Impact of Channel Switching on
MANET Routing Protocols

Previous work on multi-channel MACs has often overlooked the effect of channel switching on routing protocols. Most of the proposed protocols for MANETs, such as DSR [22], and AODV [28] rely heavily on broadcasts. However, neighbors using a multi-channel MAC could be on different channels, which could cause broadcasts to reach significantly fewer neighbors than in a single-channel MAC. SSCH addresses this concern using a broadcast retransmission strategy discussed in Section 3.2.

Figure 17: Impact of SSCH on Unmodified MANET Routing Protocols: The average time to discover a route and the average route length for 10 randomly chosen routes in a 100 node network using DSR over SSCH.

We study the behavior of DSR [22] over SSCH in the same experimental setup used in Section 4.3.2, with 100 nodes in a 200 m$\times$200 m area. However, we reduce the transmission power of each node to 16 dBm to force routes to increase in length (and hence to stress DSR over SSCH). We select 10 source-destination pairs at random, and we use DSR to discover routes between them. In Figure 17 we compare the performance of DSR over SSCH, when varying the SSCH broadcast transmission count parameter (the number of consecutive slots in which each broadcast packet is sent once).

Figure 17 shows that the performance of DSR over SSCH improves with an increase in the broadcast transmission count. The DSR Route Request packets see more neighbors when SSCH broadcasts them over a greater number of slots. This increases the likelihood of discovering shorter routes, and the speed with which routes are discovered. However, there seems to be little additional benefit to increasing the broadcast parameter to a value greater than 6. The slight bumpiness in the curves can be attributed to the stochastic nature of DSR, and its reliance on broadcasts.

Comparing SSCH to IEEE 802.11a, we see that the SSCH discovers routes that are comparable in length. However, the average route discovery time for SSCH is much higher than for IEEE 802.11a. Because each slot is 10 ms in length, broadcasts are only retransmitted once every 10 ms, and this leads to a significantly longer time to discover a route to a given destination node. We believe that this latency is a fundamental difficulty in using a reactive protocol such as DSR with SSCH. We plan to explore the interaction of other proactive and hybrid routing protocols with SSCH in the future.