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Power 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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Motivation 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

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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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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 wa

nt to send buffered packets structured the same as TIM

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Basic Idea:An Illustration Example

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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 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 sei

zed by data frame.

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Example: Immediate Response

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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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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.

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PS in Infrastructure Network

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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 del

ivered Announce periodically which stations ha

ve frames waiting for them.

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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

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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

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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.

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Buffered Frame Retrieval Process for Two Stations

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

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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.

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Buffer Transmission after DTIM

DTIM interval = 3

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(omitted)Under PCF (Infrastructure Mode) Basic Assumption:

Point coordinator uses CF-Polling to access the channel.

AP only maintains the CF-Pollable stations.

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Operations of TIM (PCF) 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.

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TIM

Poll

TIM TIM

Beacon_Interval

AP

STA 2 inPS mode

PS-poll

STA 1 inPS mode

Data

ACK

PS-poll

Data

ACK

Poll

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Operations of DTIM (PCF) All CF-pollable stations need be in Acti

ve Mode when AP broadcasts DTIM.

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

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DTIM

Broadcast Data

TIM TIM

Beacon_Interval

AP

STA 2 inPS mode

STA 1 inPS mode

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PS in Ad Hoc Mode(without base station)

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Announcement TIM (ATIM) 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.

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ATIM Usage

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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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ATIM Window If the beacon is delayed due to a traffic

overrun, the useable portion of the ATIM window shrinks.

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Operations of ATIM All stations should be in active mode durin

g 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 i

nhibited.

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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 int

erval. 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.

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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