Performance study and improvement of TCP in wireless communication networks

Wireless communications have become very pervasive as the number and variety of mobile phones and wireless Internet users have increased significantly in recent years. Third generation (3G) wireless systems are currently being implemented in many countries although different standards co-exist. Transmission Control Protocol (TCP) which has been proven to be a very efficient and reliable transport protocol in wired networks still plays an important role over the 3G wireless cellular networks. However, due to the different characteristics and prerequisites of wireless networks, TCP exhibits inefficient performance in such networks. The main reason is that the assumptions made from wired networks are inapplicable in wireless cellular networks. Therefore, it is crucial to upgrade the TCP performance in wireless networks. Two major factors that degrade TCP performance in 3G wireless networks are packet loss and delay spike which is defined as a sudden long delay usually exceeding TCP round trip time. While the former has been studied extensively in literature, the latter, which is a frequent scenario in wireless cellular networks, has just attracted the focus recently. In this thesis, a systematic performance simulation study of TCP in the presence of the delay spike is performed. This performance evaluation illustrates distorted TCP throughput. First, the responses to a delay spike of various TCP versions, Tahoe, Reno, NewReno and SACK are investigated. The delay spike causes retransmission timeout of all these TCP versions. Accordingly all TCP variants misinterpret the timeout as the congestion of network, retransmit the assumed lost packets and enter the slow start procedure. Since the packets are not lost but delayed, these retransmissions are unnecessary and a slow start is falsely triggered. Consequently, the throughput performances of all these TCP versions are degraded and the available link bandwidth is underutilized. Second, the interaction between the increased initial window of TCP, which is recommended in wireless cellular networks, and the delay spike is investigated. The different propagation delays of wireless link which represent various wireless networks are also examined. The result shows a decrease in throughput of TCP while increasing the initial window size irrespective of propagation delay. The optimized initial window size, consisting of two segments, is suggested in this study. Although there are a lot of factors resulting in delay spike, the focus of this thesis is on one cause - handover. This is because the probability of a handover occurrence during a call is very high in 3G wireless cellular networks. The performance of TCP is then investigated under various mobility models in order to avoid limiting the results on specific features of mobility models. Through extensive simulation study, the adverse effect of delay spikes on TCP is explored. Furthermore, the fairness issue between competition TCP connections is also studied. It is shown that the TCP fairness problem is enlarged under the impact of the delay spike. After quantifying the impact of delay spike on TCP performance, the DS-Agent scheme is proposed in this thesis in order to improve the performance of TCP in the presence of delay spikes. The key idea behind of this scheme is to inform the TCP sender whether the retransmission is from spurious timeout or congestion loss. If the retransmission is from spurious timeout, the sender will stop retransmitting other unacknowledged packets and continue previous transmission, thereby preserving the bandwidth and improving the performance. The proposed algorithm is very simple, easy to implement and does not need to use timestamp option as the TCP Eifel algorithm does. The simulation results demonstrate that our proposed DS-Agent algorithm improves the TCP performance in the presence of the delay spike as compared to the TCP F-RTO and TCP Reno algorithms. By using the DS— Agent scheme, the fairness problem between competition TCP connections is also mended