Switching Strategies

We implemented three switching strategies described in Section 5.3, i.e. Fixed Priority, Adaptive Buffer, and Adaptive Traffic. The test environment had a MultiNet node connected to an infrastructure and an ad hoc network. The time to switch to the ad hoc and infrastructure networks were overestimated at 500ms and 300ms respectively. The total time available for switching between networks was 1 sec. We evaluated the switching strategies when simultaneously transferring a file of size 47 MB using FTP from the MultiNet node to two nodes on the different networks. An independent transfer of the file over the ad hoc network took 80.25 seconds, while it took 54.12 seconds over the infrastructure network.

Figure 4 shows the time taken to simultaneously transfer this file over MultiNet using different switching strategies for legacy cards. We evaluated 3 different Fixed Priority switching schemes. In the `50%IS 50%AH' strategy the node stays on each network for 500ms. In the `75%IS 25%AH' scheme it stays on the infrastructure network for 750ms and on the ad hoc network for 250ms, and in the `25%IS 75%AH' scheme the node stays on the infrastructure network for 250 ms and the ad hoc network for 750ms. For the Adaptive Traffic algorithm we used a window of 3 switching cycles to estimate the Activity Periods. In this case the window is 3*1.8 = 5.4 seconds since a switching cycle is 500+300+1000 = 1800 ms.

Figure 4: Amount of time taken to complete the FTP transfer of a file on an ad hoc and infrastructure network for different switching strategies

Different switching strategies show different behavior and each of them might be useful for different scenarios. For the fixed switching strategies the network with higher priority gets a larger slot to remain active. Therefore, the network with a higher priority takes lesser time to complete the FTP transfer. The results of the Adaptive algorithms are similar. The Adaptive Buffer algorithm adjusts the time it stays on a network based on the number of packets buffered for that network. Since the maximum throughput on an infrastructure network is more than the throughput of an ad hoc network, the number of packets buffered for the infrastructure network is more. Therefore the FTP transfer completes faster over the infrastructure network as compared to the `50%IS 50%AH' case. For a similar reason the FTP transfer over the infrastructure network completes faster when using Adaptive Traffic switching. MultiNet sees much more traffic sent over the infrastructure network and proportionally gives more time to it. Overall, the adaptive strategies work by giving more time to faster networks if there is maximum activity over all the networks. However, if some networks are more active than the others, then the active networks get more time. We expect these adaptive strategies to give the best performance if the user has no priority and wants to achieve the best performance over all the MultiNet networks.