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Chapter 8 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. Introduction. Power management modes Active mode (AM) Power Save mode (PS) Power consumption of ORiNOCO WLAN Card.

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Chapter 8 Power Management in IEEE 802.11

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Chapter 8Power 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


  • Power management modes

    • Active mode (AM)

    • Power Save mode (PS)

  • Power consumption of ORiNOCO WLAN Card

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.

TIM Types

  • Infrastructure mode:

    • TIM :

      • transmitted with every beacon (for Unicast)

    • Delivery TIM (DTIM):

      • transmitted less frequently (every DTIM_interval)

      • for sending buffered broadcast packets

  • Ad hoc mode:

    • ATIM:

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

      • structured the same as TIM

Possible Access Sequencesfor a STA in PS Mode

immediate response

immediate response with fragmentation

deferred response

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

  • AP can respond immediately to the PS-Poll

  • 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

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

** note: the change of NAVs

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

PS in Infrastructure Network

Assumptions and Models

  • Assumptions:

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

      • requires time synchronization

  • Two Operational Models:

    • under DCF (contention-based)

    • under PCF (contention-free): omitted

Operations of TIM (in DCF)

  • AP periodically broadcasts beacon with TIM.

    • but STAs may have different wake-up intervals.

  • Hosts in PS must wake up to check TIM.

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

    • AP replies PS-poll with ACK or DATA.

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

Buffered Frame Retrieval Process for Two Stations

  • Station 1 has a listen interval = 2.

  • Station 2 has a listen interval = 3.

Multicast and Broadcasting: 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

  • DTIM interval = 3

PS in Ad Hoc Mode(without base station)

Announcement TIM (ATIM)

  • ATIM: a kind of “awake” announcement

    • 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 Window

  • a fixed period after beacon

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

PS in Ad Hoc Mode

  • 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.

Operations of ATIM

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

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

  • Sending beacon is based on contention.

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


3. 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

  • If the host is not in the name list,

    • go back to the PS mode.

      4. After ATIM window,

  • stations use CSMA/CA to send buffered packets

  • only those hosts who were ACKed can contend.

ATIM Example

ATIM Example (STA 1 Waking Up STAs 2, 3, and 4)

Summary of PS

  • infrastructure network

    • PCF

    • DCF (omitted)

  • ad hoc network

    • DCF

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