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Tham khảo tài liệu 'smart wireless sensor networks part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Advanced Communication Solutions for Reliable Wireless Sensor Systems 19 With the knowledge of the routes each intermediate node can now avoid using next-hop nodes which have higher cost function without increasing the number of hops to the destination. However it is possible that for a given intermediate node all of its next-hop nodes may have very high cost. To cope with this problem a back-propagation mechanism is introduced. The back-propagation logic can be described as follows. If a node sees that all its next hop nodes costs are greater than the given threshold the node will back propagate this update to its preceder so that the preceder is able to give up using this path. Once the RREQ-RREP procedure is completed the source-destination pair and intermediate nodes involved will select a single path amongst all the available local paths. 4.3 Simulation Results We implemented LMNR on ns-2 ns-2 2010 and carried out simulations to see how much gain LMNR achieves compared to AODV in practice. In the simulation scenario 50 nodes which use IEEE 802.11 radios for communications were randomly positioned on a grid. 10 source-destination pairs are randomly selected and each source generates Constant Bit Rate CBR traffic flows with the given packet rate packets second . The used NC metric was based on the size of routing tables and freshness of routes. Simulation setup is explained in Nethi et al. 2007c in detail and some of the results are depicted in Fig. 7. Fig. 7 a compares the performance of the protocols with respect to end to end delay and as we can see our scheme outperforms AODV clearly as traffic loads increase. The reason behind this is that LMNR can always find an optimal path due to the dynamic local next-hop selection mechanism. On the contrary in AODV only one route is established which means that a new route-finding procedure is initiated in case of congestion. This can be also verified by Fig. 7 b which shows the packet delivery ratios of the two .
