Wireless Ad Hoc and Sensor Networks

Distributed Fair Scheduling in Wireless Ad Hoc and Sensor Networks 309where e ij, queue is the error between the expected length of the queue andthe actual size of the queue, and e ij, delay is the error between the expecteddelay and the delay experienced by the packet so far. Note that the E ijvalue is bounded because of finite queue length and delay, as packetsexperiencing delay greater than the delay error limit will be dropped.To calculate the backoff interval and to implement the schedulingscheme, the updated weights at each node have to be transmitted in thedata frame of the MAC protocol. To enable this, changes are made to thedata packet header to accommodate the current weight of the packet.Whenever a packet is received, the current weight is used to update theweights dynamically using Equation 7.9. Then the weight field in thepacket header is replaced with the updated weight.7.3.2.2 MAC Protocol — Dynamic Backoff IntervalsThe proposed ADFS protocol follows the CSMA/CA paradigm, similarto the IEEE 802.11 protocol. Because multiple nodes in a wireless networktry to transmit simultaneously, when nodes compete to access theshared channel as in Figure 7.2, the selection of the backoff interval playsa critical role in deciding which node gets access to the channel. Toachieve global fairness, the nodes must access the channel in a fairmanner.The proposed ADFS is implemented as the MAC protocol to control theaccess of the shared medium by the wireless nodes, by adjusting thedynamic backoff intervals. ADFS calculates the backoff interval relativeto the weight of the packet. The backoff procedure is similar to DFS(Vaidya et al. 2000). However, the weights are updated via Equation 7.9;Carrier sensing rangeAB C D ETransmission rangeFIGURE 7.2Nodes contending for shared wireless channel.