Rate Control and Scheduling in Wireless Networks

Rate control is one of the most important problems in computer networks. If input rates to a network are left uncontrolled, then the offered load may exceed the network capacity causing congestion, i.e. buffers fill up and packets have to be dropped. It is well known that network congestion has a negative impact on performance, potentially leading to a throughput collapse. Rate control schemes should therefore (a) produce a predictable throughput, (b) allow a distributed implementation and (c) allocate network bandwidth among applications (flows) in a fair manner.

Rate control schemes with such important properties have already been proposed for point-to-point networks. To realize end-to-end congestion control in the current Internet, compatible rate control schemes for both point-to-point (wide area) networks and local area networks become necessary and therefore greatly desired. To this end, we consider rate control schemes that have similar properties for contention-based local area networks. Such schemes are thus widely applicable to members of this class of networks; from slotted Aloha to CSMA/CA.

In addition to rate control, we are also intersted in distributed scheduling policies for wireless networks. A key objective of these distributed policies is to achieve any throughput within the capacity region of the network. In this project we focus on Carrier Sense Multiple Access (CSMA) schedulers where nodes sense whether the channel is idle before making an attempt to transmit a packet. For networks single-hop networks where all nodes are within transmission range of each other, it is well-known that CSMA schedulers achieve network capacity as the sensing time becomes negligible (relative to the packet transmission durations). However, the analysis of the CSMA schedulers for general networks is complicated by the fact that the sizes of the idle and transmission durations are not equal which leads to an asynchronous operation of the network. In our research project, we developped a systematic framework for the analysis and design of CSMA schedulers for networks with primary interference constraints. Our results rely on a fixed point approximation, called the CSMA fixed point, to characterize the service rates of CSMA schedulers in networks with primary interference constraints. This formulation relies on an independence assumption which was also used in the analysis of loss networks. We show that under some assumptions the fixed point approximation is asymptotically accurate for large networks with a small sensing time. This is the first time that it has been shown that CSMA-type of random access policies are throughput optimal for wireless networks with primary intereference constraints.