Chapter 8 power management in ieee 802 11
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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 8 power management in ieee 802 11

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

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

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 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

  • 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

Ps in infrastructure network

PS in Infrastructure Network

Assumptions and models

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

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

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

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

Buffer Transmission after DTIM

  • DTIM interval = 3

Ps in ad hoc mode without base station

PS in Ad Hoc Mode(without base station)

Announcement tim atim

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

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

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

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.

Chapter 8 power management in ieee 802 11


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

Atim example sta 1 waking up stas 2 3 and 4

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

Summary of ps

Summary of PS

  • infrastructure network

    • PCF

    • DCF (omitted)

  • ad hoc network

    • DCF

  • Login