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

Delivering Messages in Disconnected Mobile Ad Hoc Networks 79various army camps. In our model we have five such hot-spots randomly distributedin a space of 3000m x 600m. Each hot-spot is a circular disc of radius250m. Each node is initially placed at a random position in a randomly selectedhot-spot. Nodes then, instead of selecting a destination uniformly from the wholearea, select a random position within a randomly selected hot-spot. There is alsoa small probability (10%) of choosing a destination which is not in a hot spot.4.2 ResultsWe compare the results from our protocol, Voilà, with those obtained from twoother schemes, the Source-only scheme and the Oracle scheme. In the Sourceonlyscheme, the source node buffers the messages that could not be deliveredto a disconnected host. It tries to find a route to the destinations of bufferedmessages once every FIND DST INTERVAL. In the Oracle scheme every nodeis omniscent, i.e., it has the knowledge of the present and future trajectoriesof all the nodes in the network and hence can choose the optimal intermediatenode to send the message to, if any such node exists. When a node wants tosend an message to another node and cannot find a route to the destination, therouting algorithm makes an upcall to one of the three protocols. As the numberof nodes increases from 30 to 120, the fraction of messages whose destination isdisconnected from the sender decreases approximately linearly from 91% to 62%.Figure 2 shows the fraction of upcalled messages successfully delivered to thedestinations in the three schemes as the number of nodes is varied (normalizedto those delivered by the Oracle scheme). In this experiment, FIND DST INT-ERVAL is 30 seconds and MAX HOLD TIME is 65 seconds. Voilà performsbetter than the Source-only protocol all the time except when the number isnodes is 120. We believe that this is because the network traffic in Voilà is muchhigher than in the Source-only protocol. The traffic increases as the number ofnodes increases, causing congestion in the network. We are working on variousapproaches to reduce the congestion when the number of nodes is large.Figure 3 shows the fraction of upcalled messages delivered to the destination(normalized to those delivered by the Oracle scheme) as MAX HO-LD TIME is varied from 35 to 155 seconds in a scenario of 70 nodes. Thevalue of FIND DST INTERVAL is set at 30 seconds for this experiment. AsMAX HOLD TIME increases, the probability of the source getting connectedto the destination within MAX HOLD TIME also increases and hence we cansee the rising bars for the Source-only scheme. We would expect Voilà to showsimilar behaviour but the increased network traffic arising from long HOLDMSGqueues at high MAX HOLD TIME causes important packets to get dropped andhence we see a small decrease in perfomance of Voilà asMAXHOLD TIME increases.Figure 4 shows the fraction of upcalled messages delivered to the destinationas FIND DST INTERVAL is varied from 30 to 150 seconds in a scenarioof 70 nodes. The MAX HOLD TIME is calculated according to the formula ((2* FIND DST INTERVAL) + 5) for this experiment. We see an expected drop inthe number of upcalled messages delivered to the destination as FIND DST INT-