An Energy Efficient MAC Protocol for Wireless LANs

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 2/19

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

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state  Measured power consumption   awake : transmit (1.65 W), receive (1.4 W), idle (1.15 W) doze (0.045 W) 3/19

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 4/19

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 5/19

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 6/19

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 7/19

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 : ATIM min Each allowed window : level Different nodes using different ATIM window size 8/19

DPSM Operation (cont.)

 Backoff algorithm for ATIM frame     ATIM frame transmitted using CSMA/CA mechanism Initial cw value is picked in the range [0, cw min ] 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 cw min To decrease the probability of collision 9/19

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

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marked

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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 10/19

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 ATIM min ATIM frames are transmitted in the sorted order Queues for each level of ATIM window Re-buffered packet has a higher transmission priority 11/19

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 12/19

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 13/19

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 14/19

Simulation Results

 Aggregate Throughput (Fixed network load) 15/19

Simulation Results

 Aggregate Throughput per joule (Fixed network load) 16/19

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 17/19

Simulation Results

 Dynamic network load 18/19

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 19/19