if link is poor, then the MS cannot do the periodic LU, then the MSC will perform an implicit IMSI detach. This IMSI is no more in the list of active users.
The delay between 2 periodic LU should be less than the MSC’s timer for implicit detach.
This delay can be modified in the OMCR (T3212, insn’it ?)
If a cell is not in a lac border, the amount of location update it handles is mostly due to periodic update. You can check with the QOS INDICATORS and see than the amount of SDCCH traffic it generates is rather small.