How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
It’s not a matter of not having interference, it’s a matter of keeping the level of interference low enough that the signal can be recovered at the other end. A more technical term would be “Signal to Noise Ratio” SNR. Here are a few techniques you can use to work around noise:
1. **shielding**. co-ax cables and shielded cables use a foil or mesh layer surrounding the signal wire. Outside electromagnetic interference is absorbed by the shield and never reaches the signal wires on the inside. It’s the same principle as a *Faraday Cage*, just extended over the whole length of a wire. Cable TV typically comes over a co-ax wire.
2. **twisted pairs**. Take two wires and twist them together, so that any electro-magnetic interference affects both wires equally. Send your signal down one wire. At the receiver, you subtract the value of the “dummy” wire from the signal wire, giving you a clean signal again. Telephone and ethernet cables use twisted pairs (and wires for very long distances also have shielding around the twisted pairs).
3. **repeating**. After fixed distances, receive the signal into a device, and re-transmit the signal again with more power and no noise. Since SNR is a function of distance in the wire, keeping your wires short and repeating the signal can help avoid problems.
4. **modulation**. There are three basic modulation schemes you can use to transmit a signal over a wire: amplitude, frequence and phase. The first two are used by AM and FM radio, respectively. AM can be susceptible to noise while FM is more resistant (which is part of the reason why music stations tend to use FM while talk stations tend to use AM, and why AM radio quality decreased gradually with distance from the antenna while FM tends to either be perfect or static with nothing in between).
Fiber optic cables don’t have to worry so much about electromagnetic interference. Glass fibers have multiple layers which reflect light back into the center of the fiber, and then are surrounded by shielding to keep external light out. You can get longer distances with fiber optics than you can with most metal wires, but you still need repeaters to keep the light intensity high.
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