在適應性無線系統提供TCP服務之跨層暫存器管理技術

Abstract

在下世代的無線通訊系統中，透過無線通道來傳送TCP的資料流是一個重要的議題，同時，也造成了很多挑戰。大部分的無線網路根據無線通道情形調整傳輸速率。雖然調整傳輸速率可以增加頻寬使用率，但在傳送端沒被通知無線頻寬變化的情形下，卻會也造成的一些擁塞的情形。為改善這種問題我們提出兩種方案。 在第一種方案中，我們首先分析實體層跟TCP暫存器容量的關係。然後我們得到一個關於暫存器容量、傳送速率與延遲的方程式。根據這個方程式，我們提出了一個TCP與實體層跨層的暫存器控制方法。從模擬的結果證明我們提出的暫存器管理方法可以彈性調整暫存器來權衡傳送速率與延遲的需求。 在第二種方案中，我們提出一個快速的估測可允許傳輸速率(FATRE)的方法。FATRE應用於TCP層來改善應用層在適應性無線系統中的表現。使用這一個方法可以快速的通知可允許的傳輸速率給應用層進而改善頻寬使用率與擁塞資料量。在這個方案中，我們首先推導出一個關於頻寬使用率、多餘傳送位元、封包大小與通道容積的方程式。模擬結果應證推導出的方程式的正確性，並且證明在適應性無線系統中提出的FATRE方法可以改善頻寬使用率與擁塞資料量。

Delivering transmission control protocol (TCP) traffic through a wireless channel is crucial for the next generation wireless systems and also poses many challenges. Most wireless networks adapt the transmission rates according to radio channel conditions to improve channel utilization. However, it will cause congestion if the sender is not notified with the rate change in the wireless channel. In this thesis, we proposed two methods to overcome this issue. In the first method, we suggest a TCP-PHY cross-layer buffer management mechanism based on a newly derived closed-form expression for the average throughput in terms of average number of buffered segments, queue delay and channel capacity. The derived analytical model considers the factors of bandwidth in the physical layer (PHY) and the queue capacity in the TCP layer. Our simulation results demonstrate that the proposed equation-based bandwidth-aware buffer management scheme can provide flexibility to performance tradeoff between throughput, delay and jitter in rate adaptive wireless systems when supporting TCP traffic. In the second method, we propose a fast allowable transmission rate estimation (FATRE) mechanism in the TCP layer to adjust the source rate in the application layer for rate adaptive wireless systems. The proposed FATRE mechanism can quickly notify the allowable transmission rate to the source. In this scheme, we first derive closed-form expression for bandwidth utilization and the congestion data over the link capacity in terms of packet size and channel capacity. We validate analytic results by simulations and show that the FATRE mechanism can increase bandwidth utilization and decrease congested data.