臺灣博碩士論文加值系統

在IEEE 802.11規格中，DCF模式採用CSMA/CA機制進行頻道的競爭，並透過RTS/CTS的交換機制來解決著名的隱藏點問題(Hidden Terminal Problem)。然而在實際的分散式環境中，控制封包仍然會發生碰撞的情形，且隨著網路交通負荷量(traffic load)增大，控制封包的碰撞次數也將隨之增加，不但無法期地解決隱藏點問題，更因為碰撞的產生，導致控制封包需要重傳，control overhead勢必大大增加。此外，碰撞的發生進而加大Contention Window的動作，也將造成頻道的使用率大幅下滑，因此封包碰撞所造成的負面成本將嚴重地影響網路產能(throughput)。本論文提出RCA 協定(RTS Collision Avoidance protocol)利用Pulse/Tone交換機制，將透過事先的警告措施，使得任何想傳送封包的主機在即將完成backoff procedure前，能事先偵測是否會與two-hop內的“相關主機”發生控制封包碰撞，達到控制封包碰撞的快速偵測(fast collision detection)效果。透過此觀點所提出的演算法將可強化IEEE 802.11 RTS/CTS交換機制的效能，並大幅降低RTS封包的碰撞情形，避免浪費頻寬於多次的控制封包重傳與競爭上，進而降低control overhead。此外，我們進一步分析IEEE 802.11設定的Contention Window初始值與頻道使用率的影響，並根據RCA協定的快速偵測特性，適當的調整初始的Contention Window值，以獲得更高的頻道使用率。本演算法能避免RTS封包的碰撞及提高頻道使用率，因此必能獲得更理想的產能與效益。

In ad hoc networks, the well-known hidden terminal problem severely reduces the network capacity. IEEE 802.11 DCF is based on carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. It solves the hidden terminal problem by the RTS/CTS handshaking mechanism. However, as there is still the possibility of RTS packet collisions, the aggravation of performance is inevitable. As above descried, the RTS/CTS scheme will suffer more challenge on real ad hoc network with consideration of transmission time of RTS packet. Moreover, the occurrence of RTS collisions will lead to other bad effects such as RTS false-blocking problem, CTS collisions, and ACK collisions. To address these problems, in this paper, we present RCA (RTS Collision Avoidance) MAC protocol to reduce RTS collisions efficiently by announcing the transmission in advance. RCA protocol performs a type of fast collision detection and decreases RTS collision that benefits the RTS/CTS exchange scheme. Therefore, RCA protocol could avoid wasting too much cost for multiple retransmissions and have much lower control overhead than that of IEEE 802.11. In addition, RCA protocol proposes “Channel Utilization Improvement Policy” to reduce the channel access delay and increase channel utilization. Simulation results are also provided to demonstrate the effectiveness of the proposed scheme. It is concluded that RCA protocol is superior to IEEE 802.11.

1 緒論 1
1.1 背景 1
1.2 目的 4
1.3 相關文獻 6
2 問題分析與目標說明 13
2.1 問題分析 13
2.2 本論文擬達成的目標 20
3 協定規劃與實作 22
3.1 協定運作概念 22
3.2 協定實作 27
3.2.1 RCA/2協定 29
3.2.2 RCA/1 協定 32
3.3 協定強化與修正 34
3.3.1 延遲政策(Blocking policy) 34
3.3.2 提高頻道使用率政策 37
4 模擬分析 40
4.1 格子拓樸(Grid topology) 40
4.2 隨機拓樸(Random topology) 42
5 結論與未來發展 45
5.1 結論 45
5.2 未來發展 46
參考文獻 47
英文論文稿…………………………………………………………......49
圖表目錄
圖1. 各種IFS (interframe space)關係圖 3
圖2. backoff procedure說明 4
圖3. 隱藏點問題示意圖 4
圖4. DBTMA頻道使用狀況圖示 8
圖5. 頻道S與R各自所佔的頻寬比例圖示 9
圖6. jamming 訊號運作與功能說明圖 9
圖7. 網路交通負荷(traffic load)與門檻值(threshold)關係圖[6] 11
圖8. 網路交通負荷(traffic load)對網路產能的影響[5] 12
圖9. 隱藏點問題說明 14
圖10. RTS封包傳送所需時間說明 15
圖11. 實際網路運作情形 16
圖12. RTS blocking問題描述 19
圖13. CTS封包碰撞情形 19
圖14. ACK封包碰撞情形 19
圖15. 協定運作示意圖 27
圖16. 何謂two-hop內相關主機說明 27
圖17. 實際環境解說 28
圖18. 以802.11規格運作之流程圖 29
圖19. RCA/2協定運作流程圖 30
圖20. RCA/2 協定時槽(slot)運作詳細說明圖 31
圖21. RCA/1運作細節分析圖(一) 34
圖22. RCA/1運作細節分析圖(二) 34
圖23. 延遲政策運作說明圖 37
圖24. 提高頻道使用率政策說明圖(一) 39
圖25. 提高頻道使用率政策說明圖(二) 39
圖26. 格子拓樸(Grid) 41
圖27. RTS封包碰撞次數(Number of RTS collision) 41
圖28. 網路產能(每秒成功傳送的資料量) 42
圖29. RTS封包碰撞次數(Number of RTS collision) 43
圖30. 網路產能(每秒成功傳送的資料量) 44

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