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[WWWombat]

What I'm saying is that they were frequency multiplexed channels, so if you used one for XDSL  you are stuck with a maximum data rate which corresponds to one analogue channel width, otherwise crosstalk I   This I thought was the basis of the original question.

I don't think I've heard of frequency-division-multiplexed voice channels for a long time. The transmission network switched over to TDM (with 30 channel, 2Mbps E1 connections as the basic unit) more than a couple of decades ago; the access network may have been different, but I doubt it: even ISDN is TDM-based.

If you were right, and voice channels were FDM, then the maximum data rate for one channel would, presumably be the same as the 56kbps modems that peaked just before ADSL gained ascendency.

[aesmith]

I wonder why our line was so unstable back then? Must've been a fault there instead that was later fixed.

One possibility is that originally the only residential service was 500K fixed rate.  Later BT replaced that with the 2 Meg product which offered 500K, 1meg or 2meg at the same price.  Most ISPs just migrated their users to 2meg.  This left some users on a 2meg service even though their line attenuation was outside the range, ie a new order for 2meg wouldn't have been accepted.   We were in that boat and the position from BT was that it might well work even if outside range, but if it fails they wouldn't accept it as a fault, the choice was either use it as-is or get it changed to 1meg or 500K to get it with spec.

[gt94sss2]

It's quite a long time ago now but I seem to recall that when ADSL was first released, the line test (maximum dB) for the 512K, 1MB and 2MB services was actually the same..

[William Grimsley]

From 2006 to 2012, we were always on the 1 Mb service as far as I remember (I know that long), but our line kept dropping out all the time (we had one of these whilst on this service: http://www.materiel.net/live/44362.jpg). I remember seeing the ADSL light either go out or flashing orange for long periods. LOL.

[kitz]

In the early days it was 55dB for 512k and 41dB for 1Mb & 2Mb.

• Sep 2003 - 512k was increased to 60dB
• Sep 2004 - 512k limit abolished and 1Mb limit increased to 60dB.   2Mb remained at 41dB although I seem to recall there was some discretion >43dB
• Nov 2005 - RADSL (MaxDSL) trials.
• 31 Mar 2006 - Nationwide rollout of MaxDSL

I knew it would be handy to keep this page around in case it had a use some day

[kitz]

From 2006 to 2012, we were always on the 1 Mb service as far as I remember (I know that long), but our line kept dropping out all the time (we had one of these whilst on this service: http://www.materiel.net/live/44362.jpg). I remember seeing the ADSL light either go out or flashing orange for long periods. LOL.

As I said in an earlier post you were likely on Max capped rate profile 1000.    This was applied by DLM for unstable IPStream connections.
- See SIN 485 Page 6-7.

True fixed rate 1Mbps was deprecated after MAXDSL was rolled out.

[gt94sss2]

I had ADSL first installed in 2000 - one of the first on my exchange in London (which meant we actually had 3 engineers present so 2 could learn on the job)

When first launched commercially, BT were refusing to install even the 512K service on lines unless they were 41dB or less (then the same as the 1MB/2MB) - the initial limits were relaxed later.

[William Grimsley]

Thanks, kitz.

gt94sss2, that seems crazy compared to present times! So, that means if ADSL was available in the Colaton Raleigh exchange in 2000 then we wouldn't have been able to order it!

[kitz]

No you wouldnt have.   Your line would have been too long to be able to cope.  The engineers tested whether you could get adsl based on your loop length.  (Attenuation)
Today SNRm also affects the DLM and by adjusting your Target SNRm, but back then SNR totally affected whether you could you could connect or not.  Going from memory anything under 10dB SNRm was bad and severely affected stability.
 
There was no interleaving, there was no INP (nvm G.INP) and afaik BT didn't even use RS encoding, because I cant recall ever seeing it any stats.  Trellis encoding was about your lot when it came to error protection.
As such if your line started to drop below 10dB (iirc some early sites would say 12db) then you would start racking up lots of CRCs.  Anything below 6dB and you would experience real difficulties staying connected. 

The Low SNR page was originally written prior to MaxDSL because in those days it was troublesome and a very real issue for medium length lines, nvm long lines. Lines could go for hours without being able to connect. :/

The only way BT had of ensuring they didn't provide a service that could spend a large portion of time trying and failing to connect was using limits on the loop length (atten); thus the line would have sufficient surplus SNR Margin.

When BT rolled out the rate adaptive MasDSL, this changed the ball game.  Now the line could sync as fast as it could based on a set Target SNRm.... rather than being forced to fill 'x' no of bins and hoping that there was sufficient SNRm left over for the line to sync.
They also rolled out Interleaving and FEC at the time time, which brought further stability and meant that dropping below 10dB no longer mattered as much as Interleave and FEC could recover data rather than causing the line to error.

Max capped rate is not a 'true' fixed rate product because it uses RADSL and still allows the line to sync even if it cant reach the capped rate.  The cap rate is there to give additional protection and ensure the line doesnt sync at too high a rate that could cause it to drop later on in the evenings.

[William Grimsley]

What an explanation! Noted! Thank you, kitz!