How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
they dont. well not any more back in the analog days it wasnt an issue . now they use fiber to the node then back to co-ax for the last 1000ft where there is less degradation of the signal. and even this is also digital so its all or nothing. so there far bit of over head from check suming going on as well
Analog or sound waves can be combined using a special math equation into a single sound wave that can be unmixed at the destination. It’s not like people speaking over each other, but more like how if you press multiple keys on a piano you can hear the combined sound and a maestro would know which notes are being pressed together.
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
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
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.
This primarily only applies when discussing *coaxial* cable. Some pretty good explanations have been given about how that works.
Bear in mind, if you’re talking about copper telephone lines, each end point has its own pair of (very small, like 24ga) cables. Thus, each house has its own + and – wires over which signal is transmitted to the nearest node (point at which signals are converted to fiber, generally) or CO (building that houses telephone equipment).
While this technology is growing much less common, some ISPs have, in some areas, taken to bringing fiber closer to clusters of houses, where a new node is installed so that they can send DSL over copper very short distances. At such distances, because DSL speeds are heavily dependent on the length of the copper cable, DSL bandwidth can reach speeds as high as 100Mbps.
If you’re talking about fiber optics, there *is* an analogue to the way coax works. Generally speaking, with fiber optic cable, each end point/home gets its own fiber which runs continuously to the nearest node or CO. That fiber may—and usually is—spliced at various points, so what you often have is, for example, a cable with 4 or 6 fibers running from the house to the “pedestal,” where it’s spliced to another cable which might have 12 or 24 or even 288 fibers, and these generally progress toward larger fiber-count trunk lines until reaching the CO.
However, when you need more fibers than you have, say a development is built at the end of an existing fiber line that’s already mostly or entirely in use, and installing a new line is cost prohibitive, you can install a fiber “splitter,” which allows you to send multiple (up to hundreds) of signals down the same fiber, by splitting into different wavelengths of light. Those signals are re-split at a node site to separate fibers and then sent off to the various new end points.
An example of this is the fact that the entire small town of 800ish people where I live is fed entirely by a 12 fiber cable.
They do! It’s an annoying problem.
The two types of lines you’re probably thinking of are twisted pair telephone lines (for DSL) and coaxial cable TV (DOCSIS/cable).
The first works around interference with some basic arithmetic by forming what’s called a Balanced Pair. Lets say you want to send the number 3 down the line, but there’s interference on the way that makes it look like 4. The solution is to send 3 on one wire and -3 on the other wire. When the signal arrives to you, the interference is still 1, but it’s 1 on both wires, so the signal you see is 4 and -2. Invert -2 into 2 and take the average (4+2 / 2) and you get 3 again! There’s a little more to it but this form of interference rejection is commonly used, it’s the reason a good quality microphone has 3 pins instead of 2.
On coaxial cable there’s an inside part that is basically a radio antenna. The outside part is a shield to protect against interference.
Another way to work around this problem is to cut the signal into pieces and assign each one a different frequency. Most interference isn’t at all frequencies – just a few. By segmenting things up you can use math to determine that certain frequencies are bad and avoid them. Think of this like a courier company and lanes on a road: instead of using a single massive delivery truck that takes up all the lanes, they use smaller trucks and can avoid potholes.
These days the simplest solution is to avoid interference in the first place. All signals used to come from a large centralized location (telephone central office) so you would have thousands of conversations on wires running next to each other. Today the equipment is getting moved to the neighbourhood and you might only have to deal with a hundred or so, and much shorter wires.
I’m a network maintenance engineer for everyone’s favorite cable company and some of the answers here are pretty funny. There’s absolutely tons of interference out there, and that’s one of the biggest parts of my job is isolating and mitigating it. Loose connectors, poorly shielded wire, and bad devices can put foreign RF signal (ingress) back onto the cable and it funnels back to a headend CMTS server which connects to every cable modem. Too much ingress and that CMTS is gonna start to misfire or eventually shut down.
As far as how do hundreds of modems all use one common cable to communicate data has been explained already, frequency division and wave division multiplexing allows small chunks of the RF spectrum to be used specifically for certain things, called QAMS (quatrature amplitude modulation). Our system uses 3-4 upstream QAMS which controls data being sent from your device back to our CMTS. If there’s interference in that frequency range, typically 5-42 MHz, you’re going to have your signal to noise ratio lowered, and if it’s bad enough, affecting others as well. The higher the better, more signal less noise.
These are all common to DOCSIS 3.0 standards and the newest standard DOCSIS 3.1 adds another level of complexity in its OFDM (orthogonal frequency division multiplexing) QAM. OFDM QAMs are in the higher frequency ranges of 700-800 MHz which is where LTE cellphone technology resides. Any slight bend in a hardline cable or small area of improper shielding anywhere even close to a LTE transmitter and shit hits the fan, full node outage in a second and good luck finding the crack.
As you probably noticed, phone calls are quite low quality. This didn’t use to be always the case. In the early days of telephony, when calls were manually connected by human operators they would use a patch cord to connect your phone directly to the phone of the person you’re calling, you would have a direct electrical link to an another phone.
Eventually, the demand for calls grew and it was no longer feasible to work that way. This was especially difficult for long distance calls, if there were a 100 people trying to call from Boston to New York you would need a 100 cables between the cities to carry the calls. So, phone companies started looking for ways to cram multiple calls over a single line.
The first way to multiplex (send multiple signals over a single line) was to cut off the sound frequencies just to provide the bare minimum needed to understand speech, usually limiting the calls to the range from 300 Hz to 3.4 kHz. The quality was crap, but still understandable. Now, you can take 10 calls and simply pitch up the sound by different amounts for different calls before putting it on the cable. The first call would get the 300hz-3.4kHz band, the next call would get 3.5-6.6kHz range, the next one 6.7kHz-9.8kHz range and so on, until the calls start to fizzle out because with higher frequency comes worse range. If you were to listen to such a line, you would hear multiple people speaking, some with a normal voice, some with ludicrously highly pitched voices. When the signal gets to the destination operator, it’s pitched down to the original level and sent off to the phone at home. This allowed long distance phone calls to become a lot cheaper.
With the advent of computers, more modern technologies would be used, like digital audio (you can easily cram a 1s audio sample and send it in a couple of milliseconds down a digital link). With cellular telephony a lot of issues arose, like having a 100 devices attempting to speak to a single tower at once. Time sharing is used in that case, each phone gets a couple milliseconds to say what it wants, then has to shut up and the tower will call out the next phone that may speak, splitting time evenly between customers (with the exception of emergency calls, that might get a larger timeslot to ensure reliability or just make the tower disconnect other calls to allow for it, and phones from other operators which might be treated with lower priority than networks own customers). This method is called TDMA, and has been replaced by a lot more sophisticated methods (CDMA) that would be quite difficult to explain.
In case of landlines, they are mostly moving to VoIP now, with “phones” being often just software that you download on your computer. Then the calls are transferred like any other Internet data.
Benoit Mandelbrot (of mandelbrot set fame) is largely responsible for this. Apparently he invented the “special math equation” mentioned in an earlier post which helps cancel out interference with a specific sound wave.
At least that is my thoroughly lay understanding of it. The point is that Mandelbrot and the early research into fractal geometry were instrumental in fixing this problem.
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