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
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.
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.
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