An important application for the IEEE 802.16 technology (also called WiMAX) is to provide high-speed access to the Internet where the transmission control protocol (TCP) is the core transport protocol. In this paper we study through extensive simulation scenarios the performance characteristics of five representative TCP schemes, namely, TCP New Reno, Vegas, Veno, Westwood, and BIC, in WiMAX (and WLANs) networks, under the conditions of correlated wireless errors, asymmetric end-to-end capabilities, and link congestion. The target is to evaluate how the above conditions would affect the TCP congestion control and suggest the best schemes to be employed in WiMAX networks. 1. Introduction Broadband wireless access (BWA) is a candidate for the next-generation wireless communication technology. Worldwide interoperability for microwave access (WiMAX) is the organization behind interoperability and testing for the IEEE 802.16 Specifications [2]. For WiMAX to be able to realize the objective of high speed Internet access, it must effectively support the core transport protocol of the Internet that is transmission control protocol (TCP). Standard TCP congestion control is based on the reduction of its congestion window after a packet loss [3]. Although such behavior works fairly well in the wired networks, where packets losses are almost always caused by link congestion, it becomes rather inefficient when used for data transport in WiMAX networks. In the wireless environment, the possible reasons of packet loss include fading, temporary disconnections, and handovers. Even when some losses are compensated in Data Link Layer, a part of them still appears in Transport Layer for high Bit Error Rates (BER). TCP New Reno erroneously translates the causes of packet loss as congestion and consequently reduces its congestion window, thus exhibiting significant throughput degradation in these conditions. Another phenomenon which significantly affects TCP performance is the network asymmetry. A network is assumed to be asymmetric when its characteristics in one direction greatly affect its performance in the other [4]. Asymmetry disrupts the smooth flow of ACK in the reverse direction of traffic and subsequently the TCP ACK clocking mechanism of the TCP sender, causing timer expiration, and subsequent retransmissions, even though the packets may have correctly reached the receiver. In addition, frequent ACK delays result in timeout inflates, thereby causing reduced protocol responsiveness to packet losses. There are several forms of asymmetry, such as bandwidth asymmetry,
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