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Sustainable Wireless Sensor Networks Part 12

Tham khảo tài liệu 'sustainable wireless sensor networks part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 376 Sustainable Wireless Sensor Networks complete round. It is calculated for each sensor according to its distance from the sink. A sensor that has energy below this threshold cannot act as an NM for the network. Sensors are classified according to these thresholds before NM selection into one of three categories 1 Active nodes that can act as NMs. 2 Active nodes but cannot act as NMs and 3 Inactive nodes or dead nodes. Once a node is classified as a dead node the network is considered dead according to the definition of lifetime used in this study. The sink has knowledge about the whole network and is responsible for selecting the NM and informs all other sensors about the current NM. It selects a sensor as an NM for the current round according to the following criteria. 1 The node belongs to the first category. 2 The node has energy greater than the average energy of all active nodes and 3 The sum of its distances to the active nodes is least. In this algorithm it is assumed that a node can be selected as an NM for many rounds throughout network lifetime. A simulation model is built using MATLAB MatLab with the same network parameters used in Heinzelman et al. 2002 and described above. The system is run for different values of the number of cycles C per round and the corresponding network lifetime is as shown in Fig. 1. The figure shows that there is an optimum number of cycles for which each sensor remains acting as NM before another round starts over and a new NM is selected. For the parameters considered the longest lifetime is achieved for C 3 resulting in a lifetime equivalent to 3702 cycles. Fig. 1. Network lifetime vs number of cycles per round 2.4 Algorithm II The previous algorithm selected a fixed optimum number of cycles C per round in order to achieve a longer lifetime. It is observed that with this relatively small number of cycles a sensor is chosen as an NM for many rounds. It is observed also that not all sensors act as NMs for the same number of