Wireless Ad Hoc and Sensor Networks

364 Wireless Ad Hoc and Sensor Networkslink is computed by each node. This information is treated as the link costand, along with the energy level of the neighbors, it is recorded in theneighbor table for the one-hop neighbors. Moreover, the neighbors of theHELLO message originator node (one-hop neighbor) are recorded as twohopneighbors along with the costs of their links.Therefore, after receiving the HELLO messages from all the neighbors,each node will have the following information about the one-hop andtwo-hop topology in the neighbor table: link costs to all the one-hopneighbors, energy levels of the one-hop neighbors, lists of the two-hopneighbors that can be reached from each of the one-hop neighbors, andtheir link costs.8.3.2 Multipoint Relay (MPR) SelectionThe criteria for MPR selection in the OEDR protocol are to minimize theenergy-delay cost to reach the two-hop neighbors and to consider theenergy level of the one-hop nodes to increase their lifetime. The proposedOEDR protocol uses the following algorithm to select the MPR nodes(flowchart for the algorithm is presented in Figure 8.3).MPR Selection algorithm can be detailed as follows:1. Start with an empty MPR set MPR() s of node s .2. First identify those two-hop neighbor nodes in N2 () s which haveonly one neighbor in the one-hop neighbor set Ns (). Add thesenodes of Ns () to the MPR set MPR()s if they are not alreadyin MPR()s .3. If there exists a node in N2 () s for which MPR node is not selected,do the following:• For each node in N2 () s , with multiple neighbors from Ns (),select a neighbor from Ns () as MPR node which results inminimum cost from s to the node in N 2 MPR()C s , sNs N2 , according, ( ), ( s )to Equation (8.1).• Add that node of Ns () in MPR() s if it is not already in MPR()s .The above MPR selection algorithm aims at increasing the lifetime of theone-hop neighbors and reducing the delay, besides improving the energyefficiency.The lifetime of the nodes is very critical in certain applications likesensor networks (Yee and Kumar 2003), which have severe energy limitations.The subsequent example illustrates the MPR selection algorithm.Example 8.3.1: MPR SelectionConsider the neighbor topology of node s shown in Figure 8.2. The linkcosts and the energy levels of the one-hop neighbors are also shown in