How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
If you are referring to how over a hundred people can transmit data over a few wires without getting mixed up, here is the closest explanation i can muster.
Whenever you wish to send or receive data, whether it be streaming a video or downloading a file, your PC will communicate will make a request for the data. In the case of streaming a video, your device will send a request to the server holding that video. The server will then package that data by splitting it up, associating your device’s IP address (like a home address but for computers), and will pack the segments into things called packets. The packets are transmitted through the network on a wire or by radio waves in the form of binary. Devices called routers will take this packet, read the destination IP address associated to this packet and will route it accordingly. This means the wire is filled with little segments of addressed data traveling based on their destination address, much like physical mail or cars on a highway. You should not receive anybody else’s data as your IP address and/or default gateway (your router’s IP address) is always unique. But that’s my best explanation without getting into more complicated things like the OSI reference model, TCP/UDP protocols, and ports.
Cable and telephone protect against interference in different ways. Telephone lines use twisted wire. The twists of the wire pairs block outside interference and help propogate a signal further. Than if it were untwisted.
Cable/coax cables run on a single copper conductor, surrounded by a dialectic medium and protected by aluminum shielding.
Both are limited by a rate/reach issue… The higher the frequency/data rate, the lower the distance. 128 kbps (very slow) dsl can go about 20 thousand feet, while a 100 Mbps dsl connection goes about 1000 ft max. Each home has an individual coax wire or twisted pair ran to it.
The cable/telephone company runs fiber to distribution locations and uses equipment (DSLAM in the case of DSL) to put the signal onto the copper wire to make the last mile connection… So in the case of them suddenly offering higher speed, it means they’ve ran a new DSLAM or fiber node closer to your home.
It should also be mentioned that you DO get interference. Lots of it! Most you don’t even notice, because the compounded error correction between the various pieces of hardware and software make it seem like everything is flowing smoothly.
In analog voice communications particularly, the required quality of the physical cabling (in some cases, tin or even lead) was incredibly tolerant. Many of us remember background humming, a distant-sounding busy signal, or just crackly noise during long-distance calls. Our brains did all the error correction, or we just asked the person on the other end to repeat it.
When we started using phone lines for digital communications, trying to push more than about 10Kb on the wire was unreliable. That’s when higher speeds were only realized by improving error correction. Since that time, vastly improved media (twisted copper and fiber optic are the most common now) have helped a lot, but transmission and receive errors are always present, even in communications between a processor and a hard drive.
I’m not crazy about most of the responses. Telephone lines use a pair of wires. The signal is the difference between the pair. Because they are the same length, connected the same way, and right next to each other, they tend to receive the same interference–but, this doesn’t affect the difference between the pairs. Wire pairs are actually twisted to ensure that on average, each is the same distance from interferers , such as electric lines on the same pole.
Cable TV uses coaxial cables. That’s a different kettle of fish, but it’s also constructed to mitigate interference effects.
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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