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wsanders

join:2002-09-18
Lafayette, CA
Reviews:
·SONIC.NET

2 edits

Uncapped ADSL2+ with SNR margins vs. Capped ADSL

Howdy, I've been looking through Cisco and other makers web sites for an explanation of how setting the SNR margin on the DSLAM end of a line works.

I have new Fusion service, and we have been able to wrest 6 Mbit reliable sync and 7.5 Mbit flaky sync from the crappy lines in my neighborhood and in my house. Currently, Sonic's capped me at 6 Mbit / ADSL1 which is on par with everyone else in my neighborhood, so I'm happy. The noise seems to be stray RF and impulse noise on the line. I am a ham radio operator, and the RFI in the neighborhood is terrible: arcing PG&E poles, plasma TV's and dimmers, you name it. I'm also about 9000 ft from CO, at the toe of the curve where ADSL2+ overs any real advantage of ADSL1.

I'm trying to figure if it's worth keeping the ZTE modem they sent me. I still have the venerable Speedstream 5100a PacBell sent me c.2004, and it and the ZTE perform identically with the 6 Mbit cap and ADSL1.

The question is: Is it possible to maybe wrest another 10% out of the line by uncapping the sync rate and increasing the "SNR margin", which seems to be a separate parameter that you can set on most DSLAMs? Is increasing the interleave (we do not do gaming) or enabling SRA something that might work? Is there a predictable effect on speed when bumping up SNR margin by 3 or 6 db?

Thanks for any discussions. Maybe we can find a new line profile for "guy with crappy line, lots of RFI, 2 mi from CO."

public

join:2002-01-19
Santa Clara, CA
said by wsanders:

The question is: Is it possible to maybe wrest another 10% out of the line by uncapping the sync rate and increasing the "SNR margin", which seems to be a separate parameter that you can set on most DSLAMs? Is increasing the interleave (we do not do gaming) or enabling SRA something that might work? Is there a predictable effect on speed when bumping up SNR margin by 3 or 6 db?

Thanks for any discussions. Maybe we can find a new line profile for "guy with crappy line, lots of RFI, 2 mi from CO."

SNR is a measurement, not a setting.
SNR increases by more Tx power, better receiver, or less noise on the line.
Additional interleave processing also improves snr.
Make sure you have a home run cable to the modem.

Maybe ATT can find a better pair?


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET
said by public:

SNR is a measurement, not a setting.

SNR (Signal to Noise Ratio) is indeed a measurement, however it also exists as setting on the DSLAM. Unfortunately it is not a magic button that improves SNR in some way. What it really does is reduce the data rate (speed) until the SNR reaches at least the set value. If the preference is the fastest possible speed even at the risk of errors then the DSLAM SNR setting should be lowered. If the preference is a reliable low latency connection then the DSLAM SNR setting should be higher (which will result in a lower data rate).

said by public:

SNR increases by more Tx power, better receiver, or less noise on the line.

While all of this is correct, transmit power is limited to prevent interference with other pairs in the same cable bundle and DSL is already designed to use up to the maximum power permitted. However the limit is for total transmit power over the frequency spectrum and the spectrum varies with the DSL specification used. ADSL1 and ADSL2 use the same 1.1MHz range. ADSL2+ doubles the frequency range to 2.2MHz and therefore has to use less power at a given frequency. ADSL2 Annex L (reach-extended ADSL) uses half the ADSL1/2 frequency range (about 0.5MHz) but can therefore use slightly higher transmit power at a given frequency (Annex L is only of interest when the line can't maintain at least 1.5Mbps since the smaller frequency range severely limits the maximum speed).
In ADSL1 days choosing a DSL modem with better receiver sometimes made a big difference in the maximum speed a user could receive. I haven't heard much about modem receiver differences with ADSL2+ and in fact all the modems supporting bonded Fusion are using identical chipsets. Still, it probably won't hurt to try a few different modems to see which one works best.
Noise reduction would be the best way to improve SNR, but unless the noise comes out of ones own home there is not much that can be done about it. It is important to check that the noise on the line isn't self inflicted. Most people fail to check for sources of noise in their own home because it is easier to blame the long line.
The one factor that effects SNR that is the most controllable one is the data rate. If you increase the amount of information going over the line (higher speed) the SNR drops. If you decrease the data rate (lower speed) the SNR increases.

If the SNR gets low the number of errors in the transmission increase. Switching to an interleave profile can help to deal with those errors (up to a certain level, errors can be corrected without retransmission). SRA helps with semi-random noise at different frequencies by shifting bits between frequency bins. If the SNR gets too low you loose connection due to excessive errors and will end up with lower speed on the subsequent retraining.
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DaneJasper
Sonic.Net
Premium,VIP
join:2001-08-20
Santa Rosa, CA
kudos:9
I wonder if he might see better performance in ADSL2 mode instead of ADSL2+...

-Dane


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET
said by DaneJasper:

I wonder if he might see better performance in ADSL2 mode instead of ADSL2+...

That would be my guess based on the specifications of the standards. It is definitely worth trying.

Hopefully we hear back from the poster how well that works for him.
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wsanders

join:2002-09-18
Lafayette, CA
[dupe]


wsanders

join:2002-09-18
Lafayette, CA
Reviews:
·SONIC.NET
reply to leibold
Thanks to all who replied!

Sonic has capped (fixed?) my bit rate at 6.008 Mb and ADSL (DSL1?) and and things are working better, although I am still losing sync a few times each night depending on how many neighbors have lamp dimmers running and how badly the PG&E poles are arcing nearby :0. (I'm a radio ham and I can hear the interference on MF/MF frequencies, and it is correlated with DSL performance.) I found a Cat3 RJ11 to RJ11 patch cable with seems to have helped a little.

To refine my original question: What would be the speed/stability tradeoffs of 6 Mb DSL1 and whatever the default SNR margin setting is (6db?), vs trying a DSL2 or 2+ connection, with the SNR margin bumped up to 12 or 15 dB?

OTOH Sonic very quickly "capped [my] line at 6008kbps and locked the service type to ADSL" so I assume that is the preferred approach to my situation.

-wsanders


wsanders

join:2002-09-18
Lafayette, CA
I should add - in the next couple weeks I plan to install a whole house splitter, which should be good for a few db of SNR reduction. I can also give a before/after report on the SNR as reported by my modem.


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
·SONIC.NET
reply to wsanders
Switching from the original ADSL (now often called ADSL1 to avoid ambiguity) to ADSL2 uses the same transmit power and frequency range but due to protocol enhancements is able to increase the maximum downstream data rate from 8Mbps to 12Mbps.

I'm not sure whether any of the changes make the ADSL2 signal more sensitive to noise but if not then you should see higher speed with ADSL2.

ADSL2+ is an entirely different story because of the expanded frequency range.

I suggest you pick a time when the call volume at Sonic.net is low (it shows on their website) and ask a support person to spend some time with you to try whether switching to an ADSL2 (not ADSL2+) profile will get you more speed.
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wsanders

join:2002-09-18
Lafayette, CA
Reviews:
·SONIC.NET
Thanks - I tried unchecking ADSL2+ and leaving ADSL2 enabled on my modem before Sonic locked me down. My sync rate dropped down to 6-ish from 7-1/2-ish. My CO distance is about 9K ft so this is about what you'd expect. So as far as I can tell both 1 and 2 negotiate the same speed.

However, the SNR reported by the modem for all proto versions was 6 +- a few db, which is the default for Cisco DSLAMS. As I understand it the SNR reported by the modem should more or less match the margin configured at the DSLAM end.

So as an experiment I might see if Sonic can reenable 2 or 2+ and bump up the margin to 9,12,15, or whatever dB. Based on what I can find out about the protocol, this may actually result in the circuit training more frequently, with negligible speed increase. I can't recall but either 2 or 2+ is also supposed to retrain a lot faster than 1, also.

I'll understand if they can't - they may just have a few profiles available to use. Their techs are great but they might go crazy tweaking individual params for every customer 9k ft+ from the CO.

6 Mb is a neighborhood speed record. I'm happy with even sub-6-Mb downloads (800 k uploads would be nice though), but streaming audio is the killer app at our house and we and prefer stability to speed.


DaneJasper
Sonic.Net
Premium,VIP
join:2001-08-20
Santa Rosa, CA
kudos:9
We have ASSIA DSM available on the network - rather than guess, I'd suggest you just ask our staff to push the loop into PO (performance optimization), and let is work it for four or five days. It takes some time as it gathers stats, but in many cases achieves stability and max stable rate. It's quick, easy, and works better in many cases than manual settings.

-Dane


wsanders

join:2002-09-18
Lafayette, CA
Thanks Dane, I'll give it a try. Great job you are doing with Fusion!


wsanders

join:2002-09-18
Lafayette, CA
Reviews:
·SONIC.NET
Back on ADSL2 (not +), we enabled interleaving and all the hard frame errors went away and were replaced by correctable ones. There is a negligible effect on latency. The line is synced at 8 down and 1.2 up and dropped an internet radio stream only twice last night between 10 and midnight and there were no further drops between midnight and 8 AM. We'll leave the performance monitoring stuff on for a few days and see what happens.

I've also made a few changes to my inside wiring - I cut the loop of crappy untwisted 4-pair at the computer desk wall jack and wired in a pseudo-whole-house splitter dongle in such a way that it's a straight run from the NID to the DSL modem, then downstream to all the house phones. Sooner or later I'll install a Cat-5 run from outside to the modem, and a real whole house splitter.


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Good job!