Confusion over SNR / Noise Margin

[DJCough]

Apologies for asking this basic question, but I'm getting conflicting information about the term SNR.

My current stats:

ADSL Link Downstream Upstream
Connection Speed 1321 kbps 635 kbps
Line Attenuation 52 db 17.5 db
Noise Margin 12 db 6 db

Now ADSL24 has replied and said the following:

"...your SNR is very high it should be around 6 to give best possible speeds and even lower if your line can support it. "

But looking on the ADSL24 FAQ, it says the opposite:

"For SNR margin, higher is better (meaning not much noise on the line).

The SNR margin will reduce as you connect at higher speeds. For a reliable service, the SNR margin has to be above 6, and ideally above 10."

So what's the right answer here? Is my Noise Margin of "12" too high or too low??

Thanks for your help guys,
Dennis

[_proverbial_]

The default (target) SNR margin is 6, however yours has been automatically raised most likely due to instability on the line (through several re-syncs in a short period of time) and is now 12.

I don't know if it's me but that seems a relatively slow sync for 52db attenuation (unless mine used to be abnormally high) - prior to poor weather which caused the line to go crazy and re-sync every few minutes before I could realise and turn off the router I was always getting between a 4-5meg profile depending on the time of day. However, the several re-syncs caused the target SNR to shoot up to 12 where it has stayed ever since and has left us with a 3meg profile.

Thank you BT and your useless profile system - it's nice to know their system has no problem increasing your SNR but they refuse to lower it. How nice it must be with BT-tinted glasses on. :rolleyes:

If only I could get LLU which wasn't TalkTalk or Orange. :(

[tboorman]

Take a look at http://www.kitz.co.uk/adsl/linestats.htm for information on how to interpret your line stats.

I think what ADSL24 are saying to you is that because your target SNR is 12 dB, your sync speed is lower than it would be if your target SNR was the default of 6 dB.

[grizzly]

Both answers are right. If your connection speed is the highest possible, for example I think it's 8128 kbps for adsl max without interleaving, then the higher snr margin you have the better.
For your line then the lower the snr margin the faster your connection rate is but the less stable it becomes.
Your downstream margin is high but even then I would expect the connection rate to be a lot higher than it is.
Here's mine with same attenuation:
DSL Line (Wire Pair): Line 1 (inner pair)
Protocol: G.DMT2 Annex A
Downstream Rate: 4763 kbps
Upstream Rate: 818 kbps
Channel: Interleaved
Current Noise Margin: 8.9 dB (Downstream), 6.0 dB (Upstream)
Current Attenuation: 52.9 dB (Downstream), 32.9 dB (Upstream)

Reboot mine at night when line is noisiest, but gain stability. If I reboot during the day I connect at about 5500 kbps, not so stable at night.

[DJCough]

Thanks for the info guys.

I still have a high noise margin (between 12-17 on downstream).

And I've now followed ADSL24's suggestions for reducing noise on the line. Opened all the phone faceplates in the house and removed the "unneeded" wires (ie. only leaving numbers 2 and 5 connected).

If anything, that's actually increased the problem. Now get noise margin spikes up to 17!

FYI - I've also tested it on the main socket and that's also getting a noise margin at about 12.

Although based on the replies, sounds like raising my noise margin is something BT have done automatically?

Is there anything else I can try to lower the noise margin?

Thanks again.

[spanfan]

Use DMT to reduce the noise margin which will increase your sync speed but it will probably take days for BT's IP Profile to reduce.

[James]

Also using DMT will very much kill your profile if you have any disconnections because the BT system will up your SNR stupidly high and then BT will not allow us to lower it down. Basically I would not advise using DMT as when BT see you have been and your SNR has risen as a result they will not be interested and there is then little I can do to make them reset it... Noise margin being higher means it's a better line, not worse! The only reason noise margin should be lower is when you connect at a higher speed, because lower SNR = more speed and higher SNR = less speed but more stable.

[DJCough]

Noise margin being higher means it's a better line, not worse! The only reason noise margin should be lower is when you connect at a higher speed, because lower SNR = more speed and higher SNR = less speed but more stable.

Thanks for the above info James. That makes more sense.

So to get my sync speed back to normal (eg. 2 to 2.5 mb downstream), when is the best time to reset my router? Early morning when there's less traffic?

Also, should I just do a simple "reboot" from the router admin or should I leave it unplugged for 1 hour or more?

Thanks again
Dennis

[paulbacon]

If your target snr is set to 9bd, what speeds should you be syncing at?

Im synced at 7168kbps atm and think my target snr is 9db

[James]

Early morning will give you the lowest noise and thus best SYNC :)

[paulbacon]

Is sync speed always linked to noise?

Since changing my cabling to better quality ones, my sync speed has gone up by about 1000kbps. So would that mean the cable improved the noise margin, hence could sync higher?

[DJCough]

Cheers - I'll try rebooting one early morning.

As for the Noise Margin, I was reading on the forums here that it was possible for BT to reset your noise margin in the past. But is that no longer the case with ADSL2? Do you simply have to wait for 14 days and hope they automatically reset it to 6?

[Joshua]

I believe they lower it by 3db each time.

[spanfan]

Basically I would not advise using DMT

Without DMT, BT in their wisdom allow me 500
With DMT I get 1000 and currently 1250.

It's a lifesaver until I get LLU tomorrow

[grizzly]

Cheers - I'll try rebooting one early morning.

As for the Noise Margin, I was reading on the forums here that it was possible for BT to reset your noise margin in the past. But is that no longer the case with ADSL2? Do you simply have to wait for 14 days and hope they automatically reset it to 6?

IF you maintain a steady stable connection without dropping down a profile and IF you have a reasonable amount of errors, not sure of the numbers, within 10 - 14 days your snr margin will drop by increments of 3 dB.

"Noise margin being higher means it's a better line, not worse!" is only true if you connect at the maximum rate, a high snr margin with a low sync rate means trouble.
With attenuation of 52 dB the closer to 6dB the better, conversely an attenuation of 12 dB connecting at 8128 kbps snr margin of 15 dB would be good on adslmax.

Rebooting in the morning will give you faster sync speeds but you could find the line deteriorates at night causing your line to drop.

Don't know what router you have but this http://www.vwlowen.co.uk/internet/files.htm is excellent for keeping an eye on your line.

BT might reset your margin once but if the problem isn't solved in the meantime they won't touch it again.