地面網路(Terrestrial Network)通常使用TCP-Reno來調整壅塞視窗(Congestion Window, CWND)，由於地面網路大部分是有線網路，封包的遺失大部分都來自中間節點(路由器)的壅塞，為了解決網路壅塞，傳送端會縮小CWND來降低資料傳輸率，但是非地面網路(Non-Terrestrial Network, NTN)是長距離的無線網路，因此位元錯誤率(Bit Error Rate, BER)與比起地面網路的BER還高非常多，若將Reno用於NTN，由於接收端無法判斷封包遺失是由於網路壅塞還是位元錯誤，傳送端會沒有必要的縮小CWND，這會導致資料傳輸率(Throughput)降低。為了解決此問題，本論文在NTN提出一個動態調整TCP壅塞視窗(Dynamic TCP Congestion Window Adjustment, DCWA)的機制，此機制分成三個部分，第一個部分是在衛星計算轉傳位元率(Forwarding Bit Rate, FBR)，第二個部分是在接收端根據資料鏈結層(Data Link Layer)的循環冗餘檢查碼(Cyclic Redundancy Check, CRC)與TCP層的序列號碼(Sequence Number)來判斷封包遺失的原因，第三個部分是在傳送端根據封包遺失的原因來動態調整CWND，如果封包遺失是由於位元錯誤，傳送端會維持或稍微增加CWND，如果封包遺失是由於網路壅塞，傳送端會稍微縮小CWND，如果封包遺失是由於位元錯誤且網路壅塞，傳送端會大幅縮小CWND。為了驗證我們所提出的DCWA可以有效改進資料傳輸率，我們用C語言來撰寫模擬程式，此程式有三個主要的部分，第一個部分為為產生網路壅塞與位元錯誤的比例，第二個部分為接收端判斷封包的遺失，第三個部分為傳送端根據封包的遺失來調整CWND，在模擬完成後，我們分析並比較TCP-Reno與DCWA的CWND、Throughput、封包的來回時間(Round Trip Time, RTT)、佇列延遲(Queueing Delay)、封包遺失率(Packet Loss Ratio, PLR)。

TCP-Reno is commonly used in Terrestrial Networks (TN) to adjust the congestion window (CWND). Since in TN, a large percentage of transmission medium are still wires, packet loss are mostly from network congestions. To recover from the congestion, a sender will have to reduce its CWND to decrease the transmission data rate. However, in Non-Terrestrial Networks (NTN), medium are wireless and it requires long-distance transmission. Thus, Bit Error Rate (BER) in NTN are much higher than in TN. If TCP-Reno is used in NTN, a sender may reduce its congestion window unnecessarily, since packet loss could just come from bit errors other than congestion. To solve this problem, we propose a Dynamic TCP Congestion Window Adjustment (DCWA) scheme on NTN, DCWA consists of three parts. In the first part, a satellite calculates its Forwarding Bit Rate (FBR). In the second part, a receiver can determine the cause of packet loss based on the Cyclic Redundancy Check (CRC) in data link layer and the sequence number in TCP Layer. In the third part, the sender dynamically adjusts CWND based on the cause of packet loss. If packet loss is due to bit errors, a sender maintains or slightly increase CWND. If packet loss is due to congestion, a sender slightly increases CWND. If packet loss arises from bit errors and network congestion, a sender reduces CWND significantly. To demonstrate the improvement of DCWA on throughput, we perform simulation written in C. The simulation program has three major parts. The first part generates network congestion and the ratio of bit errors. The second part determines the cause of packet loss at a receiver. The third part adjusts CWND based on the cause of packet loss. In the simulation, we compare the variations of CWND, throughput, round-trip time, queuing delay, and packet loss ratio between TCP-Reno and the proposed DCWA.

論文審定書..................................................................................................................... i
致謝................................................................................................................................ ii
摘要.............................................................................................................................. iii
Abstract ........................................................................................................................ iv
目錄................................................................................................................................ v
圖目錄.......................................................................................................................... vii
表目錄........................................................................................................................... ix
第一章 導論.................................................................................................................. 1
1.1 研究動機......................................................................................................... 1
1.2 研究方法......................................................................................................... 1
1.3 章節介紹......................................................................................................... 2
第二章 非地面網路的通訊協定.................................................................................. 3
2.1 衛星與地面傳收站......................................................................................... 3
2.1.1 NTN衛星的種類 ................................................................................. 3
2.1.2 地面傳收站的傳送與接收.................................................................. 4
2.2 非地面網路的實體層與資料鏈結層............................................................. 5
2.2.1 實體層.................................................................................................. 5
2.2.2 資料鏈結層.......................................................................................... 6
2.3 地面網路的傳輸層......................................................................................... 7
2.3.1 傳輸層控制協定.................................................................................. 7
2.3.2 TCP的壅塞控制 .................................................................................. 8
2.3.3 RTT ..................................................................................................... 10
2.4衛星緩衝器的封包累積................................................................................ 11
2.5 相關研究....................................................................................................... 12
第三章 動態調整TCP的壅塞視窗 .......................................................................... 15
3.1 低軌道衛星的網路拓樸............................................................................... 17
vi

3.2 LEO計算轉傳位元率 ................................................................................... 18
3.2.1 FBR的計算 ........................................................................................ 18
3.2.2 LEO的流程 ........................................................................................ 19
3.3 Server判斷TCP片段的遺失 ....................................................................... 20
3.3.1 TCP片段遺失的原因 ........................................................................ 20
3.3.2 Server的流程 ..................................................................................... 22
3.4 UE動態調整壅塞視窗 ................................................................................. 23
3.4.1 壅塞視窗的計算................................................................................ 23
3.4.2 UE的流程 .......................................................................................... 26
第四章 模擬結果與分析............................................................................................ 28
4.1 模擬拓樸....................................................................................................... 28
4.2 模擬程式的虛擬碼....................................................................................... 29
4.2.1 UE的虛擬碼 ...................................................................................... 29
4.2.2 LEO的虛擬碼 .................................................................................... 39
4.2.3 MOD Server的虛擬碼 ....................................................................... 46
4.3模擬結果與分析............................................................................................ 53
4.3.1 沒有背景資料流................................................................................ 55
4.3.2 有背景資料流時................................................................................ 58
第五章 結論與未來工作............................................................................................ 65
5.1 結論............................................................................................................... 65
5.2模擬遭遇的困難............................................................................................ 66
5.3 未來工作....................................................................................................... 66
Reference ..................................................................................................................... 67
Acronyms ..................................................................................................................... 72
Index ............................................................................................................................ 75

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