Speed vs Attenuation/SNR

Hello all,

I'm on Sky Unlimited; my first LLU; and was expecting at least a tiny increase in speed. We're about a mile from our exchange and were getting around a 2-3Mbit/s throughput with Vivaciti prior to changing over, and Sky's pie-in-the-sky guess was that we might see as much as 8Mbit/s when we changed to them.

Using Sky's provided Sagem router, I end up with a sync speed of 4660kbps. Downstream attenuation is 45.5dB, noise margin 6.2dB. Upstream is fine at 796kbps, 27.3dB attenuation and 14.2dB SNR.

With 45.5dB attenuation I'd expect a bit better than 4660kbps. I'm on a new filter, plugged into the master socket. There's an extension immediately after it to run the line to the Sky HD box, but removing that makes very little difference. Trying another router dropped me right back to 4000kbps, and with the occasional reset the best I've managed is just shy of 5000kbps.

The line appears normal when I make a phone call. Anyone got any idea if there's something awry here, or if there's something I should do? Would trying DLM again from scratch be likely to give me an improvement? I can live with 4660kbps as it's 50% faster than I had on non-LLU ADSL, but if I can squeeze more out I'd like to.

Thanks in advance!

Have you tried the router in the test socket ??

Sure have; the faceplate's dangling off right now, the extension upstair's disconnected (although I've left the filter in it just in case) and the Sky box is unplugged too. It's just a filter in the test socket with the router and phone connected.

Looks like that's all your gonna get from that line then (unfortunate).

You could request Sky to lower your target SNR to 3dB, and have interleaving switched on. You'll probably grab a bit more speed from the line. But if latency and instability isn't your friend, best to not do this.

And the above is assuming they could even honour this request, maybe they can't (or wont).

Edited by deleted (Sat 28-May-11 20:30:40)

In reply to a post by mixt:

Looks like that's all your gonna get from that line then (unfortunate).

You could request Sky to lower your target SNR to 3dB, and have interleaving switched on. You'll probably grab a bit more speed from the line. But if latency isn't your friend, best to not do this.

And the above is assuming they could even honour this request, maybe they can't (or wont).

Sorry to butt in, but i have to say that sky enables interleave by default, maybe if they disabled DLM then maybe a higher sync would be possible, as no doubt it will of effectively capped the max sync rate, as a result a lower snr will do little if anything to increase the sync
as for them changing the default profile of a (7-db margin interleaved ) is very doubtful they do what they call a gaming mode( reduce interleave delay level ) as they will not enable fastpath , which would be another way that you could of got a bit more out of your line

Edited by tommy45 (Sat 28-May-11 20:36:13)

In reply to a post by Elmlea:

Hello all,

I'm on Sky Unlimited; my first LLU; and was expecting at least a tiny increase in speed. We're about a mile from our exchange and were getting around a 2-3Mbit/s throughput with Vivaciti prior to changing over, and Sky's pie-in-the-sky guess was that we might see as much as 8Mbit/s when we changed to them.

Using Sky's provided Sagem router, I end up with a sync speed of 4660kbps.

You start off on 4Mb with Sky. How long have you been with sky?

About 4-5 weeks, I think. Didn't realise it might be artificially capped?

You start off on 4Mb and this should gradually raise over say, 10 days until a stable level is reached. You online account should say whether your line is still in training or not.

However, after 10 days you can ask Sky to put you on the Gaming profile which will uncap your line and lower your latency.

Ah, no, I'm past the 10 day training period. It started at just below 4, but only managed to crack 4976 or something, and since then has meaned between that and 4500 whenever a reset's been required or happened by accident.

Who do I speak to to get set on a gaming profile? What's that going to do for me?

Just found a site that checks some stats for me; this is the output.

You are synced at 4,660 kbps. This sync will have a max throughput of about 0.47 MiB/s or 3,868 kbps.

Your expected max sync depends on many things, mostly the quality of the line and the target Signal to Noise margin at the time of sync.
Expected max syncs are:
3db Fastpath: 11,182
6db Fastpath or 3db Interleaved: 10,105
6db Interleaved: 8,257


You appear to be on the standard Signal to Noise margin and with set.
Your sync is 56% of the estimated maximum for a line with and standard SNR at your line length (3,295 meters).
As your sync is not above 81% of the estimated maximum you might wish to investigate a potential problem.

Signal to Noise

You appear to have a noise issue on the line as the upstream and downstream SN margins vary by too much:
1) that has caused the DSLAM to sync you at a higher SNR on the upstream.
2) there was noise on the downstream frequencies when you synced that has now lessened. This noise is likely to return.
3) there is noise on the upstream frequencies now that was not there when you synced.
4) there could be faulty equipment on the line.

Can someone help me interpret that? I'm not sure what it's getting at when it says the SNRs vary by too much, or what I can do to try and sort that.

In reply to a post by Elmlea:

Ah, no, I'm past the 10 day training period. It started at just below 4, but only managed to crack 4976 or something, and since then has meaned between that and 4500 whenever a reset's been required or happened by accident.

Who do I speak to to get set on a gaming profile? What's that going to do for me?

Ask Sky. It will uncap your line and lower your latency.

In reply to a post by Elmlea:

Can someone help me interpret that? I'm not sure what it's getting at when it says the SNRs vary by too much, or what I can do to try and sort that.

You should take your stats with only your router connected to the phone line, not the phone.

Probably all you will get - you could have a noisy line. Not necessarily from the exchange but picked up en-route.

Use RouterStats Lite to track your SNR over a 48 hour period. Then choose a time when the SNR is around 3dB above minimum which should give you enough for the line to hold. Those times will be early morning or late evening.

Without the phone it lifts my sync to 4743, and SNR to 7.0dB.

A quick look at Sky's page says they've set my line to 4.9Mbit/s, so I think ringing them to find out about uncapping it might be the best way to start!

In reply to a post by MHC:

Probably all you will get - you could have a noisy line. Not necessarily from the exchange but picked up en-route.

Use RouterStats Lite to track your SNR over a 48 hour period. Then choose a time when the SNR is around 3dB above minimum which should give you enough for the line to hold. Those times will be early morning or late evening.

Sorry, not quite sure what you mean. What's the minimum SNR; I presume I'm set to 6dB, so look for a time when it's running at 9dB? What do I do with this info?

In reply to a post by Elmlea:

Without the phone it lifts my sync to 4743, and SNR to 7.0dB.

Looks like you have wiring issues at home

Really? What makes you think that; I didn't think a teeny change with the phone removed would signify much?

In reply to a post by Elmlea:

Really? What makes you think that; I didn't think a teeny change with the phone removed would signify much?

It shouldn't change, to me the phone wiring is suspect.

How is the upstairs extension connected to the master - is there a plug, or is it hard wired?

It's hardwired; as in when I pop the faceplate off, there's a bunch of wires there linking to the socket upstairs. It's currently disconnected though.

I should point out that while it jumped from 4660 to 4700-odd there when I unplugged the phone, it's also managed to sync at 4900 with the phone connected before.

In reply to a post by Elmlea:

It's hardwired; as in when I pop the faceplate off, there's a bunch of wires there linking to the socket upstairs. It's currently disconnected though.

It's disconnected because the faceplate is off?

Sorry about the questions, a picture would be better.

BTW, I'm not saying the phone is the problem, it just demonstrates the symptoms.

No problem at all. Like this photo here:

http://www.thinkbroadband.com/images/news/3257-maste...

I've popped off the faceplate like that and my filter's plugged into the master socket behind. The grey cable at the bottom of the shot leads to my extension upstairs.

Yes, like that, only yours.

Does your extension have 3 and 4 connected?

SNR will vary during the day on every line due to a number of factors which you have absolutely no control over - on some lines by 2 or 3 dB, others by 7 or 8 dB and some even more. There will be a minute by minute variation of 0.1 or 0.2 but ignore that.

Assuming no resyncs occur during the 24 hour periods, you will see a trend appearig with a low SNR from around 22:00 trough to 04:00 is when it will start to increase and it will continue to do so until maybe 07:00. It will vary slightly until 20:00 or so, when it starts to drop again.

http://i2.photobucket.com/albums/y31/Polchraine/TBB/... shows a typical 18 hours for my last modem.

Based on that, where the minimum is around 0.5 dB at 01:00, I would resync at about 17:30 or 04:30 which would, in my case be a 3dB SNR but yours would normally be 6dB.

It may take a couple of attempts but you want your line to be stable - if it looses sync it will do so at night and the resync will drop the sync speed and possibly profile which can take days to recover. Resyncing at a time when the line is 3dB better than the low point will allow the margin to drop below the 6dB threshold without loosing sync but you will see a margin of 8 dB or so during the day.

Does that make sense?

I think so. So my best bet is to resync at the time of day when it looks likely that the SNR will equal my 6dB target?

What's generating so much noise at 0100?

Edited by deleted (Sat 28-May-11 22:21:33)

No quite - whatever time you resync you will get an SNR of 6dB at that time.

In the extreme ... sync at 01:00 and it will be 6dB and 2400 sync speed, at 06:00 6 dB and 3500 and 15:00 6dB and 3900 but the last will drop out at maybe 00:30 and resync at 6dB 2450 ...

So, what you are doing is finding a time when the SNR is about 3 dB above minimum. The router will still sync at 6 dB but will hold down to 4 dB or 3 dB. So you are not getting the absolute best sync speed but a stable one which will avoid overnight resyncs with a significant speed drop.

Atmospheric noise from distant radio transmissions (and other causes) is the main problem overnight with additional noise from street lights &c.


You also have commented below about the phone - even with filters in place you are modifying the characteristic line impedance which will have an effect on the performance of the modem (both ends) and line attenuation. Changes of a few percent are to be expected.

Think about your hi-fi amplifier which may be designed or specified for speakers with an 8 ohm impedance. It will still work with 4, 6 or 16 ohm speakers but the performance in terms of volume and frequency repsonse will vary.

Ok; so what's my minimum, just the lowest it ever drops to if I record it? So if it falls to 0.5, I want to resync at a time where I'm normally at 3.5?

It will be the lowest you see when you record it. RouterStatsLite is freely available and useful for this. You are unlikely to get down to 0.5 dB ! it need a very good modem to achieve that, but don't worry about the number, just remember 3dB above the low point, so if your low point is 5 dB resync when it shows 8dB and your speed increase whilst the SNR should be 6 dB +/- a small variation.

So, identify the lowest value, then go 3dB above that and look at the time and plan to do it around that time the following day. Then just keep an eye on the figure in te run up - just to make sure. then resync - all quite easy.

Another quick question for the expert; should my SNRs be the same or similar upstream and downstream? That ADSL2 stats checker tells me there's an error because they're too different, which could have been caused by downstream noise that's now lessened, forcing the DSLAM to sync me at the higher SNR rate on the upstream.

Anyone available to translate that?

In reply to a post by Elmlea:

Another quick question for the expert; should my SNRs be the same or similar upstream and downstream? That ADSL2 stats checker tells me there's an error because they're too different, which could have been caused by downstream noise that's now lessened, forcing the DSLAM to sync me at the higher SNR rate on the upstream.

Anyone available to translate that?

I see you're using http://adsl.tin2tin.net/

Certainly am!

Have you tried "Tone" ?

In reply to a post by BatBoy:

Have you tried "Tone" ?

No idea what you mean?

It's an app on that site. It monitors your router stats, like Routerstats does.

Anyway, your downstream SNR shows signs of noise interference which is damaging your sync speed. Common causes are the ring wire picking up interference and the sky box being connected to the phone line without using a filter.

All phone devices connected to the phone line should go through an ADSL filter.

Everything does. The ring wire's disconnected, and there's a filter in the test socket then the router, and a splitter that leads to the home phone and the Sky box.

Even with everything disconnected they seem to have me capped at 4900kbps. They've just escalated me to tier 2 as they're not showing any line errors, and apparently tier 2 can set me to a gaming profile, uncap the line, adjust target SNRs, switch off interleaving etc which should hopefully make a difference.

Yes, Gaming profile is what you need.

A solution!

After setting me to fast path and seeing no increase, the tier 2 tech support guy very kindly arranged for an Openreach engineer to visit.

Within 2 second of plugging his machine in, he announced that the problem is I'm not on ADSL2. I'm on regular ADSL Max. Sky do have their 24mbps ADSL2 equipment in my exchange, but I'm not running through it! Seems someone didn't migrate me properly.

Back in the queue to speak to technical support again, waiting for another 24 hour callback after the engineer's report goes in, but while it may be a little irritating at least it should be an easy fix!

An update!

I've carried on this discussion in the Sky forum as I think it's more pertinent there.

Please have a look at this thread.

Edited by deleted (Sat 11-Jun-11 22:25:30)