An Energy Efficient MAC Protocol for Wireless LANs
description
Transcript of An Energy Efficient MAC Protocol for Wireless LANs
An Energy Efficient MAC Protocol
for Wireless LANs
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Contents
Introduction
Power Saving Mechanism (PSM) for DCF in IEEE
802.11
Related Work
Proposed DPSM (Dynamic PSM) Scheme
Key Features of DPSM
DPSM Operation
Rules for Dynamic ATIM window adjustment
Performance Evaluation
Conclusion
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Introduction
Energy conserving mechanisms at various layers Routing layer MAC layer Transport layer
Energy efficient MAC protocol For wireless LAN By putting the wireless interface in a “doze” state
Measured power consumption awake : transmit (1.65 W), receive (1.4 W), idle (1.15
W) doze (0.045 W)
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PSM for DCF in IEEE 802.11 Two components in IEEE 802.11
PCF (Point Coordination Function)
DCF (Distributed Coordination Function)
Power Saving Mechanism for DCF Time is divided into Beacon Interval
All nodes are in awake state during an ATIM window
All nodes use the same ATIM window size
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Related Work Adjust Beacon Interval and ATIM window [Woesner,
1998] Simulation results for the PSM
Enforce nodes to enter doze state [Cano, 2001] Use RTS/CTS for traffic indication message (per packet
basis)
Costs of doze-to-active transition
SPAN : Elects a group of coordinators [Chen, 2001] Stay awake and forward traffic for active connections
Use advertised traffic window following an ATIM window
PAMAS : use two separate channels [Singh, 1998] Separated transmission of control packet / data packet
Nodes determine when to power off and the duration
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PSM with fixed ATIM window size
Affects throughput & energy consumption
Small window size
Not enough time available to announce traffic
Degrading throughput (potentially)
Large window size
Less time for actual data transmission
Higher energy consumption
DPSM : dynamically adjust the size of ATIM window
Dynamic Power Saving Mechanism
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Key Features of DPSM
Dynamic adjustment of ATIM window
Each node uses a different ATIM window size
Longer dozing time (more energy saving)
Enter the doze state after announced packet delivery
Remained duration in the beacon interval is longer
than 1600 μs
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DPSM Operation Announcing one ATIM frame per destination
Sender Informs the number of packets pending for
Receiver
If the announced packets are not delivered in a beacon
interval Stay up in the next beacon interval
Sender delivers remained packets without ATIM frame
Enter the doze state after successful packet transmission
Increasing and decreasing ATIM window size Finite set of ATIM window sizes
The smallest ATIM window size : ATIMmin
Each allowed window : level
Different nodes using different ATIM window size
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Backoff algorithm for ATIM frame ATIM frame transmitted using CSMA/CA mechanism
Initial cw value is picked in the range [0, cwmin]
If an ATIM-ACK is not received Doubles the value of cw and selects a new backoff interval
If the ATIM window ends Use doubled cw value in the next beacon interval
i.e., cw will not be reset to cwmin
To decrease the probability of collision
DPSM Operation (cont.)
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Packet marking Set retry limit for ATIM frame in a beacon interval as
3
If ATIM-ACK has not been received after 3
transmission Transmitted packet is “marked” and re-buffered for
another try
The node is free to send ATIM frame to another node
Re-buffered packet can stay in buffer for at most 2
beacon interval
Marking => dynamic increase of ATIM window size
DPSM Operation (cont.)
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DPSM Operation (cont.) Piggybacking of ATIM window size
Each node announces its own ATIM window size
Nodes may be aware of some or all of other ATIM
window sizes
Packets pending to be transmitted are sorted the size of ATIM window at their destination
Destination node with small size of ATIM window gets
preference
If unknown, it is assumed to be equal to ATIMmin
ATIM frames are transmitted in the sorted order
Queues for each level of ATIM window
Re-buffered packet has a higher transmission priority
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Rules for Dynamic ATIM Window Adjustment
Increasing rules The number of pending packets that could not be
announced during the ATIM window If the number of pending packets is more than 10
Overheard information If neighbor’s window size is at least two levels larger
Receiving an ATIM frame after ATIM window
Receiving a marked packet
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Decreasing rules When the current ATIM window is big enough
No window increasing rule is satisfied
If a node has successfully announced one ATIM frame to
all destinations that have pending packets
Rules for Dynamic ATIM Window Adjustment
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Performance Evaluation Performance metrics
Aggregate throughput over all flows
Aggregate throughput per unit of energy consumption
Simulation model Simulator : ns-2 with the CMU wireless extensions
Number of nodes : 8, 16, 32, or 64
Simulated flows : half of nodes
Network environment : LAN (one-hop network)
Traffic : CBR, 512 bytes packet in 2Mbps channel
Beacon Interval : 100 ms
ATIM window size : 2 ms ~ 50 ms
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Simulation Results Aggregate Throughput (Fixed network load)
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Simulation Results Aggregate Throughput per joule (Fixed network load)
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Simulation Results Network load vs. ATIM window size
The number of pending packets is the main factor for a node
to increase its ATIM window
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Simulation Results Dynamic network load
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Conclusion
The ATIM window size in PSM in IEEE 802.11
Affects the throughput and the amount of energy saving
The network load is directly related to ATIM window size
Fixed ATIM window size can not achieve optimal performance
Dynamic PSM can
Adapt its ATIM window size according to observed network
conditions
Improve energy consumption without degrading throughput