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

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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

  • 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)

  • 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

  • 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

(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 ?

  • 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

  • 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)



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

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