Monitoring of user inactivity is responsibility of eNodeB. When there is no data exchange between UE and eNodeB for a specific duration (configurable value of user inactivity timer at eNodeB), eNodeB releases the UE context by initiating RRC connection release procedure.
Even after RRC connection release, UE in IDLE mode would be in cell and shows signal bar in handset since UE has already synchronized with cell earlier and it can read system information without maintaining RRC connection. If any data in downlink comes, MME does paging for UEs while in order to initiating data in uplink direction, UE initiates "Service Request" message.
