I think its fair to say that Openreach will have been placing orders for ECI cabinets before system based vectoring became a thing. ECI were actually the first manufacturer to make system based vectored DSLAMS in 2012 - From memory they was first trialled in some South/Latin American country in 2012. By this time Openreach were already installing the M41's in the UK.
The timing is rather unfortunate as ECI had a really good reputation in the likes of Germany and France when Openreach commenced rollout. ECI was favoured by Deutsche Telekom & France Telecom where they were used extensively. I remember someone saying back in 2012/2013 that the ECI DSLAMS at the time were considered superior to the Huaweis.
ECI's failure perhaps was the lack of options for modular upgrades... in part due to lack of space on the backplane. Ironically the DSLAM small size is one of the factors which originally made it popular. That said, whilst the Huawei MA5616 is upgradable it does mean replacing practically every part.
Bearing in mind that BT/Openreach
never rely on just one manufacturer, then back in ~2011 ECI probably was a good choice at the time
>> The rolling out of M41's instead of V41's also showed a lack of foresight.
Fact is that Openreach will have already placed orders for FTTC DSLAM rollout before the V41's became commercially available. As I said above - bad timing.
iirc, early versions of G.998.4 (re-tx) were downstream only and whilst technologies may have been approved at certain dates, it doesn't mean to say that manufacturers were actually making them available on their chipsets at that time. Same as with vectoring, the theory of system based vectoring was ITU approved before any DSLAM manufacturer started making it available for roll-out in the wild.
>> G.INP was approved in 2010.
It can take a few years after invention before such things become available. Manufacturers tend to wait until the ITU specifications are set before they start to manufacture equipment to be able to meet those requirements. So just because the theory and specifications were there, doesn't mean to say that such equipment is actually commercially available. Same on the retail side, heck - look how long it took even after FTTC rollout before you could purchase a VDSL2 modem/router.
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By the sounds of it, it seems like the recent failure is down to one particular ISP's CPE being incompatible with the ECI DSLAMs. Deja vu.