ADSL2+ SNR settings

Connection issues, drop outs or speed related faults for ADSL and ADSL2+ services
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Waughy
Posts: 59
Joined: Wed Aug 24, 2005 11:29 pm
Location: Fern Bay, NSW.

ADSL2+ SNR settings

Post by Waughy » Tue Oct 09, 2012 8:47 pm

Hi,
Before contacting support I thought I'd try here before wasting too much of Exetels time. I've read that Exetel set the SNR on our lines, is there a way to find out what level is set to my line? I'm getting pretty slow speeds lately, speedtest.net is showing around 3.5Mbps. I'm also finding that when watching foxtel on my xbox it seems to struggle to keep the connection (it's on wireless but hasn't had issues until recently), often stopping to buffer before continuing. This has never been an issue until the last month or so. I am borderline max distance for having a connection (was told I was lucky it was approved at the distance, which escapes me right now), I know it won't be super quick ever, but it's been quicker than it is now in the past with no changes having been made to my setup.
I have a billion 7800N modem and am looking to tweak settings and the SNR setting to attempt to improve my connection before seeking tech help if it comes to that. I just need a baseline for the SNR to give me a better idea on which way to go with the tweaking.
I'm using PPPoA with VC/MUX which I believe is correct. ADSL mode is ANNEX L (always has been, not sure if that's ok)

My current ADSL stsus shows:

DSP Firmware Version A2pB025f.d22k
DMT Status No Defect
Operational Mode ADSL2
Upstream 732
Downstream 4127
SNR Margin(Upstream) 12.1
SNR Margin(Downstream) 5.6
Line Attenuation(Upstream) 29.0
Line Attenuation(Downstream) 53.5

Any help that may assist me get a better connection and avoid hassling tech support by raising a ticket I would be very grateful for. Any further information that may be of use I can supply.
Thanks.

Dazzled
Volunteer Site Admin
Posts: 6018
Joined: Mon Nov 13, 2006 1:16 pm
Location: Sydney

Re: ADSL2+ SNR settings

Post by Dazzled » Tue Oct 09, 2012 9:44 pm

SNR (the margin for noise) can be reduced on several Billion models, but at that downstream attenuation I would expect drop outs to start appearing pretty quickly. Some models have a coding gain setting in the browser interface - I'm not sure your's has. There are online references about for Billion coding gain.

The more universal way is to enter the unit on the telnet interface and use the built-in shell - usually Busybox on an embedded Linux. Most modems have an SNR setting. The trick is to find it, as few makers publish their CLI commands, though most have a response to help or ?. I don't have a Billion so I can't look for you, but expect few Linux utilities installed. The ADSL function apps binaries are usually easily found, and they use simple command arguments to change settings.

There is a manual of CLI commands for another Billion unit at http://billion.com.au/downloads/7500G_CLI_Manual.zip which might assist. The usual thing is to lower the SNR margin a step at a time until instability occurs. Keep your eye on the transmission error rates, which can cruel your throughput. It's not unknown for power supply fluctuations to muck up insufficient margins as well as line noise. Definitely watch for RF noise interference from unshielded switching power supplies (eg 12v lights), a known speed killer.


[ps] There is reputed to be a version of the DMT modem tool for the 7800 available, which simplifies a lot of this stuff for Windows users. As it's a Windows binary executable that can control your modem check a download carefully.

Waughy
Posts: 59
Joined: Wed Aug 24, 2005 11:29 pm
Location: Fern Bay, NSW.

Re: ADSL2+ SNR settings

Post by Waughy » Tue Oct 09, 2012 10:28 pm

Thanks for the reply, I'll have a look into things a bit later, had enough for tonight. I can login at 192.168.1.254/snr.html to make changes, and have a table that shows what settings yield what SNR reductions. As you say, it's looking like a matter of trying a few until a decent stable connection is found. Some references say to aim for no lower than 3.0 dB, others say 6, but they also say it's easier if you know your SNR setting so you know what figure to input to adjust the SNR to the value you are chasing. I'm using revision g of the firmware for the modem too, so it's up to date. I also have a Telstra wall plate filter, model is C10345M.
I used to be on naked plan, things worked much better then, after being forced onto a plan with phone line things haven't been so joyful. I have a phone plugged into the landline but have never used it, would it be worth while ditching the phone and filter and plugging the modem directly into the phone socket? The house also has 3 other phone sockets, but nothing is connected to them so there shouldn't be any issues there.
I'll keep at it until I can find the sweetest possible spot, don't think I'll do much better though.

Dazzled
Volunteer Site Admin
Posts: 6018
Joined: Mon Nov 13, 2006 1:16 pm
Location: Sydney

Re: ADSL2+ SNR settings

Post by Dazzled » Tue Oct 09, 2012 10:39 pm

If you get to 6 dB margin I'd be surprised if it works at all. Unconnected telephone branches, depending on layout, can also rob you of some bandwidth. There's no reason why the modem can't have the wall socket to itself.

Waughy
Posts: 59
Joined: Wed Aug 24, 2005 11:29 pm
Location: Fern Bay, NSW.

Re: ADSL2+ SNR settings

Post by Waughy » Tue Oct 09, 2012 11:21 pm

Thanks Dazzled, appreciate the input. I'll ditch the phone and filter plate. I'll check the other sockets, don't even know if they're live. Is there anything I can do with them? Find where they lead and disconnect them? What would you suggest I aim for on the SNR? It's currently around the 6 mark, might explain the buffering issue on fox.

Dazzled
Volunteer Site Admin
Posts: 6018
Joined: Mon Nov 13, 2006 1:16 pm
Location: Sydney

Re: ADSL2+ SNR settings

Post by Dazzled » Wed Oct 10, 2012 11:30 am

How keen are you to find every little impediment? To see the line as the modem does before it agrees on a line rate with the DSLAM, you'll have to get into the telnet interface, and use the command line.

ADSL is an unappreciated miracle of mathematics hard wired into electronics. The signal is a summation of a large number of analogue frequency signals. For a brief summary of what goes on, see http://en.wikipedia.org/wiki/G.992.1. Using the telnet terminal you should extract and chart (Excel will do) all the bin values for Hlog (a measure of attenuation), QLN (quiet line noise), and SNR, which are used to compute the final BPT (bits per tone) which results in your speed. Hlog should be near linear for example; we'd rather not see spikes in QLN; and also not in SNR. Bits per tone is well explained in the Wikipedia article, and its graph will show what part of the signal isn't delivering speed for you. Even AM radio interference can often be detected when the phone line becomes an antenna. The text books are mathematical, but I was able to Google up some helpful readable stuff under Loop Analysis and the acronyms.

To convert tone number to frequency, bins are centred 4.3125 KHz apart.

[PS} Re the 3 other outlets - have you considered a central filter such as http://www.c10.com.au/products/adsl-fil ... er-c10100e to separate out the ADSL service? Unterminated branches can make issues.

Waughy
Posts: 59
Joined: Wed Aug 24, 2005 11:29 pm
Location: Fern Bay, NSW.

Re: ADSL2+ SNR settings

Post by Waughy » Thu Oct 11, 2012 10:23 am

Ideally I'm just after a more stable connection, where the fox won't keep cutting in and out as it buffers. I've ditched the phone and wall filter, and set the SNR in the moden to 25, which gives about -5.5dB attenuation. My SNR for downstream is now around 7.7 and watching fox for about an hour last night it seems more stable. I did set the SNR to 100 which is 0 attenuation and the modem was showing 12.5, so I used that as my starting point. I did note that my downstream speed dropped to around 3900 but it doesn't seem to matter. I checked and cable wise I'm about 4K from the exchange at Wallsend so it seems the speed I'm getting is decent, and the best I'll get. Speedtest.net results still give around 3.5Mbps and I noticed as it was testing it seemed to be more steady, where it was jumping up and down a bit before.
I'm possibly moving in the next few months now so probably not going to look too much harder at anything else. I'd consider the C10 if I were staying longer but it seems pointless for a few weeks, and as I said, it appears I'm getting the best speeds I can for the distance to the exchange. I'll keep an eye on things and maybe play a bit more with the SNR settings to see if it gets any better, otherwise I'll just leave it while it's looking better than it has in recent weeks.
Thanks again for the input Dazzled.

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