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.
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.
Cable tech here that works in a HFC (Hybrid Fiber-Coaxial System) system. The radio carriers are used in many different ways in many different systems, one common one is (in a very simplified way) timing the signal transmissions of equipment back to the server with each other so they dont overlap. A common system as well is equipment being assigned different portions of the radio frequency spectrum so they dont overlap with each other (except in minimal ways as to minimize it). The two most important things cable techs keep in mind for a clean and working system, is to minimize outside interference because most RF networks like cable systems overlap with cell carriers over-the-air as well as other RF sources like other over-the-air tv and communications, so we must maintain a closed and sealed system, this means no damaged lines, no open connections, and most importantly customers who try to do things themselves use terrible quality coax or connectors which allow outside RD carriers to bleed into our system which interrupts services as they can and will overlap. Sorry for terrible English, busy while answering but saw my opportunity! Cant focus on the grammar.
Edit: grammar
They do!
Interference is a big issue. When I was in cable TV, I’d disconnected numerous homes for causing interference in the rest of the plant. Leaving a note saying why, and for them to schedule an appointment to see if we could track it down.
Damaged coax, poor connectors, and even some TV have been the culprit in most cases.
It can also get in from the plant side.
If you’ve ever noticed several small antennas on your cable providers vehicles, those are connected to ~~socialized~~ *specialized* receivers known as “leak detectors” and look for a specific frequency.
My old company used a computerized system that collated all the detected transmissions, and used GPS and triangulation to determine exactly where it was coming from. Usually from animal chew, or other types of damage to the physical lines.
Squirrels apparently love the “white shit” aka dielectric, inside them…
They can’t. Nowadays most are converted to digital (or a series of ones and zeros that represent the analog signal), and these signals are transmitted. When they get close to where they want to go, and the result of the interference would be small, they’re converted back to analog.
As /u/doyouseeit suggests, older lines would multiplex the signals together. Think of it like the radio in your car. The signals from each station are separated, and interference will be minimal. There is still interference, but it could be minimal with large separation between the signals. (Note this also implies an expensive wire to transmit the data that could give you enough separation between the signals.)
There is a whole branch of mathematics related to this, called communication theory. It was really established when a smart man called Claude Shannon took work started by Harry Nyquist and really proved a lot of unexpected things. Although that work was originally complicated, many cases of the math have largely been solved to a degree that you can take off-the-shelf solutions, and guarantee communication to whatever reliability standard you want.
The data is converted into digital, electromagnetic waves. The data is then converted from time domain into what is called frequency domain. Filters can be applied to single out individual frequencies to “find” the correct information you want to look for.
Think of a radio. When you turn the dial on the radio it changes the stations. Each one of these stations, on FM, are on different frequencies. Turning the dial essentially filters out each station and finds the one you want to listen to. All of these stations are still being transmitted layered on top of one another. Digital data over cable lines and telephone lines can have thousands of available “stations” instead of the handful you can hear from FM radio. When you filter out the frequency you want, it also removes the “interference” of other frequencies.
Electrical Engineer here.
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