Overhead of Switching and Synchronizing

In this experiment, we measured the overhead of successfully initiating a CBR stream between two nodes within communication range of each other. The first node initiates the stream just after the parity slot. This incurs a worst-case delay in synchronization, because the first of the four slots will not be synchronized until 530 ms later.

In Figure 4, we graph the instantaneous throughput at the receiver node. The sender quickly synchronizes with the receiver on three of the four slots, as it should, and on the fourth slot after 530 ms. The figure shows the throughput while synchronizing (oscillating around 3/4 of the raw bandwidth), and the time required to synchronize. After synchronizing, the channel switching and other protocol overheads of SSCH lead to only a 400 Kbps penalty in the steady-state throughput relative to IEEE 802.11a. This penalty conforms to our intuition about the overheads in SSCH: a node spends 80 $\mu$s every 10 ms switching channels (80 $\mu$s/10 ms = .008), and then must wait for the duration of a single packet to avoid colliding with pre-existing packet transmissions in the new channel (1 packet/35 packets = .028). Adding these two overheads together leads to an expected cumulative overhead of 3.6%, which is in close agreement with the measured overhead of (400 Kbps/12 Mbps) = 3.3%.

Figure 4: Switching and Synchronizing Overhead: Node 1 starts a maximum rate UDP flow to Node 2. We show the throughput for both SSCH and IEEE 802.11a.

Note that the throughput of the session reaches a maximum of only 13 Mbps, although the raw data rate is 54 Mbps. This low utilization can be explained by the IEEE 802.11a requirement that the RTS/CTS packets be sent at the lowest supported data rate, 6 Mbps, along with other overheads [18].