ADSL/2/2+ Protocols

Tacitus · Jul 13, 2012, 11:19:02

Could someone give me some details on what protocols are used by ADSL/2/2+?

At present I'm on ADSL using G.DMT Annex A with a sync of 2880. The exchange is about to be upgraded to 21CN so which would be the best protocol to go for with an attn of 57dB and a 2-Wire modem?

Whilst it might be the case to ask support to try various options in order to see which works best, I would like to have some knowledge of what the options are and why.

Since my sister was upgraded her line has gone unstable which is driving the profile down from around 6000 to 512 or lower, although she is still using G.DMT Annex A. At 42dB attn I doubt she would get a significant speed increase on her normal 5500/6000, but at present she would settle for some stability, so any suggestions there before I contact support?

Thanks for any help.

Steve · Jul 13, 2012, 11:39:40

The list is here Tac - http://en.wikipedia.org/wiki/G.992.5. In my opinion adsl2+ is a non starter with BT for long lines unless your lucky enough to have a 'quiet line', Your sister's line is fairly typical of a noisy longish line on adsl2+ being hammered by BT's profile impositions. The easy fix is to be profiled as adslmax on an adsl2+ exchange. No faster but you get the extra allowances.

davej99 · Jul 13, 2012, 13:25:24

Recently converted to 21CN. I was automatically left on ADSL modulation, because of line length I think. I got a sync of 7,360kbps, 37dB attenuation and a 9dB margin. I used to get with 20CN ADSL, 8,128kbps sync (just), 35dB attenuation and 6dB margin.

After a chat with Brian in tech support we decided to give ADSL2+ modulation a try. After a couple of days I got 10,075kbps sync, 40dB attenuation, 5dB margin. Pings to fell from 42ms on 20CN ADSL, to 38ms on 21CN ADSL to 22ms on ADSL2+, so I guess interleaving is now off.

Note that attenuation is much higher on 21CN ADSL2+ vs 20CN ADSL, 5dB in my case. Also found the ADSL2+ sync was much more sensitive to noise events.

Seems to me at 57dB on ADSL you might be looking at low 60's on ADSL2+ so it seems unlikely it will help at all and will probably be unstable and crash the sync.

Suggest staying on ADSL modulation, see how the line looks, then talk to Brian. He seems to have a natural feel as to how a line will perform on 21CN in terms of ADSL vs ADSL2+ modulation.

Tacitus · Jul 13, 2012, 13:57:23

Quote from: Steve on Jul 13, 2012, 11:39:40
The easy fix is to be profiled as adslmax on an adsl2+ exchange. No faster but you get the extra allowances.

Thanks for the list Steve - never thought of looking on Wikipedia. I thought G.DMT was the adslmax profile - how would I know if the exchange is ADSL2 or 2+? BT seem to be upgrading their BT Retail customers but iDNet have not yet been informed of any upgrade although SamKnows says 21CN WBC (ADSL2/2+?) is currently available.

Tacitus · Jul 13, 2012, 13:59:54

Quote from: davej99 on Jul 13, 2012, 13:25:24
Suggest staying on ADSL modulation, see how the line looks, then talk to Brian. He seems to have a natural feel as to how a line will perform on 21CN in terms of ADSL vs ADSL2+ modulation.

This sounds the best course. I'm not expecting great things but I would like both mine and Sis lines to at least maintain their current speed and stability.

Glenn · Jul 13, 2012, 14:12:34

http://www.internode.on.net/residential/adsl_broadband/easy_broadband/performance/ will give you an idea of expected performance.

Tacitus · Jul 14, 2012, 08:22:42

Thanks for the info folks. When the exchange upgrade takes place I'll let you know how I get on.

Tacitus · Jul 17, 2012, 15:51:41

OK So far so good, the changeover took place this pm. iDNet automatically upgraded me to ADSL2; since 21CN is cheaper for them I'm guessing they're doing it automatically for all users.

After a chat with Miriam, we decided to leave it on G.DMT and see how things go. Since attn is now 55dB as against the 58dB it has been for ages, I doubt there will be anything much to gain speed wise and stability might go down the pan.

One of the more interesting things is that the VCXO Frequency Offset on the 2-wire has now dropped from 120 - the maximum I suspect it will report and which it considers a fault - to a more healthy 28.3 ppm. This leads me to think that the cause of all the problems I had with modems in the past was due to a faulty line card, something I could never pin on BT. It grieves me that I ended up having to pay £180 or so to have an SFI investigation which ended in 'no fault found' since I could get a connection on the day. I also ended up with a collection of modems, some of which might actually work.....

We'll see how we go.

Simon · Jul 17, 2012, 16:03:05

Good luck!

pctech · Jul 17, 2012, 16:29:15

I've been very lucky with the engineers that have come out to me, they have been genuinely nice people who seem to really enjoy what they do.

Tacitus · Jul 17, 2012, 19:03:23

Quote from: pctech on Jul 17, 2012, 16:29:15
I've been very lucky with the engineers that have come out to me, they have been genuinely nice people who seem to really enjoy what they do.

I've not had a problem with them as people, it's the system they have to work under. It's all water under the bridge now but at the time I tried pointing out that the VCXO offset that the 2-Wire was reporting suggested there was a problem somewhere. However, since a spare modem worked it was recorded as no fault found and I got stuck with a £180 charge.

I subsequently tried the 2-Wire on Sis iDNet connection; it worked solidly for two weeks so was obviously not faulty. The recent changeover to ADSL2 and (presumably) a new line card, has restored things to the way they should be - hence the fault would appear to have been at the exchange end all along.

Like I say, water under the bridge but it still rankles.

Niall · Jul 17, 2012, 19:56:13

I know that feeling. They've even told me/support that there was a fault at the box and at the exchange. They obviously bodged it rather than fix it properly as my line speed is 15mb for a few days, drops to 11, then 9 and then stays there. IF there is ANYTHING in the way of noise, bad weather, the line goes pear shaped. This according to BT, is acceptable. They've even replaced the line outside my house that runs to the underground opening across the road, that links to the box at the top of the estate 200 meters away, if that.

If I was still a first person gamer, I'd be fuming as my line is pathetic for gaming now. Even playing Diablo 3, I lag all over the place.

Polchraine · Jul 18, 2012, 11:25:28

Quote from: Tacitus on Jul 17, 2012, 15:51:41Since attn is now 55dB as against the 58dB it has been for ages, I doubt there will be anything much to gain speed wise and stability might go down the pan.

Which attenuation is that? The full band ADSL or the 300kHz spot attenuation? When comparing figures between services and modems, the only reliable one is the 300kHz figure as there are too many factors that affect the other and they are treated in different ways by different modems and services.

Tacitus · Jul 18, 2012, 17:04:07

Quote from: Polchraine on Jul 18, 2012, 11:25:28
Which attenuation is that? The full band ADSL or the 300kHz spot attenuation? When comparing figures between services and modems, the only reliable one is the 300kHz figure as there are too many factors that affect the other and they are treated in different ways by different modems and services.

Yes it was the 300kHz spot one that the 2-Wire reports. so the comparison is valid.

Since my original post I've been having major problems with authentication following a reboot. I left the modem off overnight and this morning got a connection but no internet. I tried the old Zyxel (which was already set up) wondering if the unlock on the 2700 had failed with the changes at the exchange. Rang support who tried a different password, but no joy so I left it with them. Service was restored sometime this afternoon, although with all the faffing around the connection speed is well down.

Interestingly the Zyexl was also labelled a 'failure' by BT even though like the 2-wire it worked perfectly on Sis iDNet line. So far so good, but I can't see any reason why it shouldn't continue working. Like I say, it still makes me angry when I think about it.

Quick vote of thanks to Miriam, Nick and all at iDNet towers. Quick, courteous and knowledgeable service. I shudder to think what it would have been like back with Demon and Bangalore.......

Tacitus · Jul 21, 2012, 08:42:00

One other point. I notice that the SNR downstream is 12dB (probably due to thunderstorms) whilst the upstream is 14dB. My memory may be playing me false, but shouldn't these be in a roughly 2:1 ratio? Or does that only apply to attn?

I have asked for an SNR reset so I'm starting with a clean sheet following the exchange upgrade, but it's not happened yet.