ADSL2 not ADSL2+?

[#1]

Hello
Been on the 21CN trial for a while.
Been with Plusnet for 8 years or so. Started with RADSL 500Mb service in 2001 and have upgraded at various times, Always been fairly happy. Must say I'm a bit underwhelmed with the 21CN trial service.
Firstly, I should explain that I am about 4km (line length) from my exchange and don't expect miracles. Right now, I have a number of specific issues that I'm trying to sort out with Plusnet support but here I'd just like to ask a question about ADSL2 v ADSL2+.
Right from the off, my Plusnet 21CN connection links using ADSL2 (G.992.3). Doesn't matter whether you select "Multimode", ADSL2+ (G.992.5) or indeed any other appropriate setting, it just defaults to ADSL2. (I wasn't aware that this mode was even being supported ... anyone?)
Leaving aside the current "stuck profile" and frequent loss of sync problems that I seem to have, normally my stats would look something like this:
Mode: ADSL2 (G.992.3)
Downspeed: 3.8MBps ( best ever was about 4.1Mbps)
Upspeed: 1Mbps (best ever was about 1.11Mbps)
SNR: 12 (varies from 10 -14)
LoopAtt: 47 (pretty constant)
Is anybody else unable to get an ADSL2+ (G992.5) link? If so, what explanation have you been given, please?
Thanks.

[#2]

Selecting multimode will allow your router and the exchange to select the best protocol. As you are only seeing sync speeds of 4Mbps ish you will not benefit from the higher tones that ADSL2+ will give so the system use ADSL2. This is normal. Believe when other users tried to force it to use ADSL2+ and they know that they will do get any increase in speed the system still was syncing using ADSL2.

[#3]

On another ADSL2+ service that I'm familiar with (Be) it's quite common for users with long lines, following a few tests of different options, having the line "locked down" to ADSL2 as it gives effectively the same speed for them but greater stability.
(and they don't have to put up with BT's weird target noise margin system  )

[#4]

Quote

LoopAtt: 47

That's not THAT long a line. I presume you have tried eliminating problems with your internal wiring by connecting to the 'test' socket ?

[#5]

Quote

and frequent loss of sync problems that I seem to have

You know, I have the same issues as you, and these frequent losses of sync are appearing to be a frequent issue on this forum.  I personally think BTs implimentation of ADSL2+ is a little flaky to say the least.....  I went from initially being fine, to having more losses of sync than line was in sync.  BT came out, and 'did something' to the line (never found out what, PlusNet didnt seem to know either......)  All fine for a couple of months, then all starts again.

[#6]

Thanks for the replies, everyone.
I read it that the consensus is not to worry about the ADSL2 vs ADSL2+ thing. That's fine. I didn't think it would materially affect my sync speeds over my line length but I did wonder if there were any other issues of benefit or detriment.
The phone line is in fact a second line that was BT engineer installed about 8 years ago specifically for broadband service. There is actually no additional (internal) wiring. I've been through all the usual test scenarios whilst trying to solve the loss of sync problems. I do remember that the Loot Attn was 44 when it was first installed and it has gradually increased over the years to the reading of 47 today.
I have tried just about everything to solve the sync drops. I'm doing some final testing now but will soon book a BT engineer visit as I cannot find any reason for it at my end.
Thanks again for your comments.

[#7]

are you monitoring the snr ect with routerstats or dmt at all

[#8]

Hi Barney
You have the same line stats as me - will Att any way of 47.  When I was on the LLU service was getting a sync of 6.8Mb (SNR6db)- using ADSL2 - now on BT21CN 3.7Mb (SNR 15db) using ADSL2.
Got a fault call with BT- now been a week - no progress.  Now every evening there are a number of re-sync but the PPP does not disconnect then reconnect.
Frankly this BT 21CN is ....... (insert your own word)    Also speaking toa BT Manager the other week - he admitted that 21CN is not going as well as they liked.  Think I will be soon time to request my MAC code then move to a LLU service again.
Phil

[#9]

Question: If BT and the LLU provider are using the same model of equipment, which is a good chance as there are not that many MSLAN manufacturers, why do they sync at different rates?
Could it be that the LLU go for the headline speed irrespective of the number of errors which will require retransmissions so reducing the effective download speeds. While BT will reduce the headline speed so very few retransmissions are seen. Therefore the download speed is the same for both LLU and BT.
Also what are the downloads speeds like with LLU at peak times when the noise levels are also higher? Could it be that they are even slower because of the high number of retransmissions?
Just thinking out aloud 

[#10]

Quote

Question: If BT and the LLU provider are using the same model of equipment, which is a good chance as there are not that many MSLAN manufacturers, why do they sync at different rates?

They do use different MSAN manufacturers, my office is on Demon whom I presume use C&W LLU these days. The manufacturer ident reported by our router is just 'P' and I've never seen that on a BT line. Also the LLU providers dont seem to use the borked ( sorry , sophisticated in BT speak! ) DLM that BT use. They just seem to go for a target SNR and leave it there, and not keep messing around with it and interleave depth at the sign of a few errors or the odd resynch...

[#11]

Think you are right there - LLU provides just use the the target SNR to control the sync and also no data profile - so your sync speed at the time is the data speed too.
@itsme:  Generally I would stay that is what we are lead to believe by BT /press etc.  But from my experience I found the LLU offering that PlusNet did via Tiscali Wholesale to be very good.
When I was on it for the start I had a Billion Router - then one day this was having problems holding on to the line use a Target SNR of 6db as most evening my SNR can drop upto 2db.
So a quick call to PN support - they could change the Target SNR on the fly to 9db.  Then my set was fine and happy again.
I got a new router - and got PN to change the SNR back to 6db -all was fine as my new router can handle the upto drop of 2db in the evenings.
Download speed on LLU in the evening was excellent was getting almost the max i could - about 90% of a daytime speed - yes there were a few re-transmits.
Phil

[#12]

The more I investigate and read about this subject, the more I realise it is no simple matter. Some folk say that on longer lines where you're never going to get the higher speeds anyway, there is some advantage to using ADSL2 (G.992.3) due to the signalling differences (less attenuation?) used by ADSL2. It seems that ADSL2+ can actually work against you at a certain point in the BT21CN implementation, although it is a different matter with a LLU service. However, there seem to be so many "ands, ifs and buts" that it's difficult to know what is good information and what is not. Anyway, unless I want this to be my full time job (!), I think I'm going to just keep prodding Plusnet (and by proxy BT) to sort my loss of sync problem. If I can get back to a stable service at a reasonable speed on 21CN, I'm not too worried. If not then I may even request a "downgrade" back to the DSLMax service I had previously. At least that service was stable and not much slower, as far as down speed goes anyway.
Back to the current situation, last evening something  was done to my line by someone "out there". Haven't had any confirmation yet but the line dropped twice in quick succession and afterwards when I checked my stats the speeds had changed from (stuck?) 3064000 down and 888000 up  to 3672000 down and 635000 up.
So no idea what's going on with the up speed which is now slower than it has ever been (on 21CN) but at least the down speed is better. SNR is 14 and LoppAtt is 47. Mode ADSL2 (G.992.3)
I'm waiting now for the next contact from Plusnet support to see what happens next. Maybe it's time for a BT engineer visit.
Thanks for the continued interest everyone.

[#13]

Quote from: BarneyMcgrew

The more I investigate and read about this subject, the more I realise it is no simple matter. Some folk say that on longer lines where you're never going to get the higher speeds anyway, there is some advantage to using ADSL2 (G.992.3) due to the signalling differences (less attenuation?)

I would not worry too much about the attenuation figures between ADSL2 and ADSL2+. The attenuation and SNRM are the average across all of the frequency bins. With ADSL2 this is approximately 256 and with ADSL2+ approximately 512 and as attenuation is related to frequency if you only do the averaging over the first 256 bins you will get a lower value. But each bin will have the same attenuation no matter whether ADSL2 or ADSL2+ is used. Not sure why the system default to ADSL2 when the higher frequencies will not add any benefit other than thinking there may be some overheads to pay in throughput.

This is from http://www.kitz.co.uk/adsl/linestats.htm#attenuation

Quote

Because the router measures against the frequencies available, some users may notice a very slight increase in attenuation if say moving from a fixed rate 512 kb connection up to 2Mb. Higher frequencies such as those used to transmit faster speeds are more likely to be attenuated (higher frequencies = higher attenuation). ADSL 2+ has an increased frequency spectrum, therefore an increased attenuation of around 3-4dB is not unusual. As a very rough guide a speed increase of 4Mb is said to increase the attenuation by 1dB. It is also important to note that different routers may load the frequency bins in slightly different ways, and on top of that some routers report the average across the frequencies actually in use, whilst some may report the average across the frequencies available - which is why sometimes using a different router may report a slightly different attenuation figures.

[#14]

Hi
I agreed with my line and Barney's ADSL2+ does not really add much - as the high bin numbers are un-usable - too much noise.  But the benifit from ADSL1 to ADSL2 is the minimum nunber of bits per tone - on ADSL1 min was 2 on ADSL2 is 1.  So on ADSL1 - if a tone could not support to 1 bit on a tone - that tone is marked un-usable. 
With ADSL2 squeezing more out the line - my line gained another 1.5Mb from ADSL1 to ADSL2.  Although this can't be a like for like test as ADSL1 was with BT and ADSL2 was LLU.  But the bit graphs do show the added usage in the tones above 190.
Phil

[#15]

ADSL2 maxes out at 12Mbps - but if your line is good enough for anything in excess of a sync of perhaps 7Mbps
then it will switch to include some of the higher ADSL2+ frequencies.
My line appears to be on a cusp with an attenuation of 34dB - but I think I have a very poor quality line.