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Power Management in IEEE 802.11. Yu-Chee Tseng @CS.NCTU Possible Access Sequences for a STA in PS Mode PS in Infrastructure Network PS in Ad Hoc Network. Motivation. Since mobile hosts are supported by battery power, saving battery as much as possible is very important.

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Power management in ieee 802 11

Power Management in IEEE 802.11

Yu-Chee Tseng


Possible Access Sequencesfor a STA in PS Mode

PS in Infrastructure Network

PS in Ad Hoc Network


  • Since mobile hosts are supported by battery power, saving battery as much as possible is very important.

  • Power management in 802.11

    • in infrastructure network vs. ad hoc network

    • PCF vs. DCF


  • Power management modes

    • Active mode (AM)

    • Power Save mode (PS)

  • Power consumption of ORiNOCO WLAN Card

Basic idea
Basic Idea

  • AP or source hosts buffer packets for hosts in PS mode.

    • AP or sources send TIM periodically.

      • TIM = traffic indication map (a partial virtual bitmap associated with station id)

      • TIM is associated with beacon.

  • Hosts in PS mode only turn on antenna when necessary.

    • Hosts in PS mode only “wake up” to monitor TIM.

Basic idea tim types
Basic Idea: TIM Types

  • TIM :

    • transmitted with every beacon (for Unicast)

  • Delivery TIM (DTIM):

    • transmitted less frequently (every DTIM_interval)

    • for sending buffered broadcast packets

  • Ad hoc TIM (ATIM):

    • transmitted in ATIM-Window by stations who want to send buffered packets

    • structured the same as TIM

Basic idea an illustration example
Basic Idea:An Illustration Example

Possible access sequences for a sta in ps mode

Possible Access Sequencesfor a STA in PS Mode

immediate response

immediate response with fragmentation

deferred response

Power saving sequences
Power Saving Sequences

  • 802.11 stations shut down the radio transceiver and sleeping periodically to increase battery life.

  • During sleeping periods, access points buffer any unicast frames for sleeping stations.

  • These frames are announced by subsequent Beacon frames.

  • To retrieve buffered frames, newly awakened stations use PS-Poll frames.

Immediate response
Immediate Response

  • AP can respond immediately to the PS-Poll

  • PS-Poll frame contains an Association ID in the Duration/ID field so AP can determine which frames were buffered for the MS.

  • Since Duration is not used, it assumes

    • NAV = SIFS + ACK

    • Although the NAV is too short, the medium is seized by data frame.

Immediate response with fragmentation
Immediate Response with Fragmentation

  • If the buffered frame is large, it may require fragmentation.

** note: the change of NAVs

Deferred response
Deferred Response

  • After being polled, the AP may decide to respond with a simple ACK.

    • although promised, AP does not act immediately

    • AP may do regular DCF activities

    • the PS station must remain awake until it is delivered

  • fig. 3-21

  • The PS station must stay awake until the next Beacon frame in which its bit in TIM is clear.

  • Fragmentation is possible too.

Power management in infrastructure networks
Power Management in Infrastructure Networks

  • All traffic for MSs must go through APs, so they are an ideal location to buffer traffic.

  • APs are aware of MSs’ power management state.

  • APs have two power management-related tasks.

    • Determine whether a frame should be delivered

    • Announce periodically which stations have frames waiting for them.

Assumptions and models
Assumptions and Models

  • Assumptions:

    • TIM interval (beacon interval) and DTIM interval are known by all hosts

      • requires time synchronization

    • Stations in PS mode are known or can be predicted.

  • Two Operational Models:

    • under DCF (contention-based)

    • under PCF (contention-free): omitted

Under dcf infrastructure mode
Under DCF (Infrastructure Mode)

  • Basic assumption:

    • use CSMA/CA to access the channel

    • RTS, CTS, ACK, PS-Poll are used to overcome the hidden-terminal problem

Operations of tim in dcf
Operations of TIM (in DCF)

  • AP periodically broadcasts beacon with TIM.

  • Hosts in PS must wake up to check TIM.

    • Check for their IDs.

  • If found having packets buffered in AP, send PS-Poll to AP (by contention?).

    • Otherwise, go back to PS mode.

  • AP replies PS-poll with ACK.

    • The receiver must remain in active mode until it receives the packet.

  • AP uses CSMA/CA to transmit to stations.

Buffered frame retrieval process for two stations
Buffered Frame Retrieval Process for Two Stations

  • Station 1 has a listen interval of 2 while Station 2 has a listen interval of 3.

Delivering multicast and broadcast frames the delivery tim dtim
Delivering Multicast and Broadcast Frames: the Delivery TIM (DTIM)

  • Frames are buffered whenever any station associated with the AP is sleeping.

  • Buffered broadcast and multicast frames are saved using AID = 0.

  • AP sets the first bit in the TIM to 0.

  • At a fixed number of Beacon intervals, a DTIM is sent.

  • Buffered broadcast and multicast traffic is transmitted after a DTIM Beacon.

Buffer transmission after dtim
Buffer Transmission after DTIM (DTIM)

  • DTIM interval = 3

Omitted under pcf infrastructure mode
(omitted) (DTIM)Under PCF (Infrastructure Mode)

  • Basic Assumption:

    • Point coordinator uses CF-Polling to access the channel.

    • AP only maintains the CF-Pollable stations.

Operations of tim pcf
Operations of TIM (PCF) (DTIM)

  • AP broadcasts beacon with TIM.

  • Hosts in PS mode checks TIM for their IDs.

    • If there are buffered packets in AP, the host must remain in Active Mode until being polled.

    • O/w, the station goes back to PS mode.

  • Then AP polls those PS stations.

  • When being polled, the station (in PS mode) sends PS-Poll to AP.

    • Then AP sends buffered packets to the station.

    • (See next page.)

  • AP must poll stations in PS mode first.

Beacon_ (DTIM)










STA 1 in

PS mode



STA 2 in

PS mode



Operations of dtim pcf
Operations of DTIM (PCF) (DTIM)

  • All CF-pollable stations need be in Active Mode when AP broadcasts DTIM.

  • Immediately after DTIM, AP sends out the buffered broadcast/multicast packets.

Beacon_ (DTIM)


Broadcast Data





STA 1 in

PS mode

STA 2 in

PS mode

Ps in ad hoc mode without base station

PS in Ad Hoc Mode (DTIM)(without base station)

Announcement tim atim
Announcement TIM (ATIM) (DTIM)

  • The ATIM frame is a message to keep the transceiver on because there is a pending data frame.

  • All stations in an IBSS listen for ATIM frames during specified periods after Beacon transmissions.

  • Stations that do not receive ATIM frames are free to conserve power.

Atim usage

Ps in ad hoc mode
PS in Ad Hoc Mode (DTIM)

  • Assumptions:

    • beacon interval & ATIM window are known by all hosts

    • Each station predicts which stations are in PS mode.

    • The network is fully connected.

  • Basic Method:

    • CSMA/CA is used to access the channel.

    • RTS, CTS, ACK, PS-Poll are used to overcome hidden terminal.

Atim window
ATIM Window (DTIM)

  • If the beacon is delayed due to a traffic overrun, the useable portion of the ATIM window shrinks.

Operations of atim
Operations of ATIM (DTIM)

  • All stations should be in active mode during ATIM window.

  • The station which completes its backoff procedure broadcasts a beacon.

    • Sending beacon is based on contention.

    • Any beacon starts the ATIM window.

    • Once a beacon is heard, the rest beacons are inhibited.

  • In ATIM window, each source station having buffered packets to be sent contends to send out its ATIM.

    • If a host finds it is in the ATIM name list,

      • send an ACK to the sender.

      • remain in the ACTIVE mode throughout the beacon interval.

    • If the host is not in the name list,

      • it can go back to the PS mode.

  • After ATIM window,

    • all stations use CSMA/CA to send the buffered packets

      • basic idea: data packet >> ATIM control frames

    • only those hosts who have ACKed the ATIM have such opportunity.

Atim example
ATIM Example to be sent contends to send out its ATIM.

Atim example sta 1 waking up stas 2 3 and 4
ATIM Example (STA 1 Waking Up STAs 2, 3, and 4) to be sent contends to send out its ATIM.

Summary of ps
Summary of PS to be sent contends to send out its ATIM.

  • infrastructure network

    • PCF

    • DCF (omitted)

  • ad hoc network

    • DCF