1. Power Management in 802.11 Presented by Sweta Sarkar June 17 th, 2002

2. Power Management Mobile devices are battery powered Enhancement of battery life enhances network lifetime Idle receive state dominates LAN adapter power consumption over time How can we power off during idle periods and yet maintain an active session?

3. Power Management Approach(Infrastructure) Allow idle station to go to sleep - stations power save mode stored in AP AP buffers packets for sleeping nodes - AP announces which station have frames buffered - TIM sent with every Beacon Power Saving stations wake up periodically - listen for Beacons TSF assures AP and Power save stations are synchronized - TSF timer keeps running when stations are sleeping

4. Infrastructure Power Management Operation Beacon-Interval DTIM Interval Time axis TIM (in Beacon): AP activity: Busy medium: DTIM: Broadcast: AP activity Poll PS station

5. Power Management Approach(Infrastructure) Broadcast/multicast frames are also buffered at AP - these frames are sent only at DTIM -DTIM = time when multicast frames are to be delivered by AP, determined by AP -this time is indicated in the Beacon frames as delivery traffic indication map(DTIM) Power Saving stations wake up prior to expected DTIM If TIM indicates frame buffered - station sends PS-Poll and stays awake to receive data - else station sleeps again

6. Power Management Approach(Ad Hoc) Before a station goes to “sleep” - has to complete a frame-handshake with”any” other station with the power management bit set in the frame header Has to wake up for every Beacon transmission - has to stay awake for ATIM window after Beacon - ATIM window is the time when other stations announce if they have buffered packets for a sleeping station If it hears an ATIM frame -it must acknowledge the frame -remain awake until the end of the next ATIM window in order to receive the frame

7. Power Management Approach(Ad Hoc) If a station desires to send a frame to another station - has to estimate the power saving state of the intended destination – how? – not mentioned in the standards If sending station determines that intended station is in PSM - sends an ATIM frame during the ATIM window - if receives an ACK of the ATIM frame, only then it transmits after the conclusion of the ATIM window.

8. Power Management Operation in IBSS ATIM Window Beacon Interval Beacon Station A Station B Transmit ATIM Receive ACK Transmit Frame Receive ACK Receive ATIM Transmit ACK Receive Frame Transmit ACK