Wireless Ad Hoc and Sensor Networks

Distributed Fair Scheduling in Wireless Ad Hoc and Sensor Networks 345the next subsection. Next, when the packet is received at an intermediatenode, the packet’s weight is updated and queued accordingly.7.6.2.2 Protocol ImplementationTo achieve fairness at the scheduling level, the proposed ADFS protocolimplements the SFQ scheme, defined as follows:1. On arrival, a packet of flow f , is stamped with start tag ( ) ,defined asp fj{ } ≥( f )= ( ( f )) ( f )Sp j vAp j Fp j−1max , , j 1,jwhere Fp ( f ), finish tag of packet p j jf , is defined as Fp ( f )=Sp ( jf ) jl+f , j≥1, φ f0where Fp ( f ) = 0 and φ f is the weight of flow f .2. Initially, the virtual time of the sensor node is set to zero. Duringtransmission, the node’s virtual time at time t , vt () is defined tobe equal to the start tag of the packet being transmitted at time t.At the end of a transmission, vt () is set to the maximum of finishtag assigned to any packets that have been transmitted by time t.3. Packets are transmitted in the increasing order of the start tags;ties are broken arbitrarily.7.6.2.3 Dynamic Weight Adaptation and Backoff Interval CalculationTo account for the changing traffic and channel conditions that affect thefairness and end-to-end delay, the weights for the flows are updateddynamically. The ADFS, which is detailed in Section 7.1 to Section 7.4,calculates the backoff interval by using the packet weight and updatedat each node due to weight adaptation.7.6.2.4 Distributed Power Control and ResetThe basic communication used to send a user packet requires a four-wayhandshake exchange which consists of four frames:1. Request-to-send (RTS) — from source to destination2. Clear-to-send (CTS) — from destination to source3. DATA frame — from source to destination4. Acknowledgment (ACK) — from destination to sourceSp fjAll these frames are transmitted over the single radio channel. Hence, thecommunication between the nodes is carried over shared half duplex medium.