With Power Save Mode

The multiple radio approach can be modified to consume less power by allowing the network card in infrastructure mode to use PSM. Figure 9 shows the energy usage when the infrastructure radio uses PSM for our application. The Beacon Period is set to 100 ms, and the Listen Interval is 4. The amount of energy consumed in the two radio case using PSM is very close to the consumption of MultiNet without PSM. However, this saving comes at a price. It is no longer possible to achieve the high throughput for infrastructure networks if the cards are in PSM. Simulated results in Table 3 show that the average packet delay over the infrastructure network with PSM is now close to the average packet delay for MultiNet. Therefore, using two radios with PSM does not give significant benefits as compared to MultiNet without PSM.

Table 3: The average packet delay in infrastructure mode for the various strategies

Scheme	Avg Delay (in Seconds)
Two Radio	0.001
MultiNet	0.157
Two Radio PS	0.156
MultiNet PS	0.167

The power consumption of MultiNet can be reduced further by allowing it to enter the power save mode for infrastructure networks as described in Section 5.3.2. In our experiment we chose the Switching Cycle to be 400ms, with `75%IS 25%AH' switching. For consistency in comparison, the Listen Interval is set to 4 and the Beacon Period to 100ms. Consequently, every time the card switches to infrastructure mode, it listens for the traffic indication map from the AP. After it has processed all its packets it goes to sleep and wakes up after 300ms. It then stays in the ad hoc network for 100ms, and then switches back to the infrastructure network. The modified algorithm results in greater energy savings as shown in Figure 9. The average delay per packet over the infrastructure network is not seriously affected, while the energy consumed is reduced by more than a factor of 3. We conclude that MultiNet beats using multiple cards for connecting to multiple networks in terms of convenience and power.

Note that we do not evaluate power saving in ad hoc mode because we are unaware of any commercial cards that implement this feature. As a result we were unable to get performance numbers when using PSM in ad hoc mode. However, we believe that if such a scheme is implemented, we will be able to incorporate it in MultiNet, and further reduce the power consumption.

Figure 9: Energy usage when using MultiNet and two radios with IEEE 802.11 Power Saving