How do the battery-conservation capabilities in Wi-Fi devices actually work?
When a client makes an initial association request with an access point, it tells the AP that it is capable of sleeping and what its sleep interval is. The sleep interval is specified in terms of number of beacon frames. For the AP to support power-save capabilities, it must have standard mechanisms called the Traffic Indication Map (TIM) and Power Save Poll (PS Poll).
When a Wi-Fi client (handset, laptop, phone) goes to sleep and reduces power, it can still receive beacon frames. When the AP discovers that it has a packet destined for a sleeping client, it notifies the TIM. The dozing client will see that it has data waiting for it by recognizing its particular bit map during the AP’s beaconing. Once it knows there is a transmission en route, it sends a PS Poll message to the AP asking it to go ahead and send the packet, and the AP complies—that is, if the wake-up process hasn’t caused so much delay that the AP has dropped the packet.
Scott Haughdahl, CTO at Wild Packets, a Wi-Fi spectrum analysis company, agrees that there might be tradeoffs to consider between battery power savings, device responsiveness, and throughput.
For example, it might take a client more than 100 milliseconds to “wake up” from sleep mode, he says, injecting latency into the network. A network manager seeing latency might presume congestion as the cause.
As noted, if the power-save sleep interval and wake-up time are too long, the AP might drop the packet aimed for the sleeping client. This is particularly undesirable in real-time voice-over-Wi-Fi environments. Haughdahl advises not shying away from power-save mode, but recommends assessing interval and wake-up times. Most Wi-Fi devices with power-saving capabilities ship with default sleep intervals, he explains.
He advises to run comparative battery-life measurements with a typical mix of applications first with no power save, then again with default power-save settings enabled, and finally with power save optimized (e.g., shorter sleep intervals) to determine if you are getting any conservation benefits.
