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.
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.
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
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.
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.
The answer is: they do.
That’s what phone calls have noise and why data needs error correction.
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.
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