Transcript Power Management in 802.11

Power Management in 802.11
Presented by
Sweta Sarkar
June 17th, 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
• How can we power off during idle periods
and yet maintain an active session?
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
Infrastructure Power Management
Operation
DTIM Interval
Beacon-Interval
Time axis
AP activity
TIM (in Beacon):
AP activity:
Busy medium:
DTIM:
Broadcast:
PS station
Poll
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
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
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.
Power Management Operation in
IBSS
Beacon Interval
ATIM Window
Beacon
Receive ACK
Transmit ATIM
Receive ATIM
Receive ACK
Station A
Transmit Frame
Receive Frame
Transmit ACK
Transmit ACK
Station B