Transcript Document 7628092

An Energy Efficient MAC Protocol for Wireless
LANs, E.-S. Jung and N.H. Vaidya,
INFOCOM 2002, June 2002
吳豐州
Agenda
Introduction
 Power Saving Mechanism in DCF
 Dynamic Power Saving Mechanism
 Simulation
 Conclusion

Agenda
Introduction
 Power Saving Mechanism in DCF
 Dynamic Power Saving Mechanism
 Simulation
 Conclusion

Introduction
Battery power is one of the critical
resources in WLAN Power Limited!!
 Battery management
 Power control
 Energy-efficiency protocol
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Introduction
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Wireless interface consumes significant
power, and can be in either the awake or
doze state
In awake state, there are three different
modes, transmit, receive, idle, and each
consumes 1.65W, 1.4W, 1.15W respectively.
As a contrast, in doze state consumes
0.045W
thus power saving mechanism (PSM) is often
putting wireless interface into a doze state
Agenda
Introduction
 Power Saving Mechanism in DCF
 New Power Saving Mechanism
 Simulation
 Conclusion

Power Saving Mechanism in DCF
Power Saving Mechanism in DCF
Time is divided into beacon intervals
 At the beginning of beacon interval,
there exists a specific time interval,
called ATIM window (Ad-hoc Traffic
Indication Message Window )
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Power Saving Mechanism in DCF
ATIM window is utilized to announce
any packets pending transmission to
nodes in doze state and every node is
awake during ATIM window
 When a node wants to transmit, it
sends ATIM frame in ATIM window
first, and then a destination node ready
to receive, it replies an ATIM-ACK
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Power Saving Mechanism in DCF
After the ATIM handshake, both source
and destination node will be stay
awake for the remaining beacon
interval to perform the data
transmission
 A node that ha not transmitted or
received an ATIM frame may enter the
doze state for saving energy after
finishing its ATIM window
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Power Saving Mechanism in DCF
During ATIM window, only ATIM and
ATIM-ACK can be transmitted, real data
transmission can only occur after the
ATIM window
 Overhead in energy consumption is
incurred for transmitting or receiving
ATIM and ATIM-ACK, and there is
overhead in time due to the ATIM
window
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Power Saving Mechanism in DCF
All nodes use the same (fixed) ATIM
window size critically affects
throughput and energy consumption,
and a fixed ATIM window does not
perform well in all situations
 If the ATIM window is chosen to be too
small, there may not be enough time
available to announce buffered packets,
potentially degrading throughput.
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Power Saving Mechanism in DCF
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If the ATIM window is too large, there
would be less time for the actual data
transmission, since data is transmitted
after the end of the ATIM window,
again degrading throughput at high
loads
Agenda
Introduction
 Power Saving Mechanism in DCF
 Dynamic Power Saving Mechanism
 Simulation
 Conclusion

Dynamic Power Saving Mechanism
Dynamic power saving mechanism
(DPSM) is similar to the IEEE 802.11
MAC protocol, we first describe how
IEEE 802.11 works
 IEEE 802.11 MAC Protocol
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When a node S wants to transmit a packet
to a node D it choose a “backoff” counter
uniformly distributed in the interval [0,cw]
Dynamic Power Saving Mechanism
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IEEE 802.11 MAC Protocol
cw = CWmin, at the beginning and also
after each successful transmission
 S waits until medium is idle, and then the
backoff counter is decremented by 1 after
each “slot time”
 When counter reaches 0, S transmit an
RTS. After D receiving RTS, D replies a
CTS to S if D can communicate with S at
the present time
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Dynamic Power Saving Mechanism
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IEEE 802.11 MAC Protocol
Absence of the CTS is taken as an
indication of congestion, and S doubles its
cw, picks a new backoff counter uniformly
distributed over [0,cw], and repeats the
above procedure
 After RTS-CTS, S sends DATA to D and
after D receiving DATA successfully, D
sends an ACK to S
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Dynamic Power Saving Mechanism
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Key Features of DSPM
Dynamic adjustment of ATIM window
 Longer dozing time (more energy saving)
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Dynamic Power Saving Mechanism
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Dynamic adjustment of ATIM window
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In the proposed DPSM scheme, each node
independently chooses an ATIM window
size based on observed network conditions
Dynamic Power Saving Mechanism
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Longer dozing time
In PSM specified in IEEE 802.11, when a
node transmits or receives an ATIM frame
during an ATIM window, it must stay
awake during the entire beacon interval
 we allow a node to enter the doze state
after completing any transmissions that
are explicitly announced in the ATIM
window
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Dynamic Power Saving Mechanism
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Longer dozing time
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there is a finite delay associated with the
doze-to-awake transition, in addition to a
higher energy consumption. Therefore, in
our scheme, a node will not enter the doze
state after completing packet
transmissions if the remaining duration in
the current beacon interval is “too small”
Dynamic Power Saving Mechanism
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In DPSM Operation, following
modifications are made
Announce one ATIM frame per destination
 Increasing and decreasing ATIM window
size
 Backoff algorithm for ATIM frame
 Packet marking
 Piggybacking of ATIM window size
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Dynamic Power Saving Mechanism
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Announce one ATIM frame per
destination
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When a node, say node A, successfully
transmits an ATIM frame to another node,
say node B, node A will not transmit
another ATIM frame to the same
destination in the same beacon interval
Dynamic Power Saving Mechanism
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Announce one ATIM frame per
destination
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If node A could not deliver all pending
packets that were previously announced to
node B, and the current beacon interval
expires, nodes A and B both stay up in the
next beacon interval, with B anticipating
the remaining packets from node A,
without node A having to send an ATIM
frame to node B
Dynamic Power Saving Mechanism
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Increasing and decreasing ATIM
window size
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We specify a finite set of ATIM window
sizes that may be used by each node, with
the smallest ATIM window size being
denoted as ATIMmin. Each allowed
window is called a level
Dynamic Power Saving Mechanism
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Backoff algorithm for ATIM frame
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while the backoff interval is being
decremented, say, at node A, the ATIM
window of node A might end. In this
event, the node will attempt to send an
ATIM frame for the corresponding
destination again in the next beacon
interval
Dynamic Power Saving Mechanism
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Packet marking
If ATIM-ACK has not been received after
three transmissions, the transmitted
packet is “marked” and re-buffered for
another try (also up to 3 times) in the next
beacon interval
 after three attempts in a beacon interval,
the ATIM frame for a given destination is
only transmitted again in the next beacon
interval
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Dynamic Power Saving Mechanism
Dynamic Power Saving Mechanism
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Piggybacking of ATIM window size
Each node piggybacks its own ATIM
window size on all transmitted packets
 The packets pending to be transmitted are
sorted by the size of the ATIM window at
their destinations
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Dynamic Power Saving Mechanism
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Piggybacking of ATIM window size
for implementing the above scheme
consists of several queues, one queue
corresponding to each allowed level of the
ATIM window, the smallest value of the
ATIM window being ATIMmin
 the packet is re-buffered in the queue
corresponding to ATIM window size
ATIMmin, to give a higher transmission
priority to such packets
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Dynamic Power Saving Mechanism
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Rules for Dynamic ATIM Window
Adjustment
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Initially, each node begins with ATIM
window size equal to ATIMmin
Dynamic Power Saving Mechanism
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Rules for increasing the ATIM window
size
Based on the number of pending packets
that could not be announced during the
ATIM window
 Based on overheard information
 Receiving a marked packet
 Receiving an ATIM frame after ATIM
window
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Dynamic Power Saving Mechanism
Dynamic Power Saving Mechanism
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Rules for decreasing the ATIM window
size
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During an ATIM window, if a node has
successfully announced one ATIM frame to
all destinations that have pending packets
and no window increasing rule defined
above is satisfied, it means that the
current ATIM window size was big enough
Agenda
Introduction
 Power Saving Mechanism in DCF
 Dynamic Power Saving Mechanism
 Simulation
 Conclusion

Simulation

Two metrics are used for evaluation
Aggregate throughput over all flows in the
network
 Aggregate throughput per unit of energy
consumption
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Simulation
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Simulation model
Duration 25 sec
 Source node generates CBR traffic, Packet
size of each flow is 512 bytes
 The initial energy for each nodes is 1000
joules so nodes do not run out of energy
during the simulations
 The beacon interval 100 ms both PSM and
NPSM
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Simulation
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Simulation model
Wireless interface consumes 1.65W, 1.4W,
1.15W, and 0.045W in the transmit,
receive, and idle modes and the doze
state, respectively
 800 μs as the doze-to-awake transition
time and a node will consume twice as
much power as the idle mode (i.e., 2.3W)
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Simulation
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Wireless LAN scenario
Network sizes are 8, 16, 32, 64 and half
the nodes are source and the other half
are destination
 Simulated network loads are 5%, 10%,
20%, 30%, 40%, and 50%, measured as
a fraction of the channel bit rate of 2 Mbps
 With a total load of 10%, and 4 traffic
sources, each traffic has a rate of 0.05
Mbps
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Simulation (fixed network load)
Simulation (fixed network load)
Simulation (fixed network load)
Simulation (dynamic network
load)
Simulation (dynamic network
load)
Simulation (dynamic network
load)
Agenda
Introduction
 Power Saving Mechanism in DCF
 Dynamic Power Saving Mechanism
 Simulation
 Conclusion

Conclusion
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The ATIM window size in PSM in IEEE 802.11
significantly affects the throughput and the amount
of energy saving
In PSM, if the ATIM window is too small, the
throughput degrades as the network load becomes
heavier. If the ATIM is too large, the energy gain
from power saving mode become small, since each
node must stay awake during the ATIM window
In DPSM, a node also can power off its wireless
network interface whenever it finishes packet
transmission for the announced packets. Simulation
results show that the proposed scheme can improve
energy consumption without degrading throughput