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 CSC8908-005 Dr. Li 09/17/2008

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

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.

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.

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

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

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

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.

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.

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.

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

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.

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.

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

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.

Energy Models Energy to receive a packet Energy to send a packet

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

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. 783-791, 2005.