| Summary: | 碩士 === 國立臺灣科技大學 === 電機工程系 === 97 === Wireless sensor networks are often used in military, agriculture, construction and other needs of large coverage and long operating hours of the applications. In order to meet the needs of such applications and to extend the network lifetime, wireless sensor network must increase the number of sensor nodes as the network coverage become bigger. However, a large number of sensor nodes will increase the probability of transmission collision, and this is the main cause to decrease entire network goodput and reduce the network's lifetime.
By IEEE 802.15.4 standard, in contention access period (CAP), sensor nodes use CSMA/CA algorithm to access channel before transmitting data, and this method will have high efficiency when the traffic load is low. Besides, this method will decrease performance when the number of sensor nodes increase and heavy traffic load. In order to meet the need of fixed bandwidth and time sensitive application, IEEE 802.15.4 standard define another transmission period called contention free period (CFP), under this period that sensor nodes can transmit data without contention. Although using CFP under low traffic load can cause lower bandwidth utilization, but this method can effectively avoid collision and increase bandwidth utilization when traffic load is heavy. According to traffic load and other parameter, if we can dynamically change sensor transmit method between CAP and CFP, we can improve whole network transmission efficiency.
In order to combine both advantages, we propose adaptive transmission using guarantee time slot (ATUGTS), that sensor nodes can adaptively request using contention free portion to transmit data. The coordinator will be in accordance with the required size of the transmission slot allocated to the most appropriate sensor nodes to enhance the overall network performance. From the simulation results, the proposed method shows better performance in goodput, power consumption, utilization of guarantee time slot which has the advantages of the combination of only using CAP and only using CFP.
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