Downstream Noise Margin

Hi guys,

I have a question about downstream noise margin... My router (Netgear DG834G) says my noise margin is 2147483647 db, That number just seems a little too big haha can anyone shed any light on this?

Thanks
Sam

Looks like it's actually -1

Found some more info

2,147,483,647 would be 2 to the 31st power, minus 1 ((2^31)-1)

So, in binary, that would be:
0111 1111 1111 1111 1111 1111 1111 1111

One more and it rolls over to:
1000 0000 0000 0000 0000 0000 0000 0000
Which is -2,147,483,648

So, using a 32 digit (bit) binary number, and devoting half of the combinations of 1s and 0s to be considered negative numbers, and the other half of the combinations to be positive numbers, that is the largest postiive number you can represent.

Edited by deleted (Thu 13-Jan-11 09:47:49)

I've got exactly the same modem and same strange snr margin. I guess if you havent already experienced it, your gonna have loads of disconnects and your bras profile sync rates gonna drop and refuse to sync just like mine. The internet will become sluggish and you'l have heavy packet loss.

http://img714.imageshack.us/img714/8118/snre.jpg

Funny thing is for me, its doing the same on the netgear dg834g and Linksys wag160n, thompson 585 v6 and bt home hub 2.

Unplugged everything, new filters and changed modem cables, same results.
Before it was rock solid no loss of sync, excellent speed.

http://www.thinkbroadband.com/speedtest/results/id/1...

Edited by deleted (Thu 13-Jan-11 10:19:30)

Thankyou for the fast response!

I have never actually experienced packet loss or lots of disconnects. I have only ever used my Netgear one its getting old though probably like 4 years now.

here are my results...

http://img684.imageshack.us/img684/7020/netgearw.jpg

http://www.thinkbroadband.com/speedtest/results/id/1...

This is where SNR and SNR margin need to be carefully considered as they are different. As you have shown 2,147,483,647 could, using 2's complement actually be -1.

If the modem needs a minimum of 6dB of Signal to Noise to work at the optimum then when the SNR is 9db the SNR MARGIN is +3db and if the SNR is 3dB the SNR MARGIN is -3dB. There has been a lot of carelessness by modem manufacturers in giving SNR and SNR margin figures and what may be happening here is that the figure is actually the MARGIN suggesting the SNR is just below optimum. I noted the 48dB of attenuation with 1.6Mb sync which don't normally match which could be an indicator of a noisy line.

http://img341.imageshack.us/img341/603/funnyi.jpg

Had my problems since November 2010 when there was a major outage at my exchange. Had loads of disconnects, and wouldn't even sync.

Tried 4 routers and lots of different filters including new ones, different cables, unplugged everything and tried just the modems in different sockets, same results.

Error logs on 3 modems show lots of line errors up and down, and my isp can see it.
Downstream snr keeps being set to -1, and only the netgear dg834g can hold onto the line when it does that.

BT wholesale have done diagnostics on the exchange and originally said it needed a firmware update, unfortuneately hasnt fixed anything. Reported 6 more times to ISP and they say they can see the fault, the errors and the connection logs.

Last Saturday engineer came out and ran some tests, and said there is no fault found. No noise or interferance on the line. I queried with him why i still dont have a master socket, but an old 1956 GPO BT20/4. Shouldn't it be replaced due to wear and tear. He replied the line from the telephone pole is also very old, but there is nothing wrong with it, so they wont change it.

I'm also under the impression I have to pay £170 for the engineer who came out because he said there is no fault found.

Today, still losing sync and my ip profile is at a record low and snr is still -1db
It's a funny old world isnt it.