X-MAC: A short preamble MAC protocol for duty-cycled wireless sensor networks Michael Buettner, Gary V. Yee, Eric Anderson, Richard HanTelvis CalhounWireless Sensor Networks CSC8908-005Dr. Li09/17/2008

OutlineDuty Cycled MACsCommon IssuesDuty Cycled MAC protocolsXMACConclusions

Duty Cycled MACSUse awake, sleep intervals to conserve energyKey parameters Sleep timeWake timeEnergy 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 ProblemIdle Listening consumes substantial energy Synchronized protocols Nodes awake on a scheduleAsynchronized protocol Uses low power listening Hybrids Combine synchronized and asynchronized.

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

Asynchronized Protocols AdvantagesUse extended preambleSender and receiver can have decoupled duty cycles. No synchronization overhead. Awake periods are much shorterDisadvantagesFrame exchange delay even if receiver awakes before preamble endsOverhearing problem Preamble latency is expensive for multihop routes

SMACSynchronizes sensor clustersNodes 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-CTSCan use adaptive listeningNeighbor briefly wakes up at the end of overheard RTS, CTSReduces one-hop latency

T-MACListen 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.

BMACUses local schedulesSend 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 MethodsWiseMACReduces the extended preamble length and energyPut next awake time in ACKs.Transmit to node only slightly before awake time specified in ACKHardware-Only MechanismLow power radio circuit that listens for the preamble.Wake-On-Radio periodically listens for preamble then wakes up main radio circuit.

XMACShort preambleReduce latency and reduce energy consumptionTarget in preambleMinimize overhearing problem.Strobed preambleReduces latency for the case where destination is awake before preamble completes.Reduces per-hop latency and energyDynamic duty-cycle algorithm

Asynchronous Duty CyclingOverhearing 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.

StrobingReduce time and energy wasted sending preamble When target wakes before the endMany 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 packetListen for early acknowledgement packetPreamble period must be greater than sleep period.

Distributed CoordinationDistributed Coordination FeatureSensor overhears strobed preamble to target.Execute random backoff longer than preamble period + dataSend to target after backoff.Target remains awake after receiving another

Packetizing Radio SupportStreaming radios transmit the raw packet from MAC layer.Packetizing radios add own preamble, header and CRC.Packetizing radios do not support extended preambles. XMACs strobed preamble packets work for packetizing radios.

Energy ModelsEnergy to send a packetEnergy to receive a packet

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

ReferencesM. 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.