In the discussion below, the term downstream refers to the signal transmitted by the cable operator at the UBR to be received by your cable modem, and upstream refers to the signal transmitted by your cable modem to be received at the UBR.

The downstream figures displayed will only be meaningful if the modem is shown to be Locked onto a downstream frequency (typically 403000000 or 402750000 for NTL, 331000000 for blueyonder). If instead the words In Progress are shown beside the frequency, then a downstream frequency has not yet
been locked onto, corresponding to a flashing Receive light: in this case the signal strength and SNR values below will be meaningless, and might refer to some random cable TV channel that is being scanned. If In Progress persists , then perhaps the downstream signal on the CATV coaxial cable is too weak for the cable modem to detect.

The downstream power level should lie in the range -15 to +15 dBmV. A value of -15 or worse is unacceptable, and indicates a poor downstream signal path. There is no truth in the urban myth that this level must be within -2.5 to +2.5. Having said this, it is likely that a technician tweaking your CATV levels would aim for a value close to the optimal 0 dBmV, but a good cable modem should be capable of working within the broader range of -15 to +15 dBmV, provided the downstream Signal to Noise Ratio remains good enough. A reading of 0 dBmV might indicate that the cable modem does not support downstream
power measurement, or that it is disabled. Note: the 3Com Tailfin, by default, has downstream power measurement disabled, and will always show 0.0 dBmV.

The downstream Signal to Noise Ratio (SNR) [not displayed by 3Com ] mus t be 23.5 dB or higher, and should ideally be 30 dB or higher. The lower the SNR, the more noise is present, and the poorer the performance, as the CM will have to keep requesting retransmissions of packets with uncorrectable
errors. There is no sudden cut-off point when the CM just stops working; performance just degrades as the SNR gets worse. The figure of 17 dB appears to be the minimum that is capable of being reported, and I guess that can be taken to mean "so much noise that no signal can be decoded".

Provided the downstream SNR is at an acceptable level, the precise downstream signal level within the acceptable range does not matter.

The upstream figures displayed will only be meaningful if the modem is shown to be Ranged on an upstream frequency. If instead it says In progress or Down, then an upstream channel has not yet been locked onto: this corresponds to a flashing Send light.

The upstream signal level should be in the range +8 to +58 dBmV, and is set by commands from the UBR so that the signal strength received by the UBR from your modem reaches an acceptable level. The lower this figure is, the better the upstream path to the UBR is. A value within the range +25 to +55 dBmV is within spec, and figures in the forties are the most common.

A value at +58 dBmV indicates a poor upstream signal path: some cable modems cannot transmit any more strongly than 58 dBmV, and 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, if you are having problems such as the cable modem going offline, and the upstream signal strength is at +58dBmV, then a poor upstream path is a likely contributor to the problem. On the other hand, an upstream signal strength of +58 dBmV
without any other symptoms need not be an immediate cause of alarm: just keep a watching brief.

The upstream SNR can only be sensed at the UBR, not in the cable modem, so it is available only to the cable operator's technicians.

The external cable infrastructure is affected by weather conditions, so you may expect signal levels to vary according to the weather, particularly temperature.