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Power Management in IEEE 802.11

Yu-Chee Tseng

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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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Access Sequences

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.

NAV = SIFS + ACK Although the NAV is too short, the

medium is seized 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 delivered Announce periodically which stations have fram

es waiting for them. Stations only need to power up

receiver to listen to the buffer status and transmitter to transmit polling frames after bein

g informed.

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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)

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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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Listen Interval Listen Interval

The number of Beacon periods for which the mobile station may choose to sleep.

One of the key parameter used in estimating the resources required to support an association.

Longer listen intervals require more buffer space on the AP.

If a MS fails to check for waiting frames after each listen interval, they may be discarded without notification.

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PS-Poll Frame Retrieval

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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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Traffic Indication Map (TIM) To inform stations that frames are buffered, AP

s periodically assemble a TIM and transmit it in Beacon frames.

The TIM is a virtual bitmap compose of 2,008 bits.

Offsets are used to transmit only a small portion of the virtual bit-map.

Each bit in the TIM corresponds to a particular AID.

Setting the bit indicates that the AP has buffered unicast frames for the corresponding station.

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TIM (cont.) MSs must wake up to listen for Beacon frames to re

ceive the TIM. By examining the TIM, a station can determine if th

e AP has buffered traffic on its behalf. To retrieve buffered frames, MSs use PS-Poll contr

ol frames. When multiple stations have buffered frames, stati

ons use the random backoff algorithm before transmitting the PS-Poll.

The buffered frames must be positively acknowledged before it is removed from buffer.

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TIM (cont.) If multiple frames are buffered for a MS, then t

he More Data bit in the Frame Control field is set to 1.

MSs can then issue additional PS-Poll to AP until More Data bit is set to 0.

After transmitting the PS-Poll, a mobile station must remain awake until either The polling transaction has concluded or The bit corresponding to its AID is no longer set in t

he TIM

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Delivering Multicast and Broadcast Frames: the Delivery TIM (DTIM) Frames are buffered whenever any station a

ssociated with the AP is sleeping. Buffered broadcast and multicast frames ar

e saved used AID = 0. APs set the first bit in the TIM to 0. At a fixed number of Beacon intervals, a DTI

M is sent. Buffered broadcast and multicast traffic is t

ransmitted after a DTIM Beacon.

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Buffer Transmission after DTIM

DTIM interval = 3

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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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IBSS Power Management Power management is less

efficient. Far more of the burden is placed

on the sender to ensure that the receiver is active.

Receiver must also be more available and cannot sleep for the same lengths of time.

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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:

ATIM interval (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 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 interval. If the host is not in the name list,

it can go back to the PS mode.

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After ATIM window, all stations use CSMA/CA to send the buffered p

ackets basic idea: data packet >> ATIM control frames

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

The ATIM window is the only IBSS-specific parameter. setting it to 0 avoids using any power managem

ent.

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An Example of ATIM Window If the beacon is delayed due to a traffic

overrun, the useable portion of the ATIM window shrinks.

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ATIM Effects A station is permitted to sleep only if it neit

her transmits nor receives an ATIM. When a station stays up due to ATIM traffic,

it remains active until the conclusion of the next ATIM window.

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ATIM Example

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ATIM Window (cont.) Only certain control and management fram

es can be transmitted during the ATIM window. Beacons, RTS, CTS, ACK and ATIM frames.

ATIM frames may be transmitted only during the ATIM window.

ACKs are required for unicast ATIM frames. ACKs are not required for multicast ATIM fra

mes.

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ATIM Window (cont.) Buffered broadcast and multicast frame

s are transmitted after the conclusion of the ATIM window.

After that, a station may attempt to transmit unicast frames announced with an ATIM and for which an ACK was received.

Then, stations may transmit unbuffered frames to others that are known to be active.

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Effects of ATIM

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PS Summary infrastructure network

PCF DCF

ad hoc network DCF

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Open Problem 1:PS for multi-hop MANET ATIM design in

IEEE 802.11

Problems: clock drift

(asynchronous) network partitioning

Ref: Tseng et al. INFOCOM 2002

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Open Problem 2:ATIM window length

ATIM window length has performance impact on an ad hoc network.

How to choose a proper size of ATIM window?

Ref: E. S. Jung and N. Vaidya, “An Energy Efficient MAC Protocol for Wireless LANs”, INFOCOM 2002.

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Open Problem 3: for soft handover:

802.11 only allows hard handover searching for new AP only after a STA loses its previous AP thus, causing long delay (e.g., VoIP services are delay-sensiti

ve) to support soft handover, a STA must frequently scan new

AP’s approach:

using PS mode as an excuse to search for neighboring APs (through such as an full frame with PS bit = 1)

the old AP will buffer frames during the PS period.