CS Pagings goes from MSC to SGSN (Gs interface), then from SGSN to MFS(PCU) (Gb interface), then from here:
– either MS is in PTM : CS Paging is sent on PACCH
– or MS is not in PTM : CS Paging is sent to the BSC (which will send it to the PCH)
It is done this way because the BSC doesn’t know which ms is in ptm, and which one is not.
If the BSC knows, then this is called the “BSC Paging coordination”, and here there is no need of gs interface anymore.