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A friend of mine asked me this and I wasn't able to come up with a good answer. Basically, you could be watching Comcast television in one room but the internet router is in another room. TV is fine but then the internet cuts out.

With his case, internet and TV come from the same source but a splitter splits internet on one coaxial cable and TV on another.

Any ideas?

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They're independent services/signals sharing the same cable -- at least up to your splitter. – martineau Jan 5 '11 at 10:40
It sounds like a case for the "turn it off and turn it on again" remedy. Sometimes the modem (the device that dials out and receives data from the internet) simply needs resetting for whatever reason. Turning the modem and/or router off for 30 seconds normally solves a connection problem. Remember to switch it back on again. – Jay_Booney Jan 5 '11 at 18:24
up vote 3 down vote accepted

The Internet connection depends on more than a physical link to the cable company's HQ. Their network systems, mostly routers and authentication devices, need to be working. If the router one level up from you (or higher) fails, Internet connectivity will be down. Meanwhile the perfectly good physical cable can still carry TV signals, which don't use IP (at least on many systems).

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The cable modem handles getting you on the Internet, and often (because of nonpayment of bills ?) the Internet services are cut off by denying the cable modem authentication details and preventing you from joining the backbone which connects to the Internet

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TV usually works, for the most part, on a downstream channel though some pay-per-view, and off-site DVR features may use upstream to communicate with head-end controllers.

Cable systems were originally designed to have a few large transmitters broadcast to several thousand receivers. To have such a system work in reverse requires a much cleaner path, and simply does not have the same amount of bandwidth allocated for upstream communication.

Furthermore, while a bad splitter or run of coax might degrade signal slightly near a home's television sets, since the noise is not amplified or converted to light to run on fiber, and the inbound signal is strong to begin with, the degradation mightn't be noticed. However, in the reverse-path situation, all the ingress noise created at the customers' homes gets amplified and combined before getting back to the CMTS; enough noise to drown out one cable modem at one house might be enough noise to drown out all cable modems on the whole node.

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