Slower Than Expected - The SVBN Effect?

Hey all,

Brand new to Sky, previously on cable, also took UK Online delivered over Easynet's Lucent kit way back when.

This line appears to be underperforming quite considerably.

ADSL Link Downstream Upstream
Connection Speed 16381 kbps 1101 kbps
Line Attenuation 17.0 dB 11.4 dB
Noise Margin 5.9 dB 8.9 dB

I've had a 14Mb sync on a 34dB line and 23Mb along with 2.3Mb up on a 21dB line, both with Be, and I'm a bit confused as to why the shortest line I've ever had barely beats a line twice as long and doesn't touch one 300 metres longer.

I'm also quite looking forward to seeing what happens in the winter when the RF interference levels increase. Either the line sucks or the SVBN in combination with the Sky modem sucks, any opinions and experience welcome.

Plugging into the test socket does nothing incidentally, there are no extensions anyway.

It's a jittery crapsack too.

http://www.pingtest.net/result/68200234.png

Just had my latency and jitter outperformed by someone nearly 200 miles north of me on the end of a 71dB attenuation line.

Did your line go through DLM line testing or have you had someone manually apply a line profile?

In reply to a post by Oliver341:

Did your line go through DLM line testing or have you had someone manually apply a line profile?

Both, was briefly taken off DLM then put back onto it. There's no fixed profile on there at this time, the line is rate adapting to that level with a 6dB target margin downstream and some random high figure upstream.

The line sucks and you're probably on full interleave. The best you can do is ask for a Gaming profile.

In reply to a post by BatBoy:

The line sucks and you're probably on full interleave. The best you can do is ask for a Gaming profile.

Also perhaps break the terms and swap a different router with different line chips. Just a thought.

Ignore batboy.

the line is not bad.

Its not far off being the same as mine.

Ive got a 11db attenuation and i get 18mb sync speed.

But ive tweaked my line down to 2db snr to get the full 24mb sync speed.

I do wonder what attenuation/snr combo will give the full sync speed.

I would of thought yours and my line would get the full 24mb but they dont, witch i find odd

Perhaps plugging in a router with decent ADSL stats (e.g. Technicolor or Netgear+telnet) will give more clues, e.g. power output. Don't worry about extracting user/pass and such, you don't need to auth to pull the stats.

Ignore Seansmit17.

ADSL Link Downstream Upstream
Connection Speed 16381 kbps 1101 kbps
Line Attenuation 17.0 dB 11.4 dB
Noise Margin 5.9 dB 8.9 dB

Ok. Looking at those stats there's a good attenuation. Based on that you can probably get a few more Mbps.

The noise margin, ignore it. Basically whatever speed DLM sets it reduces the power... So even though the noise margin is bang on target it doesn't mean you can't sync higher and still maintain the same noise margin.

If DLM had set your line to 14000 kbps that SNR would still be a similar value no doubt.
Here sky is different to most other ISPs who would maintain a higher noise margin.
This saves Sky money as it means their equipments not blasting unnecessary power down a line. It's part of their eco initiative

Sooooo looking at the stats you've almost certainly gone via DLM. That's because the sky profiles wouldn't give you that upload at 16Mbps

Rightio. Ring Sky up and firstly ask them to put you on an 18Mbps profile. NOT A 16 Mbps profile.
18Mbps is the first profile that maintains full upload.
See if the speed increases. If it syncs at the full 18Mbps ask for a complete uncap to 20 Mbps.

SKY nowadays can only set the following profiles manually
4Mbps, 10Mbps, 16Mbps, 18Mbps and 20Mbps.
4 - 16 Mbps has an upload around 765 and 18 & 20 have full upload.

DLM can set any sync speed. Hence why it's often the best route. Also DLM will keep a high upload even on low profiles.

So yep ask for the 18Mbps profile and then the 20 if you achieve full 18. Don't worry about interleaving for now.

To check you are in the master socket? You have disconnected the ring main on any extension sockets? You have tried in the test socket and got similar stats with similar noise margin levels?

Should add that Sky don't do a 2Mbps upload. So don't expect that to get any better. Uploads already maxxed out.

Edited by ukhardy07 (Sat 18-Aug-12 23:48:03)

It's not jittery crapsack as you say. That's fine. Also no packet loss. With that pingtest you'll have absolutely no problems with anything.

The 71db line may be better on the pingtest but a few ms jitter will make barely any difference. The terrible speed they have will make things much worse.
The ms pings is down to interleaving. Having a high ping isn't great for gaming but it means lower error build up and less chance of the line dropping out.
Gaming profiles often result in a worse connection due to error build up and plenty of lines will disconnect on a gaming profile, whereas on standard interleaving they would be perfectly stable.

So really ignore that pingtest it's all fine.

Edited by ukhardy07 (Sat 18-Aug-12 23:46:49)

You should be getting the full 20 Mbps without any tweaking. You have an issue if you're having to tweak it down further. How's the setup?

I have seen plenty of Sky lines with a 19, 20 and 21db attenuation achieving the full 20Mbps speed using skys equipment. The issues not across the board that's for sure.
Anything less than 18db should be getting the full speed ideally.

Edited by ukhardy07 (Sat 18-Aug-12 23:54:56)

I'm aware of interleaving etc, I've worked on broadband access networks a fair bit, it's more concerning that on a serialised link like DSL I'm seeing that level of jitter, it should be pretty much zero.

So, yes, 8ms of jitter isn't great given the access technology and the distance between me and the test server, I'd find it more understandable on cable.

In reply to a post by BatBoy:

The line sucks and you're probably on full interleave. The best you can do is ask for a Gaming profile.

I'm not on full interleave, the line may suck but it doesn't suck as badly as those stats suggest as it was happily functioning on a set 18Mb profile with a 7dB SNR margin, suggesting it's Sky's SVBN DLM controlling the output power.

That was exactly what I was looking for confirmation of when I mentioned the SVBN effect, it's Sky's DLM being too clever it would seem.

As I mentioned below when put onto a fixed 18Mb profile briefly I was achieving that with a 7dB SNR margin, which was likely the target margin so it was reducing output power accordingly.

I'll poke Sky on this, selling 'up to' 20Mb and artificially throttling through design or incompetence extracts the urine.

It will be interesting to see if they blame "errors on the line". I had success with them by requesting Gaming mode via their web form even though they responded with the stock excuse. I noticed a much higher upload sync speed over 1Mb when moved to MER rather than always 700k on VC-MUX.

In reply to a post by BatBoy:

It will be interesting to see if they blame "errors on the line". I had success with them by requesting Gaming mode via their web form even though they responded with the stock excuse. I noticed a much higher upload sync speed over 1Mb when moved to MER rather than always 700k on VC-MUX.

Won't wash, managing 18Mb with 7dB margin before their DLM got its mitts on the line then dropping to 16Mb with 6dB doesn't suggest anything other than their dodgy management being to blame.

Seems Sky now set up to 22 Mb via DLM
Here's my previous stats:

ADSL Link Downstream Upstream
Connection Speed 19506 kbps 997 kbps
Line Attenuation 21.0 db 9.5 db
Noise Margin 7.2 db 8.0 db

Then noticed a speed increase looked on Sky and

Testing on your broadband connection began on 26/07/2012 and will be completed within 10 days

ADSL Link Downstream Upstream
Connection Speed 21194 kbps 992 kbps
Line Attenuation 20 db 9 db
Noise Margin 3 db 8 db

Line connection testing completed on 02/08/2012, resulting in the following settings:

Download speed setting
Your connection has been set to a download speed of up to 22.0Mbps .

Upload speed setting
Your connection has been set to an upload speed of up to 1.3Mbps .

In reply to a post by Ignitionnet:

I'll poke Sky on this, selling 'up to' 20Mb and artificially throttling through design or incompetence extracts the urine.

It's not 'artificial.' DLM has capped your line specifically for one reason or another. Most people are not affected much.
DLM has an objective. This objective is NOT to get the best speed. DLM aims to give you a broadband service that will not drop out. This is why lines that have gone via sky DLM will often not have a broadband drop out for months and months on end.

Most lines are fine anyway. Most lines have little error build up and will sync at near enough full speed anyway with DLM. Others get throttled severely like yourself.

It also means that people using an extension socket and a cheap £2 30m white, flat DIY telephone extension also get a broadband service that' totally reliable. Sadly because so many people have incorrect setups DLM has taken this route.

When Sky used to just give whatever speed a 7db noise margin gave, so many people had constant drop outs due to incorrect internal setups. It lost them customers. So here's the solution.
Sadly the more advanced users are a minority. Most people are clueless and when their connection drops out in their extension socket they blame sky & not their setup.

Edited by ukhardy07 (Sun 19-Aug-12 23:38:08)

Just wondering... Do you sync with a 3db SNR or has that just dropped through the day. I've heard that DLM is potentially setting 3db SNR targets now.

It seems that DLM is less strict that what it once was. Although there are still some extreme cases (like this thread).

Syncs at 3 Db has done since retraining and never dropped,have rebooted a couple of times and syncs at 3Db from start now.

In reply to a post by boysteve:

Syncs at 3 Db has done since retraining and never dropped,have rebooted a couple of times and syncs at 3Db from start now.

That's the key point. Your lines totally and utterly stable. Would probably go months and months without a drop. Hence DLM is happy.

Good to know they now offer 3db. This brings them into direct competition with other providers.

Something I didn't mention.

DLM does sometimes change things. Eg DLM may set your line to 16Mbps
After say 1.5 months of absolutely no drop outs and 24/7 connection DLM often increases the speed.
This being said it's not a definite. So best to ring up. Its not all bad

In reply to a post by ukhardy07:

In reply to a post by Ignitionnet:

I'll poke Sky on this, selling 'up to' 20Mb and artificially throttling through design or incompetence extracts the urine.

It's not 'artificial.' DLM has capped your line specifically for one reason or another. Most people are not affected much.
DLM has an objective. This objective is NOT to get the best speed. DLM aims to give you a broadband service that will not drop out. This is why lines that have gone via sky DLM will often not have a broadband drop out for months and months on end.

Sky are artificially throttling, they aren't allowing the line to rate adapt. The only time the connection has dropped is when DLM is changing profiles, I've noted zero packet loss.

I'll give it until it's done tinkering, if it still is relatively low I'll reach out to Sky. There's zero reason why the line can't manage at very least the 18Mb it was handling with room to spare before DLM starting working its magic.

Download speed setting
Your connection has been set to a download speed of up to 16.4Mbps .

Upload speed setting
Your connection has been set to an upload speed of up to 1.3Mbps .

This is on a line with 17dB downstream attenuation that was, without DLM, managing 18Mb with 7.xdB of SNR margin but now apparently can't do more than 16.384Mb with a 6dB margin in the space of, well, the time it took DLM to kick back in.

Sky saving money with power management, lovely.

17dB should give you much faster than that, more like 21Mbps so I would repeat you have a bad line. It's very usual for a line to go faster before DLM kicks in as DLM is concerned with errors more than speed and reduces the speed to make the line stable.

I would still ask for Gaming mode.

I would in turn repeat that without DLM the line managed 18Mb with a higher margin than, immediately after DLM kicked in, it could manage at 16.384Mb.

It was like flicking a switch. 18Mb no problems, DLM comes on, 16.384Mb with a lower margin where it's remained ever since.

That or a bizarre noise kicked in on the line which has been omnipresent and it just happened to coincide perfectly with Sky's DLM beginning.

As I said, that's what happens. Before DLM kicks in, your line runs at it's max producing all sorts of errors. When DLM kicks in, it analyses your error statistics and reduces the line speed accordingly.

Lines on Sky are normally provisioned at 4 meg. Was yours provisioned at 18 meg?

In reply to a post by BatBoy:

As I said, that's what happens. Before DLM kicks in, your line runs at it's max producing all sorts of errors. When DLM kicks in, it analyses your error statistics and reduces the line speed accordingly.

Interesting those all sorts of errors corresponded with a higher SNR margin and showed zero packet loss while this lower, more stable data rate shows a lower SNR margin.

FYI: http://helpforum.sky.com/t5/Sky-Broadband-Speed/why-...

Edited by deleted (Mon 20-Aug-12 16:28:56)

In reply to a post by Oliver341:

Lines on Sky are normally provisioned at 4 meg. Was yours provisioned at 18 meg?

It was provisioned at 4, I got them to provision it up to 18.

Normally when they set a manual profile, DLM leaves the line alone. That's been my experience, anyway.

Here's the line being taken from 4Mb to an 18Mb profile manually:

http://www.thinkbroadband.com/ping/share-thumb/dac16...

Here's the DLM kicking in.

http://www.thinkbroadband.com/ping/share-thumb/d64a7...

Every resync is the DLM.

http://www.thinkbroadband.com/ping/share-thumb/2423f...
http://www.thinkbroadband.com/ping/share-thumb/2f404...
http://www.thinkbroadband.com/ping/share-thumb/f12d2...
http://www.thinkbroadband.com/ping/share-thumb/8d4c3...

Skipped 18th as I was playing with the line plugging it into the test socket.

It's been stuck at between 16,381kbps and 16,384kbps since DLM first kicked in with the same SNR margin after running perfectly stably and packet loss free with a higher SNR margin at over 18Mb.

Sky's DLM is misfiring, I reckon they are employing power management to save a few pence on powering lines, it's been mentioned this is likely the case.

I'm going to plug a more diagnostics friendly modem into the line and take some statistics from it. I suspect I'll see a low output power, not really the way to go if concerned about stability, lowering the transmit power so putting the signal closer to the noise floor.

The worst thing about instability is that the IP address will change if the line drops and this can mess up any downloads currently taking place.

In reply to a post by BatBoy:

The worst thing about instability is that the IP address will change if the line drops and this can mess up any downloads currently taking place.

Not anymore, uses 'real' DHCP pools delivered via the Broadband Manager rather than crazily dynamic local allocations.

Edited by deleted (Mon 20-Aug-12 16:51:56)

In reply to a post by BatBoy:

The worst thing about instability is that the IP address will change if the line drops and this can mess up any downloads currently taking place.

Surely as long as DHCP is active (on MER) the same IP will be granted to the asking MAC address. I read that DHCP leases are hourly on MER and thus renew every 30mins.

In reply to a post by jchamier:

Surely as long as DHCP is active (on MER) the same IP will be granted to the asking MAC address. I read that DHCP leases are hourly on MER and thus renew every 30mins.

I think Sky are moving ADSL2+ users back to PPPoA and reserving MER for FTTC.

In reply to a post by Oliver341:

I think Sky are moving ADSL2+ users back to PPPoA and reserving MER for FTTC.

Strange as they've already done the work and MER is more efficient use of the line and doesn't require the PPP encapsulation (and a RAS server to de-encapsulate)

In reply to a post by jchamier:

Strange as they've already done the work and MER is more efficient use of the line and doesn't require the PPP encapsulation (and a RAS server to de-encapsulate)

I thought it was strange too, to go through all the effort of rolling out new firmware for all their legacy routers and reconfigure their network, only to roll back to PPPoA, exchange by exchange.