An Energy-Efficient MAC Protocol for Wireless Sensor Networks

1. An Energy-Efficient MAC Protocol for Wireless Sensor Networks Wei Ye1, John Heidemann1, Deborah Estrin2 1USC Information Sciences Institute 2UCLA and USC/ISI IEEE INFOCOM 2002

2. Introduction • Wireless sensor network • Special ad hoc wireless network • Large number of nodes w/ sensors & actuators • Battery-powered nodes energy efficiency • Unplanned deployment self-organization • Node density & topology change robustness • Sensor-net applications • Nodes cooperate for a common task • In-network data processing IEEE INFOCOM 2002

3. Primary Secondary Medium Access Control in Sensor Nets • Important attributes of MAC protocols • Collision avoidance • Energy efficiency • Scalability in node density • Latency • Fairness • Throughput • Bandwidth utilization IEEE INFOCOM 2002

4. 0.018 0.016 Flooding 0.014 0.012 0.01 (Joules/Node/Received Event) Average Dissipated Energy 0.008 Omniscient Multicast 0.006 Diffusion 0.004 0.14 Diffusion 0.002 0.12 Flooding Omniscient Multicast 0 0.1 50 100 150 200 250 300 0 0.08 Network Size Average Dissipated Energy (Joules/Node/Received Event) 0.06 Over energy-aware MAC Over always-listening MAC 0.04 0.02 0 0 50 100 150 200 250 300 Network Size Energy Efficiency in MAC • Major sources of energy waste • Idle listening • Energy consumption of typical 802.11 WLAN cards idle:receive — 1:1.05 to 1:2 (Stemm 1997) • Example: directed diffusion (Intanagonwiwat 2000) IEEE INFOCOM 2002

5. Dominant in sensornets Common to all wireless networks Energy Efficiency in MAC • Major sources of energy waste (cont.) • Idle listening • Long idle time when no sensing event happens • Collisions • Control overhead • Overhearing • We try to reduce energy consumption from all above sources • Combine benefits of TDMA + contention protocols IEEE INFOCOM 2002

6. Latency Fairness Energy Sensor-MAC (S-MAC) Design • Tradeoffs • Major components in S-MAC • Periodic listen and sleep • Collision avoidance • Overhearing avoidance • Message passing IEEE INFOCOM 2002

7. sleep listen listen sleep Energy Latency Periodic Listen and Sleep • Problem: Idle listening consumes significant energy • Solution: Periodic listen and sleep • Turn off radio when sleeping • Reduce duty cycle to ~ 10% (200ms on/2s off) IEEE INFOCOM 2002

8. Node 1 sleep sleep listen listen Node 2 sleep sleep listen listen Schedule 1 Schedule 2 Periodic Listen and Sleep • Schedules can differ • Prefer neighboring nodes have same schedule • — easy broadcast & low control overhead Border nodes: two schedules broadcast twice IEEE INFOCOM 2002

9. Periodic Listen and Sleep • Schedule Synchronization • Remember neighbors’ schedules — to know when to send to them • Each node broadcasts its schedule every few periods of sleeping and listening • Re-sync when receiving a schedule update • Schedule packets also serve as beacons for new nodes to join a neighborhood IEEE INFOCOM 2002

10. Collision Avoidance • Problem: Multiple senders want to talk • Options: Contention vs. TDMA • Solution: Similar to IEEE 802.11 ad hoc mode (DCF) • Physical and virtual carrier sense • Randomized backoff time • RTS/CTS for hidden terminal problem • RTS/CTS/DATA/ACK sequence IEEE INFOCOM 2002

11. Overhearing Avoidance • Problem: Receive packets destined to others • Solution: Sleep when neighbors talk • Basic idea from PAMAS (Singh, Raghavendra 1998) • But we only use in-channel signaling • Who should sleep? • All immediate neighbors of sender and receiver • How long to sleep? • The duration field in each packet informs other nodes the sleep interval IEEE INFOCOM 2002

12. Energy Msg-level latency Fairness Message Passing • Problem: Sensor net in-network processing requires entire message • Solution: Don’t interleave different messages • Long message is fragmented & sent in burst • RTS/CTS reserve medium for entire message • Fragment-level error recovery — ACK — extend Tx time and re-transmit immediately • Other nodes sleep for whole message time IEEE INFOCOM 2002

13. ... ... ... Data 1 Data 3 Data 3 Data 17 Data 1 Data 19 CTS 2 RTS 21 CTS 20 RTS 3 ... ACK 2 ACK 0 ACK 0 ACK 2 ACK 18 ACK 16 Msg Passing vs. 802.11 fragmentation • S-MAC message passing • Fragmentation in IEEE 802.11 • No indication of entire time — other nodes keep listening • If ACK is not received, give up Tx — fairness IEEE INFOCOM 2002

14. Platform Motes (UC Berkeley) 8-bit CPU at 4MHz, 8KB flash, 512B RAM 916MHz radio TinyOS:event-driven Implementation on Testbed Nodes • Compared MAC modules • IEEE 802.11-like protocol w/o sleeping • Message passing with overhearing avoidance • S-MAC (2 + periodic listen/sleep) IEEE INFOCOM 2002

15. Source 1 Sink 1 Sink 2 Source 2 Experiments • Topology and measured energy consumption on source nodes • Each source node sends 10 messages • — Each message has 400B in 10 fragments • Measure total energy over time to send all messages IEEE INFOCOM 2002

16. Conclusions • S-MAC offers significant energy efficiency over always-listening MAC protocols • Future Plans • Measurement of throughput and latency • Throughput reduces due to latency, contention, control overhead and channel noise • Experiments on large testbeds • ~100 Motes, ~30 embedded PCs w/ MoteNIC • URL: http://www.isi.edu/scadds/ Thank You! IEEE INFOCOM 2002