An Energy-Efficient MAC Protocol for Wireless Sensor Networks Tijs van Dam

Quick recap MAC protocol determines next node to use the medium Evaluate MAC protocols based on energy usage Use realistic communication patterns for evaluation

Classic CSMA If air is clear, send; if receiving, wait How do we know when to receive? Listen always! Listening costs 200x more energy than sleeping Not energy-efficient

S-MAC protocol Fixed active/sleep duty cycle Synchronization of active times Not optimal for fluctuating message load time (events) location (important nodes)

T-MAC protocol Make duty cycle dynamic End active time with time-out All messages at start of frame

T-MAC Protocol (2) RTS/CTS/DATA/ACK exchange Synchronization not critical No increasing backoff: fixed contention interval Overhearing avoidance

Early sleeping problem Ready-To-Send A BCD

Early sleeping problem B overhears CTS Clear-To-Send ! A BCD

Early sleeping problem Data ? A BCD B must remain silent A times out and goes to sleep

Early sleeping problem Ready-To-Send A BCD When B can send, A is sleeping B must wait until next frame (problem!)

Early sleeping problem A node goes to sleep while another node still has a message for it Occurs mostly in asymmetric communication: uni-directional flow (nodes-to-sink) edge of active area Throughput decreases dramatically

Solutions After overhearing a CTS, quickly send a Future-Request-To-Send Target node will be awake Short pause between CTS and DATA When receiving an RTS, ignore and send own RTS only when running out of buffers only after two tries Solutions increase throughput with 100%

Simulation setup 10 x 10 grid network Patterns: local unicast, nodes-to-sink Active areas simulate real-world events Realistic nodes energy radio properties DAS2 cluster 32 – 64 cpus 2 mins / simulation, > 40,000 msgs

Simulation results energy [ mA / node ] load [ msg / node / s] Nodes-to-sink pattern, msglength = 20 CSMA T-MAC

Simulation results (2) energy used [avg. mA / node] peak load [msg / node / s] Event-based local unicast, msglength=20 CSMA S- MAC T- MAC

Real implementation Implemented T-MAC on EYES nodes FRTS and priority not implemented (yet) Seems to work well, no data loss yet When idle, radio is off 97.5% of the time

Conclusions T-MAC reduces energy. A lot. Simple implementation Simple idea can lead to dramatic improvements Paper submitted for SENSYS’03 Infrared demo