Support for sta power saving in 802 11v
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doc: IEEE 802.11-05/xxx3r0. May 2005. Support for STA power saving in 802.11v. Joe Kwak, Marian Rudolf (InterDigital). Motivation for power saving. Long stand-by and usage time before recharging are very important for battery-operated client devices (any wireless technology)

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Support for sta power saving in 802 11v

doc: IEEE 802.11-05/xxx3r0

May 2005

Support for STA power savingin 802.11v

Joe Kwak, Marian Rudolf (InterDigital)

Kwak, Rudolf

Motivation for power saving
Motivation for power saving

  • Long stand-by and usage time before recharging are very important for battery-operated client devices (any wireless technology)

  • Basic factors affecting STA battery time,

    • Battery capacity itself

    • Air interface design (TDMA vs. CDMA vs. CSMA / FDD vs. TDD)

    • Usage scenarios and statistics (idle time vs. web browsing vs. VoIP)

    • Chipset power consumption (different for Tx and Rx mode and Idle, very implementation and form-factor dependent)

  • In terms of battery efficient operation, 802.11 STAs do rather badly compared to other wireless technologies

    • Compared to Cellular, most 802.11 chipsets consume order of magnitude more power in Idle Mode and factor 2-5 more in Rx and Tx modes

    • Idle Mode: 802.11 stand-by time is O (hours), but 2G Cellular is O (days)

    • Tx and Rx Modes: 802.11 operation time is O (several hours), but 2G cellular is usually 2x-3x better than that

    • In some laptop models, WLAN chipset consumes ~25% of overall power

Kwak, Rudolf

Motivation for power saving cont d
Motivation for power saving (cont’d)

  • Reasons for relatively unfavorable battery performance with 802.11

    • Basic mode of operation of the CSMA air interface is “always-on”

      • “Always listen, receive packet, look at address, then discard or decode”

    • Baseline 802.11 power-saving features in the early days were not well enough standardized, interoperability problems hindering widespread use

      • However, the basic approach taken is simple and good enough, “Agree when NOT to expect packets, go to sleep in-between”

  • Several more considerations on battery efficiency with 802.11

    • Less a problem for some particular types of portable devices and WLAN usage scenarios (“unpleasant, but still tolerable”)

      • Example: laptops have a greater battery capacity than many other devices

    • More a problem for small-form factor devices and usage scenarios where 802.11 does cellular-like applications (“on the edge of unacceptable”)

      • WLAN VoIP client devices, both home / public and enterprise usage

        • User expects stand-by and talk-time to be at least ~equal to cordless, ideally like cellular

    • Implementation-specific (proprietary) battery saving tweaks and twists are efficient (and fortunately widely used), but run into limitations when it needs to be coordinated between AP and STA

  • Good potential for more battery-efficient operation with 802.11 WLAN clients

  • Need of standardization to coordinate across the air interface

Kwak, Rudolf

What is possible today
What is possible today

  • Chipset/firmware implementation-specific PS features

    • Inactivity timeout’s

    • Socket-activity detection

    • Full or partial power-down of WLAN chipset or drivers

    • Several Rx mode chipset optimizations

  • Standardized support for power-saving in 802.11

    • Baseline 802.11 PS features

    • 802.11e APSD

      • Optional feature in 11e, not part of WMM

      • Wi-Fi Alliance: investigating certification program for Battery-efficient operation

    • 802.11k has some PS-relevant features, such as “Mini-Beacons” as by D2.0

      • Not yet clear if mandatory/optional

      • Mainly intended for battery-efficient scanning for BSS selection

    • 802.11n proposed PS features

      • Couple of proposed features in both TGnSync / WWise, mostly Tx/Rx mode

Kwak, Rudolf

High level approaches to power saving
High-level approaches to power saving

(1) STA-internal PS optimizations- for all Scanning, Idle mode and Tx/Rx mode



DTIM Interval






(2) Deep-Sleep PS Mode


Tx and Rx



(3) Short-term PS optimization

Data frames to other STAs


Data frames to STA

Kwak, Rudolf

What could 11v do about power saving
What could 11v do about power saving ?

  • STA-internal PS optimizations (1)

    • Scanning and BSS discovery phase

      • 802.11k has introduced more PS features for that purpose

      • Much more may not be needed, not much potential to get to more savings beyond what 11k can do ?

    • Chipset/firmware

      • Nothing can be done, completely out of scope for 11v (or any standard for that purpose)

  • Long-term PS optimizations (2) of orderseveral 100’s ms’s to sec’s

    • STAs are usually idle much longer than Tx/Rx

    • Idle Mode is particularly power-consuming in 802.11 because of “Continuous Rx” mode

    • Area where Cellular traditionally has had the edge (DRX and long paging cycles), but we could introduce simple PS functionality into 802.11 WLANs with the same PS gains

    • AP-initiated PS with long DRX cycles and Paging as new resource management tool (in addition to frequency, power, load, QoS…) – complementing STA-initiated PS today (see example)

    • Different from the PS features proposed in 11n TGnSync / WWise

    • Recommendation: significant PS gains still possible, area of most interest to 11v

  • Short-term PS optimizations (3) of order one to several tens of ms’s

    • Still potential to achieve better battery-efficiency

    • Simple and more sophisticated approaches are possible

      • Simple: Look-ahead traffic schedules (see example)

      • Sophisticated: Air interface tweaks and TDMA-like overlays (TxOp or polling schedules)

    • However, many approaches could be too air-interface-specific, we should not duplicate 11n

    • Recommendation: potential for PS gains, simple PS techniques may be interesting for 11v if not covered by 11n

Kwak, Rudolf

Example tgv ap initiated ps mode
Example – TGv AP-initiated PS mode

  • Basic idea is to introduce Discontinuous Reception (DRX) and network-controlled PS features such as typical for Cellular networks today, STA Power-saving mode network initiated and controlled

  • When STA is idle, i.e. no Tx and Rx, very good potential for PS when in DRX

    • STA Receiver-On-Time would be maximum several ms’s per DRX cycle

    • DRX cycle lengths 100’s ms’s up to several sec’s

  • Benefits of AP-initiated PS as new management tool

    • Battery management of attached STAs becomes an extension to traditionally managed settings for Frequency, Power, QoS, Load, …

    • Particularly in Enterprise/Office deployments, the network monitors ESS-wide traffic patterns, usage statistics and can base a PS decision on it (STAs usually cannot)

    • Allows additional, STA-vendor independent PS for WLAN networks (complementary to STA-internal proprietary PS optimizations)

    • For home WLAN usage, AP PS management can remove the need for the user to manually shut-down or micro-manage PS status of the client

  • What about setup delays (example: VoIP) due to long DRX cycles ?

    • Ideally, with STA-initiated calls, none – STA could revert to Tx any time on its own

    • For incoming calls, there is an acceptable max upper bound, same order of what is acceptable for Cellular should be acceptable for us

Kwak, Rudolf

Example tgv ap initiated ps mode cont d
Example – TGv AP-initiated PS mode (cont’d)



Step 1: DRX PS capability and DRX PS-on

Step 2: DRX command (wake-up interval 100ms’s …~5sec)

DRX Interval


Step 3: Idle


DRX STA sleep mode

Resume Tx/Rx of data frames

Step 4: Revert to Tx/Rx mode

If paged (incoming call)


4a: Incoming call



4b: Outgoing call


STA allowed to revert to Tx/Rx any time (outgoing call)

Data frames to other STAs

Beacons or Paging frame

Data frames to STA

Kwak, Rudolf