無線感測網路(wireless sensor network ; WSN)經常被使用於軍事、農業、建築等需要大涵蓋範圍及長運作時間的應用。為了滿足此類應用需求並延長網路壽命，無線感測網路中的感測節點數量必須隨著涵蓋範圍增大而隨之增加。然而，大量密集配置感測節點將提高傳輸時的碰撞機率，此一現象將大幅降低無線感測網路的有效產能並減少網路整體的使用時間。
在IEEE 802.15.4的競爭存取週期(contention access period ; CAP)中，感測節點使用CSMA/CA機制進行資料的傳輸，此種方式在網路流量負載較低的環境中具有高傳輸效率。然而，當感測節點數量增加及網路負載增加時，此種方式將提高傳輸時的碰撞機率，因此造成有效產能大幅下降。此外，為了滿足固定頻寬需求及時間急迫性的應用，IEEE 802.15.4 標準定義了免競爭週期(contention free period ; CFP )，使得感測節點得以使用免競爭的方式進行傳輸，此種方式在中低網路負載下有較低的頻寬使用率，但在高網路負載有效避免碰撞，因而具有較高頻寬使用率。因此，若能依照目前網路的負載及相關參數，讓感測節點動態的於此二種方式間進行切換，將可有效的提高網路的整體效率。
為了能夠結合兩者優點，我們提出適應性使用保證時槽傳輸機制(adaptive transmission using guarantee time slot; ATUGTS)，讓感測節點可以適應性向協調者要求使用免競爭週期傳輸資料。並且協調者將優先分配免競爭週期給保證時槽使用率最高的感測節點，進而提升整體網路效能。從模擬結果得知我們所提出的方法在有效產能、能量消耗、保證時槽使用率均結合了兩者的優點，且都有較好的表現。

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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