Brooktree 2B1Q bitpumps were intended primarily for building HDSL T1/E1
terminals, and Brooktree made several HDSL framer / PCM bus interface chips for
this purpose.  Most SDSL flavors, however, did not use the HDSL framing format
and ran the bitpumps in the "no channel unit" mode.

In the case that some SDSL flavor does use the HDSL framing format and you need
an HDSL framer, IFCTF strongly recommends that you do NOT buy chips from
Mindspeed and implement the HDSL framer yourself in an FPGA.  Mindspeed is a
seriously fucked up company and an enemy of the people; they withhold essential
software and support materials from the users of their chips.  It took us 5
months of brute force cracking to recover the source code for the BT software
package, which the miscreants had encrypted and for which they had refused to
tell the key.  This package also contains the channel unit code.

We have, however, gathered the datasheets for the HDSL framer / channel unit
chips and the channel unit software manual and posted them here for
completeness.