Kitz Forum

Broadband Related => Broadband Technology => Topic started by: Weaver on January 05, 2016, 08:11:35 PM

Title: 3dB downstream target SNRM
Post by: Weaver on January 05, 2016, 08:11:35 PM
I've been set to a 3dB downstream target SNRM for a week or so. The results have been really great, not too unstable, that I've noticed, and a downstream sync rate of 2700 - 2900 kbps at 65 dB d/s attn. on each of my three lines now that line #a.3 has been set to 3 dB too. That line was ill, but may now have been mended.

Various posters rightly warned me about possible serious instability at 3 dB, but it seems I'm ok thus far. I read that Andrews and Arnold had a trial group of TalkTalk LLU users who were doing this, I didn't realise to begin with that I would be able to choose 3dB when on BTW 21CN, or ADSL2. (Not sure which is the controlling factor?)


Title: Re: 3dB downstream target SNRM
Post by: burakkucat on January 05, 2016, 09:18:51 PM
Just to clarify the situation (for anyone speed reading this thread), you are configuring the SNRM target for each of the three circuits via the controls provided by your ISP/CP, A&A, and not by tweaking the configuration via each modem.

{Edited to replace the word post with thread.]
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 05, 2016, 09:48:30 PM
Oh sorry. I used the controls on A & As website clueless.aa.net.uk I think there was a screenshot showing the control panel UI in an earlier thread somewhere?

I don't know if there is an SNRM-tweaking facility in this modem, the DLink DSL-320B-Z1.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 12:43:52 AM
Another Andrews and Arnold user who has an attn of a mere 49 dB iirc said that he could only tolerate 3 dB d/s target SNRM during the summer, and had to revert to a full 6 dB during the winter months. I'm not amazed at this, but I wonder exactly why?

It's a good job then that I started this experiment in the teeth of winter weather. The weather was worse before and during Xmas with high winds for days. It may be though that with the benefit of multiple lines I will get away with 3 dB even during really bad weather periods. We'll have to see.

Title: Re: 3dB downstream target SNRM
Post by: loonylion on January 06, 2016, 01:04:57 AM
I used to get away with 1dB, it depends entirely on the line.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 01:22:40 AM
@loonylion - that was what you set? Not what it drooped to? Wow that's good going. What speed did you get and what environment did you have?
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 01:27:14 AM
I'm not sure what to look out for in assessing reliability. I don't have any direct stats as many of  you do, just the modem and line status reports from A & A's CQM (http://aa.net.uk/kb-broadband-cqm.html (http://aa.net.uk/kb-broadband-cqm.html)) and clueless.aa.net monitoring and control server. It sometimes reports a possible resync explicitly. But I'm not sure I am seeing all resyncs on the CQM graphs though, if there are any that is, because if they are short duration I don't know that they would be picked up.
Title: Re: 3dB downstream target SNRM
Post by: c6em on January 06, 2016, 09:07:48 AM
The summer/winter aspect is due to the solar wind from the sun and how it impinges on the earth in different seasons - and in particular the longer nights of winter. The solar wind also causes the day/night variation.

This 'wind' on the side of the earth facing the sun causes the "Heviside Layer" in the upper atmosphere to rise up.  This is a radio-reflective layer and results in most notably radio stations from further away being receivable during the night time.  It also means a LOT more general noise in the atmosphere on the night time side during the hours of darkness and this effect is multiplied during the winter periods.
Your old AM band radio amply demonstrates this as during the night other stations appear from other continents while the general hiss on the radio on all wavelengths gets much worse.

All this noise is picked up by phone lines which act as gigantic aerials and results in the SNRmargin reducing at night and becoming more variable and jagged.  Interestingly the SNRmargin (with me anyway) usually starts to rise again in the early morning before it has got light in the winter months.
For those lines which are on the margins as to whether they can work on a 3 or 6 dB target margin the extra interference during the winter periods might be enough to tip them over into requiring a 6dB margin.
Title: Re: 3dB downstream target SNRM
Post by: sevenlayermuddle on January 06, 2016, 09:34:31 AM
I'd predict that the degree of instability would vary, depending on the services you use.

As the margin reduces, the error rate will increase.   So far as I recall, 0dB margin is simply the margin at which the BER exceeds a standardised threshold, 1 in 10^7 I think(?).  As the BER increases, some non-recoverable errors will take place, leading to data retransmission.

For many services, these retransmissions go unnoticed.   For time-critical stuff like video streaming, it might lead to freezes.   For true IPTV, which does not benefit from TCP retransmissions at all, the lost data would simply lead to picture corruption.

Above is written on the assumption that TCP handles the re-tx.   If using Physical retx, things are not as bad.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 10:27:07 AM
Guys is there physical Retx in ADSL2 itself?
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 10:29:03 AM
I didn't know that 0 dB was some arbitrary level, I never really thought about it to tell the truth, never knew what the definition was in this context.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 10:35:47 AM
@c6em thanks for making me aware, I should have thought about that. I have a (bad) degree in Theoretical Physics, so a <doh />  :-[ there  ;D
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 10:38:26 AM
So 7lm, negative SNRM is entirely possible? I seem to remember some old Netgear modem-routers showing huge positive numbers because of bugs in mishandling of negative numbers
Title: Re: 3dB downstream target SNRM
Post by: sevenlayermuddle on January 06, 2016, 10:49:00 AM
I believe it is perfectly possible to maintain a connection with a BER that is worse than the 0dB BER .  But I suspect the modem might be supposed to retrain at 0dB, on the assumption that such a connection was undesirable.

Please note careful, politician-style, choice of wording in above.. 'I suspect', 'I believe', etc.  Deduce that I'm not 100% sure of my facts... and not volunteering to download ITU specs and spend the rest of the week researching to confirm.    ;)

Re PhyR, I thought it was optional though probably rare, in ADSL2.   Somebody might know better. :-\
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 10:54:36 AM
> volunteering to download ITU specs and spend the rest of the week researching to confirm.

Well that goes for me too. I'm just lazy and hoping the more knowledgeable in our community had gone there before me.
Title: Re: 3dB downstream target SNRM
Post by: loonylion on January 06, 2016, 11:21:15 AM
weaver: I was running a billion 7800N on a long (~3.3km, underground except the dropwire) ADSL2+ LLU line (IIRC maybe operating in G.DMT) with the target margin set on the modem. It allowed me to get 5Mbps when the rest of the street (and indeed, village) was stuck on 3Mbps or less. It dropped about once a week on average.
Title: Re: 3dB downstream target SNRM
Post by: d2d4j on January 06, 2016, 12:35:55 PM
Hi

I'm not sure if this is relevant, but we have an adsl2+ line running at snrm of 2 att loop 25 on a draytek 2820 on g992.5 I believe with download speed of approx 18mb I think from memory, which shown an uptime of 110 days if I remember correctly, but I do have a picture of service status.

If you would like to see the picture, I would just need to blank any identifying details first

Many thanks

John
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 01:37:45 PM
@d2d4j - thanks for that John ;D You can tell how long your connection has been up, which is a great advantage over my situation. I have no idea, apart from hoping I can spot resync or reconnection events.

And yes please, pic would be good.
Title: Re: 3dB downstream target SNRM
Post by: d2d4j on January 06, 2016, 02:08:11 PM
Hi Weaver

Please see pic attached (sorry, I read the post over posting attachments, but this is so small)

Also, we have many lines, from adsl2 to FTTC, LOS, VPN bonded etc... but too many to get all details on sorry

Many thanks

John
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 02:21:08 PM
John, many thanks for pic.
Title: Re: 3dB downstream target SNRM
Post by: NewtronStar on January 06, 2016, 06:04:31 PM
Which modem/router are you using weaver ?
Routerstats lite is compatible with loads of routers

http://www.vwlowen.co.uk/internet/files.htm (http://www.vwlowen.co.uk/internet/files.htm)

If you want I can post my old Dlink 2640B to you free of charge think that works with DSLstats
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 07:52:56 PM
@newtronstar that's incredibly generous of you. I need to work out how to do the NATing runes necessary to map 192.168.1.1 in its /24 for each modem into something passed through the Firebrick router so that each modem's admin I/f is accessible. Accessible both to me and to Andrews & Arnold remotely should they wish to help diagnose things via the Firebrick. I've never done NAT with firewall rules before and I'm going to need some help from A & A. I also don't want the modems to be generally accessible to the LAN otherwise I will have to remember to set a strong password on the modem admin UI each time I install a replacement modem, at the moment because they are completely inaccessible then defaults will do fine. I might deal with this in another thread, not this one.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 08:02:59 PM
Oh and I'm using three DLink DSL-320B-Z1 modems configured especially to order by Andrews & Arnold to be locked in ADSL2-only mode, not auto-ADSL2+.
Title: Re: 3dB downstream target SNRM
Post by: NewtronStar on January 06, 2016, 08:28:22 PM
@newtronstar that's incredibly generous of you.

Well it's here if you need it, just a pity your stats are confined to A&A as you know we here love a gander into 24/7 dsl stats graphs  ;D
Title: Re: 3dB downstream target SNRM
Post by: currytop on January 06, 2016, 09:48:47 PM
Quote
I've never done NAT with firewall rules before and I'm going to need some help from A & A

I use masquerading rules in iptables on a Billion router to access the modem stats via LAN1 rather than setting up static routes. The modem remains blind to both the internal network and the Internet that way, but can be accessed from a specified local subnet or host.  I can't see the equivalent iptable commands in the Firebrick manual so not sure how you would do that or even if it supports iptables.
Title: Re: 3dB downstream target SNRM
Post by: burakkucat on January 06, 2016, 10:14:37 PM
I can't see the equivalent iptable commands in the Firebrick manual so not sure how you would do that or even if it supports iptables.

I have a vague feeling, from my reading of the FireBrick documentation, that they use an operating system specifically written by Watchfront / A&A.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 06, 2016, 10:18:32 PM
I don't know that it is a *nix box, so no iptables. I suspect that you're right about an original o/s, although I don't know why. Looking at the FB boot loader system for example, it suggests originality somehow.
Title: Re: 3dB downstream target SNRM
Post by: Goofair on January 08, 2016, 01:39:56 AM
Well, I don't even from UK, so I don't know if my experience will be someway relevant for you
I live in a pretty bad area for ADSL (the telephone cables here is more than 10 years old and I never see any routine maintenance, just if something really fail)
Fortunately I'm close from the DSLAM, like 1 km, but my att is 32, maybe because of the age of these cables...

I'm testing a 3 snr target for 3 days and all is going fine until now (there is a lot of Super Frame and HEC errors, but no forced resyncs at all)

My actual stats:

(http://i.imgur.com/omuxCVg.png)

I made a modem restart 2 hours ago to try some others tweaks to reduce the errors, but had no success. and I think it's because the line is much closer to max attainable Line speed
9184 is my line cap, so I can't get more than that anyway

I have chosen ADSL 1 (G. DMT) because I get a way better ping in this mode, like 19ms vs 50ms from ADSL 2+ with 64 bytes of interleave
The modem shows my G.DMT line as Interleave, but in fact there is not Interleave at all

Speed test:

(http://www.speedtest.net/result/4977866885.png)

I tried to make this text the better possible (even used a spell checker, but English is not my best language)  :-\
btw i'm from southeast part of Brazil.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on January 08, 2016, 01:50:03 AM
Welcome Goofair!   ;D
Title: Re: 3dB downstream target SNRM
Post by: Goofair on January 08, 2016, 02:01:29 AM
Welcome Goofair!   ;D

Thank you sir  ;D
Title: Re: 3dB downstream target SNRM
Post by: kitz on February 02, 2016, 05:44:21 PM
Guys is there physical Retx in ADSL2 itself?

Its possible to do it on adsl2+,  but I think just on the downstream (dont quote me on that Im going from memory re upstream).   
So yes it is available, but BTw havent implemented it.
Title: Re: 3dB downstream target SNRM
Post by: Weaver on February 02, 2016, 05:52:31 PM
Wow. Seems a shame, an opportunity missed. I can imagine it complicating the software somewhat because of the requirements of managing the buffering. It might be faster to disable the system in a world with no errors.
Title: Re: 3dB downstream target SNRM
Post by: kitz on February 02, 2016, 05:57:19 PM
I believe it is perfectly possible to maintain a connection with a BER that is worse than the 0dB BER .  But I suspect the modem might be supposed to retrain at 0dB, on the assumption that such a connection was undesirable.

Please note careful, politician-style, choice of wording in above.. 'I suspect', 'I believe', etc.  Deduce that I'm not 100% sure of my facts... and not volunteering to download ITU specs and spend the rest of the week researching to confirm.    ;)


I think that it is more to do with the number of errors recorded rather than the actual value of the SNRm.   
The modem should go into an alarm state when 'x' no of errors per second is recorded.  If you look at LOS (http://www.kitz.co.uk/adsl/linestats_errors.htm) you can see the type of errors and alarm states which cause the modem to resync.

So yes you are correct in that it is possible to go into negative SNRm  - as some of the old netgears used to.   The E/Sec rate would get pretty high though and the connection would get really laggy.   This was back in the days before DLM monitored Errored seconds,  today's DLM would kick in pretty quick, and bump up things like interleaving - not just the target SNRm
Title: Re: 3dB downstream target SNRM
Post by: kitz on February 02, 2016, 06:05:46 PM
Wow. Seems a shame, an opportunity missed. I can imagine it complicating the software somewhat because of the requirements of managing the buffering. It might be faster to disable the system in a world with no errors.

It may be something to do with the MSANs.   Bear in mind that some of BTs MSANs (such as the Marconi's) have now been in a long time and may not support a technology that is much newer than the equipment .   BT have lots of different makes of equipment for the MSANs so it could cause complications trying to update it on all.    The MSANs hold a lot more users than what the cab DSLAMs do, so it could also require a lot of buffering for those MSANs.

I think Sky have implemented retx on their ISAMs.  Certain SPs stick to certain equipment.  Funny enough I was in the exchange today and saw some Stingers.   I immediately knew that those would belong to Easynet... and they were.    Same as TT tend to use Huawei. 
Title: Re: 3dB downstream target SNRM
Post by: kitz on February 02, 2016, 06:19:05 PM
@d2d4j - thanks for that John ;D You can tell how long your connection has been up, which is a great advantage over my situation. I have no idea, apart from hoping I can spot resync or reconnection events.

And yes please, pic would be good.

Here's something from me.  I think its from about 7yrs ago.   My line would sit at 3dB for months on end without any errors and sync at 24/2.2Mbps (Annex_M)

As an experiment I ran it at 2dB for a while and it would also perform quite happily without errors.  I then pushed it further to 1.5dB.   Here you can see I ran it on 1.5dB of SNRm for about 6 weeks.

This was a BE line so no automatic DLM, and the tweaks were done at a router level.  After that I couldn't be bothered to tweak and left it at the default 3dB.   I'll also attach my errored seconds for the same period.  These are cumulative and as you will see, after I cleared the totals in March, the amount of ErrSecs was practically zilch..  therefore I could probably got away with even lower than 1.5dB