Chapter 4 - Congestion Control in Broadband Wireless Networks

Abstract

This chapter discusses congestion control in broadband wireless networks. This work was motivated by finding solutions to the performance problems that TCP Reno has with wireless links. Because the algorithm cannot differentiate between congestion losses and losses caused by link errors, decreasing the window size whenever a packet is lost has a very detrimental effect on performance. This chapter describes TCP Westwood, which solved this problem by keeping an estimate of the data rate of the connection at the bottleneck node. Thus, when Duplicate ACKs are received, the sender reduces its window size to the transmit rate at the bottleneck node (rather than blindly cutting the window size by half, as in TCP Reno). This works well for wireless links because if the packet was lost because of wireless errors, then the bottleneck rate my still be high, and this is reflected in the new window size. This chapter also describes techniques, such as Split-Connection TCP and Loss Discrimination Algorithms, that are widely used in wireless networks today. The combination of TCP at the end-to-end transport layer and Link Layer Retransmissions (Automatic Retransmissions [ARQ]) or Forward Error Correction (FEC) coding on the wireless link is also analyzed using the results from Chapter 2. The large variation in link capacity that is observed in cellular networks, has led to problem called bufferbloat. We describe techniques using Active Queue Management at the nodes as well as end-to-end algorithms to solve this problem.