Effect of Flow Duration

SSCH introduces a delay when flows start because nodes must synchronize. This overhead is more significant for shorter flows. We evaluate this overhead for maximum rate UDP flows with different flow lengths. In the first experiment the flow duration is chosen randomly between 20 and 30 ms, while for the second experiment it is between 0.5 and 1 second. In both the experiments, each node starts a flow with a randomly selected node, discards all packets at the end of the designated sending window, pauses for a second at the end of the flow, and then starts another flow with a new randomly selected node. The decision to discard enqueued packets at the end of the flow duration is designed to model a highly delay-sensitive application. This process continues for 30 seconds. We run these experiments for both SSCH and IEEE 802.11a, and vary the number of nodes from 2 to 16. We present the ratio of the average throughput achieved by SSCH to that achieved by the flows when using IEEE 802.11a in Figure 13.

Figure 13: Effect of Flow Duration: Ratio of SSCH average throughput to IEEE 802.11a average throughput for flows having different durations.

Figure 13 quantifies this tradeoff. For small numbers of sufficiently short-lived flows, IEEE 802.11a offers superior performance; short flows do indeed suffer from a more pronounced synchronization overhead. However, as soon as there are more than 4 simultaneous flows in the network, the ability of SSCH to spread transmissions across multiple channels leads to a higher total throughput than IEEE 802.11a in both the short and long flow scenarios.