An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC) Wei Ye, John Heidemann, Deborah Estrin

Sensor Network MAC Differences Energy Consumption  Difficult to recharge  Request long lifetime of work

Major source of energy waste Collision Overhearing Control Overhead Idle Listening  Listening to possible traffic that is not sent  50%-100% energy drain compared with receiving

Main Contributions Periodic listen and sleep Collision avoidance Overhearing avoidance Message passing

What is the main idea this paper propose?  Duty Cycles

Listen for SYNC tdtd Listen/Sleep Schedule Assignment Choosing Schedule (1) Sleep Listen Go to sleep after time t Sleep Listen Broadcasts A B Go to sleep after time t- t d Synchronizer Listen for a mount of time If hear no SYNC, select its own SYNC Broadcasts its SYNC immediately Follower Listen for a mount of time Hear SYNC from A, follow A’s SYNC Rebroadcasts SYNC after random delay t d

Listen for SYNC Go to sleep after time t2 tdtd Listen for SYNC Listen/Sleep Schedule Assignment Choosing Schedule (2) Sleep Listen Go to sleep after time t1 Sleep Listen A Broadcasts Sleep Listen C Broadcasts B Only need to broadcast once 1.B receive A’s schedule and rebroadcast it. 2.Hear a different SYNC from C. 3.Adapt both schedules Late sleep results to longer listening period Early sleep results to early wake up in the next schedule Nodes only rarely adopt multiple schedules

Listen/Sleep Schedule Assignment Maintaining Schedule Reason: Clock drift Solution: Sent SYNC periodically  Listen interval is divided into two parts SYNC RTS/CTS/DATA/ACK No data transmission during synchronization Node B listen Node A listen Node B listen

Collision and Overhearing Avoidance Collision Avoidance  Similar to RTS/CTS Virtual carrier sense (NAV) Physical carrier sense Overhearing Avoidance  Problem: A node picks up packets that are destined to other nodes  Solution: Interfering nodes go to sleep when overhear RTS, CTS or ACK.

Example (To sleep, or not to sleep, this is a question) A is talking to B D receives CTS from B -> sleep D’s transmission will collide B’s C receives RTS from A -> sleep C cannot receive CTS/DATA from E All immediate neighbours of transmitting node sleep How long should they sleep? C and D update their NAV Keeping sleeping until NAV count down to zero

Message Passing How to transmit long message? One long packet Many small packets with RTS/CTS/DATA/ACK for each S-MAC: Divide into fragments, transmit all in burst RTS/CTS reserve medium for the entire sequence Fragment-errors recovery with ACK - no control packets for fragments

Acknowledgment to Pro. Jun Yang Why use ACK?

Hidden Terminal Example C DATA AB C ACK AB

Why utilize message passing?

Message Passing Advantages: Energy saving: immediate node go to sleep when sense transmissions Reduces control overhead by sending multiple ACK Disadvantage: Node-to-node fairness reduces However, message-level latency reduces

Experiment Listen time: 300ms Sleeping time: 1s SYNC: every 13s (10 listen/sleep period) A, B, C use the same schedule

Heavy Traffic Light Traffic Energy save due to avoiding overhearing by using message passing Energy save due to periodic sleep OA SMAC

OA: In light traffic status, sources nodes keep listening for quite a long time

Heavy TrafficLight Traffic SYNC overhead Overhearing avoidance still benefit

Discussion What is the tradeoff by using duty cycles? How can we make the schedule more intelligent?

Acknowledgment to Jun Yang (CS, Duke) and Romit Roy Choudhury