Chapter 8
Power Management in
IEEE 802.11
Yu-Chee Tseng
@CS.NCTU
1.
2.
3.
Possible Access Sequences
for a STA in PS Mode
PS in Infrastructure Network
PS in Ad Hoc Network
1
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
2
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.
3
TIM Types

Infrastructure mode:

TIM :


Delivery TIM (DTIM):



transmitted with every beacon (for Unicast)
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
4
Possible Access Sequences
for a STA in PS Mode



immediate response
immediate response with fragmentation
deferred response
5
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.
6
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.
7
Immediate Response with
Fragmentation

If the buffered frame is large, it may
require fragmentation.
** note: the change of NAVs
8
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
9
PS in Infrastructure Network
10
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
11
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.
12
Buffered Frame Retrieval Process
for Two Stations


Station 1 has a listen interval = 2.
Station 2 has a listen interval = 3.
13
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.
14
Buffer Transmission after
DTIM

DTIM interval = 3
15
PS in Ad Hoc Mode
(without base station)
16
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.
17
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.
18
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.
19
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.
20
(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.
21
ATIM Example
22
ATIM Example (STA 1 Waking Up
STAs 2, 3, and 4)
23
Summary of PS

infrastructure network



PCF
DCF (omitted)
ad hoc network

DCF
24