wlan power save mode in linuxlinuxwireless.sipsolutions.net/attachments/en/...linux wireless power...
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Linux WirelessLinux Wireless
WLAN Power Save Mode in Linux
Kalle [email protected]([email protected])
FUDCon Berlin 2009
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Linux Wireless
Outline
1 Introduction
2 Status
3 Future
Linux Wireless
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Linux Wireless
IntroductionIntroduction
Linux Wireless
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Linux Wireless
IntroductionWhy?
• Total system power consumption of Nokia N800 (stlc45xx,mac80211, 2.6.29-omap-wl):
0
200
400
600
800
1000
1200
PSM off PSM on PSM + beacon filter WLAN off
pow
er (
mW
)
Linux Wireless
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Linux Wireless
IntroductionPower Save Mode (PSM) simplified
• client informs Access Point (AP) of the PSM status with a flag inFrame Control header
• AP buffers all unicast, broadcast and multicast frames• AP informs of buffered unicast frames with TIM bits in beacons• AP sends broadcast and multicast frames immediately after the
DTIM beacons with a special TIM bit set• AP sends buffered unicast frames either when client wakes up or
requests them with a PS-Poll frame
Linux Wireless
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Linux Wireless
IntroductionPSM example
Linux Wireless
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Linux Wireless
IntroductionDownsides
• few ms latency in transmission due to firmware wakeup (dependson hardware)
• hundreds of ms latency in receive, depends on:• beacon interval• DTIM period• how often client wakes up for beacons
• broken APs→ strange packet loss• broken firmware and/or HW, timing is very delicate
Linux Wireless
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Linux Wireless
StatusStatus
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Linux Wireless
StatusDynamic and Static PSM
• timeout > 0→ Dynamic PSM• stays awake a certain period after a last transmission• wakes up with Nullfunc• should not affect throughput• in certain cases increases latency considerably, but user won’t
notice in practise• timeout = 0→ Static PSM
• goes immediately to sleep after transmitting a frame• fetches buffered frames with PS-Poll frame• decreases throughput• increases latency significantly
Linux Wireless
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Linux Wireless
StatusUser space interface
• Wireless Extensions have power and power timeout:• iwconfig wlan0 power timeout 200m• the dynamic PSM timeout value
• no support in nl80211, yet• network latency PM QoS interface
• /dev/network_latency• how often hardware wakes up for beacons
Linux Wireless
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Linux Wireless
Statusmac80211 status
• supported HW types:• HW supports PSM (IEEE80211_HW_SUPPORTS_PS)• Stack must create Nullfunc and PS-Poll frames
(IEEE80211_HW_PS_NULLFUNC_STACK)• HW has timers for dynamic PSM transitions
(IEEE80211_HW_SUPPORTS_DYNAMIC_PS)• drivers supporting PSM:
• ath9k• rt2x00• iwlwifi (was disabled due to bugs, just re-enabled today)• stlc45xx (staging, patches pending)• wl1251 (patches pending)
• PSM enabled by default for the drivers which claim to support it,default timeout 500 ms
Linux Wireless
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Linux Wireless
StatusFullmac driver support
• unknown, any comments?
Linux Wireless
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Linux Wireless
StatusBeacon filtering
• in softmac beacons are normally forwarded to the host CPU• wakes up CPU unnecessarily• some HWs support a feature to only forward beacons which have
chaned in relevant parts• HW sends an event when beacons are lost
Linux Wireless
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Linux Wireless
FutureFuture
Linux Wireless
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Linux Wireless
FutureUnscheduled Automatic Power Save Delivery (U-APSD)
• also known as WMM Power Save• needs QoS support from the applications, otherwise legacy PSM
is used
Linux Wireless
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Linux Wireless
FutureImprovements to Dynamic Power Save
• an interface for the applications to influence the timeout value• use PM QoS network_latency somehow?
• more advanced algorithm for adaptation• academic papers available
Linux Wireless
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Linux Wireless
FutureUser space
• nl80211 support• wpa_supplicant:
• support to disable PSM?• configure timeout value?
• more QoS information from applications?
Linux Wireless
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Linux Wireless
Thanks for listening.
Questions?
http://wireless.kernel.org/