companies started looking for ways to cram multiple calls over a single line.
The first way to multiplex (send multiple signals over a single line) was to cut off the sound frequencies just to provide the bare minimum needed to understand speech, usually limiting the calls to the range from 300 Hz to 3.4 kHz. The quality was crap, but still understandable. Now, you can take 10 calls and simply pitch up the sound by different amounts for different calls before putting it on the cable. The first call would get the 300hz-3.4kHz band, the next call would get 3.5-6.6kHz range, the next one 6.7kHz-9.8kHz range and so on, until the calls start to fizzle out because with higher frequency comes worse range. If you were to listen to such a line, you would hear multiple people speaking, some with a normal voice, some with ludicrously highly pitched voices. When the signal gets to the destination operator, it’s pitched down to the original level and sent off to the phone at home. This allowed long distance phone calls to become a lot cheaper.
With the advent of computers, more modern technologies would be used, like digital audio (you can easily cram a 1s audio sample and send it in a couple of milliseconds down a digital link). With cellular telephony a lot of issues arose, like having a 100 devices attempting to speak to a single tower at once. Time sharing is used in that case, each phone gets a couple milliseconds to say what it wants, then has to shut up and the tower will call out the next phone that may speak, splitting time evenly between customers (with the exception of emergency calls, that might get a larger timeslot to ensure reliability or just make the tower disconnect other calls to allow for it, and phones from other operators which might be treated with lower priority than networks own customers). This method is called TDMA, and has been replaced by a lot more sophisticated methods (CDMA) that would be quite difficult to explain.
In case of landlines, they are mostly moving to VoIP now, with “phones” being often just software that you download on your computer. Then the calls are transferred like any other Internet data.
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