Downstream Target SNR

[Ardua]

I am a little confused by something that I read on another forum. I always thought that the downstream target SNR was set by BT's DLM depending on the quality of the line. And, normally the lowest downstream target SNR was 6. My downstream SNR sits happily at 3 (varying between 4 and 2) and I get the occasional re-synch (about once every 5 to 10 days) that sometimes drops the connection speed by a few hundred or raises it by a similar amount. My inclination is to do nothing but I have seen suggestions on other forums that the router firmware may be at fault. Thoughts/comments?

[Steve]

You have to measure the margin immediatley following sync,the default margin for adslmax is 6 which gives enough overhead for minimal noise variation and the adsl signal. However we have seen adsl2+ running on a margin of 3 which I think means the levels of noise on that particular line are at a constant level to allow a stable connection.

[Lance]

Indeed. My own ADSL2+ target margin was dropped to 3db a few months back.

[Ardua]

You have to measure the margin immediatley following sync,the default margin for adslmax is 6 which gives enough overhead for minimal noise variation and the adsl signal. However we have seen adsl2+ running on a margin of 3 which I think means the levels of noise on that particular line are at a constant level to allow a stable connection.

Sadly, neither my Mac nor modem/router will allow me to use RouterStats so I shall just take some comfort from the fact that I am now getting the best speed possible on my line utilising ADSL2+ and accept that the occasional re-synch is a small price to pay. Thanks to all.

[jezuk1]

My line is going through the 10 day (re)training at the moment and the downstream noise margin dropped to 5.4db 48 hours ago, not sure if it will stay there though, I rekon I've got a fairly noisy phone line 

The sync speed also stepped up a little but interleaving was activated and that added almost 10ms to my latency. The line is erroring a lot, but most of it is FEC'd so that's far less of an issue. I think this is pretty much as far as the line can be pushed, option is to run it at a lower sync speed in fast mode (that's how the training process started) or about 1000kbps higher and it seems to drop into interleaved mode.

[Rik]

I agree with you.

[Ardua]

Looking at Jezuk1's stats, what determines the upstream connection speed? I am running at 17240 down and 1059 up. The upside having a LA and SNR of 6. 

[Rik]

Essentially it's the noise picked up by the line in the upstream frequencies.

[Ardua]

Essentially it's the noise picked up by the line in the upstream frequencies.

Thanks Rik. Just reading my local rag and BT has confirmed that FTTC is on track to be installed in my area by the end of the year. No doubt that will throw up even more questions and answers.

[Rik]

They've been saying that about FTTC for the past year here - since they missed the original date.

[Steve]

I was originally due FTTC March 2011. I have this week seen 2 new cabinets so who knows maybe anytime soon for me.

[Simon]

Make the most of your reliable connection while you can then, Steve. 

[Steve]

I may not take up FTTC but that would be out of character for me.

[Simon]

[arobertson676]

The DLM usually decreases the target SNR to 3dB on lines that:
don't resync much (lines that are very stable)
low error lines (e.g. low error second and CRC error lines)