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

On-demand Routing in MANETs 41(1) Under an ideal model, most packets are dropped because a target node is out oftransmission range. With a shadowing model, more packets are dropped because theInterface Queue is full. The routes found during a Route Discovery process are notvalid because of power fluctuation. These invalid routes will trigger a new RouteDiscovery process, which significantly increases the number of control messages,which have higher priority in the interface queue than data packets. Then data packetshave less chance to be sent out. When more new data packets are coming, packetshave to be dropped.(2) Because of power fluctuation, a packet may need multiple retransmission to beforwarded to the next node. Sometimes, the receiver actually received the packet, butthe transmitter did not receive an ACK from the receiver. So the transmitter assumesthis link is broken. In DSR, the transmitter will salvage this packet if it can find anotherroute to the destination from its route cache. Each salvaging introduces oneRoute Error message to be sent to the source node. But the previous receiver stilltransmits this packet based on the source route in the packet, so the same packet maytake several paths to the destination. It will consume network resources and increasepacket latency.(3) A packet may take more or less hop counts than the “optimal” number as determinedby General Operations Director in NS2, which is based on the ideal environment,with different β values. For example, when β=2.0, the mean transmission rangewill be much longer than 250 m, as listed in Table 2. Even if the shortest path needs 3hops based on the ideal environment, sometimes the source can send the packet to thedestination directly. Especially for DSR, there are more paths shorter than the “shortest”paths pre-calculated in NS2 because of shortening mechanism. So for β=2.0, thePDR of DSR could be higher than the ideal model. On the other hand, the signalpower fluctuation can cause a high rate of transmission failures and bigger hop countsthan under an ideal model.(4) In DSR, there exist Route Request messages that are not sent by the source of datapackets. It happens when a node wants to send a Route Reply or Error message andfinds that the source route is not valid and also it cannot find any route in its routecache. Another source of protocol overhead is due to extra Route Error messages.When Route Error and Route Reply messages are salvaged, a Route Error message issent to the source node.3.3 Improving Routing Performance under a Shadowing ModelFrom the last section, we can see that both routing protocols have low PDR and longpacket latency, caused by power fluctuation, in shadowing model. The performance isnot acceptable in real applications. In this section, we present some proposals to increasethe PDR based on our studies. However, before we discuss the specific proposalsand results, we need to address another issue first: how to fairly compare resultsfor different β values, which determine the average transmission range and thereforeaverage network connectivity.3.3.1 Mean Transmission Range and Required Node DensityDifferent β values correspond to different average transmission ranges; we can changeEquation (3) to: