Cable Modem Signal Levels

[Ignition]

OK, so you've browsed to http://root:root@192.168.100.1 and gotten your signal levels... to save any posts asking if their levels are OK here's a quickie guide as to real world acceptable ranges, rather than those dictated by the specifications for DOCSIS and EuroDOCSIS.

Downstream Receive Power: Ideally +/- 3dBmV, cause for concern at around +/- 8dBmV, different modems handle the higher and lower levels differently however I'd expect to see service degredation at that point.

Downstream SNR: Minimums are 24.5dB on the Langley platform and 26dB on the Bromley platform. Again these aren't the minimums required to function but when you're down this low you're teetering on the brink of losing your service.


Upstream Transmit Power: Anything between 28dB and 53dB is fine. Up to 55dB is also fine so long as it's fairly stable. Above 55dB suggests issues. Depending on your modem connectivity is jeopardised when transmit power hits 58dBmV or in the case of Pace set tops 61dBmV.

Edit: 18/02/2006

Both Telewest and ntl are now using the more efficient, but demanding 256QAM modulation in some areas. This gives more bandwidth but required better networks. For this modulation the recommended power range is -6dBmV to +15dBmV with a recommended SNR of 33dB.

In addition to this, Telewest are using the 16QAM upstream modulation in some areas. This requires an upstream transmit power below +55dBmV.

[BBKing]

Nice one Ig.

You can have Ambits at a transmit power of 61 too, and it's only Pace 1000 and 2000 series boxes that go > 58, Samsungs and 4000s don't.

Also, viewing the modem directly can only give you half the picture, each parameter you read there has an equivalent at the UBR for the opposite direction

CM
DS receive power
DS SNR
US transmit

UBR
US receive power
US SNR
DS transmit (although this is a totally pointless figure)

That gives us five meaningful parameters of which three are readable off your modem - the other two are readable by NTL TS.