Dear, kamal kotecha! When network works in NMO1 (Gs-interface between MSC/VLR and SGSN) and MS is in both GPRS-attached and Imsi attached states then Implicit Detach Timer in MSC/VLR must be blocked by MSC/VLR and it must relys on Mobile Reachable Timer in the SGSN (MRT) 3,5???. Also there is no Periodic LA update in this situation – but only Periodic RA Update (PRU). You need to set the correct values for MRT and PRU timer (T3312) in SGSN!!! – not in BSS. See also 23.060 for overall description of combined procedures.