Performance in Multihop Mobile Networks

We now study the impact of mobility in a network using DSR over IEEE 802.11a and SSCH. In this experiment, we place 100 nodes randomly in a square and select 10 flows. Each node transmits packets at 21 dBm. Node movement is determined using the Random Waypoint model. In this model, each node has a predefined minimum and maximum speed. Nodes select a random point in the simulation area, and move towards it with a speed chosen randomly from the interval. After reaching its destination, a node rests for a period chosen from a uniform distribution between 0 and 10 seconds. It then chooses a new destination and repeats the procedure. In our experiments, we fix the minimum speed at 0.01 m/s and vary the maximum speed from 0.2 to 1.0 m/s. Although we have studied SSCH at higher speeds, the results are not significantly different. We performed this experiment using two different areas for the nodes, a $200m\times 200m$ area and a $300m\times 300m$ area. We refer to the smaller area as the dense network, and the larger area as the sparse network - the average path is 0.5 hops longer in the sparse network. For all these experiments, we set the SSCH broadcast transmission count parameter to 6.

Figure 18: Dense Multihop Mobile Network: The per-flow throughput and the average route length for 10 flows in a 100 node network in a $200m\times 200m$ area, using DSR over both SSCH and IEEE 802.11a.

Figure 19: Sparse Multihop Mobile Network: The per-flow throughput and the average route length for 10 flows in a 100 node network in a $300m\times 300m$ area, using DSR over both SSCH and IEEE 802.11a.

Figure 18 shows that in a dense network, SSCH yields much greater throughput than IEEE 802.11a even when there is mobility. Although DSR discovers shorter routes over IEEE 802.11a, the ability of SSCH to distribute traffic on a greater number of channels leads to much higher overall throughput. Figure 19 evaluates the same benchmarks in a sparse network. The results show that the per-flow throughput decreases in a sparse network for both SSCH and IEEE 802.11a. This is because the route lengths are greater, and it takes more time to repair routes. However, the same qualitative comparison continues to hold: SSCH causes DSR to discover longer routes, but still leads to an overall capacity improvement.

DSR discovers longer routes over SSCH than over IEEE 802.11a because broadcast packets sent over SSCH may not reach a node's entire neighbor set. Furthermore, some optimizations of DSR, such as promiscuous mode operation of nodes, are not as effective in a multi-channel MAC such as SSCH. Thus, although the throughput of mobile nodes using DSR over SSCH is much better than their throughput over IEEE 802.11a, we conclude that a routing protocol that takes the channel switching behavior of SSCH into account will likely lead to even better performance.