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