Yup I was a member of the "24Mbps club" for a year or so until they added O2 to the MSANs and I gradually started to lose some speed due to cross-talk. I can't recall now but I think by the time I left after the Sky takeover I was more like somewhere around 21Mbps. I could still get 24Mbps but I'd have to tweak my SNRM down to something like 1.5dB.. but I cba because I still had the 2.6Mbps upload and the line was rock steady.
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I'm not certain why it took BT so long to roll out Annex_M, but I would think that their existing network would have had a heck of a lot to do with it.
Nationwide ADSL only really started to take off in about 2002 and it was 2003 that the first BB4 campaigners started to see their exchanges go live. BT used their
MSiP network to backhaul to the exchanges which were mostly limited to 155Mbps pipes.
Whilst we can laugh now at being limited by
STM-1 / OC3 fibre cable, but that was the norm 20 yrs ago and most of the world was using SDH/SONET. STM-4 brought out an increase to 622Mbps and I can recall Plusnet being very proud to be one of the first UK ISPs to purchase a Juniper STM-4 edge router with 622Mbps capability. BT also upgraded some of the larger exchanges to STM-4 622Mbps.
So basically there wasn't at that time any viable way that the BT backhauls could ever support the bandwidth requirements for Annex_M ADSL2+.
WDM was _the_ big game changer when it came to increasing capacity for fibre bandwidth and BTw set about building the 21CN network to replace MSiP which we all know how long that took to fully complete.
BE* was able to go straight in building a new network based on the most up to date technology and only target the viable exchanges. In fact they didn't have to worry about WDM either, because of the number of customers they were able to get away with just using STM-4 and satellite exchanges. If you think about it.. say they had 50 customers at an exchange, then multiple 155's or a 622 would be sufficient. I'm having a memory block, but I'm sure that our exchange backhaul for BE was 155 because I remember joking on this forum with another member who had load balanced 2 BE lines that the 2 of us plus one other would be able to practically max out BE's backhaul to our exchange. I'm sure someone will correct me if I'm wrong.
Things weren't quite so simple for BTw (pre Openreach) whose client base at each exchange would be multiple times over those of BE's... not to mention business connections and leased lines. Prior to WDM adding more backhaul capacity fibre was no mean feat and why BTw was always so cautious in what they gave the consumers. Only the 21CN exchanges got adsl2+ whilst those exchanges still using 20CN MSiP were stuck with 8Mbps. :/