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

td

Listen/Sleep Schedule AssignmentChoosing Schedule (1)

Sleep

Listen

Go to sleep after time t

Sleep

Listen

Broadcasts

A

B

Broadcasts

Go to sleep after time t- td

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 td

Listen for SYNC Go to sleep after time t2

td

Listen for SYNC

Listen/Sleep Schedule AssignmentChoosing 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 AssignmentMaintaining 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 listenNode A listen

Node B listen

Collision and Overhearing Avoidance Collision Avoidance

Similar to 802.11 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 BD 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 NAVKeeping 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

CDATA

A B

CACK

A BACK

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: 300msSleeping time: 1sSYNC: 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 802.11

OA

SMAC

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

Heavy Traffic Light 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