Thursday, May 22, 2008

Tips: Cable Modem Signal Troubleshooting

I've had a few problems with fluctuating signal levels on my cable modem, which result in losing connectivity for a period of time. If you have a cable modem and the connection has ever been flaky for no apparent reason, you might be able to use a few tips I've picked up.

First, most cable modems have an internal management/monitoring system you can use to see what's going on, and it's conveniently exposed via HTTP so you can look at it with your browser.

This web page has a great collection of information on accessing the modem status and what numbers and error messages to look for. This may sound obvious, but download all of the info before you have an outage unless you have a redundant connection. (Also, don't be scared off by the old dates on the page ... most newer models have the same interfaces, and of course DOCSIS is a standard so it doesn't change year to year).

The modem typically listens on So if you have a router (e.g. a WiFi access point) between your PC and modem, you may need to adjust the subnets or routing, or remove it, because this subnet may be downstream/local/wrong-side-o-the-box for you. If subnets and routing aren't your thing, the easiest alternative is to unhook the router and connect directly to the modem.

With cable modems, it's not just an issue of having a "strong enough" signal -- the signal has to be consistent within a certain band, because the modem determines its uplink transmit power level based on the downstream level it observes. In other words, if the downstream level comes in too strong, then the modem will reduce its upstream power in response, and you can lose connectivity because of that.

If you've had some issues, you see signal fluctuations, and your cable operator doesn't have any reports of problems (their system can remotely observe other people in your area), here are a couple of remediation steps to try:

First, isolate the cable modem on your wiring -- meaning unhook other devices, TVs, DVRs, etc., which might be on your line. Clearly not a long-term solution, but if this makes a difference then you'll have info that will help if and when you do need the cable company to make adjustments.

If that doesn't make a help, follow the cable line (assuming you can get to it) from the modem all the way back to where it comes into your house. Remove any unneeded splitters, barrel connectors, 20m tangles of  coax, etc. To the extent that you do need these parts, they should be ideally be 5GHz capable (marked that way right on the device). If you have older ones that say, e.g., 1000MHz, replace them with newer ones.

Splitters and the like are not normally weatherproofed, and apparently (I have no proper data to support this, but technicians have told me), they show deterioration after a few years of exposure to heat, cold, condensation, etc. in basements and crawlspaces -- enough to cause a high-frequency signal to get flaky. And they can end up producing effects that vary with temperature.

Last, on my line, a technician also chopped out the connectors where the cable company's own line transitions to the house wiring, put new connectors on both sides, and hooked it back together. If you want to make this swap, or replace any of your connectors (as opposed to parts that the connectors attach to, like splitters), you'll need a coax tool for cutting and stripping the wire and then crimping on a new connector.

These primitive coax tricks won't solve all problems (even in my own installation, these changes resolved maybe 85% of the trouble) but they may make a difference and, at the least, they'll remove a bunch of unknowns from your equation.


javieth said...

I like to fix things all the time, and i always have a lot of cables around me, around my house. But i believe they are very useful, like this blog. I liked it very much.

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