How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
It does, all the damn time, and we go out damn near daily fixing that stuff. Its typically refered to ingress/egress. And is common on coax systems. Fiber systems arnt susptibe to rf intereference like coax is. Most hardline on the poles is extra thick with lots of emf shielding to help. But damn squirels here the freq sent on the lines and think their bugs and try chewing to get a tasty treat making us fix your line to your house called a drop as thats usually soft rg6 or rg11 soft coax cable.
short answer: they don’t
long answer because of stupid bot: there are always interference by electromagnetic waves, as from radio transmission, the sun oder other parallel wires.
that’s why data connection, wired or not have a signal noise ratio, which determine the quality of a connection.
both sides need to be able to differentiate the signals from the background noise. if that’s not possible they try alternative frequencies. that’s how one wire can transport multiple data connections simultaneously, each using their own frequencies and encryption.
that’s called the handshake
From every signal (like a song) you can always know how it changes **through time** (with the graph of a sound wave) **AND** how its **frequencies** changes (with a graph that has frequencies from 0 to infinity in x and their “loudness” or amplitude in y). You can always pass between these two graphs interchangeably.
Now, imagine that this is the graph of the **frequencies** of a song: |n____. The “n” covers the frequemcies of hearing range, so from 20Hz to 20000Hz. If you add other spectrums on it, you would not be able to separate them again, because they **overlap**. If you don’t want to overlap them, you can shift every spectrum a bit to the right and then sum them togheter, getting something like this: |nnnn___. After receiving this signal, you can isolate a single n that you are interested in and reposition it in its original position. From the graph of the frequencies you can then get the graph of the soundwave
It’s similar to how AM and FM radio work. When your analog audio signal from your phone is modulated onto another signal you can fit millions of different audio signals.
The analog signal from connection point to your house is not very long. And it actually not uncommon to get some powerline interference (50 or 60 Hz hum).
From the connection point to the core network it will be digital and not susceptible to interference in the same way.
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.
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.
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…
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
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.
Latest Answers