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Power Management in 802.11. Presented by Sweta Sarkar June 17 th , 2002. Power Management. Mobile devices are battery powered Enhancement of battery life enhances network lifetime Idle receive state dominates LAN adapter power consumption over time

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Power management in 802 11 l.jpg

Power Management in 802.11

Presented by

Sweta Sarkar

June 17th, 2002


Power management l.jpg
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?


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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


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Infrastructure Power Management Operation

DTIM Interval

Beacon-Interval

Time axis

AP activity

AP activity:

Broadcast:

TIM (in Beacon):

Busy medium:

DTIM:

PS station

Poll


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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


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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


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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.


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Power Management Operation in IBSS

Beacon Interval

ATIM Window

Beacon

Receive ACK

Receive ACK

Station A

Transmit Frame

Transmit ATIM

Receive ATIM

Receive Frame

Station B

Transmit ACK

Transmit ACK


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