IEEE 802.11e 無線區域網路之進階分散式頻道存取機制的效能提升

Abstract

IEEE 802.11e進階分散式頻道存取機制具有能夠提供服務分類以及支援即時性應用的優勢。然而，它在高傳輸負荷下可能會大大地降低產量以及增加碰撞機率，這是由於它在每次封包傳輸成功之後會固定地重設競爭視窗的大小。於本論文中，我們提出了一個新的協定以提升進階分散式頻道存取機制的服務品質，這個協定稱為改良型進階分散式頻道存取機制，是為了要解決上述進階分散式頻道存取機制的缺失。改良型進階分散式頻道存取機制在每次封包傳輸成功之後會根據平均的碰撞機率來動態地調整競爭視窗的大小，而不是像進階分散式頻道存取機制去重設競爭視窗的大小。此外，一旦發生虛擬碰撞，改良型進階分散式頻道存取機制在每次封包傳輸失敗之後會維持較低優先權存取類別的競爭視窗大小，而不是像進階分散式頻道存取機制去加倍競爭視窗的大小。它也提供向後相容於原本的802.11分散協調機制。模擬結果顯示我們的改良型進階分散式頻道存取機制就平均產量而言，比適應性進階分散協調機制、時間相關的後退機制、進階分散式頻道存取機制分別高出9%、11%、以及15%。另外，我們的改良型進階分散式頻道存取機制就高優先權資料的平均封包延遲時間而言，比時間相關的後退機制、適應性進階分散協調機制、進階分散式頻道存取機制分別縮短了22%、26%、以及40%。因此，我們所提之改良型進階分散式頻道存取機制對於在IEEE 802.11e無線區域網路下支援服務品質是很有效的。

The EDCA in IEEE 802.11e has advantages of providing service differentiation and supporting real-time applications. However, under high traffic load, its throughput performance may degrade and the collision rate may increase significantly due to its statically resetting the contention window size after each successful transmission of packets. In this thesis, we propose a new protocol for quality of service (QoS) enhancement to the EDCA, called Improved Enhanced Distributed Channel Access (I-EDCA), to resolve the above shortcomings of EDCA. I-EDCA dynamically adjusts the contention window size according to the average collision rate instead of resetting the contention window like EDCA after each successful transmission of packets. Besides, once a virtual collision occurs, I-EDCA retains the contention window size of the Access Category (AC) with lower priority instead of doubling the contention window size like EDCA after each unsuccessful transmission of packets. It also provides backward compatibility to the legacy 802.11 DCF. Simulations results have shown that the average throughput of our I-EDCA is 9%, 11%, and 15% better than AEDCF, ADB, and EDCA, respectively. In addition, for high priority traffic, compared to ADB, AEDCF, and EDCA, I-EDCA shortens the average packet delay by 22%, 26%, and 40%, respectively. Therefore, I-EDCA is effective in supporting the QoS of IEEE 802.11e wireless LANs.