Thanks for the info.

My major gripe is the fact I can't force ADSL2. My line doesn't handle 2+ well at all, resyncs every could of hours.

On 21CN WBC I can set whichever modulation mode I want and also the SNR is 3db (default).

Got a feeling I could lose a few Mb in sync speed moving to Sky.

I know this wasn't part of your original question but be aware if you use Usenet Sky do not have any Newsgroup servers so if you don't already do so you will have to subscribe to a Usenet service eg Newshosting.

In reply to a post by TJNewton:

I know this wasn't part of your original question but be aware if you use Usenet Sky do not have any Newsgroup servers so if you don't already do so you will have to subscribe to a Usenet service eg Newshosting.

Hardly any ISPs host their own Usenet servers these days; it's not exactly unique to Sky.

In reply to a post by wolvesmad:

Thanks for the info.

My major gripe is the fact I can't force ADSL2. My line doesn't handle 2+ well at all, resyncs every could of hours.

On 21CN WBC I can set whichever modulation mode I want and also the SNR is 3db (default).

Got a feeling I could lose a few Mb in sync speed moving to Sky.

Basically if the line drops on ADSL2+ but not on ADSL2 DLM may set it to use ADSL2.
However even if the lines not dropping out on ADSL2, there may be error build up.

DLM WILL NOT leave the sync at a speed that causes drop outs every 2 hours. It wont even leave it at a speed that results in drop outs every 12 hours. DLM wants perfection in terms of stability.

Also bear in mind that SKY use 7db noise margin not 3db. However it's not a 'target' in the sense that BTs system is.

The target becomes the sync speed.

DLM may set a speed of 2000 kbps.
If you sync at 3pm, it may sync at 2000kbps with a noise margin of 7db.

If you sync at 10am, it may sync at again 2000kbps, this time with a noise margin of 10db.
The sync becomes the fixed factor & the noise margin is whatever it comes out at given that fixed DLM speed.

SOMETIMES DLM sets a sync optimistically.

E.g dlm sets the speed to 2000kbps.
You sync at 10am at 2000kbps, with a noise margin of 7db.
You sync at 7pm at 1700 kbps with a noise margin of 7db.
To sync at 7pm at 2000kbps may give a noise margin of 5db. This is TOO LOW. Here sky WILL NOT allow you sync at 2000kbps as it would result in a noise margin below 7db.

GENERALLY though...

DLM sets a sync speed which means whenever you sync, the sync speed is about the same. E.g in my house, if I sync at 10am I get a noise margin of 9db. Sync at 7pm I get a noise margin of 7db. Whenever I sync the router I reconnect at 15.5mbps regardless. The noise margin varies on every resync but my downstream rate is fixed. This is unlike BT where the sync speed changes on every sync (provided you are not at the highest rate)

On my FTTC things are quite different.

Hope I helped slightly.

If your line is unstable. Sky don't increase the target noise margin like other ISPs. It doesn't go 3db, 6db, 9db, 12db etc.

Line syncs at 2000 kbps say... but it drops out every 2 hours. Here at sync the noise margin is 7db on average.
Sky then cap the sync at say 1900kbps. Every time you resync the average noise margin may only be 8db.
The sync gets capped. Not the noise margin.

If the sync means the noise margin will go below 7db, then the sync is reduced. This is generally not the case.

SKYs system is very unique to say the least. Sky also reduce output power quite substantially which can make the noise margin misleading.

Edited by ukhardy07 (Tue 31-Jul-12 16:18:52)

Thanks for the info, an excellent insight into how Sky's DLM works.

Unfortunately, I do not like the sound of it

I don't like how the DLM caps the sync speed, so to speak. As my line is generally quiet during the day, I use my Netgear to slash the SNR margin in half so I can gain some speed when doing big downloads.

On Sky it seems I would gain nothing from this.

Unfortunately at night, on ADSL2+ things go pear shaped quite quickly and I was syncing at ADSL1 speeds (around 4500kbps).

On ADSL2 it will remain stable at around 6500kbps, with interleaving turned on.

Second problem is obviously the fact Sky use a 7db target SNR, whereas on 21CN if the DLM feels your line is stable, the DSLAM drops it to 3db.

It seems on my particular line, I could end up on ADSL2+ by Sky's DLM and this would be a major disadvantage - especially at night.

Thanks for the info, an excellent insight into how Sky's DLM works.

Unfortunately, I do not like the sound of it

It works well given that plenty of people don't use the master & use cheap DIY extensions. It gives them a stable service that otherwise would be dropping and probably make them leave.

I don't like how the DLM caps the sync speed, so to speak. As my line is generally quiet during the day, I use my Netgear to slash the SNR margin in half so I can gain some speed when doing big downloads.

On Sky it seems I would gain nothing from this.

This is correct. Say DLM sets your line to 6000 kbps... It will sync at this speed pretty much every time you resync the router.

If you slash the noise margin in the netgear router. Well guess what, you would get 6000kbps again. The noise margin wouldn't go lower like on BTs system as there's a physical cap on the line.

Unfortunately at night, on ADSL2+ things go pear shaped quite quickly and I was syncing at ADSL1 speeds (around 4500kbps).

What DLM seems to do is start the line at a slow speed. Usually around 4000 kbps. It then monitors the line for around the 1st 72 hours. If there are no drop outs and very little errors building up, it will then increase the sync a little bit more. Say to 5000 kbps. It then seems to monitor for around 24 - 48 hours, again if it's stable it will increase. If it drops out / errors build up it will drop the speed again.

It continues with the 24 hour monitoring until a stable speed is found that means it's stable 24/7 without drop outs.

It won't leave the line in a state where things go pearshaped so you may loose some speed.

On ADSL2 it will remain stable at around 6500kbps, with interleaving turned on.
Second problem is obviously the fact Sky use a 7db target SNR, whereas on 21CN if the DLM feels your line is stable, the DSLAM drops it to 3db.

Yep it will automatically be slower because of this.

It seems on my particular line, I could end up on ADSL2+ by Sky's DLM and this would be a major disadvantage - especially at night.

You COULD this is correct but DLM will set you two whichever modulation it thinks is best.

If ADSL2+ is lots more unstable I doubt it will keep you on that.

What's your current line attenuation?

Attenuation on ADSL2 is 48db / ADSL2+ 51db.

I would expect between 5 and 7 meg on that atten.
Given everything you have said I think you'll end up with ADSL2.

You never know though. I switched my line rental over to Sky from BT. As soon as I did this my line stopped intermittently dropping which it used to do 10+ times a day. My sync shot up over 3000 kbps due to the new found stability.

The drop outs MIGHT not be noise related but equipment related. Who knows.

Are you taking calls and line rental over too? If so just see how things get on. Whatever you do don't turn the router off during the 1st 10 days.

EDIT: I would also expect marginal difference between ADSL2 and ADSL2+ speeds, although 2+ I would expect to be every so slightly quicker. So ADSL2 via DLM is very possible.

Edited by ukhardy07 (Tue 31-Jul-12 17:06:13)

In reply to a post by ukhardy07:

Are you taking calls and line rental over too?

New customers have to, there is no choice.

Oh yes of course!

A good thing in a way. When I had issues with broadband drop outs sky kept saying they couldn't touch BTs equipment & that their stuff was fine... BT wouldn't send anyone out for the voice as it only affected the broadband. My voice did sometimes drop out mid call...

Having it all with Sky means they can do whatever it takes.
Moving the calls and line rental solved me issues so it proved the issue was on the BT voice side. Which I didn't believe at the time.

I've also found the full LLU network to give me better sync speeds & it's never gone down once.. I had a few outages on the other network. Could all be related to my fault though.