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.