Power Management in IEEE 802.11

1
Power Management in IEEE 802.11

• Yu-Chee Tseng
• _at_CS.NCTU
• Possible Access Sequencesfor a STA in PS Mode
• PS in Infrastructure Network
• PS in Ad Hoc Network

2
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

3
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
4
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 want
  to send buffered packets
• structured the same as TIM

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Basic IdeaAn Illustration Example
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Possible Access Sequencesfor a STA in PS Mode

• immediate response
• immediate response with fragmentation
• deferred response

8
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.

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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
  seized by data frame.

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Example Immediate Response
11
Immediate Response with Fragmentation

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

note the change of NAVs
12
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

13

• 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.

14
PS in Infrastructure Network
15
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 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

17
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

18
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.

20
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.

21
Buffer Transmission after DTIM

• DTIM interval 3

22
(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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Beacon_ Interval
TIM
TIM
TIM
Poll
Data
Poll
Data
AP
STA 1 in PS mode
PS-poll
ACK
STA 2 in PS mode
PS-poll
ACK
25
Operations of DTIM (PCF)

• All CF-pollable stations need be in Active Mode
  when AP broadcasts DTIM.
• Immediately after DTIM, AP sends out the buffered
  broadcast/multicast packets.

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Beacon_ Interval
Broadcast Data
TIM
TIM
DTIM
AP
STA 1 in PS mode
STA 2 in PS 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
30
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 during 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
  inhibited.

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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.
• After ATIM window,
• all stations use CSMA/CA to send the buffered
  packets
• basic idea data packet gtgt ATIM control frames
• only those hosts who have ACKed the ATIM have
  such opportunity.

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ATIM Example
35
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