An Energy Efficient MAC Protocol for Wireless LANs

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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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Dynamic Power Saving Mechanism

• 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

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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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DPSM Operation (cont.)

• 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

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DPSM Operation (cont.)

• 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 gt dynamic increase of ATIM window size

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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 neighbors window size is at least two levels
  larger
• Receiving an ATIM frame after ATIM window
• Receiving a marked packet

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Rules for Dynamic ATIM Window Adjustment

• 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

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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