1. A mobile communication system in which an Idle-mode Signaling Reduction (ISR) is active, the mobile communication system comprising:an User Equipment (UE);
an evolved Node B (eNB);
a Radio Network Controller (RNC);
a Mobility Management Entity (MME);
a Serving General Packet Radio Service Support Node (SGSN); and
a Serving Gateway (S-GW),
wherein when the S-GW receives a downlink data packet for the UE during a tracking area update procedure or a routing area update procedure with a change of the MME or the SGSN, while the ISR is still active, the UE is paged by the eNB or the RNC based on a paging message of only one of a new MME and a new SGSN.