1. A dynamic packet relay method of a personal area network (PAN) coordinator and a device associated with the PAN coordinator in a sensor network, the dynamic packet relay method comprising:receiving, at the PAN coordinator, a Child_Register message that includes information about a device associated with a coordinator and building or updating a network hierarchical structure;
monitoring, at the PAN coordinator, reception of a packet or a beacon from the coordinator and determining whether the coordinator is defective or not;
in response to a determination that the coordinator is defective, changing, at the PAN coordinator, a current coordinator to one of devices belonging to a PAN as a new coordinator
wherein during an inactive period of the RAN coordinator, the device associated with the coordinator transmits a packet to the coordinator and receives a beacon from the coordinator, and
wherein, when the coordinator is defective, one of the devices belonging to the PAN that received the packet transmitted during the inactive period of the PAN coordinator is determined as the new coordinator.