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[William Grimsley]

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?

[WWWombat]

Because the first iteration of ADSL only worked at fixed rates.

[JGO]

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  ?

[gt94sss2]

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..

[William Grimsley]

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?

[WWWombat]

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".

[JGO]

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 !

[William Grimsley]

Well, something must've changed then!

[kitz]

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.....

---
There's some photos on this page 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

[William Grimsley]

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).

[kitz]

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.

[William Grimsley]

Makes sense.

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

[WWWombat]

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.

[Black Sheep]

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.

[JGO]

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.