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snr margin drops after max upgrade

Community Veteran
Posts: 1,229
Thanks: 1
Registered: 30-07-2007

snr margin drops after max upgrade

On BB+, just noticed my Voyager 105 USB modem is showing 8128 downstream so I assume I've been Maxed in the past few days (although I have not received notice from PN about this at all).
Prior to u/g sync was 2272 as expected with attentuation 9dB and SNR Margin of 34dB (yes thats the right way round,I'm 400m from the xchange)
Now atten is still 9dB but SNR has dropped to 13.5dB.
Could anyone explain the mechanism that may have caused the SNR drop, or drop a link to an explanation on t'web or in these forums.
tia
9 REPLIES
prichardson
Grafter
Posts: 1,503
Registered: 05-04-2007

snr margin drops after max upgrade

Hi,

As syncronisation speed increases, SNR margin decreases. This is the expected result.

The idea is that the DSLAM and modem self negotiate the speed to reach a target SNR margin.

If for example noise increaed on your line, your SNR would drop too. If it went below the target mark, your modem would negotiate a new speed, to increase the SNR back to the target.

For the record, a SNR of 13.5dB is more than enough, and is pretty much a rock solid line you should expect.

Do not however take this as gospell, as there is some way to go to proove this yet.

Kind Regards,
N/A

snr margin drops after max upgrade

Why then do lots of MaxDSL people like me get put on a target SNR margin of 6db, that doesn't increase even after 6 weeks of line drops? What's wrong with itHuh
Community Veteran
Posts: 1,229
Thanks: 1
Registered: 30-07-2007

snr margin drops after max upgrade

Thanks Phil. I understood the general principles anyway but that's helped me.
So the idea is I guess that the increased b/w is a "larger" signal, but the base noise level and attenuation stays broadly the same (as the copper to the exchange hasn't actually changed), so the margin must of necessity drop ... or maybe that the increased b/w takes up more of the "available" margin ... makes sense when i says it but when i draws it as a graph it looks counter-intuitive.
you'd never guess that i once passed a c&g in electronics would you Shockedops:
Community Veteran
Posts: 14,469
Registered: 30-07-2007

snr margin drops after max upgrade

Because 6dB is the target SNR that BT try to aim for to maximise your sync speed. If after a test period the ecvhange sees to much data loss, then it resets the target at 9dB which the causes your max sybc speed to drop. If it still sees problems it sets a 12dB SNR figure and correspomding sync speed.

IN addition to the above, the option of turning on interleaving is available which is a way to get a higher sync speed on lines with an improved error correction system to counteract the data loss.

Its an on-going process where your line is constantly being checked and adjustments made to achive the best stable connection.
prichardson
Grafter
Posts: 1,503
Registered: 05-04-2007

snr margin drops after max upgrade

Quote
Why then do lots of MaxDSL people like me get put on a target SNR margin of 6db, that doesn't increase even after 6 weeks of line drops? What's wrong with itHuh


I am unsure what you are trying to say with that.

If your line has a target of 6dB, then the idea is that Max will see your modem attempt to gain the fastest rate without the margin dropping below 6dB.

The ideal situation is that once the speed is found, the SNR should not fluctuate.

As such, there is no such increase that you refer to.
N/A

snr margin drops after max upgrade

Quote
Quote
Why then do lots of MaxDSL people like me get put on a target SNR margin of 6db, that doesn't increase even after 6 weeks of line drops? What's wrong with itHuh


I am unsure what you are trying to say with that.

If your line has a target of 6dB, then the idea is that Max will see your modem attempt to gain the fastest rate without the margin dropping below 6dB.

The ideal situation is that once the speed is found, the SNR should not fluctuate.

As such, there is no such increase that you refer to.


I have been on MaxDSL since 19th April. Every synch since then has been at the 6db margin - which tends to give me a synch speed of 5mbps. Generally 6db is too low for my line, and it will drop if it dips to 1-2db, which typically happens within 1-2 hours. The exchange hasn't "learnt" to increase my target SNR to a higher amount and thus give me a stable line. We are talking 6 weeks into masdsl here, and I still get line drops, so what's up with the sytem?
Community Veteran
Posts: 1,817
Thanks: 11
Registered: 30-07-2007

snr margin drops after max upgrade

Here are my stats is the downstream margin, 6db, the snr mentioned in this posting. Does the 24db on the upstream make a difference.

My sync speed is always above 7000 Kbps my upload speeds are always reported to be around 360 to 380kbps but speedtest report my line speed as more less always around 2500 to 3000kps

Any feedback is much appreciated.

A

DSL Status : Up
DSL Modulation Mode : GDMT
DSL Path Mode : FAST
Downstream Rate : 7744 Kbps
Upstream Rate : 448 Kbps
Downstream Margin : 6 db
Upstream Margin : 24 db
Downstream Line Attenuation : 47
Upstream Line Attenuation : 24


downstream margin is now at 5db previously 6db will this make a big difference?
greypound
Newbie
Posts: 7
Registered: 21-08-2007

Lower noise margins at busy times

I have had Max DSL since 18/04 and suffer regular disconnections at busy times - evenings and afternoons on weekends.

Typical Downstream Stats are :

Mornings

Connection Speed 3200kb/sec
Line Attenuation 44db
Noise Margin 10db

Evenings/busy

Connection Speed 2900kb/sec
Line Attenuation 14db
Noise Margin 44db

Upstream stats hardly change - typically being
Connection 448 kb/sec Line Attn 14db Noise margin 21db

I am approx 2 km from the exchange and never have speeds greater than
4100 kb/sec - which is quite acceptable. What I do need is stability.

Is the lower Noise Margin due to increased traffic (contention?)
Would stability improve if I were to pay for a change to 2 or 3 Mb/sec max?
Will the line eventually self adjust?

I am using a Netgear DG834G router but my desktop has a direct LAN connection - my laptop (wireless) has similar results. Sad
greypound
Newbie
Posts: 7
Registered: 21-08-2007

Correction to last!!

My post makes no sense because I put in wrong stats for evenings! Apologies.

They should have read

Line Attn 44db
Noise Margin 5db occasionally 3db