利用訊號變化與遮蔽效應之802.11MAC碰撞避免協定

Collision Avoidance 802.11 MAC Protocol with Signal Variation and Masking Effect

在以IEEE 802.11DCF為標準的多跳躍無線網路中，傳輸對的鄰居節點會進入NAV狀態，並且在傳輸對完成傳輸時結束NAV狀態以避免與傳輸中的節點發生碰撞。然而當一個節點在NAV狀態時，如果此時其鄰居開始進行資料傳輸，當此節點離開NAV狀態後，可能會與正在傳輸資料的節點產生碰撞而影響網路效能。這個狀況在Ad-Hoc Network和網路環境密度高的情況下更為嚴重。為了解決此種碰撞問題，在本篇論文中我們提出一個MAC層碰撞避免協定。主要分成三個部分，當某一節點處於NAV狀態時，1.如果可以收到其鄰居節點的控制訊號並且不會收到原傳輸對的訊號時，其要監聽其鄰居節點是否發出控制訊號來調整其NAV狀態時間以避免在NAV狀態結束後發生產生碰撞。2.如果節點同時會收到原傳輸對與其鄰居節點訊號時，並且鄰居節點的控制訊號強度比上原傳輸對訊號強度大於接收控制訊號的門檻值時，雖然節點處在同時收到兩者的訊號，但是根據遮蔽效應還是可以收到鄰居節點的控制訊號進而調整其NAV時間以避免碰撞發生。3.當節點會同時收到兩者訊號，並且接收的鄰居節點的控制訊號強度不足以遮蔽原通訊對的訊號而無法變遮蔽原通訊對的訊號而無法辨識時，此時利用偵測訊號強度變化來預測是否有鄰居節點傳輸對正在傳輸來改變NAV時間。實驗模擬結果顯示我們提出的協定碰撞發生比率較傳統802.11DCF協定減少60％，同時比起其他的研究也減少約41％。除此之外，傳輸延遲以及重傳次數也能有較好的表現，整體網路輸出量也因此有所提昇。

IEEE 802.11 DCF is widely used and able to avoid the collisions of the hidden terminal problem in a wireless network. However, there may be collisions existing in the DCF MAC protocol. That is, when a node in NAV state does not receive any control signal, a collision may occur after the node ends the NAV period. In this paper, to solve this collision problem, a detailed analysis about the problem is given. Afterwards, according to the analysis, we utilize the 'masking effect' and the detection of signal variation to design the MAC collision avoidance protocol. With the received signal strength and the variation of received signal strength of a node, the node can determine whether there are other transmissions occurring in the transmission range. Therefore, a node can determine whether the period of NAV state can be extended to avoid collisions. Simulation results show that our proposed MAC protocol can efficiently reduce 60% and 41% collisions than the IEEE 802.11 DCF protocol and OCS-MD. In addition, our proposed MAC protocol also shows shorter transmission delay and lower retransmissions than the 802.11 DCF and OCS-MD to have better network throughput.