Cable Troubleshooting GuideHowto fix speed issues and connection loss on cable
2006-04-26 (updated: 2009-11-09) by Rocky_Grim
Tags: troubleshoot, splitter, cable modem, tap, SNR
This guide contains information for fixing issues with cable, mostly compiled from a friend who works for Adelphia cable here in New Castle, Pa. This guide will aim at resolving lost connection and slow speed issues by yourself. You should also try tweaking in addition to those suggestions. After going through the steps in this guide, if you are still having problems with your speed, you may need your cable company to come and replace the cable line from the pole to your house. The cable run from the pole could be damaged, old, or have water in the line. If you are still having problems staying connected after that it is most likely on your ISP's end. Contact them and have them look into the problem. If they don't resolve the problem in a timely manner, or can't, changing ISP's is usually possible. Anyway here are some steps you can take to ensure the best signal levels at your end:
Remove unnecessary splitters
Once you have all the correct parts, follow these steps.
Find your cable signal level
Many cable modems have web based self-diagnostics showing the signal levels. To find out how to access the signal levels on your particular brand/model, either find the manual from the manufacturer of your modem, or check the SG Hardware Database
For example, for the Motorola SurfBoard series modems, navigate to this page: http://192.168.100.1/ . You can post what your signal levels are in the forums, and someone can interpret them for you. Generally, power level should be between -5db and +5db for best performace. That can tell if there is a problem on your end with the wiring and signal that you modem is recieving.
What do those signal levels mean ?
For diagnostic purposes, the cable modem can measure and report the Downstream Received Power and the Downstream Signal-to-Noise Ratio (SNR). The SNR is important because if there is too much noise on the line, data cannot be decoded correctly, even with downstream power levels within acceptable limits. If the SNR is good enough so that the cable modem is working correctly, the exact power level, even if slightly outside normal ranges, does not matter.
Downstream SNR: should be 30 dB or higher: the higher the better. As the SNR decreases below 30 dB, performance will steadily decrease, and errors will increase. The cable modem might stop working properly if the SNR drops below 23.5 dB.
Downstream Received Power: the DOCSIS specification requires cable modems to function correctly with downstream power levels in the range -15 dBmV to +15 dBmV: power readings at or close to those extremes are likely to be unacceptable. However, when connected to a real-life CATV network, a cable modem might be functional over a narrower range than this. Accordingly, cable ISPs will specify an even narrower target range when commissioning their network: this target range will differ according to ISP.
Provided the Downstream SNR is acceptable, the downstream power should be satisfactory if your cable modem reports a figure either within these specified ranges or close to them.
If a cable modem reports a downstream receive power of exactly 0.0 dB, this means that measurement of downstream power is disabled, and no information is available. In this case, you will need to judge downstream quality by SNR alone.
Upstream Transmit Power: The cable modem's Upstream Transmit Power (or return power) is set by commands from the UBR so that the UBR hears the same signal level from all cable modems on that upstream channel. Because of different cable losses for each cable modem in the area, each cable modem will tend to settle onto a different upstream transmit power level. The better the return path is, the lower the upstream transmit power will be.
The upstream transmit power will lie within the range +8 to +58 dBmV, with many ISPs specifying a target commissioning level below +55 dBmV. Values in the forties are the most common. Most residential cable modems are not able to transmit any higher than +58 dBmV. One cannot tell how far this is below the figure that the UBR would need to see a strong enough signal at its end to maintain satisfactory performance, so a figure as high as +58 dBmV is normally a sign of an unacceptable return path. If other issues are also present, an upstream transmit power of +58 dBmV would constitute valid supporting evidence for requesting technical support from your Cable ISP.
The upstream SNR can be sensed only at the UBR, it cannot be discovered by end-users.
The external cable infrastructure is affected by weather conditions, so it is normal to see power levels fluctuating slowly with temperature. If your cable signal conditions are marginal, the CM might stop working in certain weather conditions, and recover in others.
However, rapidly fluctuating power levels might be a sign of a failing amplifier in the ISP's network, or a bad cable connection, and should normally be investigated.
If you have any questions please contact me and I will do my best in helping in any way I can. If anyone notices any errors in this guide please let me know and I will update it. Feel free to use the comment system to add to the above information.
Thanks for reading and I hope it helps.