you are talking about cell reselection during CS Idle Mode ?
1. the MS performs measurements on the BCCH of serving cell and neighbor cells. If the C1 / C2 of a neighbor is better than C2 of serving cell, then it decides to perform reselection.
2. the MS doesn’t give an update to the BSC. Actually only in case the reselected cell belongs to another LAC, then the MS will perform a LOCATION UPDATE procedure, to update its LAC in the VLR/HLR. This is done on the SDCCH channel.
3. MS in idle mode does not send measurements to the BSC.