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Support for STA power saving in 802.11v

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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  1. doc: IEEE 802.11-05/xxx3r0 May 2005 Support for STA power savingin 802.11v Joe Kwak, Marian Rudolf (InterDigital) Kwak, Rudolf

  2. 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

  3. 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

  4. 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

  5. High-level approaches to power saving (1) STA-internal PS optimizations- for all Scanning, Idle mode and Tx/Rx mode STA AP DTIM Interval Beacon-Interval APSTA Idle Mode STAAP (2) Deep-Sleep PS Mode APSTA Tx and Rx Mode STAAP (3) Short-term PS optimization Data frames to other STAs Beacons/TIMs/DTIMs Data frames to STA Kwak, Rudolf

  6. 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

  7. 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

  8. Example – TGv AP-initiated PS mode (cont’d) STA AP Step 1: DRX PS capability and DRX PS-on Step 2: DRX command (wake-up interval 100ms’s …~5sec) DRX Interval Beacon-Interval Step 3: Idle APSTA DRX STA sleep mode Resume Tx/Rx of data frames Step 4: Revert to Tx/Rx mode If paged (incoming call) APSTA 4a: Incoming call STAAP APSTA 4b: Outgoing call STAAP 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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