How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?


How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?

In: Engineering

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.

The answer is multiplexing.

[There are many ways to do it.](

In really simple terms, you can put multiple individual ‘data streams’ into the same signal by various means. You can give each data stream its own frequency or its own time slice on the channel etc and then reassemble them back into the individual streams on the receiving end.

A really simple eli5 example of multiplexing over fiber would be to give a different colour laser to each stream and shoot them down the fiber. The receiver could then use colour filters to single out the individual colours again to recover each stream.

In metro areas data is transmitted mainly via one of two different cable types: Fiber Optic and Coax. Get back to these in second.

Interference in data transmissions come from Radio Frequency (RF). The biggest source of RF noise that causes inference is power or electricity. One of the easiest ways and common to cut down on interference is to keep power lines away from data transmission lines. This is why if there are power poles in an area then the data lines are buried. Or the other way around if there are telephones poles then the power lines are buried.

However keeping power lines away from data lines is not always possible. So that is where the two different cable types come into play and they treat interference completely differently.

Coax is older technology and has been around for awhile. This is the same cable type that brought TV and Cable to the home since the 1960’s. Coax addresses interference basically through a ton of shielding wrapped around the central core cable. If you ever cut into a coax cable. There is a central copper wire surrounded by thick plastic and then a metal jacket and then more plastic. It is only the central copper wire that carries data. The plastic and metal surrounding the copper wire protect the copper wire. There are several grades of Coax some with more metal and plastic protection that limit interference even more. Coax is cheap and easy to produce and the transmission of tried and technology.

Fiber is newer technology and is based on the transmission of light (lasers). Great thing about Fiber is that the light is virtually immune to RF interference because light is different then radio frequency. Power lines literally do not change the direction of light therefore you can for the most part ignore interference when it comes to fiber lines.

So why not do everything over fiber? Well for the most part that is the direction data transmission is going. Fiber used to be very expensive both in the cabling and the equipment needed to use fiber. It has only been in the last decade or so that fiber has become cheap enough to be used everywhere. Coax has been around for a long time and there is a lot of it. It will take time to replace the Coax with Fiber. This still may not happen completely because Coax technology is still being improved.

This plays into the getting fiber to the home or getting cable to the home.

Probably still way more then an ELI5, but what I could come up with. RF is hard to explain. Think of it as static on a radio station. If there is too much static then you can’t hear the music. Electricity is the main source of this static when it comes to data transmission.

We are talking about billions of possible frequencies. Think of it like this, you have a bag of sand. Only one size grain is meant for you though. So you have two sifters, one that lets through all grains all grains smaller than yours, one that holds back all grains bigger.

So you pour the sand through the first sifter, and boom, no grains smaller than yours get through. You pour the leftover contents through the other sifter, and only the ones meant for you get through. Boom. Out of billions of grains, you get the ones meant for you, no matter how mixed they were before.

Edit, since I apparently wasn’t clear enough. The information is split into different frequencies. Each frequency being the grains of sand. They can be mixed together, yet still singled back out through bandpass filtration, aka, the holes in the sieve here.

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

The interference part of your question.

Edit: Actually a different side of it to consider. The other answers already cover the signals interfering with each other.

There is a lot of interference.

Cracks in cable, bad connectors, faulty hardware and other issues can all lead to signal egress and ingress that can lead to interference with external RF signals from leakage and internal interference from outside sources getting in. These issues increase the amount of signal noise in the system, which essentially makes the signal dirty by reducing the amount of signal above the noise floor (SNR). It is maintained by technicians in the field and an office crew that monitors the plant for those and other issues. The long range work is done almost entirely on fiber-optic cable, but that still requires a lot of work. Fiber splicing is hard work that requires a clean room to prevent dust and other debris from getting inside of the splice and blocking or (even slightly) redirecting the light.

With a coax network, every piece of cable, connector, splitter, directional coupler, amplifier, mini-bridger, and literally any other piece of hardware can cause interference. Even electrical issues in homes can cause problems. I can’t tell you how many intermittent area outages I’ve seen that were caused by people using old electronics that were causing interference. Everything has to be perfect, because there is just so much on these networks.

Basically, it’s done with a lot of work. A lot.

The other answers regarding multiplexing and the like should explain the parts that I would have to Google.

The answer is: they do.

That’s what phone calls have noise and why data needs error correction.

Eli5 answer: There is interference. But the content can be transformed into a format that is easier to send. Additionally the sent signal is processed to minimize the impact of the interference.

They can and do. Especially with the old analog lines. Now there is a lot more digital lines. Over the digital, the interference is ignored. But over analog some times you can hear someone else’s convo. Also, a strong enough radio signal can go over the phone line and be heard. For instance, a ham operator may be heard in a near by home over the phone when transmitting.

They do get interference. Look up ingress and egress for cable. The FCC is very strict about this and signal leaks are almost immediately taken care of

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.

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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