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Chapter 8Power Management in

IEEE 802.11

Yu-Chee Tseng@CS.NCTU

1. Possible Access Sequencesfor a STA in PS Mode

2. PS in Infrastructure Network3. PS in Ad Hoc Network

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Introduction Power management modes

Active mode (AM) Power Save mode (PS)

Power consumption of ORiNOCO WLAN Card

Transmit mode

Receive mode

Idle mode Doze mode

1400mW 900mW 700mW 60mW

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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.

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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

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Possible Access Sequencesfor a STA in PS Mode

immediate response immediate response with fragmentation deferred response

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Power Saving Sequences 802.11 stations shut down the radio transce

iver 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.

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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.

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Immediate Response with Fragmentation If the buffered frame is large, it

may require fragmentation.

** note: the change of NAVs

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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

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PS in Infrastructure Network

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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

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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.

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Buffered Frame Retrieval Process for Two Stations

Station 1 has a listen interval = 2. Station 2 has a listen interval = 3.

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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.

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Buffer Transmission after DTIM

DTIM interval = 3

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PS in Ad Hoc Mode(without base station)

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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.

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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.

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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.

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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.

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(cont.)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.

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ATIM Example

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

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Summary of PS infrastructure network

PCF DCF (omitted)

ad hoc network DCF