How do cable lines on telephone poles transmit and receive data along thousands of houses and not get interference?
In: Engineering
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.
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.
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 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.
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.
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.
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.
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.
The answer is multiplexing.
[There are many ways to do it.](https://www.elprocus.com/what-is-multiplexing-types-and-their-applications/)
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.
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.
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