How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
From what I remember (which may be not completely correct), when you’re dialing a number, you’re setting switches along a route to the destination number, and the phone provider sorts incoming traffic to the proper destination.
I see a lot of discussion about multiplexing but the answers seem tangential to the original question.
For coaxial cable, interference is absolutely a concern especially from cellular bands that use the same frequencies. The cable had an outer sheath that looks like aluminum foil that provides shielding from interference. However outside energy can still sneak in from bad or unterminated connectors, among other things. This is called ingress noise.
A good cable tech will connect a test set at the curb to compare the signal at the curb to the energy coming from your house. If ingress noise is present, expect them to start replacing connectors, wallplates, etc.
Also, the cable network can tolerate a fair amount of interference using a technique called Forward Error Correction. Basically extra redundant data is transmitted, and this extra data can correct a certain amount of bit errors from interference.
EDITED AFTER POSTING BUT I’M NOT GONNA EDIT MORE TO REMOVE REDUNDANCIES.
Big picture.
Analog computer modems were using digital info compressed into an audio signal that sounded like hissing static sounds.
Phone signals and internet over phone lines using an old analog modem connected to your telephone line, that’s highly compressed analog information – high pitched sound like what you could hear when the modem first connects, or if you pick up the phone and listen when someone was logged in to AOL.
The sound you hear is two modems “singing” data to each other.
Phone lines carry not only compressed voice but also could carry that “conversation” of computer data with the internet service provider using that same basic analog signal. So, text and pictures digitized but then converted to sound.
Modems were listed at 14k/sec then 28k 33k and 56k, but I think the max actual transmission speed was 28.8k/sec plus super compression (like WinZip files) for 56k.
Then DSL came around, a digital signal running at a frequency high above all phone audio. So I think DSL is still “somewhat audio” but totally different from phone audio and modem audio.
Analog modem was one call at a time. You have to hang up the phone to login online, and log off the internet to make a phone call.
DSL can slide in side by side with regular analog phone calls.
VOIP is phone voice that is digitized and flowing as DSL data, the opposite of data flowing as analog signal.
Over the air TV also compressed video and audio into a radio wave that is decoded by the TV tuner into human level information.
Cable transmits digitized voice, digitized video, and digitized audio (plus computer data) over cable. Connections are established and packets that are communicated are addressed to their destination. THERE’S NOTHING ANALOG ABOUT CABLE SIGNALS until the receiver decodes it.
So your home cable signal is kinda like personal point-to-point, because of digital addressing, but all together within a stream, but with digital addressing to separate signals from your neighbor’s point to point connection. (Not audio multiplex.)
You can SEE the difference. When a discrete digital TV signal gets stopped or corrupted, you see missing square chunks or a frozen screen or blank.
When an analog signal is imperfect you see and hear increasing levels of static speckling and may see bleed-through as the tuner tries to decode two nearby signal frequencies, tuning in on one and tuning our others, but not being very successful.
I guess it depends on what sort of setup you are talking about. I am a telecommunications engineer in the UK.
How things are fed here is you get dial tone from the exchange in a pair of wires that are twisted together. The twists help resist any interference from other circuits.
These cables generally go to a street cabinet which, again generally speaking, will be close to your house.
At the green cabinet there is a DSLAM which is a box that had a fibre connection in it that your phone line runs through fibre ports which then when it comes out it has your dial tone and broadband service on it.
This is then on a pair of wires to your house via different connections. In your house you should have a micro filter which is really a splitter that splits the different frequencies the one you can hear for the phone and one that’s beyond your hearing range for broadband.
I have worked on lines that have a lot of cable above ground on poles and when using my test phone I can hear the radio on the line. But this can be filtered out by phone sockets.
TLDR. Basically from the exchange you have one pair of wires all they way to your house.
Having them twisted makes a big difference in reducing any interference.
There is a high level of software detection called data error analysis. Basically, there is no way for a computer to know if the data sent was received without and infinite loops of checks and confirms that would slow computing to a halt.
For networks, this means that systems are designed to have data sent in scattered arrays that will verify if that signal was interfeared with, wait, and then send again if priority is low.
3 things send data at the same time, priority 1,2,3 respectively, one send first and 2 and 3 wait. So on and so on.
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.
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.
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.
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.
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.
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