Praise for Fundamentals of WiMAX

262 Chapter 7 • Networking and Services Aspects of Broadband Wireless50Congestion Window (# of packets)454035302520151050Timeout leads to drop intransmission rate.Slows-StartThreshold0 10 20 30 40 50Round-Trip TimeTimeoutSlow-StartThresholdFigure 7.17 TCP congestion controlonly every round-trip time. When wireless networks have large latencies, this also leads tothroughput reduction. Large latencies coupled with high data rates can also mean that at a giventime, large amount of data is in transit. This can lead to TCP’s assuming that the receiver buffer isfull and slowing its transmission rate.TCP performance is particularly bad under conditions of burst errors. Fast retransmit andfast recovery improve the throughput of TCP under sporadic random losses only if such lossesoccur only once within an RTT. Consecutive failed attempts will cause the TCP sender to exponentiallyback off its retransmission timer. A loss of a series of packets can therefore cause thetimer to be set very long, leading to long periods of inactivity and underutilization of the availablelink bandwidth [64].Clearly, running TCP over wireless channel leads to unnecessary degradation in throughput,inefficient utilization of scarce resources, and excessive interruptions in data transmissions. Inmobile systems, these problems are exacerbated during handover. Given these problems, a lot ofresearch into methods to improve TCP performance in wireless networks has occurred over thepast decade or so.A number of simple tricks can be used to improve TCP performance in wireless networks.For example, increasing the maximum allowed window size, using selective repeat-ARQ insteadof the go-back-N ARQ for retransmission [23, 39], and using an initial window size larger thanone segment [3] have all been shown to improve TCP performance over wireless links. Althoughthese optimization methods do provide marginal improvements, they do not mitigate all theproblems of TCP in wireless.