Overhead of Clock Drift

As we described in Section 3.3, SSCH tries to synchronize slot begin and end times, though it is also designed to be robust to clock skew. In this experiment, we quantify the robustness of SSCH to moderate clock skew. We measure the throughput between two nodes after artificially introducing a clock skew between them, and disabling the SSCH synchronization scheme for slot begin and end times. We vary the clock skew from 1 ns ($10^{-6}$ ms) to 1 ms such that the sender is always ahead of the receiver by this value, and present the results in Figure 8. Note the log scale on the x-axis.

Figure 8: Overhead of Clock Skew: Throughput between two nodes using SSCH as a function of clock skew.

The throughput achieved between the two nodes is not significantly affected by a clock skew of less than 10 $\mu$s. These practical values of clock skew are extremely small to impact the performance of SSCH. The drop in throughput is more for larger clock skews, although the throughput is still acceptable at 10.5 Mbps when the skew value is an extremely high 1 ms.

These results provide justification for the design choice we made not to require nodes to switch synchronously across slots, as described in Section 3.3. For example, a node will delay switching to receive an ACK, or to send a data packet if its channel reservation is successful. In the 100 node experiment described in Section 4.3.2, we measured the skew in channel switching times for a traffic pattern of 50 flows to be approximately 20 $\mu$s. Figure 8 shows that this is a negligible amount.