S-MAC Synchronized Sleep Animation

Step through synchronized listen/sleep schedules, packet handshakes, energy savings, and latency trade-offs

Interactive S-MAC workbench with synchronized sleep/listen windows, SYNC/RTS/CTS/DATA/ACK stages, energy and latency calculations, schedule drift, mobile-safe reference cards, and technical accuracy notes.

Animation S-MAC Duty Cycling WSN MAC

S-MAC Synchronized Sleep Animation

Step through how Sensor-MAC saves energy by letting neighboring wireless sensor nodes share a synchronized listen window, exchange control packets, send data, and return to sleep. The important trade-off is visible immediately: lower duty cycle saves current, but queued packets wait longer for the next listen period.

2.19 mA Average Current

387 ms Worst Path Wait

Aligned Schedule State

Fixed duty cycle

Every cycle has a listen window and a longer sleep period. Nodes only communicate while awake.

SYNC aligns neighbors

Nodes exchange schedules so nearby nodes wake together instead of wasting idle listening.

RTS/CTS protects data

Unicast traffic can reserve the medium inside the active window before DATA and ACK.

Latency is the price

A packet generated during sleep waits until the next listen window, and each hop can add delay.

1

Sleep

Radio is off; current is low, but new packets must queue.

2

SYNC listen

Neighbors wake and exchange schedule beacons.

3

RTS

The source requests the channel for a receiver.

4

CTS

The receiver replies and nearby nodes set NAV.

5

DATA

Payload moves while both endpoints are awake.

6

ACK + sleep

The receiver acknowledges; nodes return to sleep.

1 SleepRadio off, packet waits for next listen period.

2 SYNC listenNeighbors wake and advertise schedule.

3 RTSSource reserves the medium.

4 CTSReceiver confirms and neighbors defer.

5 DATAPayload transfers inside the active window.

6 ACK + sleepACK completes the exchange; nodes sleep again.

Synchronized Sleep Field

Sleep: the radio is off for most of the cycle, so new traffic waits in a queue.

Sleep

Energy: fixed sleep schedule Average current is dominated by the selected duty cycle.

Latency: packet is queued A packet generated during sleep waits for the next listen window.

Next: SYNC listen Step forward to see neighbors wake and exchange schedule information.

Controls

Change the schedule, traffic, and drift to see when S-MAC helps and when it becomes risky.

Duty cycle

12%

Listen time as a percentage of each S-MAC cycle.

Cycle length

500 ms

Longer cycles reduce wake-ups but increase queueing delay.

Nodes shown

5

Extra nodes show overhearing and schedule alignment effects.

Path hops

3

Store-and-forward paths can add one sleep wait per hop.

Traffic load

8 pkt/min

Higher traffic uses more of the listen window for handshakes and DATA.

Schedule drift

4 ms

Offset between neighbor schedules. Large drift can remove rendezvous overlap.

Duty cycle math Listen time is cycle length multiplied by duty cycle; sleep time is the rest of the cycle.

Idle listening S-MAC reduces idle listening by turning the radio off during long sleep periods.

Schedule overlap SYNC packets are useful because sender and receiver must be awake at the same time.

Per-hop delay A queued packet can wait until the next listen window at each hop in a path.

Overhearing avoidance RTS/CTS lets non-participating neighbors set NAV and sleep instead of listening to DATA.

Fixed schedule limit S-MAC is simple, but fixed active windows waste energy under very light traffic.

Formula Trace

The active stage updates the equations and units behind the visual schedule.

listen_ms = cycle_ms * duty_cycle / 100
sleep_ms = cycle_ms - listen_ms
I_avg ~= duty * I_listen + (1 - duty) * I_sleep + tx_fraction * (I_tx - I_listen)
energy_saved_vs_always_listen = 1 - I_avg / I_listen
average queue wait to next listen ~= sleep_ms^2 / (2 * cycle_ms)
worst_path_wait ~= hops * sleep_ms
rendezvous_overlap_ms = max(0, listen_ms - schedule_drift_ms)

Reference Material

Use these quick cards to connect the animation to real S-MAC design decisions.

S-MAC Quick Reference

Listen periodFixed active portion of the cycle where nodes can receive SYNC and data traffic.

Sleep periodRadio-off interval that saves energy but queues new packets.

SYNC packetSchedule advertisement used to align neighboring nodes.

Virtual clusterGroup of neighbors following the same or compatible schedules.

RTS/CTSOptional reservation exchange for unicast data inside the active window.

NAVDuration hint that lets overhearing nodes sleep or defer during another exchange.

Energy and Latency Checklist

Lower duty cycleReduces average current but increases sleep waiting time.

Longer cycleFewer wake-ups per minute, but worse worst-case wait.

High trafficConsumes more of the listen window and can reduce the energy advantage.

Multi-hop pathsDelay can accumulate because forwarding often waits for a later listen window.

Schedule driftClock error reduces overlap unless SYNC refreshes are frequent enough.

Always-on baselineEnergy savings are compared with keeping the radio continuously listening.

Technical Accuracy Notes

Current modelThis page uses a simple radio current model: listen 18 mA, transmit 28 mA, sleep 0.03 mA.

Queue waitAverage queue wait assumes packet generation is uniformly distributed across the cycle.

Worst caseWorst waiting time is bounded by the sleep period before the next listen window.

Handshake timeSYNC, RTS, CTS, DATA, and ACK durations are simplified for teaching visibility.

CollisionsS-MAC still needs contention handling; synchronized wake-up does not remove MAC contention.

Protocol comparisonT-MAC adapts the active period; X-MAC uses strobed preambles instead of synchronized schedules.

Related Practice

Open these next to compare S-MAC with adaptive and preamble-sampling approaches.

T-MAC Adaptive Duty CycleCompare fixed listen windows with adaptive early sleep. X-MAC Strobed PreambleStudy asynchronous low-power listening with early ACKs. Sleep-Wake Duty CycleReview current, battery life, and latency fundamentals.