Windows CE Networking Team WebLog : Author: Kevin Chin

Simplified WLAN (WiFi) Power States

cenet — Thu, 01 Mar 2007 20:18:00 GMT

General Current/Power consumption relationships:

OFF < ON
Waiting to Connect < Trying to Connect (e.g. ACTIVE SCAN)
No User Data < Data
VoIP data < File transfer data
802.11 Power Save Mode << Active Mode

The key to power savings is QUICKLY MOVING TO MOST RELIABLE LOW POWER STATE.

Specifically, when associated the use of 802.11 PSM is critical.

When not associated, keeping in a low power state (while waiting to re-try to connect) is also important. This is for instances the user has left a WLAN coverage area without turning off the WLAN interface.

802.11 WLAN WiFi : Power Mini FAQ - Windows CE

cenet — Thu, 14 Dec 2006 22:16:00 GMT

Mini Frequently Asked Questions related to WLAN, Power, and Windows CE.

Why is it important?

WLAN can use significantly more power than other wireless technologies. For this higer battery consumption, you may get much higher performance. WLAN is not inherently poorer performer with regard to power; many times it is the use case that needs to be refined.
WLAN is being used in small battery powered devices. High power usage and small battery leads to poor battery life.
WLAN is being used for new scenarios beyond web browsing. This introduced assumptions and problems not previosuly anticipated in the original use case.

How much power does WLAN use on CE?

While associated to an Access Point, it can use between 5-250 mA.
The power consumption is (current X duration).
It is useful, but sometimes difficult to isolate WLAN from the CE host processor.

Why is there such a range in power consumption?

WLAN can be in several modes, those modes correspond to what the user would like the WLAN to do next.
The use cases can be very complex; recall wireless does not necessitate mobility. WLAN also operated in a unlicensed frequency band.
On the lower range, the WLAN chip, while associated, stationary, and in 802.11 Power Save Mode, with certain DTIM settings can be extremely power efficient.

What are the power modes for WLAN?

There are multiple modes across different vendors but we can assume 4 modes - Active Mode, Low Power Mode, Unconnected Mode, and Off.
The names may differ but the functionality is similar.

Can you describe Active Mode?

Active mode consumes the most power.
Active mode is generally used when connected to an AP.
Active mode provides the lowest latency.
Active mode provides the best performance.
Active mode is useful for transfering data.
Active Mode (w.r.t. Power) may be used for Scanning

Can you describe Low Power Mode?

Low Power Mode consumes significantly less power than Active Mode.
Low Power Mode can used when connected to an AP.
Low Power Mode may introduce more latency.
Low Power Mode will reduce performance.
Low Power Mode is useful for timer based operations - when you know when you need to wake up again.
Low Power Mode is useful for associated, idle, and in 802.11 Power Save Mode.
Low Power Mode is a good state to be in when not transferring data.

Can you describe Unconnected Mode?

Unconnected Mode consumes significantly less power than Low Power Mode.
Unconnected Mode is when you are unconnected and not trying to connect.
Unconnected Mode needs to be woken up by another process -- not timer based.
Unconnected Mode does not enable a timer.
Unconnected Mode can not be used to SCAN.
Unconnected Mode can not be used to CONNECT.
Unconnected Mode provides some power to the WLAN NIC.
Unconnected Mode provides minimal difference over OFF.

Can you describe Off?

Off implies the WLAN NIC is not powered.
Off means the WLAN is not connected.
Off implies the operating system may not see/notice the device.
Off is the lowest power consumption.

Also See Diagram in blog titled:

"Simplified WLAN (WiFi) Power States"

802.11 WLAN WiFi Power Golden Rule: Minimize Duty Cycle

cenet — Thu, 14 Dec 2006 21:53:00 GMT

IEEE 802.11 WLAN, a.k.a. WiFi, is a dynamic technology being added to a number of CE devices. Increasingly, these devices are battery powered. Under these circumstances driver developers have to trade-off between POWER and PERFORMANCE. Making the wrong assumptions lead to horrible battery life, poor performance, or both!

Q: How should a developer/ODM/system integrator optimize battery life for a CE device with WLAN?

A: Minimize the required WLAN duty cycle.

In many ways, the answer is quite obvious -- "Turn off WLAN or put WLAN in idle mode when not needed". The more difficult exercise is determining why, when and how to accomplish this optimization. Some simple steps which help determine what you want to optimize:

Understand WLAN and how the device is going to be used
Test the device for reliability
Test the device for baseline power consumption
Test the device for WLAN power consumption
Refine and Optimize

This is a dynamic research area and will continue to evolve. In the meantime, I'll add tips and suggestions as time permits.

I will also update a FAQ on WLAN.