臺灣博碩士論文加值系統

當網路電話還在初發展的階段，人們只把它當作是個免費的服務，並不太在意通話品質的好壞。但如今，網路電話已越來越盛行，並且大有取代傳統電信網路的架勢。網路電話能否持越來越受歡迎的關鍵絕對不是在於它是否廉價，而是在於它的通話品質能否持續的提升。
這幾年來，配有多重無線網路介面的用戶端設備變得越來越受歡迎，原因當然是各式無線網路的方便性和越來越便宜的售價。多重無線設備可以包括固定式多重無線設備(像是802.11a/b/g多模多網卡之無線閘道器)和可配有如2G/3G/無線區域網路之多重無線設備(像是PDA或是smartphone)。使用多重無線設備的主要原因有增加流量，降低成本，還有增加便利性和可靠性。最近幾年，因為無線網路介面卡價格越來越便宜，每個人都負擔得起一台可攜式多重無線設備。本論文首先提出採用重覆平行傳輸方式來改善無線頻道傳送VoIP封包之延遲和遺失情形，並提出了頻道選擇演算法和頻道切換演算法，只要在多重無線設備上實現這此二演算法，就能夠有效運用多個無線通道間的互補功能，改善網路電話的通話品質。
為了驗證我們演算法的效能，我們用C語言寫了模擬程式，並使用IEEE802.11a當作我們實驗的頻道。模擬的結果顯示在無線通道的傳輸品質不是太差且背景資料流量不是持續嚴重的情況下，執行我們演算法的多重無線設備可提供網路電話可以接受的通話品質。

When VoIP was in the initial developing stage, people thought it as a free service and did not care the voice quality too much. VoIP is more and more popular around the world and is going to replace a part of traditional telecommunication. The key that VoIP can keep gaining popularity in the future absolutely not lies in its cost effectiveness but in its the improvement of voice quality.
In recent years, multi-radio device has become more and more popular because its convenience and cost down of such radio modules. Multi-radio devices include fixed multi-radio devices (such as 802.11a/b/g multi-mode multi-radio gateway) and mobile multi-radio devices (such as 2G/3G/WLAN PDA and smartphone). The main purposes of multi-radio devices are to increase the total throughput, cost effectiveness, convenience, and the reliability. Because the cost of wireless network interface modules have been decreased rapidly in recent years, everyone should be able to afford such a mobile multi-radio device. In this thesis, we use duplicated parallel transmission to reduce the delay and packet loss condition, and then propose the Channel Selection Algorithm and the Channel Switching Algorithm deployed in multi-radio devices to improve the VoIP call quality by exploring the complementary properties among parallel channels.
In order to verify the performance of our algorithms, we execute the simulations in C language and use IEEE 802.11a as the wireless channel environment. The simulation results show that under the circumstances that the wireless channels quality is not too bad and the background traffic is not constantly heavy, the proposed multi-radio device system dose provide an acceptable VoIP call quality.

Chapter 1 Introduction 1
1.1 Motivations and Goals 1
1.2 Related works 2
Chapter 2 System Architecture and Algorithms 5
2.1 System Architecture 5
2.1.1 System Architecture 1: Parallel Transmission Between Two Wireless Multi-radio Gateways 5
2.1.2 System Architecture 2: Parallel Transmission Between a Wireless Multi-radio Gateway and a Mobile Multi-radio Device 7
2.2 The System Queueing Model and System Flow Chart 8
2.2.1 System Queueing Model 8
2.2.2 System Flow Chart 10
2.3 Algorithms 11
2.3.1 ITU-T E-Model 11
2.3.2 Channel Selection Algorithm 14
2.3.3 Channel Switching Algorithm 23
2.4 The power saving issues in Channel Selection Algorithm 26
Chapter 3 Simulation Results 29
3.1 Simulation scenario 29
3.2 Simulation Environments 30
3.2.1 Traffic source 30
3.2.2 Channel Type 32
3.3 Simulation Results 33
3.3.1 Case 1: A VoIP call 33
3.3.2 Case 2: A VoIP call with background traffic 41
Chapter 4 Implementation direction, conclusions, and future works 45
4.1 Implementation direction in multi-radio gateway 45
4.2 Conclusions 47
4.3 Future Works 48
References 49

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