Consider following example, u have 1 base station having 3 sectors n u have 9 TRX (freq). Therefore 9TRX will be divided in 3sec. Hence 1 sec wil have 3TRX means 3×8=24 time solts. out of 24 TS 1 will be used for BCCH,1 for SDCCH n remaining 22 FR TCH.
Same applies for other sectors.
Note;Only first TRX of that particular sector will have BCCH or SDCCH remaining all 8 TS of other TRX can be used for TCH.Also in some cases there can be more then 1 SDCCH (say2),then number of TCH will be 21.