Line or modem?

[#1]

I've started having issues with my broadband - when I set my modem to ADSL2+ Auto(G 992.5) The speed goes up and down very erratically and tends to hover around the 0.50k level!
If I set it to ADSL2 Auto(G992.3) I achieve around 7Mb at a constant speed.
Just to make it clear, It was running at ADSL2+ @ around 14mb for months completely fine!
So.. line or modem? 
I've run the BT SpeedChecker - results below: (Running at ADSL2 Auto(G 992.3)
Download speedachieved during the test was - 6372 Kbps
For your connection, the acceptable range of speeds is 4000-21000 Kbps.
Additional Information:
Your DSL Connection Rate :9638 Kbps(DOWN-STREAM), 1019 Kbps(UP-STREAM)
IP Profile for your line is - 8503 Kbps
And here's the current settings for my modem running at ADSL2 (Auto G 992.3) : (It's a Draytec Vigor2800 running 2.8.2)
System Status System Uptime: 0:15:16
LAN Status Primary DNS: 212.159.6.9 Secondary DNS: 212.159.6.10
  IP Address TX Packets RX Packets
  192.168.2.1 36626 24051
WAN Status GW IP Addr:   195.166.128.48
  Mode IP Address TX Packets TX Rate RX Packets RX Rate Up Time
  PPPoA 212.159.nn.nnn 21770 46 35753 12 0:14:22
ADSL Information      (ADSL Firmware Version:  E.67.36.1 )
  ATM Statistics TX Blocks RX Blocks Corrected Blocks Uncorrected Blocks
  120921 833178 0 17
  ADSL Status Mode State Up Speed Down Speed SNR Margin Loop Att.
  ADSL2 SHOWTIME 1019900 9638500 3.0 24.0
And these are the modem settings at ADSL2+ (G 992.5)

System Status System Uptime: 0:1:53
LAN Status Primary DNS: 212.159.6.10 Secondary DNS: 212.159.6.9
  IP Address TX Packets RX Packets
  192.168.2.1 985 1063
WAN Status GW IP Addr:   195.166.128.237
  Mode IP Address TX Packets TX Rate RX Packets RX Rate Up Time
  PPPoA 212.159.69.222 85 115 48 150 0:00:59
ADSL Information      (ADSL Firmware Version:  E.67.36.1 )
  ATM Statistics TX Blocks RX Blocks Corrected Blocks Uncorrected Blocks
  696 258 0 3246
  ADSL Status Mode State Up Speed Down Speed SNR Margin Loop Att.
  ADSL2+ SHOWTIME 1035800 13802400 2.5 25.5
No errors are shown at any time on the modem..... It just doesn't work 😞
Any help would be brilliant! Thanks 🙂

[#2]

ADSL2+modem stats show a low (2.5) SNR Margin, also a very high (3426) Uncorrected blocks for under 2 minutes up-time.
Maybe the DLM has decided your line can handle a 3dB target margin and high error rates have resulted.

[#3]

I was thinking that the SNR looked a little low.
but I didn't really know what the uncorrected blocks bit meant to be honest 
And sorry.. but DLM? Digital Line Management? or something random? lol
What do you suggest that my next step should be?

[#4]

Hi, it's Dynamic Line Management, equipment at the exchange that causes a lot of moans and groans.
If you have a spare modem/router or can borrow one that would be the best way to rule out a router issue.
If DLM has got it wrong then keeping connected on ADSL2+ (reboot maybe once or twice a day) and it should eventually sort itsself out if there's no router or line fault.

[#5]

I'll see if I can borrow another router first as connecting with ADSL2+ simply isn't practical. (It's unusable basically!)
Thanks for your help!

[#6]

Corrected blocks is a reference to the Reed Solomon error correction routines.
For them to exist the line must be interleaved.
Interleaving improves the stability of longer lines and enables error correction of data packets which have acquired errors within them between the exchange and your router due to noise on the line.
It's a complicated mathematical procedure/algorithm using lots of prime numbers and checksums etc to enable re-creation of a damaged packet into is correct form from the surviving  parts which have been interleaved among other packets. All the unscrambling is done in microseconds in the router and resulting in only a few milliseconds of delay on the line.  They are sometimes called FEC errors (forward error correction)
Short lines are 'Fast path' and have no error correction on them so the corrected blocks are always zero - and are a little faster as a result.
Uncorrected blocks are those which the interleaving unscrambling routines find so damaged they are unable to re-create them - or alternatively the line is 'fast  path' so no error correction is being done yet there are still scrambled blocks being received.
The standard fix for instability on a line is therefore to switch interleaving on if it is not already.

[#7]

I'm guessing that this interleaving isn't something that is switched on / off at the modem?
I'm still a bit none the wiser to be honest!
ADSL2+ is completely unstable, ADSL2 is fine.
ADSL2+ used to be perfectly fine and had been for months. Nothing has changed in my home, but now ADSL2+ doesn't work.
it doesn't *appear* to be my modem - could there be a line fault so should I raise a ticket, and if I did, what on earth should I put in it? lol! 

[#8]

No, interleaving is switched at the exchange.
I'm not too sure about the current state of play as to when it can be changed, and by whom, on what authority under what circumstances
Originally it was intended that BT Openreach would determine the interleaving state of the line at first switch on and there it would stay, unless it was a 'fast line' that proved unable to handle the fast path protocol in practice.
(There are actually various levels of interleaving - but you "really" don't want to know about that!)
The previous poster 'walker23' might be right.
Need some input from plusnet staff on the forum really  - daytime tomorrow
For your attenuation of 24, the "std graphs" give a sync of around 18000 odd on a good line without any faults on ADSL2+
so going at 83% sync for max achievable download speed would be around 14900 odd
So your figures tally with the expected achievable under proper conditions.

[#9]

Ok thanks, fingers crossed that Plus Net will have a nice simple answer - like "Oh that's just a 2 point adjustment of the widget valve!" lol

[#10]

See my last para added to my previous post  - I cross posted while you were doing your reply - sorry

[#11]

Yeah, about 14900 was what I used to get..
But quite a bit of what's been said has gone right over my head! lol..
Ah well..  😉

[#12]

Harmless bump to get the attention of any plus net staff that are floating around that might be able to offer an explanation or advice? 🙂

[#13]

I've just run some diagnostics and that isn't highlighting any major issues so I'm going to advise that you try a different router out if you can.
Adam