Power Save Mechanismin80211 Protocol Part1


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This presentation represents the basic pictorial functionality of various power save mechanisms supported in IEEE 802.11 protocol.

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Power Save Mechanismin80211 Protocol Part1

  1. 1. <ul><li>Power Save Mechanisms Supported by IEEE 802.11 Protocol </li></ul><ul><li>Tutorial - 1 </li></ul><ul><li>Shashank Tadakamadla </li></ul>
  2. 2. Outline <ul><li>Association State Machine </li></ul><ul><li>Power Save Mechanism Matrix </li></ul><ul><li>Operation of Power Save Mechanisms </li></ul>
  3. 3. Terminology <ul><li>Traffic Indication Map (TIM) </li></ul><ul><ul><li>The TIM shall identify the STAs for which traffic is pending and buffered in the AP. </li></ul></ul><ul><ul><li>AP can buffer all types of traffic such as unicast & multicast/broadcast traffic. </li></ul></ul><ul><li>Delivery Traffic Indication Message (DTIM) </li></ul><ul><ul><li>DTIM = ( Integer ) * Beacon Interval </li></ul></ul><ul><ul><li>DTIM period is used to deliver broadcast/multicast traffic to STAs in PS mode. </li></ul></ul><ul><li>Listen Interval </li></ul><ul><ul><li>Listen Interval = ( Integer ) * Beacon interval </li></ul></ul><ul><ul><li>This field is advertised in Association Response of AP. </li></ul></ul><ul><li>Automatic Power Save Delivery (APSD) </li></ul><ul><li>Spatial Multiplexing (SM) </li></ul><ul><li>Power Save (PS) </li></ul><ul><li>Power Save Multi-Poll (PSMP) </li></ul>
  4. 4. Points To Remember <ul><li>STA should wakeup at every Listen Interval, DTIM Beacon period. </li></ul><ul><ul><li>Listen Interval is determined at the AP based on its buffering capability. </li></ul></ul><ul><li>How does STA collects data from AP </li></ul><ul><ul><li>When STA is in awake mode, it listens to APs Beacon and looks at TIM IE. If it finds any traffic for STAs AID, STA responds actively w.r.t power save mechanism enabled on that STA. </li></ul></ul><ul><ul><li>STA wakes up early enough to listen to its first Beacon after PS mode. </li></ul></ul><ul><li>QoS APs supporting U-APSD shall signal its capability through APSD subfield present in the capability information of a Beacon. </li></ul><ul><li>Access Category (AC) </li></ul><ul><ul><li>Traffic can be categorized into four categories </li></ul></ul><ul><ul><ul><li>AC_VI - Video </li></ul></ul></ul><ul><ul><ul><li>AC_VO - Voice </li></ul></ul></ul><ul><ul><ul><li>AC_BE - Best Effort </li></ul></ul></ul><ul><ul><ul><li>AC_BK - Background </li></ul></ul></ul><ul><ul><li>APSD mechanism is used only when such AC is enabled by STA. Prerequisite is that both AP & STA should support QoS (WMM). </li></ul></ul>
  5. 5. Need for Power Save Mechanisms <ul><li>To reduce the power consumption of Wi-Fi radios when a device has limited power. </li></ul>
  6. 6. Basic Association Block Diagram Probe Request Probe Response Authentication Authentication Association Request Association Response Basic Association Procedure <ul><li>In the above diagram, I have assumed that AP is configured with open security settings. </li></ul>Access Point Station
  7. 7. Types of Power Save Mechanisms √ √ √ √ X √ √ √ HT X X X X X √ √ √ QoS X X X X X X √ √ Non- QoS S-PSMP U-PSMP Dynamic Static S-APSD U-APSD Non-PSPoll PS-Poll PSMP SM APSD Legacy Power Save Mechanisms Station Types
  8. 8. PS-Poll Power Save Mechanism <ul><li>STA transmits a PS-Poll frame with Power management bit set to 0 to AP. </li></ul><ul><li>ACK is optional. </li></ul><ul><li>w.r.t to PS-Poll frame received by AP, AP transmits Data frame for the AID present in PS-Poll frame. </li></ul><ul><li>If in the last transmitted data frame transmitted by AP contains More Data field set to 1, STA follows the procedure present in stage 1. </li></ul><ul><li>STA goes to sleep mode until data from AP buffer is empty. </li></ul>PS-Poll Acknowledgement Acknowledgement Data Basic Association Procedure PS-Poll Acknowledgement Acknowledgement Data 1 2 3 4 Stage 1 5 Access Point Station
  9. 9. Non-PS-Poll Power Save Mechanism <ul><li>STA transmits a Data/Null frame with Power management bit set to 0 to AP. </li></ul><ul><li>w.r.t to Data/Null frame received by AP, AP transmits all Data buffered to the STA. </li></ul><ul><li>If the More Data field is set in the last data frame transmitted by AP, STA waits for data from AP until More Data field is 0. </li></ul>Null/Data Frame Acknowledgement Data Basic Association Procedure 1 2 3 Data Data Acknowledgement Acknowledgement Acknowledgement 4 5 Access Point Station
  10. 10. <ul><li>? </li></ul><ul><li>For any queries mail me at </li></ul><ul><li>[email_address] </li></ul>