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Question about SNR Margin

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Question about SNR Margin

I was always under the impression that the target SNR for DSL Max was 10db and Tiscali LLU was 6db. Could someone confirm this before I continue with my question?
17 REPLIES
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Question about SNR Margin

Nope, other way around. BT is 6db LLU is 10dB. BUT if the (BT) DSLAM doesn't like you it may be 9dB 12dB or 15dB (and you will probably notice you have interleaving on if this is the case.)

Paul.<><
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Question about SNR Margin

The reason I ask is that I'm on MaxDSL and my SNR fluctautes between 10db and 10.5db with interleaving turned off on my 6.5mb connection and I'm 600m from the exchange, and I never have any problems.

A friend who had MAXDSL until July never used to have problems with his connection until he was switched over to LLU against his wishes. The LLU was a nightmare and he was swiched back over to BT about 12 days ago but gets regular disconnections in the evenings. His SNR Margin is 5.7db which I thought might have been too low. Is there a way of getting this increased? Basically Plusnet don't want to know saying that the connection will adjust itself. He lives about 2km from the exchange and the router is a DG834 PN. Online gaming is impossible in the evening due to the regular disconnections.

Downstream SNR Margin

Should be at least 10 dB to get adsl - ideally above 12dB.
Anything less than this will see frequent disconnections and other problems.

This was taken from http://www.kitz.co.uk/adsl/linestats.htm
shellsong
Grafter
Posts: 2,191
Registered: 03-08-2007

Question about SNR Margin

kittz quote was re the BT pre MaxDSL situation-- the MaxDSL default target SNR is 6 dB and the Tiscali one is more like 12-15 dB or "ridiculously high" as it is known in the trade!

The BT target can be shifted in, I think, 3 dB increments when problems are experienced and I understand that Tiscali can do the same in either direction if they can be persuaded to!
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Question about SNR Margin

When I was on *iscali, my SNR always started at 12db in the morning, but ended up being 5db by 10pm and required a router reboot to force a retrain and get anymore traffic through.

Now I'm on BT, my SNR always starts at 6db and remains constant. No router reboots are required and no logged retrains are done.

Last I heard about *iscali, PN was requesting some additional profiles, but *iscali were ignoring the requests...

Best thing I ever did was move back to BT - higher speeds & much more stable, despite a lower snr!
Community Veteran
Posts: 5,877
Thanks: 1
Registered: 05-04-2007

Question about SNR Margin

shellsong; the tiscali line profile tools that PN use are ace. my line has been put on a higher SNR profile a couple of times by PN and Dave has been able to sort it immeadiately. Also I was under the impression that the BT ones weren't so good...
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Question about SNR Margin

Does anyone know how BT or Tiscali measure the SNR at the user end?

Is there a common commend on all routers and modems to return the SNR?

The SNR at the exchange will not be the same as the user end so how does the setting of the 6db or 12db work?
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Question about SNR Margin

I've no doubt that most people who enjoy sync speeds of 8128 have margins much lower than 10/12 dB and a fair number will be on 6 dB.

Assuming your own system - filters, extensions etc are in good order then its down to the line conditions.

You seem to have good speed with a fairly high SNM, modest SNR variation and it would seem stability. You can determine your Target SNM by checking what its value is when you connect the Router, either the log or other Router Stats.

I guess your Target SNM is either 9 or 12 dB. Either way this would normally suggest some instability, frequent loss of sync, during or after the Trial. If there has been no significant instability, and interleaving being off would suggest not, there could be a problem remote from yourself ie Plusnet or BT. Your distance from the exchange and your experience would suggest to me that 8128 should certainly be achievable.

I'm approx 2.5 KM from the exchange, have a Down Attenuation of 43 dB and, following a BT fault a Target SNM of 15 dB. This fault is now repaired and I would love PN/BT to drop my Margin back down to 6 dB. I've had the same response from PN that your friend has about MaxDsl adjusting itself which in his circumstances it often does. In my situation the response is simplistic in the extreme since BT rarely reduces the Target SNM even though as in my case it was a BT fault which caused the increase in the first place.

I use a Speedtouch Router and have reduced the Target SNM with DMT - German software.
This software works with other Routers and can also be used to increase the Margin - I think 5 dB is the limit in either direction. Setting the Target SNM to 10 dB, as low as I can get it, gives me a sync speed of 7456.

If your friends Router is compatible it could be a better option than the Margin being increased by BT since it can be very difficult to get it down again.

This link http://dmt.mhilfe.de/ lists versions up to 8.04 and if you check, each version quotes the Routers it works with - your friend might be lucky.

Other Routers can achieve the same effect using different techniques. ADSLGuide is a good place to look - enquire.

There are other more basic ways of increasing the Margin but they are rather heavy handed, unpredictable and difficult to reverse.
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Question about SNR Margin

This situation sounds very similar to mine.

I first got ADSL Max in May when the target SNR was 6db. I guess this was too low for the training period and it resynced quite often. It was increased to 9db and interleaving was turned on. This seemed more stable but then a week or so later due to either line faults or problems with my internal extension wiring it would occasionally sync at ~256kbps so I'd have 3 days of 128kbps download. Sometime after that my target SNR was increased to 15db and interleaving was left on. According to Plusnet when I lost my broadband connection for 3 weeks BT fixed at least 2 faults on my line so who knows Smiley

I now have a really stable connection, I don't think my router (DG834PN) has dropped a connection due to low SNR for a couple of months. However I think it's overkill and I could have a lower target SNR with higher speeds while still being very stable.

I live 2.15km straight line distance from the exchange but I've no idea how long the line is. I could loop round the town twice for all I know. Is there any way of finding out the line length? Is the downstream line attenuation of 54db a good indicator?

Here are my current line stats:
ADSL Link Downstream Upstream
Connection Speed 2848 kbps 448 kbps
Line Attenuation 54.0 db 31.5 db
Noise Margin 17.7 db 22.0 db

The problem I have is that during the hours of ~6am to some time in the evening (probably when the street lights come on) if I reboot my router it syncs at about 3700kbps with a SNR of 15db. The SNR remains fairly stable all day until about 4-5pm when it starts to reduce down to 10db by 7pm and then the router sync rate lowers automatically without losing connection to about 2848 with SNR of 15db. The SNR may flucuate during the night but doesn't get lower than 12db. First thing in the morning it's usually back up to 18db. I'm using "Router Stats" at the moment which can draw a nice graph of the SNR over a period of time.

So the annoying thing is, even though my line can perfectly handle 3.0 Mbps data rate during the day because the sync rate always drops below 3424 during the night my data rate is always at best 2.5 Mbps. Occasionally it has a bad night and the sync rate drops to between 2272 - 2847 so my data rate is sometimes 2.0 Mbps Sad

Do anyone know how to find out what the Maximum stable rate on your line is?

Does anyone think my line should be able to maintain a higher sync while still being stable? It would be great to have the target SNR reduced to 12db or turning interleaving off as I've heard this can increase sync rate, or maybe both.

What I do know is that the target SNR is not going to reduce automatically as Plusnet have said in the past. My connection has been very stable for the last 2 months so surely it should have reduced by now.

Is it worth opening a ticket to get Plusnet to reduce my SNR or trying to force my router to do it manually? Have Plusnet created this button in Connection settings to turn off interleaving yet?

Regards,
Tim
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Question about SNR Margin

Unless you are into Gaming and have very good line conditions you may well be better off with interleaving on. You may in fact sync higher and achieve better performance with interleaving on than off.

Your attenuation of 54 dB is quite high and is indicative of the length of your line although this can on occasion be high due to poor BT Line conditions.

If I understand correctly you tend to lose sync of an evening down to 2848.
With a SNM of around 15 dB this suggests either your Router is re-syncing at a relatively high SNM, possibly well above zero dB or your margin is dropping to zero which means a rather substantial variation in SNR.

You can set RouterStats to sample every 5 seconds and it may pay to do so one evening and check exactly what is happening. If your Router loses sync at high SNM values you could try another Router.

If your variation in SNR is or exceeds 15 dB and is causing loss of sync then you have a problem.
You could try to eliminate your own side. Connect the Router direct to the BT Test Socket behind BT's Faceplate for an evening and see if that gives an improvement.

If not then the variation in SNR could simply be down to electrical interference on the line somewhere which you can probably do nothing about or it could be a BT fault of course.

If PN undertake some diagnostic testing the results tend to be on your ticket which includes MSR and Fault Threshold. If you haven't raised a Ticket associated with previous BT faults you may simply have to ask them - I assume they would tell you. I would be inclined to ask that one simple question initially otherwise they will tend to wander off the subject and never get round to telling you.

I think you will have considerable difficulty getting PN/BT to reduce your Target SNM at the best of times let alone if you are having problems.

You can check DMT that I referred to but I don't think it will work with your Router.
I don't think you can reduce your SNM using the SNM tweak with this Router.

I think you need to investigate exactly what is happening and why before tweaking anything otherwise your connection is liable to become less stable.
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Question about SNR Margin

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His SNR Margin is 5.7db.......Is there a way of getting this increased?

In my first comments I should have added that in terms of increasing your SNM, using a Router which allows you to set the Maximum Sync Rate rather than tweaking the SNM is by far the better option.

One advantage is this allows you to set a maximum Sync Rate to match the lowest sync speed of a given profile therefore giving maximum SNM for a particular profile and associated Data Rate - Download Speed.
This offers much greater control than tweaking the SNM.

Unfortunately Routers having this option are not that common although some of the BT Voyager, Billion, Draytek and Belkin models do.
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Question about SNR Margin

Ok well Im a gamer and pings etc are critical to me, I dont really care about bandwidth in all honesty, 512k down would be fine for me, but anyway i digress.

My line attenuation is 57DB consistently, my snr hovers at 6 all the time. My con is 4 megs down and 448 up 24/7 as far as I can tell, I do get crc errors and the odd hiccup, the router may discon but this is rare, I didnt get any last night and I was playing SW Galaxies all evening.

Now the ventrilo ports are sorted I ping at sub 30ms to that all the time and swg is a solid 50 ping or lower all the time.

My line atten is very high cos I am 4-5km from the exchange, under the old BT guidelines I would have been refused 2meg cons let alone anything else, so to me Ill accept the odd hiccup once an evening. Apart from PN fannying around sorting out ventrilo, Im quite happy with my service right now, famous last words I know

:shock:
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Question about SNR Margin

I'm not into gaming atm and I need a stable connection for work. It's just I read quite recently on here that someones sync rate increased when interleaving was turned off. But if it can go both ways then who knows. That's why we need an easy on/off button that users can control.

I've set RouterStats to sample every 5 secs now. I just rebooted the router and the sync is now 3680 with exactly 15db SNM. I'll monitor it this evening to see what happens. When the router sync rate drops to 2848 which it probably will this evening I don't think it actually drops the connection because the WAN PPPoA uptime doesn't change. What I assume is happening is that the DLM allows the SNM to reduce slightly while taking no action but once it drops below a certain threshold (~9-10db) it then reduces the sync rate to 2848 to maintain a 15db SNM.

This is normal I presume except the target SNM will be different for other ppl. If only it let the SNM get lower before changing the sync rate it may manage to maintain that speed. Or if my target SNM was 12db then the sync rate may stay above 3424 even during the night so I could have a data rate of 3.0 Mbps.

I've had my router plugged into the BT test socket for about 2 months now with no extensions connected at all so it's as good as possible.

Tim
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Question about SNR Margin

Your Router should not lose sync until it gets to very low values of SNM.

My old Netgear DG834v1 would sit at zero dB for extended periods and not lose sync.
Some argue that Netgear's displayed values are 'pessimistic' and that the actual values are higher whilst others are adamant that Netgear Routers can hold sync at very low values of SNM. I've used several Routers and am not aware of any of them losing sync until the SNM dropped to zero dB.

There are reports of people losing sync for values just below say 6 dB but other things being equal this shouldn't normally happen.

To my knowledge BT can/will do nothing after the margin is set by way of attempting to maintain that margin - until you lose sync of course.

Either your Router is at least part of the problem or -
your SNR variation is so great that your SNM is dropping so low ie zero db, that you lose sync.
RouterStats graphs/logs will hopefully confirm if your Router is hanging in at low values of SNM. If you never see low values of SNM then it could point to your Router as the problem.

If its not conclusive I would try another Router if possible.

In principle Fast Path is capable of providing the maximum possible speed provided your line conditions allow. Interleaving is regarded by some - many, as desirable for lines which are some way short of ideal. No doubt you will give further thought to it once you've sorted your current problems out.

Even if you could I wouldn't be inclined to get it turned off at the moment since it is liable to make your current stability worse - you may well have to pay to get it changed.

Good luck

Edit :
Quote
if my target SNM was 12db then the sync rate may stay above 3424 even during the night so I could have a data rate of 3.0 Mbps.

Assuming the problem is a reducing SNR, which it probably is - the lower your Target SNM the greater the likelihood of a loss of sync, and therefore the greater the instability, because it would take a smaller reduction in SNR to initiate a loss of sync. Whatever reduction in SNR is taking place you would simply tend to lose sync sooner and more frequently therafter.

You might argue you could live with that but in practice following a loss of sync, due to reducing SNR, you will often still have a relatively low SNR when you re-sync, which means by the time the Target SNM is applied you may well sync at quite a low speed. Also if it happens often enough BT would simply push the Margin back up to 15 dB anyway - its the way their system works.

I hope that makes sense.
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Question about SNR Margin

Well the same thing happened yesterday evening. I've looked at the router stats for every 5 seconds. My router had been happily syncing at 3680kbps with ~14db SNM for over 6 hours until about 5pm when the SNM started slowly flucuating on a downward trend until it reached 8.4 at 18:52:44. Then, I don't know whether it's my router or the DLM but the sync rate reduced to 2720 and the SNM jumped back to 15db. There are a couple of strange readings inbetween.

Thu 19 Oct 2006 18:51:34 Rx-Noise= 9.1 Tx-Noise= 23
Thu 19 Oct 2006 18:51:39 Rx-Noise= 8.9 Tx-Noise= 23
Thu 19 Oct 2006 18:51:44 Rx-Noise= 8.8 Tx-Noise= 23
Thu 19 Oct 2006 18:51:49 Rx-Noise= 8.8 Tx-Noise= 23
Thu 19 Oct 2006 18:51:54 Rx-Noise= 8.8 Tx-Noise= 23
Thu 19 Oct 2006 18:51:59 Rx-Noise= 8.8 Tx-Noise= 23
Thu 19 Oct 2006 18:52:04 Rx-Noise= 8.9 Tx-Noise= 23
Thu 19 Oct 2006 18:52:09 Rx-Noise= 8.9 Tx-Noise= 23
Thu 19 Oct 2006 18:52:14 Rx-Noise= 8.8 Tx-Noise= 23
Thu 19 Oct 2006 18:52:19 Rx-Noise= 8.6 Tx-Noise= 23
Thu 19 Oct 2006 18:52:24 Rx-Noise= 8.4 Tx-Noise= 23
Thu 19 Oct 2006 18:52:29 Rx-Noise= 8.5 Tx-Noise= 23
Thu 19 Oct 2006 18:52:34 Rx-Noise= 8.4 Tx-Noise= 23
Thu 19 Oct 2006 18:52:39 Rx-Noise= 8.5 Tx-Noise= 23
Thu 19 Oct 2006 18:52:44 Rx-Noise= 8.4 Tx-Noise= 23
Thu 19 Oct 2006 18:52:49 Rx-Noise= 12 Tx-Noise= 8
Thu 19 Oct 2006 18:52:54 Rx-Noise= 7 Tx-Noise= 8
Thu 19 Oct 2006 18:52:59 Rx-Noise= 17 Tx-Noise= 8
Thu 19 Oct 2006 18:53:04 Rx-Noise= 15.3 Tx-Noise= 23
Thu 19 Oct 2006 18:53:09 Rx-Noise= 15.3 Tx-Noise= 23
Thu 19 Oct 2006 18:53:14 Rx-Noise= 15.1 Tx-Noise= 23
Thu 19 Oct 2006 18:53:19 Rx-Noise= 15.1 Tx-Noise= 23
Thu 19 Oct 2006 18:53:24 Rx-Noise= 15.1 Tx-Noise= 23
Thu 19 Oct 2006 18:53:29 Rx-Noise= 15.1 Tx-Noise= 23
Thu 19 Oct 2006 18:53:34 Rx-Noise= 15.1 Tx-Noise= 23
Thu 19 Oct 2006 18:53:39 Rx-Noise= 15.1 Tx-Noise= 23
Thu 19 Oct 2006 18:53:44 Rx-Noise= 15.1 Tx-Noise= 23
Thu 19 Oct 2006 18:53:49 Rx-Noise= 15.1 Tx-Noise= 23

I wasn't monitoring the sync speed at the time as I was more interested in the SNM but I can say for definite that is dropped to 2720 between 18:52:49 and 18:53:04. Also note that the router definitely definitely did NOT lose sync at the this point. I had 4 putty ssh sessions running at the time the sync rate dropped and all 4 remained connected. I know in the past if there was a power outage even for a second which caused the router to lose sync then the putty sessions always disconnectd. So the router did not lose sync.

If what you're saying is true and the BT side does not do anything to alter the sync rate until loss of sync then maybe it is my router. But I can't understand how. If it was then I'd expect this is happen randomly, not roughly the same time every evening when the noise increases.

Could a Plusnet comms person please comment on this? Does the BT DLM dynamically alter the sync rate to maintain the connection if the SNM drops too low. I thought that how ADSL Max worked. It is quite a drop is SNM, from 15db down to 8db. Is this not normal?

Thanks for your help,

Regards,
Tim