Wireless Ad Hoc and Sensor Networks

Optimized Energy and Delay-Based Routing 361advantage of reduced latency in route discovery due to its proactivenature. OLSR is an optimization over the classical link state algorithmthat is tailored for mobile ad hoc networks. The important concept introducedin the protocol is the multipoint relays (Qayyum et al. 2002) whereonly selected nodes, referred to as RNs or MPRs, forward the broadcastmessages during the flooding process. The purpose of MPR nodes is tominimize the overhead of flooding messages in the network by reducingthe number of duplicate retransmissions while forwarding a broadcastpacket. According to MPR selection criteria presented in Qayyum et al.(2003) and Clausen and Jacquet (2003), each node in the network selectsa set of nodes from its one-hop neighbors as MPRs. These nodes are chosensuch that they reach the maximum number of uncovered two-hop neighborsuntil all of them are covered.The OLSR protocol uses HELLO messages for neighbor sensing and TCmessages to declare the MPR information. MPR nodes periodicallyannounce information about the neighbors that have selected it as anMPR, by broadcasting a TC message. In contrast to the classic link statealgorithm, only a small subset of links between neighbors (only the MPRselectors) is declared and, therefore, the overhead due to the control messagesis reduced. Upon receiving the TC message, each node in the networkstores the information in the topology table, and only the MPR nodesforward the TC messages to the next-hop neighbors until all the nodes inthe network receive the message. Whenever the topology table changes,routing table entries are calculated at each node, using a shortest pathalgorithm (number of hops is the metric used in OLSR) to determine theroutes to all the destinations.An example of the MPR selection algorithm used by the OLSR protocolis shown in Figure 8.1. Consider a node s with one-hop neighbors:p 2p 3p 1p 4n 1 n2 p 5MPRsp 6sn 3n 5n 4p 7p 8FIGURE 8.1MPR selection using OLSR protocol.