How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
Some of the answers here are interesting but don’t actually address the question.
The answer is that data transmitted along cable lines DOES get interference, BUT several important things happen:
1. Information being carried down a line is some kind of analog signal. It might use various voltages and frequencies, and splice many “data streams” together in a process called multiplexing, but it’s an analog signal of some sort. However, what it represents is usually digital data, and as long as you can uncover that original digital representation okay at the other end, the interference is irrelevant.
2. Interference is okay, so long as the Signal to Noise ratio is high, meaning we can clearly distinguish some light noise from a strong signal, and ignore it. If we have to do this over really long distances maybe we put repeaters every once in a while. This way the interference over the whole distance doesn’t sum up: each repeater can strip out mild noise then repeat a clean signal down the line. Also, if we expect interference at e.g. 2MHz because e.g. it’s known to be widely used for something, maybe we transmit at 3MHz to avoid it.
3. At some point we convert back to digital. Let’s just make up a simple scenario that the signal is represented by voltage and 5v is a binary 1 and 0v is a binary 0. Well if we have about 1v of noise from interferenece and we can still easily say “anything from 0-2v is a binary 0 and anything from 3-5v is a binary 1”, then that 1v of noise doesn’t matter and the digital data we can recover will be the exact same one we put in.
4. Sometimes errors happen. Usually higher-level protocols can detect them. Say we send 011010 but we receive 011000. Well we might want a way to know something went wrong. A really simple example is called parity, and so e.g. we can just make a rule that for every 7 bits of data we add an extra bit to make the number of 1’s an even number. So we send 0110101, and we receive 0110001. We know there was an error because there are three 1’s, and that’s not an even number. The communcations protocols handling the data exchange can send a signal down the wire asking for that data to be sent again.
So the answer is that there is interference, you try to keep it low relative to the signal, you might engineer your signal characteristics to avoid certain expected interference, you might use repeaters, you represent data in a way that is less affected by interference, and you detect interferenece and handle error correction and retransmission as needed.
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