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

80 R. Shah and N.C. HutchinsonOracle Voilà Source-onlyOracle Voilà Source-only10.80.60.40.2030 50 70 100 120Number of nodes10.80.60.40.2035 65 95 125 155MAX_HOLD_TIME ( in seconds )Fig. 2. Fraction of messages delivered vs.number of nodesFig. 3. Fraction of messages delivered vs.MAX HOLD TIMEOracle Voilà Source-onlyOracle Voilà Source-only10.80.60.40.2030 60 90 120 150FIND_DST_INTERVAL ( in seconds )10.80.60.40.20500 525 550 575 600 625 650Distance in metersFig. 4. Fraction of messages delivered vs.FIND DST INTERVALFig. 5. Fraction of messages delivered vs.hot-spot separationERVAL increases. As in Figure 3, Voilà performs better than the Source-onlyprotocol for all parameter values.Any scheme for delivering messages across disconnected networks is sensitiveto the sizes of the hot-spots and their relative positions. The size of a hot-spotdetermines the diameter of the network partition in the hot-spot and hence thenumber of nodes selected to hold a message. The relative positions of hot-spotsdetermine how connected they are under the movement of nodes and hence howmuch a scheme like ours is warranted. It is easy to see that any such scheme is notuseful for hot-spots that are completely connected at all times. To determine thesensitivity of the protocols to the positions of hot-spots we experimented witha scenario of 30 nodes and three circular hot-spots. The hot-spots are arrangedsuch that their centers lie in a straight line. The centers of the adjacent hotspotsare separated by a distance which is varied from 500m to 650m. Since theradius of each hot-spot is 250m, the circumferences of adjacent hot-spots toucheach other when distance is 500m. The number of messages whose destinationsare not connected to the source increases as the distance between the hot-spotsincreases, from 27% at 500m to 38% at 650m. Figure 5 compares the performaceof the three schemes.