Page 2 Lecture Notes in Computer Science 2865 Edited by G. Goos ...

46 L. Qin and T. KunzFig. 5. Average Hop Counts (left) and Average Packet Delay (right) for Different Beta ValuesDifferent mobility patterns do not show an apparent impact on the packet deliveryratio. The shadowing effect is the main cause for the reduced PDR. For DSR, theaverage improvement for PDR is at least 31 percentage points and 24 percentagepoints for AODV. For total number of control messages, the new threshold can reduceat least 77% for DSR, and 65% for AODVFor different mobility patterns and different β values, AODV has higher packet deliveryratio than DSR with and without the new threshold. Part of the reason is that inDSR a node can reply to a Route Request from its route cache. Though the newthreshold has blocked the weak links, the rest of the route still cannot be verified.Under a shadowing model, the cached routes are more likely out of date.When the new threshold is applied, the route reply ratio decreases rapidly, especiallyin DSR, as low as one tenth of original DSR’s. This means that a number of weaklinks have been blocked. Also, DSR has higher route reply ratio than AODV, becausein DSR nodes are set in promiscuous mode, so that they can copy the routes from thepackets to their route caches.The average hop count increases faster in AODV than in DSR with the newthreshold, but its average packet latency is comparable to DSR. This is because ofDSR’s salvaging mechanism. A node might try several times to send a packet to thenext hop, if it failed, then it will try another route in its route cache to send the packet.The packet delivery rate decreases with increasing β value, which means shortertransmission range. A packet requires more hops to reach the destination and it hashigher possibility to drop because of the power fluctuation.5 Conclusions and Future WorkWith a shadowing model, the signal strength will fluctuate by several order of magnitudes,so the links appear to become unstable. The consequence is decreased packetdelivery ratio, increased control messages and longer packet latency. In this paper, wepropose a new signal strength threshold applied in the Route Discovery process toselect reliable links, then the route can resist to fluctuation caused by shadowing effect.Also unnecessary control messages are eliminated for DSR. After these modificationsto DSR and AODV, in most cases the packet delivery ratio significantly increasesand the number of control messages and packet latency are considerablyreduced.