I don't get cable/coax networking protocols.

I’m having problems getting a clean Comcast signal to one room in my house. Used to work fine but at about the time of the digital transition, the signal started to fail. Comcast can see and query the cablecard but we can’t get any channel signals through. All the coax and cat5 cabling in our house goes back to a central wiring closet; the ethernet network in the room in question works fine at 1 gigabit but for some reason the coax/cable network fails.

Why does the protocol/modulation scheme for cable fail? Why can the device be addressed and queried but we can’t see channels? Why do they need to put a signal amplifier on the line — i never need to do this for ethernet? Is comcast still using some analog scheme to send the signals across? This just seems odd and ridiculously archaic. And the crazy pairing nonsense for cablecards with all kinds of identifiers needed to be traded back and forth — it makes DHCP and mac addresses look positively simple.

I know I could go read about 64QAM and 256QAM and Cablelabs and all kinds of other stuff to get all smartened up about this but I am frankly tired of dealing with it. I’ve been ignoring the verizon fios offering in our neighborhood but if it would let me junk the coax and move to all cat5/ip i might consider….

7 thoughts to “I don't get cable/coax networking protocols.”

  1. on some modems will give you some fairly detailed diagnostic information. If you scrape that page and put it on a graph you can begin to see how it varies over time and can get an idea as to what’s happening. Chances are you have poor wiring out to the street – I doubled my neighbors connection speed using some new RG6 to the street after Comcast said nothing was wrong with their wire!

  2. With digital cable, you have a much higher susceptibility to interference. Put another way, the same coax that worked for analog cable could be less than stellar for digital cable.

    I have Comcast and had the same kind of problem. Invariably the issue turns out to be either a misplaced cable retainer, one that is either too tight or has nicked the braid. I had to replace the one cable that was damaged, luckily the room that had the bad cable was literally above the utility panel where the cable entered the house. All the best!

    A suggestion that may help. If you know someone who is a HAM radio operator (ELMER), they may be able to help you.

  3. Of course they are using an analog scheme… when it comes right down to it, the whole world is analog. QAM and it’s varients are merely ways, albeit complex ones, to impose a stream of digital bits onto an analog carrier suitable for transmission over long distances.

    Cable distribution uses a high frequency carrier to send to you all of the cable channels you may wish to watch. Every channel is there, all the time, whether you choose to watch it or not. Each channel is allocated it’s own little slice of the frequency spectrum. So, if your cable distribution does not have a flat frequency response across the entire band you will get some channels “better” than others. In the analog world this translates to picture quality (snow, ghosting, etc). In the digital world, it’s all or nothing. either you get the channel or you don’t. In the comcast world, this can appear to be random channel outages due to the fact that they virtualize their channel numbers in the cable box or cable card. Just becuase 104 and 105 are adjacent numbers displayed on your TV does not mean that they occupy adjacent physical channels in the spectrum allocation.

    So, what can you do? Pretty much nothing other than replace any components that may be suspect, kinked cables, loose connectors etc. Any one of these issues can have huge impact on impedance matches and return losses. Oh and also, review your whole distribution scheme to make sure you don’t have any unnecessary splitters, etc. If you are like me, your system has evolved over time and sometimes I find a splitter I installed for a reason I cannot fully remember. Don’t even get me started on splitters and amplifiers… cost of these items has absolutely no correlation to quality.

    Good Luck

  4. FiOS TV uses Coax (QAM) for TV, and IP over Coax (MoCA) for the IPG and VoD. Best HD resolution … but let me know if you have specific questions and I can dig them up.

  5. Ah ok thanks. basically that tells me that, no matter what, i have to improve the coax cable plant in my house, as both comcast and FiOS use it for video distribution. thanks

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