Virgin Media Modem

Useful Information

The modem provides transmit and receive power for each channel, in addition to the SNR (signal to noise ratio) and errors encountered on the channel.

Data is pulled in through the hub3_exporter, which provides information from the modem.

In this setup, the exporter runs on a Raspberry Pi in a Docker container with the Modem IP overridden as the hub is not in modem mode.

The exporter doesn’t currently aggregate the T1-T4 timeouts, which would be a useful statistic to include in the future.

Pre/Post-RS Errors

These are Reed-Solomon errors and are a form of error correction. If the signal is weak, or there is a low SNR, then we would expect to see some pre-RS errors and depending on how error correction is setup, quite a lot less post-RS errors.

A few instantaneous errors are nothing to worry about, but a continual climb would indicate an issue with the coaxial cable or CPE and would quite possibly require an engineer visit.

Channels

Following a visit from a Virgin engineer, I was informed that there should be 5 upstream channels and 24 downstream channels connecting back through their fibre network. Before their visit, there were only 2 upstream channels showing, suggesting a lot of loss somewhere on the cable.

Power Tolerances

The engineer informed me that the downstream channels should be between -6 and +10 dBmV each, and the upstream should be between 34 - 49 dBmV up. Outside these tolerances, if seeing network issues, a visit from an engineer would be a good idea.

Acceptable SNR

This website provides useful information on a range of tolerances for signals, broadly in line with what the Virgin engineer told me. SNR should be ideally no less than 30 dB and issues would most likely be found if the SNR dropped below 25 dB.