In alcatel system, the OMC-R operator defines which timeslots in the cell are capable of becoming SDCCH or TCH.
In the cell, the operator chooses a certain timeslot, and defines it as “SDD”, which means this TS can decide dynamically to be either TCH or SDCCH (in case there are a lot of SDCCH requests).
It is not the BTS which chooses which TS can become SDCCH. It is statically configured in the OMC-R.
Now, if you define 2 SDD TS in a cell, there might be some priority, and the BTS might choose a certain TS among the 2 TS to become a SDCCH. Insuch case, the BTS will choose the TS which is located on a TCU with the less load (other secondary criteria are used as well, but none about the interference band measurements).