TCP (Transmission Control Protocol)大部份使用在可靠的傳輸協定，主要是針對有線網路量身設計，有線網路的隨機位元錯誤率(bit error rate ; BER)是可以被忽略，封包遺失的原因主要是因擁塞所造成。不像有線網路，無線網路因較高的位元錯誤率所造成的隨機封包遺失是不可以被忽視。現有標準的TCP方法，如Reno皆假設封包的遺失原因為擁塞所造成，它並沒有任何機制去判斷封包遺失的原因是無線鏈路錯誤還是擁塞所造成，結果將會不正確且過度頻繁的去觸發TCP Reno擁塞控制，進而導致TCP效能的降低。
本論文中，我們提出一個以封包來回時間為基礎之擁塞控制(round trip time based congestion control；RCC)機制，來改善TCP在有線無線鏈路之異質網路上的效能。RCC利用封包來回時間(round trip time ; RTT)的變化，去判斷封包遺失的原因為擁塞或隨機造成，然後去調整擁塞視窗以適應無線環境。當擁塞遺失發生，傳送端為了避免擁塞而須減少擁塞視窗；當隨機遺失發生，傳送端則須遞增擁塞視窗藉以提高網路頻寬的利用率。透過模擬的方式，將我們所提出的方法與現有TCP方法互相比較，從模擬結果顯示，RCC確實能得到較佳的效能在異質網路上。

Transmission Control Protocol (TCP), the most widely used reliable transport protocol, was designed mainly for the wired networks. In the wired network, random bit error rate (BER) is negligible and congestion is the main cause of packet loss. Unlike wired networks, random packet loss due to higher bit error rate is not negligible in wireless networks. The current standard TCP scheme, such as Reno, assumes that packet losses are caused by congestions, and it has no mechanism to differentiate packet losses due to wireless link errors form those due to congestions. As a result, frequent TCP Reno congestion controls are incorrectly triggered and lead to serve TCP performance degradation.
In this paper, we propose a round trip time (RTT) based congestion control (RCC) mechanism to improve TCP performance over heterogeneous networks with wired and wireless links. RCC utilizes RTT variation to distinguish the loss caused by congestion or random loss, and then adjusts the congestion window to adapt to the wireless environment. When congestion loss occurs, sender with RCC reduces congestion windows in order to avoid network congestion. When random loss occurs, it keeps increasing congestion windows in order to enhance network bandwidth utilization. By way of simulating, RCC compares with existing TCP schemes. The simulation results show that it can achieve much better performance over the heterogeneous network.

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