System Evaluation

We simulate SSCH in QualNet and compare its performance with the commonly used single-channel IEEE 802.11a protocol. In Section 4.1, we present microbenchmarks quantifying the different SSCH overheads. In Section 4.2, we present macrobenchmarks on the performance of SSCH with a large number of nodes in a single hop environment. In Section 4.3, we extend the macrobenchmark evaluation to encompass mobility and multihop routing. Our results show that SSCH incurs very low overhead, and significantly outperforms IEEE 802.11a in a multiple flow environment.

Our simulation environment comprises a varying number of nodes in a $200m\times 200m$ area. All nodes in a single simulation run use the same MAC, either SSCH or IEEE 802.11a. We set all nodes to operate at the same raw data rate, 54 Mbps. We assume 13 usable channels in the 5 GHz band. SSCH is configured to use 4 seeds, and each slot duration is 10 ms. All seeds are randomly chosen at the beginning of each simulation run. The macrobenchmarks in Sections 4.2 and 4.3 are averages from 5 independent simulation runs, while the microbenchmarks in Section 4.1 are drawn from a single simulation run.

We primarily measure throughput under a traffic load of maximum rate UDP flows. In particular, we use Constant Bit Rate (CBR) flows of 512 byte packets sent every 50 $\mu$s. This data rate is more than the sustainable throughput of IEEE 802.11a operating at 54 Mbps.

For all our simulations, we modified QualNet to use a channel switch delay of 80 $\mu$s. This choice was informed by recent work in solid state electronics on reducing the settling time of the Voltage Control Oscillator (VCO) [7]. Switching the channel of a wireless card requires changing the input voltage of the VCO, which operates in a Phase Locked Loop (PLL) to achieve the desired output frequency. The delay in channel switching is due to this settling time. The specification of Maxim IEEE 802.11b Transceivers [3] shows this delay to be 150 $\mu$s. A more recent work [17] shows that this delay can be reduced to 40-80 $\mu$s for IEEE 802.11a cards.