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