An Energy Efficient MAC Protocol for Wireless LANs

1. An Energy Efficient MAC Protocol for Wireless LANs

2. 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

3. 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)

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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 => dynamic increase of ATIM window size

11. 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

12. 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

13. 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

14. 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

15. Simulation Results • Aggregate Throughput (Fixed network load)

16. Simulation Results • Aggregate Throughput per joule (Fixed network load)

17. 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

18. Simulation Results • Dynamic network load

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