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toro
join:2006-01-27
Scarborough, ON

toro

Member

How to interpret cable modem levels ?

Can someone point me to an article where I can learn how to interpret cable modem levels ? Thanks !
vincom
join:2009-03-06
Bolton, ON

vincom

Member

»Cable Modems and Wiring Issues »What kind of signal levels do I want on my cable modem?

sbrook
Mod
join:2001-12-14
Ottawa

sbrook to toro

Mod

to toro
The numbers quoted in that FAQ are a little geenrous.

Downstream should be from -12 to +12dBmV

Upstream should be less than 52 dBmV

While they may work outside that range, depending on Signal to Noise they may or may not.

Generally with cable modems, there is a very small range where operation is "degraded" ... Most often it's do or die!

TwiztedZero
Nine Zero Burp Nine Six
Premium Member
join:2011-03-31
Toronto, ON

4 edits

TwiztedZero to toro

Premium Member

to toro
said by toro:

Can someone point me to an article where I can learn how to interpret cable modem levels ? Thanks !

As for modem error codes you might find in your CM logs, google Cisco CMTS manuals and have a read. Its helpful at times once you kind of understand whats going on.

Periodic Ranging & Signals (T3 & T4)

CMTS Error Messages .pdf

Uncorrectable Codewords , & Volpe Docsis Codeword Errors

CM trouble shooting tips

This will get you started & further research with your google-fu will of course as always help fill the gaps.

Keep in mind the codes found in your logs are things that have allready occurred, and that there may be variances between the different CMTS routers at the headends (Phubs/POI's). I'm not clear on exactly what equipment specifically Rogers uses at the interconnects all we can do is work on a best guess basis following what we can discover from the various CMTS manuals and guides to shine a little light on what our cable modems are telling us in the logs.

At least after this you won't be feeling quite so blind and naive, anymore.

Happy hunting

toro
join:2006-01-27
Scarborough, ON

toro

Member

Thanks a lot everyone, this is really helpful.

TwiztedZero
Nine Zero Burp Nine Six
Premium Member
join:2011-03-31
Toronto, ON

TwiztedZero

Premium Member

said by toro:

Thanks a lot everyone, this is really helpful.

Moar helpful , corrected the .pdf link.
Adam220
join:2013-04-17
Stouffville

Adam220 to toro

Member

to toro
Is there such thing as an upstream signal level that is too low? From what I've read a lower upstream signal level means the modem doesn't have to communicate at a high level to reach the CMTS. The lower the better kind of thing as it signifies a clean connection to the CMTS. I have an upstream signal level of 25 dBmV but I've also heard that too low isn't great either. Any thoughts?

sbrook
Mod
join:2001-12-14
Ottawa

sbrook

Mod

Yup, there is ... it would normally coincide with a downstream that's too high. Usually the result of too low a tap attenuator being installed on your line.

The problem with both low upstream or high downstream is that it results in overloads in the detection circuits of the thing receiving the signals and as a result corrupted data ... although that gets detected by the modem or CMTS and results in a rejected packet.
Adam220
join:2013-04-17
Stouffville

Adam220

Member

Interesting.. Is 25 dBmV considered too low? My downstream channels are between +4-6 dBmv. I haven't really noticed any issues with my connection but I do have DHCP renew errors and "critical" ranging response timeout errors showing in the modem log. Also seeing about 1500 uncorrectable codewords on each channel over 2 days up time. Is this normal?

sbrook
Mod
join:2001-12-14
Ottawa

sbrook

Mod

When you reboot / restart your cable modem, it's common to get a host of unusual errors and abotu 1500 errors. Thereafter the number of errors stays about the same.

+4 to +6 dBmV is not too high.