TCP Performance over SSCH

We now study the behavior of TCP over SSCH. SSCH allows a node to stay synchronized to multiple nodes over different slots. However, this might cause significant jitter in packet delivery times, which could adversely affect TCP. To evaluate this concern quantitatively, we run an experiment where we vary the number of nodes in the network from 2 to 9, such that all nodes are in communication range of one another. We then start an infinite-size file transfer over FTP from each node to a randomly selected other node. This choice to use non-disjoint flows is designed to stress the SSCH implementation by requiring nodes to be synchronized as either senders or receivers with multiple other nodes. In Figure 14 we present the resulting cumulative steady-state TCP throughput over all the flows in the network.

Figure 14: TCP over SSCH: Steady-state TCP throughput when varying the number of non-disjoint flows.

Figure 14 shows that the TCP throughput for a small number of flows is lower for SSCH than the throughput over IEEE 802.11a. However, as the number of flows increases, SSCH does achieve a higher system throughput. Although TCP over SSCH does provide higher aggregate throughput than over IEEE 802.11a, the performance improvement is not nearly as good as for UDP flows. This shows that jitter due to SSCH does have an impact on the performance of TCP. A more detailed analysis of the interaction between TCP and SSCH, and modifications to support better interactions between TCP and SSCH, is a subject we plan to address in our future work.