Congestion Control in Wireless Cellular Networks

Through the introduction of the third generation of mobile cellular network technologies a major step towards ubiquitious and wireless access to the Internet has been taken. There are however still challenges due to the different characteristics and prerequisites of wired and wireless networks. An important network element is congestion control. The purpose of congestion control is to ensure network stability and achieve a reasonably fair distribution of the network resources among the users. TCP is a well-established protocol, which offers reliable transport of data and applies congestion control. With regards to TCP it is of interest to follow up on proposed changes to the protocol and to learn how to tune wireless networks for optimal TCP performance, since its usage is wide spread. We have performed a study of buffer management for TCP with High Speed Downlink Packet Access (HSDPA) and evaluated the effect of reducing the lower bound of the retransmit timeout interval in an environement with varying capacity. A number of the features that TCP consists of introduce arbitrarily delay, therefore reliable transport is sometimes traded for less delay variations by applications with strict timing requirements. Until recently UDP has been the main alternative to TCP. UDP does not provide any service guarantees, nor congestion control, but does on the other hand not introduce any delay in itself. There are however concerns that increased usage of UDP would cause network instability and starve the TCP flows that reduce their send rate when competition intensifies. Therefore a new transport protocol called the Datagram Congestion Control Protocol (DCCP) is being designed to provide applications that do not desire the service model of TCP with an alternative. Currently, DCCP includes two profiles for congestion control, TFRC and TCP-like. For these new algorithms verifying the design, identifying weaknesses and suggesting improvements, as I have done, is important in order to drive the development forward. Through the studies that comprise this thesis, I contribute to the stable operation of the future Internet and the merging with wireless cellular data architectures.