The purpose of of this scheme is to minimize the transmission energy consumption summed by all master- slave pairs and to serve as many slaves as possible in order to operate the network with longer lifetime and better performance. Two schemes, single-phase clustering and double-phase clustering, are proposed in [15]. In singlephase clustering, initially every master node will page slave nodes with the allowed maximum energy. For each slave that receives one or multiple paging signals, it always sends an acknowledgment message back to the master from which it receives the strongest paging signal. Since a master node can serve only a limited number of slaves, it first allocates channels for slaves that only receive a single paging signal from itself. If any free channels remain, other slave nodes, which receive more than one paging signal, are allocated channels in the order of the power level of the paging signal received from the master node. For those slave nodes, which do not receive a channel from a master in the channel allocation phase, are dropped in the further communication phase