The cable modem will choose or be allocated to one of the available upstream channels at your particular location. If cable signal levels become unacceptably poor, the LEDs on the cable modem will start indicating problems:. If the above occur, an immediate call to your ISP's technical support is justified. There is no point trying to get the PC working while the above conditions persist. Alternatively, in conditions of poor signal levels, the cable modem might spontaneously reboot.
Other possible causes of spontaneous reboots are given below. When signal levels are poor but not poor enough to provoke the above symptoms, you might observe degraded throughput, or packet loss , or poor latency , or failure to acquire or renew a DHCP lease. But poor signal levels are not the only cause of these symptoms, so some diagnosis is required before you blame the signal levels.
If the cable modem is spontaneously re-booting, and there is no evidence of signal-related problems, then you should check on electrical conditions near to the cable modem. Possible causes of these problems are:.
The SNR is important because if there is too much noise on the cable, the 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 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.
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.
Values in the forties are the most common. 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.
Cable modems maintain counters of various error conditions, some of which can be more definite indicators of signal problems than the power and SNR measurements. Only a few models of cable modem report these counters in the diagnostic web pages available to end-users, but the advanced diagnostics can usually be read by the DocsDiag program unless the ISP has denied the use of SNMP to the end-user , using its -v verbose keyword.
All the error counters are cumulative counts of errors since the last cable modem restart. Therefore, some non-zero counts might relate to short-lived problems which cured themselves long ago. What matters is whether the counters are increasing now. To sense this, you should take the advanced diagnostics twice, and look at the differences in the counters. The downstream data transmitted from the UBR is split into codewords usually between 16 and bytes. Each codeword is packaged with extra Forward Error Correction FEC data bits, which enables the cable modem to reconstruct the original codeword if no more than a few bits of it are in error when received.
The algorithm used for generating the FEC bits and correcting the received codeword is known as Reed-Solomon. Here is an extract from a verbose DocsDiag report:. The received without error count is the number of codewords received on the downstream channel whether destined for this cable modem or not with no errors detected by the FEC system.
This counter increases very rapidly, and exhausts the capacity of a bit integer after only a few hours, after which it starts from zero again. It can however be safely used when taking differences between two successive readings a few minutes apart. The correctable errors count is the number of codewords which were detected to be damaged but could be repaired by the FEC system.
In a perfect world, this would be zero. This low rate of error roughly 2 per million can be tolerated. The uncorrectable errors count is the number of codewords which were detected to be damaged beyond possibility of repair by the FEC system. If this count is increasing, then downstream signal conditions are unacceptable, no matter how good the SNR and power figures look.
In the above example, the current rate of uncorrectable codewords is zero, which is acceptable. Indeed, no uncorrectable errors have been seen in the last 2 days.
If the T3 time-out counter is incrementing, in the absence of any downstream codeword errors, this is an indicator of upstream problems. Details specific to certain models will be discussed below. The following models are known not to provide diagnostics on web pages:. If you are unable to browse the above address, see the article Inability to connect to cable modem diagnostic address below.
To include the advanced diagnostics, use the keyword -v. To print the event log, use the keyword -log. You will see a page similar to:. The power levels are highlit above. The downstream power level and SNR will be valid only if the downstream frequency is shown to be Locked. The upstream power level will be valid only if the upstream frequency is shown to be Ranged. The log is useful when diagnosing sporadic problems, and can reveal the reason for the cable modem having previously gone offline.
Not every line in the log represents an error, even if it is flagged as an error. Interpreting the log is not easy. The log is presented in reverse chronological order, so the most recent log entry is at the top. The timestamp is replaced by asterisks if the cable modem has not yet acquired the time from the UBR. Note the name of the configuration file flagged above on the D See Cable Modem configuration file for more about this file.
In some cases, after i. Errors flagged during this process should not be a source of concern for the end-user. Here is an example:. The X Here is another case with a longer delay of 2 minutes at the end of the boot sequence:. The username and password for this service are also shown here, but I have chosen to not let you see that! Paging down to see the rest of the configuration reveals the internal Network settings. Below this are the details for your domain and hostname. I use my own domain and have the hostname of e-smith set to gatekeeper.
I do not use an external proxy as NTL's transparent proxy will trap traffic heading for Port 80 anyway. To avoid the NTL cache specify an external, public, proxy to use and ensure it runs on any other port rather than I also choose to send status reports to e-smith, hence reports is set to on.
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