Archive for July, 2017

What to do when your cable modem doesn’t work

I got a call from my sister about her internet connection dying.  Here’s what I do in this situation – my internet is cable modem (CM) and my ISP is comcast.

  1. Attempt a reset by the ISP.  Through remote reset, the ISP can re-initialize your router and this may help.  If they *can’t* remotely reset your router, that seems like an indication that they may need to physically send someone to your home, doesn’t it?
    1. Gory details: Cable modems function on a standard called DOCSIS.  You probably don’t hear about this or think about it much but periodically your ISP may start nagging you insisting you need to upgrade your cable modem to a newer model to get the best service.  This probably means there is a new DOCSIS standard out, or they are offering support for a new standard and unless you get a newer modem, they won’t be able to sell you more expensive service.  You’ll probably only find yourself in this situation if you choose to buy your own cable modem rather than leasing one from your ISP – and you should almost definitely do that.  Newegg offers a bunch of DOCSIS 3.0 Cable Modems for under $60 and that’s what you’d probably pay in lease fees after 1 year.  Plus, why pay Comcast for this?
  2. Attempt to reset it yourself through power cycle.  Simply unplug the CM, wait a minute, then plug it back in.
    1. Waiting a minute may or may not be a magic amount of time (depending on your ISP, your CM, or the service you’re getting from your ISP with your CM).  Go ahead and wait a full minute though.
  3. Attempt to get in to the configuration for the cable modem and figure out what’s wrong from the logs.
    1. If you lease your modem, then your ISP has probably locked this down and you won’t be able to reach it (so go back to 1 or 2).
    2. Otherwise, it might be http://192.168.100.1 – so give that a try.
    3. Or, it might be printed on the bottom of your CM so get a flashlight and go try to take a peek.
    4. Or, you can do a web search for your CM model and probably find its default address that way.
    5. Assuming you were able to reach your cable modem’s configuration page, as it boots, you should be able to see a few useful bits of data:
      1. Where, in the connection negotiation process, it is failing.  This may or may not help
      2. Signal strength information.  The two most interesting values will probably be signal-to-noise (should be ~32dB or higher) and power (should be between ~-10dBmV to +10dBmV).  If your numbers aren’t in those ranges, you probably have an issue with your line connection somewhere and a technician will have to come to the house / line to assess where the problem is
      3. Full logs.  If you’re hitting a problem, you’ll probably find your logs end when they hit that snag and you may be able to research based on that information.

I hope this is useful!

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