I might be in big trouble at times when I couldn't achieve 6dB DS SNR?
Yep. At least maxdsl would let you sync at a lower rate and give you say 1.7Mbps. On 2Mb fixed you'd be completely without any service at all.
Fixed rate also doesnt have interleaving and error correction and its all FAST path, so things could become very tricky indeed.
Going back several years ISPs used to transfer some troublesome lines that were say syncing & flapping at around say 700kbps to 512kb fixed. The advantage was at least the EU was getting what they paid for and because fixed doesnt do interleaving & Error Correction, then there was no DLM flapping either.
However these days costs have come down dramatically for rate adaptive DSL & its actually the cheaper product of the 2 types because it has a fixed price no matter what the sync. ISP bulk purchase reductions and kit for rate adaptive technologies means that once the cost of rate adaptive came down to meet a similar cost of 512kb fixed line then there is little incentive for the ISP to offer fixed rate. Its one of those things that the more people buy a product then the cheaper it becomes.
Also the DLM is (believe it or not) better than it was in the early days of maxdsl, so its easier and cheaper for the ISP to request a '
max capped profile 512/1000/2000'. This product is basically rate adaptive (max) dsl set to sync at maximum speed of 512 and mimic a fixed rate line. Doing it this way: -
(1) stops most of the DLM flapping
(2) reduction in speed usually gives more SNRm to play so line is less likely to need interleaving etc
(3) Customer knows what their line speed should be at each sync
(4) If for any reason it cant sync at 512 then at least it can sync at a lower speed to get some sort of connection
(5) Its cheaper for the SP & therefore the customer.
Thats why not many ISPs now retail a fixed rate product.
