An EnergyEfficient MAC Protocol for Wireless Sensor Networks

An Energy-Efficient MAC Protocol for Wireless Sensor Networks Wei Ye 1, John Heidemann 1, Deborah Estrin 2 1 USC Information Sciences Institute 2 UCLA and USC/ISI IEEE INFOCOM 2002 1

Introduction v. 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 v. Sensor-net applications • Nodes cooperate for a common task • In-network data processing IEEE INFOCOM 2002 2

Medium Access Control in Sensor Nets v Important attributes of MAC protocols 1. 2. 3. 4. 5. 6. 7. Collision avoidance Energy efficiency Scalability in node density Latency Fairness Throughput Bandwidth utilization IEEE INFOCOM 2002 Primary Secondary 3

Energy Efficiency in MAC v. Major sources of energy waste • Idle listening 0. 14 0. 12 0. 1 Diffusion Flooding Omniscient Multicast 0. 08 0. 06 0. 04 0. 02 00 50 100 150 200 250 300 Network Size Over always-listening MAC IEEE INFOCOM 2002 Average Dissipated Energy (Joules/Node/Received Event) Average Dissipated Energy üEnergy consumption of typical 802. 11 WLAN cards idle: receive — 1: 1. 05 to 1: 2 (Stemm 1997) üExample: directed diffusion (Intanagonwiwat 2000) 0. 018 0. 016 0. 014 0. 012 0. 01 0. 008 0. 006 0. 004 0. 002 00 Flooding Omniscient Multicast Diffusion 50 100 150 200 250 Network Size Over energy-aware MAC 4 300

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

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

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

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

Periodic Listen and Sleep v. 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 9

Collision Avoidance v. Problem: Multiple senders want to talk v. Options: Contention vs. TDMA v. 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 10

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

Message Passing v. Problem: Sensor net in-network processing requires entire message v. 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 v. Other nodes sleep for whole message time Fairness IEEE INFOCOM 2002 Energy Msg-level latency 12

Msg Passing vs. 802. 11 fragmentation v S-MAC message passing Data 19 RTS 21 ACK 18 CTS 20 . . . Data 17 ACK 16 Data 1 . . . ACK 0 v Fragmentation in IEEE 802. 11 • No indication of entire time — other nodes keep listening • If ACK is not received, give up Tx — fairness Data 3 RTS 3 CTS 2 Data 3 ACK 2 IEEE INFOCOM 2002 . . . ACK 2 Data 1 . . . 13 ACK 0

Implementation on Testbed Nodes v. Platform Motes (UC Berkeley) 8 -bit CPU at 4 MHz, 8 KB flash, 512 B RAM 916 MHz radio Tiny. OS: event-driven v. Compared MAC modules 1. IEEE 802. 11 -like protocol w/o sleeping 2. Message passing with overhearing avoidance 3. S-MAC (2 + periodic listen/sleep) IEEE INFOCOM 2002 14

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

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