Hello Arinze Chidi!
HR/FR is a multiplexing mechanism on timeslot taken by the BSC in other to fit with capacity requirement. This rate change is guided by parameters that enable the BSC to take decision on the rate to set for new incoming calls in a particular cell.
Now half rate is 2 users multiplexed on a single TS every frame and each of them haS his SACCH channel on frame 12 and 25 for MR report purpose, across multiframe26