Simulation Environment and results

We use NS-2 [9], an event-driven simulator, as our simulation tool. The simulation network contained 100 nodes placed randomly in a map of L×L units, where a unit is the length of communication radius set to 250 meters. The random waypoint model for mobility patterns was adopted. The topologies were generated randomly by the “setdest” program supported by NS-2. The moving speed was randomly distributed from 0 to 20 (m/s), and the pause time was set to 0. The MAC layer was constructed using the IEEE 802.11 standard, which is implemented in NS-2.

Broadcast data packets are gathered from the broadcast requests in 200 second simulation time. Hello packets are sent periodically during the entire simulation period. The simulation results were averaged by the results of 15 simulation runs. The detailed parameters are summarized in Table I.

Table1. Simulation parameters

We consider the following performance metrics：

．Coverage – the percentage of nodes which can be covered as compared with blind

flooding

．Energy consumption – the total energy consumed by hello and broadcast packets in Joule.

．Collision – the average number of collisions per transmitted packet

．Hello packet number – total number of hello packets generated during entire simulation period

To achieve more saving in the hello packets than effective hello period (EHP), we further enhance EHP by sending hello packets only if no broadcast data packets transmitted during the effective hello period instead of sending hello packet at end of each effective hello period, We refer to it as enhanced effective hello period (E_EHP) in the following simulations. We simulate 1 second, 3 second, and 7 second of hello periods with 25, 50, and 100 broadcast requests. The effective hello periods are summarized in Table II.

Table 2. The effective period used in simulation effective_period

Figures 8, 9, and 10 illustrate the coverage and total number of hello packets under different broadcast requests with hello periods equal 1, 3 and 7 seconds. The bar charts show the coverage performance, while the line charts indicate the percentage of hello packet number used by EHP and E_EHP compared with original hello period. Periodical hello sent fixed number of hello packets, the actual numbers are 20000 for 1 second, 6700 for 3 second and 2900 for 7 second during 200 second simulation time. Compared with EHP and E_EHP, it has the most accurate 1-hop neighbor list, so the coverage performance is the best, as we can observe in the first bar in these figures. The EHP we proposed performs compatible with periodical hello.

There is only about 2% degradation in coverage performance. However, compared with total savings in hello packets, in Figure 8, when the number of requests number is 25, the EHP can only save about 8% of hello packets, and this is because the piggybacked hello packets are fewer. However, when number of requests is 100, the saving can reach 32%. In Figure 10, the E_EHP can save even more hello packets as 88%, 93% and 96% with only about 5% degradation of coverage performance.

Figure 8. Coverage and hello number on 1s period

Figure 9. Coverage and hello number on 3s period

Figure 10. Coverage and hello number on 7s period

Figures 11, 12, and 13 illustrate energy consumption and number of collision under different hello/data packet size ratios. The bar charts show the energy consumption, while the line charts indicate the collision number. The number of broadcast requests is fixed to 50. We change the hello and data packet size ratio by fixing the size of hello packet and adjusting size of broadcast data packet. The size of hello packet is 40 bytes and the ratios of hello and data packet size are 1/2, 1/4, and 1/8 respectively. When the ratio decreases, (the data packet size increases) the energy consumption increases. Since the objective of our effective hello period is to reduce the number of hello packets, the total amount of energy consumption will decrease as well. In Figure 11, when the broadcast requests is 25, since the reduction of hello packets is fewer, the saving of energy is lower. With the growing of the number of broadcast requests, the savings incline. Figures 12 and 13 indicate

similar saving trend.

The reduction of hello packet also influences the number of collisions directly, since the contenders for the communication channel are less. When the ratios of hello and data packet size become smaller (the size of data packet become larger), the transmission time of data packets becomes longer. Therefore, the collision probability rises. Hence, as shown in Figure 11, 12 and 13, the number of collision tends to increase with the growth of data packet size. However, the average collision per transmitted packet is lower when the period is shorter. This is because of total number of transmitted packets are much more in short hello period case.

However, short hello period still causes the largest number of actual collisions.

Figure 11. Energy and collision performance on 1s period

Figure 12. Energy and collision performance on 3s period

Figure 14 and 15 shows the energy consumption of effective hello period under different broadcast requests in 5x5 and 7x7 maps. We can observe that the energy consumptions are less in EFP and E_EFP compared with flooding in most cases.

Only under such a circumstance the use of neighbor knowledge broadcast schemes do make sense. Notice that the energy consumption in 5x5 is larger than in 7x7. This is because the average number of neighbors in 5x5 map are more than in 7x7, and thus 5x5 map consumes more receiving power.

Figure 13. Energy and collision performance on 7s period

Figure 14. Comparison of energy consumption on different broadcast requests under 5x5 map

Figure 15. Comparison of energy consumption on different broadcast requests under 7x7 map

Chapter4: Conclusion

In the first part, we analyze the probability that a node leaves its original transmission range. According to this model, we can deduce the link change rate that the variation of the neighbor of a node. Lastly, we discuss the proper transmission period of hello packets under certain QoS constraint.

Second, we analyzed the most frequently used topology information—1-hop neighbor list in wireless MANETs broadcast schemes. This topology information can be obtained by periodical hello packets. Many researchers consider the overhead derived from hello packets is negligible, since the size of a hello packet is small. We believe that it is certainly the most popular delusion about hello packets. It results in abuse usage of hello packets. This situation leads to extra energy consumption and collisions. We showed that the overhead can be significant through our quantification process. We further proposed the effective hello period to reduce the hello packet overheads. From the simulation results, we can show that our proposed method can reduce the overhead notably while still maintaining high coverage performance. We believe this analysis is likely to be essential for a real world MANET implementation.

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