Wireless Ad Hoc and Sensor Networks

Distributed Fair Scheduling in Wireless Ad Hoc and Sensor Networks 305The main contribution of the proposed scheme is the dynamic adaptationof weights as a function of delay experienced, number of packets in thequeue, and the previous weight of the packet. The initial weights areselected by employing the user-defined QoS. Moreover, weights areupdated for each packet when it reaches the front of the queue. Theupdated weights are used in deciding which packet to place in the outputqueue and also in the calculation of the backoff interval if there is apotential collision.7.2 Weighted Fairness CriterionIn wired and wireless ad hoc networks, the fair scheduling schemes implementthe distributed algorithm to achieve certain local criterion, whichaffects the overall global fairness of the network. Observe a typical nodeshown in Figure 7.1, where it maintains several input queues (belongingto several flows) for storing incoming packets to be transmitted on theoutput link. A fair queuing algorithm is used to determine which flow toserve next. Consider several such nodes trying to access a shared wireless(or wired) channel in CSMA/CA paradigm. The node that has to getaccess to the channel in such a scenario is determined by the backoffinterval at each node.Therefore, a fair scheduling protocol for wired and ad hoc wirelessnetworks should be able to implement the fair queuing algorithm alongwith a fair backoff algorithm to satisfy the required fairness criterion andto achieve global fairness.Intuitively, allocation of output link bandwidth is fair if equal bandwidthis allocated at every time interval to all the flows. This conceptgeneralizes to weighted fairness, in which the bandwidth must be allocatedin proportion to the weights associated with the flows. Formally,if φ f is the weight of flow f and Wf ( t1, t2)is the aggregate service (in bits)Input flowsNodeOutput queueFIGURE 7.1A node with several contending flows sharing a link.