Flaky Ambit modem?

[GeoffW]

I've been on the 10MB L service for a while and it's been stable, but over the past few weeks I started to lose the signal on the modem (RDY Light flashing). First once a week, then every few days, now every day - usually in the evenings.

A power cycle of the modem a few times brings it back. The levels look OK to me when working:

Downstream Receive Power Level : 5.30 dBmv
Downstream SNR : 31.35 dB
Upstream Transmit Power Level : 32.00 dBmv

Downstream Lock : Locked
Downstream Frequency : 402750000 Hz
Downstream Modulation : 256 QAM
Downstream Receive Power Level : 5.27 dBmv
Downstream SNR : 29.65 dB

When it doesn't work:
Downstream Receive Power Level : 4.77 dBmv
Downstream SNR : 31.83 dB
Upstream Transmit Power Level : 31.00 dBmv

Cable Modem Status
Task Status
Search for a Downstream Channel Done
Search for a Upstream Channel Done
Obtain Upstream Parameters Not done
Establish IP Connectivity using DHCP Not done
Establish Time Of Day Not done
Transfer Operational Parameters through TFTP Not done
Register Connection Not done

So before i suffer VM customer service, is this a network problem or does my modem need replacing?

(just noticed whilst posting these levels, the SNR is changing about 2 db)

[Sephiroth]

You were right to home in on the SNR.

If it's below 33dB you are guaranteed problems. If it's above 35dB signal will usually overcome noise.

Maybe your coax needs screwing in properly at one or t'other end. maybe damp got into your external wall box. Maybe there's a loose coax at the street cabinet. Maybe the coax cable to the modem is shot in some way.

There was one case I noticed here recently where a replaced modem cured the low SNR.

So, after checking the physical stuff, it'll be a service call IMHO.

[GeoffW]

Thanks, I thought the SNR was OK to be a bit lower so it's good to know 33db is what I should aim for.

I have a 10db loss pad on the modem to attenuate the signal, I was told this was to avoid my transmit signal overloading the ntl card and it's been in there since the modem was installed 5 years ago, I've taken that out so I'll see how that goes.

New levels:

Downstream Receive Power Level : 15.83 dBmv
Downstream SNR : 32.43 dB

[Sephiroth]

Thanks, I thought the SNR was OK to be a bit lower so it's good to know 33db is what I should aim for.

I have a 10db loss pad on the modem to attenuate the signal, I was told this was to avoid my transmit signal overloading the ntl card and it's been in there since the modem was installed 5 years ago, I've taken that out so I'll see how that goes.

New levels:

Downstream Receive Power Level : 15.83 dBmv
Downstream SNR : 32.43 dB

You should aim for much higher than that. Above 35dB makes sense. A good SNR is 37dB or higher.

With regard to the attenuator that you removed, if it was to deal with upstream power, I would expect now to see 40dBmv upstream (transmit) power - which is perfect. What is your upstream power now?

But I notice that the downstream power has jumped by 10 dBmv and now the modem will be struggling.

The common factor here is a steady poor SNR.

[GeoffW]

There is no change in the upstream power I'd have to get VM to look at their kit for that?

[Sephiroth]

Well there was a conflict between your statement that the attenuator had been put in to deal with the transmit side. But from what you've published, it's the receive side that's jumped 10 dBmv. You should put that attenuator back in and call the engineer to deal with the SNR issue.

[GeoffW]

I was told the attenuator was because my modem signal was too high into the mux, so my output 32dbmv plus attenuator was ok. It says 10db forward path attenuator - but I'd always assumed that it added 10 db lossto both ways as my receive goes down by 10dbmv and it's passive.

Maybe I had it the wrong way round, is the 15db receive power too much?

[Sephiroth]

I was told the attenuator was because my modem signal was too high into the mux, so my output 32dbmv plus attenuator was ok. It says 10db forward path attenuator - but I'd always assumed that it added 10 db lossto both ways as my receive goes down by 10dbmv and it's passive.

Maybe I had it the wrong way round, is the 15db receive power too much?

The forward path attenuator attenuates the downstream. Put it back!

[GeoffW]

Too lateeeeeeeeeeeeeee......

:)

[Sephiroth]

Too lateeeeeeeeeeeeeee......

:)

Meaning?

[GeoffW]

Well I had the delight of calling "customer service", who after waiting 20 minutes to talk to someone then decided to put me on hold while my details loaded and probably take his lunch break. I gave up after waiting another 10 minutes for him to return.

Why are these people such turkeys, do VM realise how much damage they do to their customers and reputation?

Anyway after redialling, waiting another 20minutes I got through to someone else who predictably said my poor SNR was due to my modem so they'd send another out. Meanwhile the problem seems to have got worse and I was without BB most of the weekend.

I'm dreading another call to "customer service" to get the new modem activated. In fact the whole thing just makes me want to ditch VM altogether and get DSL, but I guess I've got to suffer this process first and least give them a chance to fix it.