Life was simple in the good old days when 2B1Q was the only game in town. The Brooktree HDSL chipset provided perfect transceivers and framers for that signal format, and it appears that its original Brooktree owners were much nicer than the current Conexant/Mindspeed gang.
There were competing chipsets for HDSL from other vendors, but as far as we know they only support the HDSL standard, not SDSL/2B1Q which is basically Brooktree's invention. See an example of a non-Brooktree HDSL chipset.
This is a classic, so chips are or at least used to be available from many different vendors:
Nowadays there are several modern
chipsets which implement
a much higher degree of integration than the simple 2B1Q bitpump
and support the new G.shdsl standard,
but the new chipsets also exhibit a new problem.
The fundamental problem is the attitude adopted by the DSL chip companies
that DSL product design is the exclusive realm of the gods and that mere
mortals are not worthy of the privilege.
DSL transceiver chips are very complex and require a lot of support materials: detailed hardware descriptions, firmware images (they all require firmware it seems), detailed descriptions of firmware versions and capabilities, honest and accurate descriptions of the errata and limitations, application notes on how to connect various interfaces coming off the chips, how to command the firmware for various specific applications, interoperability notes, etc. The chip companies argue that they are being exclusivist about whom they divulge these materials to because they don't have the bandwidth to provide personal support to every Joe Amateur Hacker, but they could have just put all their materials up on their FTP or web sites. I wouldn't mind if they were selective about whom they babysit with personalized hand-holding support if the complete set of self-support materials was available to all and everyone, but it is the chip companies' refusal to publicly share their static support materials that makes them enemies of the People.
For the plain old SDSL/2B1Q we are building our OSDCU based on the RS8973 bitpump, but for SHDSL we are aware of the following chipsets:
All three vendors have nothing but marketing crap on their websites, all three require NDAs to get even the most basic datasheets, and all three chipsets require firmware and other support materials beyond the basic datasheet in order to actually use them. M289xx is the only one for which we currently have the firmware images and enough documentation to build an actual STU (hopefully). For the others we currently have too little concrete information and no firmware.