1. Michael Buettner, Gary V. Yee, Eric Anderson, Richard Han
X-MAC: A short preamble MAC protocol for duty-cycled wireless sensor networks
Michael Buettner, Gary V. Yee, Eric Anderson, Richard Han
Telvis Calhoun
Wireless Sensor Networks
CSC
Dr. Li
09/17/2008

2. Outline Duty Cycled MACs Common Issues Duty Cycled MAC protocols XMAC
Conclusions

3. Duty Cycled MACS Use awake, sleep intervals to conserve energy
Key parameters
Sleep time
Wake time
Energy consumed during the awake state and the sleep state.
The period of a duty cycle is equivalent to its sleep time plus awake time.

4. Idle Listening Problem
Idle Listening consumes substantial energy
Synchronized protocols
Nodes awake on a schedule
Asynchronized protocol
Uses low power listening
Hybrids
Combine synchronized and asynchronized.

5. Low Power Listening
Sender uses longer preamble to allow the receiver to awake periodically.

6. Asynchronized Protocols
Advantages
Use extended preamble
Sender and receiver can have decoupled duty cycles.
No synchronization overhead.
Awake periods are much shorter
Disadvantages
Frame exchange delay even if receiver awakes before preamble ends
Overhearing problem
Preamble latency is expensive for multihop routes

7. SMAC Synchronizes sensor clusters
Nodes periodically wake-up to receive synchronization info from its neighbors.
Mitigates need for system wide synchronization.
Nodes can belong to more that one virtual cluster.
Communicate using RTS-CTS
Can use adaptive listening
Neighbor briefly wakes up at the end of overheard RTS, CTS
Reduces one-hop latency

8. T-MAC Listen for a short time after awake period. Sleeps if IDLE.
Improves on S-MAC by shortening the awake period if IDLE.
For variable payloads, T-MAC uses 20% of energy used in S-MAC.

9. BMAC Uses local schedules
Send preamble that is slightly longer than the sleep period.
Long preamble assures that the neighbor will receive packet.
Provides API to adjust sleep period.
Suffers from overhearing problem.

10. Other Methods WiseMAC Hardware-Only Mechanism
Reduces the extended preamble length and energy
Put next awake time in ACKs.
Transmit to node only slightly before awake time specified in ACK
Hardware-Only Mechanism
Low power radio circuit that listens for the preamble.
Wake-On-Radio periodically listens for preamble then wakes up main radio circuit.

11. XMAC Short preamble Target in preamble Strobed preamble
Reduce latency and reduce energy consumption
Target in preamble
Minimize overhearing problem.
Strobed preamble
Reduces latency for the case where destination is awake before preamble completes.
Reduces per-hop latency and energy
Dynamic duty-cycle algorithm

12. Asynchronous Duty Cycling
Overhearing Energy loss is proportional to number of receivers in range.
Short preamble packets.
Non-receivers return to sleep more quickly.
Bad in high density sensor networks.

13. Strobing Reduce time and energy wasted sending preamble
When target wakes before the end
Many successive transmitters send full preamble to a single target a single target.
XMAC uses strobed preamble.
Addresses problem when target wakes before the end.
Send preamble packet
Listen for early acknowledgement packet
Preamble period must be greater than sleep period.

14. Distributed Coordination
Distributed Coordination Feature
Sensor overhears strobed preamble to target.
Execute random backoff longer than preamble period + data
Send to target after backoff.
Target remains awake after receiving another

15. Packetizing Radio Support
Streaming radios transmit the raw packet from MAC layer.
Packetizing radios add own preamble, header and CRC.
Packetizing radios do not support extended preambles.
XMAC’s strobed preamble packets work for packetizing radios.

16. Energy Models
Energy to receive a packet
Energy to send a packet

17. Conclusions Evaluation shows savings over low power listening.
Gains continue as network density increases.
Unable to schedule sufficiently small listening periods

18. References
M. Buettner, G. V. Yee, E. Anderson, and R. Han, "X-MAC: a short preamble MAC protocol for duty-cycled wireless sensor networks," in Proceedings of the 4th international conference on Embedded networked sensor systems Boulder, Colorado, USA: ACM, 2006.
G. P. Halkes, T. v. Dam, and K. G. Langendoen, "Comparing energy-saving MAC protocols for wireless sensor networks," Mob. Netw. Appl., vol. 10, pp , 2005.