Kitz Forum
Chat => Chit Chat => Topic started by: William Grimsley on July 13, 2016, 05:22:34 PM
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Hi all,
I've been thinking about this recently and can't think why but it's interested me.
Why were customers connected at a fixed rate and not at what their line could achieve?!
For example, when we moved to this property in 2006, we were connected at the 1 Mbps fixed rate service (1024/288). But, what confuses me is why were we not connected at the maximum speed our line could achieve?
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Because the first iteration of ADSL only worked at fixed rates.
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Because the first iteration of ADSL only worked at fixed rates.
And I suggest that was because it was the way telephone voice circuits were stacked i.e. all 300 -3000 Hz wide ?
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There are technical and marketing reasons why BT did this - not least that BTs backbone would not have been able to cope if BT had introduced rate adaptive DSL from day one..
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Wait, so even though ADSL Max was available, customers were still connected at fixed rates? Or, am I wrong in saying that it was just ADSL Max right from the start?
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And I suggest that was because it was the way telephone voice circuits were stacked i.e. all 300 -3000 Hz wide ?
They still are, aren't they? Well, 300Hz - 3.4kHz. ADSL just uses "the rest".
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There are technical and marketing reasons why BT did this - not least that BTs backbone would not have been able to cope if BT had introduced rate adaptive DSL from day one..
Exactly; when voice lines are stacked in frequency to share a single physical line you can't dedicate the whole bandwidth ( REAL bandwidth in Hz) to one digital line without B-- ing the system. Well you can in a lab but not for paying customers !
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Well, something must've changed then!
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It was pre RADSL (rate adaptive DSL).
All the DSLAMs needed upgrading for RADSL. (MaxDSL). Prior to that there was no such thing as Target SNRm and the modem would either sync.. or not in the case of long lines. A large proportion of lines were unable to sync at 2Mbps.
I could be wrong but isn't it more likely to do with the type of modulation. RADSL uses G.DMT modulation rather than (the older) DMT.
I can recall several lines which could only get 1Mbps fixed rate and being incapable to hold sync at 2Mbps.. yet they could get say 4 Mbps or more using G.DMT.
Technologies advance and change all the time.
DMT became G.DMT. ADSL became rate adaptive.
ADSL2 used a more efficient type of error protection which reduced overheads allowing higher bit stream.
ADSL2+ built on ADSL2 and opened up more frequencies.
Now we have VDSL/VDSL2
Next.....
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There's some photos on this page (http://www.kitz.co.uk/adsl/telephone_exchange.htm) of the old Westell MUXes* which were installed in the early days of ADSL. Alcatel was also a supplier of first gen DSLAMs in some exchanges. Those had to be updated/replaced for MaxDSL with newer MSANs.
*Note the term MUX/Multiplexer used back in those days rather than DSLAM/MSAN. MSANs are in effect 3rd generation Multiplexors
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Thanks for that detailed explanation, kitz. That explains why our very long ADSL line would drop out frequently most days and would only sync at 1024/288 (guessing from wired speed test results).
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Doubtful.
BT had replaced practically all their 1st Gen DSLAMs by 2006. In a lot of exchanges they went straight to 3rd generation preferring to buy the newer MSANs in preparation for ADSL2+. They also started building the new 21CN network at about that time so it made sense to go for MSANs.
True fixed rate lines were later deprecated and replaced with Max Capped Lines.
Max Capped rate lines used RADSL and were artificially restricted either by :
1) DLM to keep a line stable.. or
2) by the ISP. -
In respect of 2) ISPs used to charge a premium for 'up to 8Mbps' and it wasnt unknown for certain ISPs to use MAXDSL yet make up their own product speed and capping throughput at their end. In those days bandwidth was very expensive for the ISP.. worked out at about £1 per GB.... so you can see someone who was say using 20GB or more of bandwidth each month would soon eat into profits.
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Makes sense.
I wonder why our line was so unstable back then? Must've been a fault there instead that was later fixed.
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Around 2000, I thought the hardware was also a very significant limitation. Certainly my first fixed-rate 2Mbps Alcatel modem (received in the 2000 trial) got frying-pan hot, so I'd hate to think what the racks were like in the exchange.
Exactly; when voice lines are stacked in frequency to share a single physical line you can't dedicate the whole bandwidth ( REAL bandwidth in Hz) to one digital line without B-- ing the system. Well you can in a lab but not for paying customers !
Sorry - by "stacking" you were talking about multiplexing multiple voice channels onto one physical line?
Yes - that happened in the access network - through use of DACS equipment. I didn't think it was in particularly widespread use, but it was enough to get a few headlines when it didn't work with ADSL.
IIRC, a DACS line was totally incompatible with ADSL of any form (a form of pt-to-pt ISDN?), so I doubt that "stacking" was a reason for the early use of fixed-rate ADSL.
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No idea about 'stacking', never heard of the term TBH ............. but to answer your question about DACS, yes you are right ..... they are totally incompatible with DSL.
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Sorry - by "stacking" you were talking about multiplexing multiple voice channels onto one physical line?
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.
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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.
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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.
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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..
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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.
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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 (http://www.kitz.co.uk/adsl/history.htm) around in case it had a use some day :)
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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 (http://www.sinet.bt.com/sinet/sins/pdf/485v1p3.pdf) Page 6-7.
True fixed rate 1Mbps was deprecated after MAXDSL was rolled out.
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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.
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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!
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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 (http://www.kitz.co.uk/adsl/lowSNR.htm) 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.
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What an explanation! Noted! Thank you, kitz! :D