What causes a bad SNR/How to fix a bad SNR?
 

Hello all,

About a month ago we had cable TV installed and ever since the SNR on the modem has been significantly lower than normal, causing random cut offs, mainly around 11pm - 6am period, never other times.

Downstream Receive Power Level : 5.53 dBmv
Downstream SNR : 28.11 dB
Upstream Transmit Power Level : 47.50 dBmv

Those are my levels at the moment. The SNR varies between 26dB and 32dB. Before the TV was installed, we had an SNR of 36dB+

If my broadband hasn't dropped out, you can see live SNR stats on http://m3ezw.no-ip.com:85/routerstats.htm

I have checked the cables and done my own trouble-shooting, and I have done the following:

1. Removed the TV cable
2. Removed the splitter
3. Tried with and without the attenuator on the modem
4. Replaced the connector on the end of the cable for the modem
5. Reseated all connectors in their plugs

None of those seemed to have any effect on the SNR level.

Does anyone have any clue as to what else could be causing this problem? It only started when we had the TV installed, yet the TV doesn't seem to have any effect on it (since it was tested with it removed).

If I need a technician out I take it that the call out would be free? I assumed I could correct this on my own but it seems not in this case.

Seems this is goodbye to 6 years flawless service from VM!

All help appreciated.

Thanks.

Re: What causes a bad SNR/How to fix a bad SNR?
 

You need to call Tech Support to report your problem as the is no way that a user can adjust SNR, they will check you details and run some tests and probably book an engineer visit.

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Re: What causes a bad SNR/How to fix a bad SNR?
 

SNR (Signal to Noise Ratio) is caused by one of three things..

Too low a signal (power needs jacking up at the box int he road)
Interference (again power needs raising to get over it)
Too high a power causing distortion and noise (The power needs to be lowered at the box unless it's already at low power, this is where the attenuator comes into play but it's hard to get right)

WhydoIneedatech is perfectly right in saying that a tech should be prought in to adjust it as it's ever so easy to think you've cured it only for a slight fluctuation to suddently make it worse. A tech who does his job correctly should get your levels mid-range so variance wither way will NOT effect the signal.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Apologies for hi-jacking this thread as it were, but as the topic is about SNR, what is an "optimal" SNR to have on a SACM?

For info, I only have 1x STB and 1x SACM connected.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Thanks, I think it is time to call out a technician then. I'll speak to my Dad and get him to call (being the account holder he has to).

Re: What causes a bad SNR/How to fix a bad SNR?
 

Ideal level around 40dB not to high not to low.

Re: What causes a bad SNR/How to fix a bad SNR?
 

... (ignore, wrong thread)

Re: What causes a bad SNR/How to fix a bad SNR?
 

:LOL:

Re: What causes a bad SNR/How to fix a bad SNR?
 

Thats a signal level not an SNR?

Did you mean 40dB?
A very high SNR with an acceptable signal level would be best.

Re: What causes a bad SNR/How to fix a bad SNR?
 

40dB is optimum and nicely in the middle, and yes I put mv in error.:dunce:

Re: What causes a bad SNR/How to fix a bad SNR?
 

lol hiya mate :)

Re: What causes a bad SNR/How to fix a bad SNR?
 

I just phoned up and the guy said because its working to call back when it wasn't! So I phoned back anyway and got another guy who has booked me an engineer because he knows there is a problem.

Amazing how some know how to do their job and some don't eh? The first guy said ex NTL was capable of working on an SNR down to 20dB! He might be right but its certainly not working that low for me.

I have an engineer booked for Monday anyhow so that should be fixed quickly. I just hope the engineer is not a "thickie" (sorry to all techies!) and actually knows what he's doing and listens to me when I tell him what is wrong instead of dismissing everything and claiming its fine!

I will have graphs to prove the SNR randomness and as of today I am going to record all the times it disconnects.

Lets hope it gets fixed!

Re: What causes a bad SNR/How to fix a bad SNR?
 

Thanks whydoIneedatech. That helps me, now to check if mine tallies up. It was 42 the other week, and was 33 last night... oddly enough, I've not knowingly had any problems.

Rep++

Re: What causes a bad SNR/How to fix a bad SNR?
 

Mine is 38dB at present and is rock solid.

Re: What causes a bad SNR/How to fix a bad SNR?
 

No 'high' limit to SNR for best performance though when it gets too high it's probably a misreading.

33dB+ is ok but higher the better.

Re: What causes a bad SNR/How to fix a bad SNR?
 

NTL have a lower limit than Telewest and are still stable.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Well tonight it is fluctuating between 39.8 and 40.2dB, so it seems to have settled down for tonight at least.
I'll keep my eye on it anyway... thanks for all your responses :)

Apologies to AbyssUnderground - I seem to have completely robbed your thread :(


really? My mum and sister are both on Telewest (Knowsley area) but I've never seen the SNR on there before. They've always had less problems than me over the last 7 years :)

Re: What causes a bad SNR/How to fix a bad SNR?
 

No problem :p:

I was told by the guy on the phone that the ex NTL area can go as low as 20dB but the ex Telewest will have problems below 28dB because of the difference in the network (e.g. not the same company deployed the network all over the country)

Re: What causes a bad SNR/How to fix a bad SNR?
 

Telewest is on the Knowsley Platform

NTL is on the Langley and Bromley Platform I think 20dB is a bit to low even for NTL.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Are you suggesting noise levels are higher on Telewests network? :) If the SNR needs to be higher (for the same type of customer premise equipment) then thats exactly the case.

Re: What causes a bad SNR/How to fix a bad SNR?
 

The Knowsley Platform is a completely separate Network to NTL has completely different power levels.

Langley and Bromley have differences between them, and they are part of the same Network.

I presume you are unaware of this?

Re: What causes a bad SNR/How to fix a bad SNR?
 

LOL. I used to work at the DMC.
Langley and Bromley are still two distinct platforms, two very different networks. Bromley and Knowsley were very similar as they both initially used Thomson encoding and Multiplexing equipment, very much due to Anderson Consulting taking any designs they picked up from the Bromley Network over to Telewest. Langley used a completely different Divicom setup.
All the equipment in customers homes will be very similar, with similar required power levels, I can't see why the Telewest boxes need a better SNR than the ntl networks unless the network is noisier.

Re: What causes a bad SNR/How to fix a bad SNR?
 

We now both use the Ambit 256 modem on all networks.

Telewest SNR is supposed to be between 29db and 50dB.

NTL can go below 26dB and sometimes can get lower and still work.

Three different platforms within what is now one company.

Knowsley, Langley and Bromley all have differences, such as SNR and also the Downstreams differ between all 3 platforms.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Same modems all using same modulation (QAM or QPSK) means from your figures above that the noise must be higher on the Telewest network. The same signal levels are required for the modems across all networks, but the lower SNR requirements of the ntl side must mean that noise is lower on those networks.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Thats it 3 networks one company and different Downstreams as well, its all good fun.

You cannot use your knowledge of one network to troubleshoot one of the others, you must know the differences.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Its not that difficult, given a knowledge of basic RF, and signal level measurements.
When you say different downstreams, do you mean different frequencies, different modulation or something else?

Re: What causes a bad SNR/How to fix a bad SNR?
 

As in this for NTL

Below is the recommended Downstream power levels for both NTL Platforms

Bromley (exCable & Wireless) Downstream power levels +6dB to +8 (QAM 64) +2.5 to +5.5 (QAM256)

Langley (NTL area) Downstream power levels -2.5dB to +2.5 (QAM 64) -1.5 to +2.5 (QAM256)

And this is for Telewest is Downstream power is -12dB to +12dB

This what you see in your config pages of your modem.

Re: What causes a bad SNR/How to fix a bad SNR?
 

I graphed my friends SNR level yesterday and here is the result. You can see mine definitely has a problem.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Depends on the area and modulation. Ex-CWC areas need a higher SNR than original ntl and Telewest, however Langley ntl and Telewest have identical SNR requirements as they are the same equipment either end of the network.

I wasn't talking about any limits on troubleshooting guides or the documents you might have I was talking about operational limits from experience and the standards.

I suspect the 26dB you refer to refers to 64QAM original ntl downstreams. Neither ex-CWC nor 256QAM downstreams will be happy at 26dB. 256QAM just won't happen and ex-CWC will be showing errors even on 64QAM.

The power is irrelevant to the SNRs, and the guidelines you copy/pasted above while they might be what is aimed for are not required. On Telewest and original ntl so long as network is clean modems will be quite happy to +/- 15dBmV receive power, Bromley -17dBmV - +13dBmV on 64QAM and -13dBmV - +17dBmV on 256QAM.

BTW The ntl Langley and Telewest networks run identical kit. Telewest just took their 'recommended' power straight from the DOCSIS specs while ntl apparently picked theirs from a hat. I'm sure in their dreams they could run every modem on their network within a 3dBmV power range but in reality not going to happen.

Re: What causes a bad SNR/How to fix a bad SNR?
 

How come we get many customers losing connections if anywhere near those levels you posted above then.

How can you you say that a modem can run perfectly well at those extreme levels, but we find in practice during normal use that they will drop the connection, and then the customer will call Tech Support.

Broadbanding please remember that I am only asking questions about this issue not your obvious knowledge on the subject.

The NTL part was cut and pasted from my own documents.

Re: What causes a bad SNR/How to fix a bad SNR?
 

I'm quoting the DOCSIS standards. A signal hitting the modem at those high levels failing is an issue with the modems assuming the SNR is ok.

SNR is the overwhelming thing. High power levels of the wanted signal can indicate high power levels of the noise carried on the network as well resulting in a low SNR, high power levels can cause distortion at the modem, same as with a guitar amp, the distortion is caused by high power levels coming into the modem.

I would bet you'll find some modems are perfectly happy at those levels.

Knowsley / Langley whichever the power level guidance you've been given is a misnomer. So long as the signal is clean the power tolerances, assuming the modem is the same, should be equal. Whatever the network is doing it's how the modem handles it that's the main thing. Modems don't care what the name of the headend is, they only care about how high or low powered the signal is and the difference between the signal and the background noise. There's no reason why the ntl areas should struggle with some power levels while the Telewest areas are ok.

It seems a bit odd that the Telewest areas can apparently not cope with an SNR below 29dB which is actually below the DOCSIS standard for 256QAM which states 31dB is desirable, while they have a far wider power range they can tolerate. It's all about what the cable modem does with the signal, some cope with high power better than others, all will struggle with low SNR.

Re: What causes a bad SNR/How to fix a bad SNR?
 

Its the question of the downstream power levels I asked about in my previous post, could you please clarify why the modems should be able to handle the much higher power levels you say they are capable of, but tend to go off line and require a Tech visit if they actually reach such levels.

Re: What causes a bad SNR/How to fix a bad SNR?
 

So long as the modem is good enough and fully compliant to standards no reason why it can't deal with the power levels I mentioned.

High power levels can cause issues with modems, not so much from the power of the cable modem signal but the overall power of the RF hitting the modem's receiver causing distortion by overdriving it.

Not so much the cable modem signal but the other few hundred MHz of RF hitting the modem receiver and how well it isolates channels it's not tuning.

Re: What causes a bad SNR/How to fix a bad SNR?
 

The technician came this morning at 12.05pm, he said between 12pm and 4pm so he was very early! I offered him a drink, to which he accepted with a lot of thanks! He must be deprived by most people, the poor soul!

He hooked his test equipment up and it gave him a flat battery warning... What a great start! Back to the van for his charger. He also brought a new modem in with him, a lovely black 256 modem. He hooked that up, phoned to get it activated and et viola, my SNR is now 39-40dB and rock solid like it should be.

So, it seems that the modem was to fault. After 8 years of 24/7 service its finally given up the ghost. And the first time we've ever had Virgin Media technicians to come to the house too and it was certainly a good experience.

The techy was also interested in the software I used to graph the SNR etc. It turns out he'd never even heard of the modems having a web interface! Ah well, I guess his hand held equipment does pretty much the same job doesn't it.

Anyway, all in all a great turnout. 5 stars to VM and the guy who visited me today!