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Lowering SNR to improve line speed

Posted: Mon Feb 25, 2013 4:34 pm
by kash
Hi
I want to try and change snr to see if I can get better line speeds. My Billion 7300 is reporting the following:

DSP FirmwareVersion DMT FwVer: 3.7.6.89_A_TC, HwVer:T14F7_1.0
DMT Status Up
Operational Mode ADSL2+
Upstream 812 kbps
Downstream 6173 kbps
Noise Margin (Upstream) 13.6 db
Noise Margin (Downstream) 9.2 db
Attenuation (Upstream) 23.9 db
Attenuation (Downstream) 41.0 db

Is there anything that can eb changed to improve speeds?

Re: Lowering SNR to improve line speed

Posted: Mon Feb 25, 2013 7:34 pm
by Dazzled
41 dB attenuation is about 3 km of line length. Is that about right? If it isn't, ask for a line check. If so, you can't expect much better. It makes it important though for you to remove noise sources that are under your control.

With some Billions you can modify a setting they call coding gain, which is essentially fiddling with the noise margin when the modem computes bits per tone. Billion don't recommend varying the default, and neither do I, but the experiment is simple enough to make. The downside risk is drop-outs. 9 dB isn't a lot.

If you want to study your line in technical detail, you'll need to get into the modem on telnet. It runs an abbreviated Linux. There may be a list of commands about; there is certainly one for the 7402 models. The things to look for and graph are HLog, QLN, SNR and bits per tone (BPT), all of which are arrays. There are notes online about this stuff - loop analysis.

Re: Lowering SNR to improve line speed

Posted: Mon Feb 25, 2013 9:30 pm
by kash
Thanks .. yeah just over 3km is the distance. May just leave as is then if it cannot be improved much more that current speeds. I'm guessing switching ISPs makes no difference because same line etc.

Re: Lowering SNR to improve line speed

Posted: Mon Feb 25, 2013 9:35 pm
by Dazzled
Moving to another ISP never changes the line, and the phone line's the weak link.

Re: Lowering SNR to improve line speed

Posted: Mon Feb 25, 2013 10:18 pm
by casio7131
if you can manually adjust the snr settings on your modem (dunno if your model allows it), then you can probably get better speeds. i have a billion 7800vdpx and i set snr=1 and i get these stats:

Downstream Upstream
Line Coding (Trellis) On On
SNR Margin (0.1 dB) 70 137
Attenuation (0.1 dB) 420 242
Output Power (0.1 dBm) 0 128
Attainable Rate (Kbps) 9088 912
Rate (Kbps) 7932 812

with the default snr setting (snr=6?), i got this:
Downstream Upstream
Line Coding (Trellis) On On
SNR Margin (0.1 dB) 121 138
Attenuation (0.1 dB) 415 240
Output Power (0.1 dBm) 0 126
Attainable Rate (Kbps) 7736 904
Rate (Kbps) 6630 812

as you can see, adjusting snr made quite a difference to me.

Re: Lowering SNR to improve line speed

Posted: Tue Feb 26, 2013 12:30 pm
by kash
Thanks, where do you change the snr value? I can't find anything on my modem menu.

Re: Lowering SNR to improve line speed

Posted: Tue Feb 26, 2013 1:23 pm
by CoreyPlover
kash wrote:Thanks, where do you change the snr value? I can't find anything on my modem menu.
On some Billion modems, it is called "Coding Gain" so you can try an explicitly lower value there to see if there is any change (by default, I think it is set to Auto)

Re: Lowering SNR to improve line speed

Posted: Tue Feb 26, 2013 1:25 pm
by Dazzled
Coding gain was mentioned in my reply. I just skip read the 3700 manual, and didn't spot a gain setting on the browser interface. I may have missed it, but perhaps it isn't being encouraged.

That leaves telnet. Few manufacturers publish their telnet interface command manuals, but there is one for a model 5100 at http://au.billion.com/downloads/BIPAC-5 ... Manual.pdf which may be similar. It's simple enough to find out. The manual is worth a look through anyway to get an idea of what is in a modem.

I don't have a Billion, and it's a while since I did this with someone else's, so....
In a terminal or command window, telnet the modem address, and you will be asked to log in to a rudimentary Linux shell with very few built-in commands other than those used by networking and the iptables firewall. The user and password will be the same as the browser interface. You may have to be a little creative to navigate about, eg echo *, or echo /bin/* will usually get a directory listing. If the commands in the 5100 manual work in yours they will have already been given the path. Telnet is available on all operating systems, but some versions of Windows think it should be hidden away, and you might have to enable it.

There is also an article regarding telneting the 7800 model for SNR adjustment at http://forums.whirlpool.net.au/archive/1438256. Before you start executing the adsl command with arguments, as this writer does, run it without arguments and you will usually be given an error message listing the valid arguments for your device,

A zip file containing the 7500 manual as doc files is online also.

Re: Lowering SNR to improve line speed

Posted: Tue Feb 26, 2013 8:05 pm
by kash
Thanks plenty to do on the weekend :-)

Can Exetel adjust my line though and drop the snr to say 6db?

Re: Lowering SNR to improve line speed

Posted: Tue Feb 26, 2013 9:26 pm
by casio7131

Re: Lowering SNR to improve line speed

Posted: Tue Feb 26, 2013 9:38 pm
by CoreyPlover
kash wrote:Can Exetel adjust my line though and drop the snr to say 6db?
No. The modem negotiates the SNR margin.

Some background for your interest can be found at sites such as http://www.kitz.co.uk/adsl/linestats.htm.

Basically, SNR is the relative measure of signal to noise. The higher the better and the biggest determinant of this is the distance of cable between you and your phone exchange. SNR margin is the "buffer" that your modem has negotiated. While your modem could potentially measure the background noise, and negotiate a signal that can *just* be heard above that noise, this won't work at all in practice because the background noise fluctuates. So what you modem does is determines a "margin" above the background noise (often about 6 or 7dB higher) and so negotiates a signal that can be "easily heard" above the background noise. The downside to this is "more easily heard" equates to "slower". So a little speed is sacrificed to make the connection stable.

Lowering the SNR margin forces the modem to be less conservative in the signal that is being negotiated. This will eke out a signal which is slightly faster (the rule of thumb is about 450kbps per 3dB) but the stabiltiy of the connection may be reduced; it depends on how much fluctuation in background noise there is on your line. If you notice more modem drop outs at the lower SNR margin, this is the reason.

What Exetel can do is add "stability profiles" to a line that has unusual background noise fluctuations. This forces the negotiated ADSL signal to be "at least as loud as x dB" and actually slows down your connection speed in order to ensure a high SNR margin and thus a more stable connection. These stability profiles usually target speeds of 1.5Mbps, 8Mbps or 12Mbps and besides which, it won't achieve what you want, which is to have your modem target the opposite (i.e lower the negotiated signal strength to get a higher speed)

Re: Lowering SNR to improve line speed

Posted: Wed Feb 27, 2013 8:52 am
by Dazzled
Further to Casio7131's Whirlpool thread reference, take a look also at http://www.computer-aid.com.au/blog/200 ... -dropouts/

Re: Lowering SNR to improve line speed

Posted: Wed Feb 27, 2013 12:26 pm
by kash
Thanks guys really appreciate the help.