Protocol timeline
The target is asleep at first. The sender repeats addressed strobes until the target's wake check overlaps a strobe.
Addressed short strobe
Early ACK
Data frame
Long-preamble reference
X-MAC Strobed Preamble Animation
Trace how short addressed strobes, receiver wake checks, and early ACKs reduce preamble waste in duty-cycled wireless sensor networks
animation
xmac
mac
duty-cycling
low-power-listening
wsn
energy
A learner-ready X-MAC workbench with duty-cycle timing controls, early ACK animation, B-MAC comparison, bystander overhearing evidence, and technical accuracy notes.
Animation
Wireless sensor networks
Low-power listening
MAC protocol
X-MAC: stop the preamble as soon as the sleeping receiver wakes
Move through a duty-cycled radio exchange. The sender does not know when the target will wake, so it sends short addressed preamble strobes with listening gaps. When the target hears one, it returns an early ACK and the sender switches to data instead of transmitting a long continuous preamble.
Short strobesCurrent MAC behavior
ACK at 194 msReceiver response timing
8 strobesAddressed preambles sent
71% less TXVersus long preamble airtime
Problem
A long-preamble sender must keep transmitting long enough to cover a full receiver check interval, even if the receiver wakes early.
X-MAC Move
Short preamble packets carry the target address and leave gaps so the receiver can answer before the worst-case wait is over.
Receiver Test
The intended receiver sends an early ACK. A non-target receiver can read the address and return to sleep.
Design Lever
Longer check intervals save sleeping energy but increase worst-case latency and the number of strobes before a target wakes.
Sleep gap 1
The receiver saves energy by sleeping. The sender must bridge the uncertainty until a wake check happens.Addressed strobes 2
Each short preamble names the target, so non-target nodes do not need to stay awake for the whole exchange.Early ACK 3
The target answers during a gap. The ACK tells the sender to stop strobes and move to the payload.Data transfer 4
Data begins before the long-preamble worst-case duration, reducing latency and preamble transmit airtime.Sender energy model
144 / 500 ms X-MAC still spends energy while sending strobes, but it can stop once the early ACK arrives. Model compares preamble transmit airtime only; actual radios also spend energy in gaps, ACK reception, contention, and data transfer.Latency model
294 ms earlier Compared with waiting for a full long preamble before sending the data frame. Worst case approaches the check interval when the target wakes late or an ACK is missed.Bystander overhearing
Address mismatch A bystander can read the target address in a strobe and go back to sleep instead of listening until data. This is why the animation keeps the non-target lane separate from the target lane.What to watch first
One exchange
Follow the vertical time cursor. The important moment is not the first strobe; it is the first addressed strobe that overlaps the target's wake check.
Then change timing
Move the target wake slider later. More strobes are needed, preamble TX grows, and data starts closer to the long-preamble reference.
Technical accuracy notes
What "preamble" means here
In this teaching page, a strobe is a short MAC-level preamble packet carrying the receiver address. It is not BLE advertising and it is not only a PHY sync bit pattern.
What the energy number means
The savings metric compares preamble transmit airtime against a long-preamble reference. It is a teaching model, not a full radio energy budget.
Common mistakes
Long interval is not free
A long check interval reduces receiver wake frequency, but the sender may need to bridge a longer unknown sleep gap when it has data to send.
Non-target behavior matters
X-MAC's addressed strobes help bystanders return to sleep after an address mismatch instead of overhearing every following frame.
Terms
- Check interval: time between receiver wake checks in asynchronous low-power listening.
- Strobe: short addressed preamble packet, repeated until a target ACK or timeout.
- Early ACK: receiver response sent during a gap, telling the sender the target is awake.
Teaching formulas
long preamble = check intervalX-MAC TX = strobes sent x strobe lengthTX saving = 1 - X-MAC TX / long preamble
Design tradeoffs
- Shorter checks reduce sender waiting but wake the receiver more often.
- Longer strobes can be easier to catch but cost more airtime.
- Lost ACKs push the exchange toward the long-preamble worst case.
Try this
Set the target wake time near the end of the check interval. Explain why X-MAC still helps bystanders even when sender savings shrink.
Diagnose
Choose "First ACK lost". Which part of the exchange repeats, and why does the sender not send data after the first attempted ACK?
Compare
Use a 300 ms and a 700 ms check interval with the same wake percentage. Which design feels better for bursty event traffic?