An Ultra Low Power MAC Protocol for the Downlink of Infrast.ppt

1、WiseMAC: An Ultra Low Power MAC Protocol for the Downlink of Infrastructure Wireless Sensor Networks,Presented by Angel Pagan November 27, 2007,A. El-Hoiydi and J.-D. Decotignie CSEM, Swiss Center for Electronics and Microtechnology, Inc. Computers and Communications, 2004. Proceedings. ISCC 2004. N

2、inth International Symposium Volume 1, Issue , 28 June-1 July 2004 Page(s): 244 - 251 Vol.1,2,Outline,Introduction Infrastructure Network WiseMAC ZigBee Comparison Power-delay characteristics Conclusion,3,Introduction,Focus on infrastructure topologyPropose WiseMAC (Wireless Sensor MAC) for the down

3、linkTrade-off power consumption and transmission delay.WiseMAC is compared to ZigBee.,4,Power consumption,Energy efficiency is important in the sensor nodes Power consumption of transceiver in receiver mode is considerable Minimize energy waste Idle listening active listening to idle channel. Overhe

4、aring reception of a packet or part of a packet destined to another node.,5,Infrastructure WSN,Composed of a number of access points (AP). Each access point serves a number of sensor nodes. AP is energy unconstrained Can listen continuously Can send any amount of signaling traffic Exploited by WiseM

5、AC protocol,6,Traffic direction,Focus on low traffic situations Downlink From AP to sensor nodes Transmit configuration data and query requests Transmit without requiring sensor node continuously listening Uplink From sensor node to AP Transmit acquired data AP can listen continuously with unlimited

6、 power Only issue is multiple access of medium,7,WiseMAC,Medium Access Control protocolBased on CSMA with preamble samplingSampling minimizes idle listeningExploit sensor nodes sampling schedules to minimize length of the wake-up preambleData frames are repeated in long preambles to mitigate overhea

7、ring,8,Sampling,Sensor nodes regularly sample the medium listen to the radio channel for a short duration If medium found busy listen until frame is received or until idle again Sensor node sample with constant period Tw Schedule offsets are independent of each other and constant,9,Preamble,AP trans

8、mits wake-up preamble of duration Tp in front of every data frameEnsures the receiver will be awake when the data frame arrivesProvides low power consumption when channel is idleTp is minimized by exploiting knowledge of sensor node sample schedule,10,Sampling schedules,AP keeps an up-to-date sampli

9、ng schedule of all sensor nodesSample schedules acquired from every acknowledgment packetACK specifies the remain time until next scheduled sampling,11,WiseMAC sampling activity,Diagram from IEEE Computer Journal feature article, WiseNET: an ultra low-power wireless sensor network solution, publishe

10、d by IEEE Computer Society, August 2004,12,Preamble duration,Tp must compensate for drift between the clock at the AP and the sensor nodePreamble duration must be 4L if both quartz have a frequency tolerance of and L is the interval between communications,13,Drift Compensation,AP may be late, while

11、node may be early, start the preamble 2L in advance Because the sensor node may be late while the AP is early the duration of preamble must be 4L,Diagram from presentation slides of Real-Time Networking Wireless Sensor Networks by Prof J.-D. Decotignie. http:/lamspeople.epfl.ch/decotignie/RTN_WSN.pd

12、f,14,Drift Compensation (contd),In cases where L is very large and 4L is larger than the sampling period Tw, the preamble length of Tw is used.,Tp = min (4L, Tw),15,WiseMAC is adaptive,In high traffic, the interval L between communications is small In low traffic, the interval L between communicatio

13、ns is large, with maximum equal to Tw WiseMAC is adaptive to the traffic; per packet overhead decreases in high traffic conditions,Diagram from presentation slides of Real-Time Networking Wireless Sensor Networks by Prof J.-D. Decotignie. http:/lamspeople.epfl.ch/decotignie/RTN_WSN.pdf,16,High traff

14、ic conditions,When traffic is high overhearing is mitigated due to the preamble sampling technique and minimized preamble Short transmissions are likely to fall in between sampling instants of potential overhearers,17,Low traffic conditions,When traffic is low Tp can exceed the length of the data pa

15、cket In which case the wake-up preamble is composed of padding bits and repetitions of the data frame,18,Frame pending bit,In the header of the data packetIf set, the sensor node will continue listening after having sent acknowledgmentThe AP will send the next data packet after receiving the acknowl

16、edgementPermits a larger wake-up interval and reduces queue delay at APCost of preamble is shared among multiple data packets,19,IEEE 802.15.4 ZigBee,WiseMAC is compared to the power save MAC protocol in ZigBeeUses central coordinator labeled access point (AP) in this documentAP buffers incoming tra

17、fficAP sends periodic beacon every TwBeacon contains address of sensor node for which data is buffered,20,ZigBee Power Save Protocol,All sensor nodes wake-up regularly to receive beacon Sensor node polls AP for the buffered data if the beacon contains its address Also uses frame pending bit in data

18、packet header,21,Optimize Zigbee,For fair comparison, consider optimized version of ZigBeeIn practice polling procedure consist of POLL-ACK-DATA-ACKInterested in performance of basic protocol that uses beacon indicationFor low power consumption, consider POLL packet followed by DATA packetACK is pig

19、gy-backed on following POLL packet,22,Performance Analysis,Model transition delays between transceiver states and power consumption in each state Transceiver states DOZE The transceiver is not able to transmit nor receive, but is ready to quickly power-on into the receive or transmit state RX The tr

20、ansceiver is listening to the channel possibly receiving data TX The transceiver is transmitting data,23,Radio Model,Ts the setup time required to turn on the transceiver from DOZE state into the RX or TX state TT the turn-around time required to switch the transceiver between RX and TX Pz, PR, PT p

21、ower consumed, respectively, in the DOZE, RX, and TX states PR = PR PZ ; the increment in power consumption caused by being in the RX state PT = PT PZ ; the increment in power consumption caused by being in the TX state,24,Traffic Model,Population of N sensor nodes Downlink Poisson traffic arrives a

22、t the AP at global rate Average packet inter-arrival time at sensor node is L = N/ Data packet duration is TD Control packet (pollings, acks, beacons) duration is Tc Assume low traffic conditions,1/ TD + TT + Tc,25,WiseMAC Power Consumption,Average power consumed by WiseMAC,Power consumed in DOZE st

23、ate,Power consumed by sampling activity,Power consumed while receiving the packet and ACK it,Power consumed overhearing the packet by N-1 neighbors,Duration destination node listens to preamble prior to detect of start of the data frame,Average duration a potential overhearer listens to a transmissi

24、on,26,ZigBee Power Consumption,Average power consumed by ZigBee,Power consumed in DOZE state,Power consumed while listening to cover the drift between AP and node,Power consumed to power on and listen to the beacon length Tc,Power consumed while polling and receiving of data packet every L seconds,2

25、7,Transmission delay,The time elapsed between the arrival of a packet at the AP and the end of its transmission to the destination,Transmission delay with WiseMAC,Transmission delay with ZigBee,28,Radio Transceiver,Consider the transceiver used for WiseNET low power radio transceiver,29,Power consum

26、ption and delay,Trade-off between consumed power and average transmission delay,WiseMAC consumes less power than ZigBee,30,Power-delay characteristics,Ideal delay,Ideal power consumption,Combine power plot with delay plot and draw power-delay characteristics for varying Tw,31,Compare wake-up schemes

27、,WiseMAC wake-up scheme consumes less power than the one of ZigBee As L approaches infinity the power consumption of WiseMAC and ZigBee becomesWiseMAC node powers up every Tw with a duration of a radio symbol ZigBee transceiver periodically receives a beacon with a duration larger than a radio symbo

28、l,32,Sensitivity Analysis,Vary the traffic and the number sensor nodesCompare WiseMAC, ZigBee, and WiseMAC*WiseMAC* - a sub-optimal version where long wake-up preambles are not composed of repeated data frames,33,Varying traffic,WiseMAC has low power consumption in both high and low traffic conditio

29、ns,WiseMAC* has more power consumption than WiseMAC for medium traffic overhearing is maximized for L 4000,34,Varying number of sensor nodes,Power consumption of ZigBee is independent of the number of nodes,Power consumption of ZigBee is independent of the number of nodes no overhearing, scales bett

30、er than WiseMAC,WiseMAC suffer from overhearing component overhearing component is proportional to the number of nodes,35,Conclusion,Proposed WiseMAC for the downlink of infrastructure wireless sensor networks Analyzed power consumption-delay trade-off in low traffic condition and analytically compared it against ZigBee WiseMAC is more power efficient than ZigBee up to hundreds of nodes WiseMAC can provide a lower power consumption than ZigBee for the same delay,36,Observations,Repetition of data frames in wake-up preamble explained?,