IEEE 802.16e及時通訊頻道品質感知的最大傳輸單元調整機制

Abstract

隨著科技越來越普及，人們對於網路的依賴也越來越深，從早期的窄頻撥接，到現在的xDSL寬頻上網、光纖上網等，頻寬的成長似乎是無止境的， 只因為人們總是有新的應用，把提升的頻寬作有效的應用。 無線網路技術的進步，讓網路的使用更方便。無線網路技術較為人知的有紅外線、藍芽和IEEE 802.11無線區域網路等。而近期的IEEE 802.16，很多國際大公司像北電，英特爾等，更把它視為網路電信產品的明日之星。 和有線網路比起來，無線網路的環境較不穩定，也因此，無線網路實際的頻寬使用率相較於有線網路低，在開發高頻寬技術的同時，有效的利用現有的頻寬也是一項選擇。 本篇論文針對IEEE 802.16e系統提出了一套即時動態頻道相依的流量分析方法，並利用分析結果，提出一套可以根據不同Bit Error Rate (BER)，動態改變Protocol Data Unit (PDU) 的Maximum Transmission Unit (MTU) 機制，以減少資料拋棄，並增加有效頻寬使用率。 如上面所述，藉由修改MTU，可提高有效頻寬使用率，但修改MTU會衍生出另一個問題，那就是當MTU變小時，可能會增加資料傳輸的Overhead (如MAC header、Cyclic Redundancy Check等) 的數量 ，進而可能會造成部分資料延遲傳送。因此，本論文亦提出一個Lazy MTU 調整的方法，以避免因為修改MTU而產生的資料傳送延遲。

The advance of technologies has changed the way people access the Internet - from narrow band modem dialing to xDSL broadband access. This change stimulates more and more new applications, including multimedia audio/video services, and thus increases the demand of network bandwidth. The development of wireless communication technologies makes it more convenient for people to network access anytime, anywhere, and even when moving. Several well-known wireless technologies, such as Bluetooth and IEEE 802.11 standard, have already been equipped with modern Internet or telecommunication products. Among them, IEEE 802.16 is the most promising wireless communication standard and is regarded as the most important technology for the next-generation wireless networks. Compared with wired networks, wireless networks are more sensitive to the environmental interferences and is relatively unstable. Moreover, bandwidth utilization of a wireless network is considerably lower than that of a wired network. One intuitive way is to develop new wireless technologies with higher bandwidth. Alternatively, increasing the utilization of available bandwidth is also a good approach to extending the throughput of a wireless network. In this thesis, we propose a Channel-Quality-Aware Maximum Transmission Unit (MTU) adjustment mechanism for real-time communication in IEEE 802.16e networks. The proposed method can dynamically adjust MTU of Protocol Data Units (PDUs) in accordance with the current Bit Error Rate a mobile station experienced. By decreasing the MTUs when BERs are high, a mobile station can help to reduce the packet loss ratios and thus increase the system throughput. However, decreasing the size of MTUs may introduce extra framing overhead, such as MAC headers and Cyclic Redundancy Check codes. As a consequence, a mobile station may need an extra cycle to complete the transmission of the PDUs that the mobile station can transmit in one cycle before adjusting the size of MTUs. Therefore we also propose a Lazy MTU adjustment scheme to eliminate such delay. Performance results show that our proposal is very effective.