Power control takes place in the same way as it does for TCH.SDCCH on BCCH TRX is recommended as BCCH is ought to be much more cleaner than Hopping frequencies and to increase the MS accesibility to the Network as it has more foot print.
1 SDCCH ts must be located on the BCCH carrier, so that if only 1 TRX remains alive (all others = dead), it would contain all necessary ts for call management (BCCH + SDCCH + TCH)
(last TRX alive would be the BCCH TRX… remapped dynamically on the live HW)
Other SDCCH can be allocated on other TRX, on whichever TS you need. The recommendation is : “at least 1 SDCCH ts on the BCCH carrier”.
Not “ALL sdcch ts on the BCCH carrier”.