电子科大-无线传感器网络ppt课件

1、0University of Electronic Science and Technology of ChinaWireless Sensor NetworksChapter 5:MAC Protocols1University of Electronic Science and Technology of ChinaWireless Sensor Networks物物 理理 层层数据链路层数据链路层网网 络络 层层传传 输输 层层应应 用用 层层能能 量量 分分 配配移移 动动 管管 理理应应 用用 优优 化化无线传感器网络协议Application LayerTransport Laye

2、rNetwork LayerLink LayerPhysical LayerPower Management PlaneMobility Management PlaneTask Management Plane2University of Electronic Science and Technology of ChinaWireless Sensor Networks数据链路层的主要作用是经过校验、确认和反响重发等手段将该原始的物理衔接改呵斥无过失的数据链路其主要义务是加强物理层传输原始比特的功能，使之对网络层显现为一条无错线路。两个设备同时在网络的共享媒体(有线或无线)上发送信息时，会相

3、互关扰而导致传输都无效因此对共享媒体的访问需求有效的管理3University of Electronic Science and Technology of ChinaWireless Sensor Networksq链路管理 q帧同步帧定界q帧的透明传输 q流量控制 q过失控制 q寻址数据链路层协议 :为实现数据链路控制功能而制定的规程或协议。4University of Electronic Science and Technology of ChinaWireless Sensor Networks局域网数据链路层的两个子层 由于局域网中传输介质的多样性，为了使数据链路层能更好地顺应多

4、种局域网规范，802 将局域网的数据链路层拆成两个子层：逻辑链路控制 LLC (Logical Link Control)子层媒体访问控制 MAC (Medium Access Control)子层。与接入到传输媒体有关的内容都放在 MAC子层，而 LLC 子层那么与传输介质无关，不论采用何种协议的局域网对 LLC 子层来说都是透明的 无线传感网传输介质相对采用单一的射频，因此无需分层。WSN依然是多点共享传输介质，必需提供相应机制来控制对传输介质的访问。5University of Electronic Science and Technology of ChinaWireless Sens

5、or NetworksObjectives of MAC Protocols控制设备允许访问物理层传输数据的方式和时间Collision AvoidanceEnergy EfficiencyScalabilityLatencyFairnessThroughputBandwidth Utilization6University of Electronic Science and Technology of ChinaWireless Sensor NetworksChallenges for MAC in WSNs1. WSN ArchitectureHigh density of nodesI

6、ncreased collision probabilitySignaling overhead should be minimized to prevent further collisionsSophisticated and simple collision avoidance protocols required7University of Electronic Science and Technology of ChinaWireless Sensor NetworksChallenges for MAC in WSNs2. Limited Energy ResourcesConne

7、ctivity and the performance of the network is affected as nodes dieTransmitting and receiving consumes almost same energyFrequent power up/down eats up energyNeed very low power MAC protocolsMinimize signaling overheadAvoid idle listeningPrevent frequent radio state changes (activesleep)8University

8、of Electronic Science and Technology of ChinaWireless Sensor NetworksPOWER CONSUMPTIONSENSORCPUTXRXIDLESLEEPRADIO9University of Electronic Science and Technology of ChinaWireless Sensor Networks * Idle Listening* Transmitter* ReceiverOBJECTIVE: Reduce energy consumption !Major Sources of Energy Wast

9、eCommon to all wireless networks10University of Electronic Science and Technology of ChinaWireless Sensor NetworksChallenges for MAC in WSNs3. Limited Processing and Memory CapabilitiesComplex algorithms cannot be implementedConventional layered architecture may not be appropriateCentralized or loca

10、l management is limitedSimple scheduling algorithms requiredCross-layer optimization requiredSelf-configurable, distributed protocols required11University of Electronic Science and Technology of ChinaWireless Sensor NetworksChallenges for MAC in WSNs4. Limited Packet SizeUnique node ID is not practi

11、calLimited header spaceLocal IDs should be used for inter-node communicationMAC protocol overhead should be minimized5. Cheap Encoder/DecodersCheap node requirement prevents sophisticated encoders/decoders to be implementedSimple FEC codes required for error controlChannel state dependent MAC can be

12、 used to decrease error rate12University of Electronic Science and Technology of ChinaWireless Sensor NetworksChallenges for MAC in WSNs6. Inaccurate Clock CrystalsCheap node requirement prevents expensive crystals to be implementedSynchronization problemsTDMA-based schemes are not practical7. Event

13、-based NetworkingObserved data depends on physical phenomenonSpatial and temporal correlation in the physical phenomenon should be exploitedBOTTOMLINE: Existing MAC protocols cannot be used for WSNs!13University of Electronic Science and Technology of ChinaWireless Sensor NetworksOverview of MAC Pro

14、tocols for WSNs1. Contention (RANDOM/CSMA)-Based MAC ProtocolsSleep-MAC, BMAC, T-MAC, X-MAX, CCMAC, etc2. Reservation-Based (TDMA BASED) MAC ProtocolsTRAMA, DMAC, FLAMA, etc3. HYBRID (CSMA/TDMA) MAC ProtocolsZMAC, .14University of Electronic Science and Technology of ChinaWireless Sensor NetworksMAC

15、 Protocols for WSN?-MAC (pick your letter!)-MAC, AI-LMAC, B-MAC, Bit, BMA, CC-MAC, CMAC, Crankshaft, CSMA-MPS, CSMA/ARC, DMAC, E2-MAC, EMACs, f-MAC, FLAMA, Funneling-MAC, G-MAC, HMAC, LMAC, LPLMMAC, MR-MAC, nanoMAC, O-MAC, PACT, PEDAMACS, PicoRadio, PMAC, PMAC, Q-MAC, Q-MAC, QMAC, RATE EST, RL-MAC,

16、RMAC, RMAC, S-MAC, S-MAC/AL, SCP-MAC, SEESAW, Sift, SMACS, SS-TDMA, STEM, T-MAC, TA-MAC, TICER, TRAMA, U-MAC, WiseMAC, X-MAC, Z-MACst.ewi.tudelft.nl/koen/MACsoup/15University of Electronic Science and Technology of ChinaWireless Sensor NetworksContention (Random)-Based MAC ProtocolsChannel access th

17、rough carrier sense mechanism. Provide robustness and scalability to the network. Collision probability increases with increasing node density.16University of Electronic Science and Technology of ChinaWireless Sensor NetworksIEEE 802.11 IEEE 802.11, “Wireless LAN medium access control (MAC) and phys

18、ical layer (PHY) specifications, 1999Originally developed for WLANs17University of Electronic Science and Technology of ChinaWireless Sensor NetworksREMINDER BASIC KNOWLEDGEBASIC CSMA/CA (FLOWCHART)(Distributed Foundation Wireless Medium Access Control - Distributed Coordinated Function CSMA/CA (DFW

19、MAC-DCF) for IEEE 802.11)18University of Electronic Science and Technology of ChinaWireless Sensor NetworksBASIC CSMA/CASlot TimeDirect access if medium is free IFStMedium BusyIFSIFSNext FrameContention Window(Randomized Back-off Mechanism)Stationsensesthe channel and it is idleIFS: Inter-frame Spac

20、es19University of Electronic Science and Technology of ChinaWireless Sensor NetworksDFWMAC-DCF CSMA/CA with ACKtSIFSDIFSContention WACKWaiting TimeOtherStationsReceiverSenderDataDIFSContentionWindow20University of Electronic Science and Technology of ChinaWireless Sensor NetworksProblems with CSMA/C

21、AHidden terminal problemExposed terminal problem21University of Electronic Science and Technology of ChinaWireless Sensor NetworksABCDHidden Terminal Problem当A和C检测不到无线信号时，都以为B是空闲的，因此都向B发送数据，结果发生冲突。这种未能检测出介质上已存在的信号的问题叫做隐蔽终端问题(hidden station problem) 隐藏终端带来的冲突使得通讯节点需求重发已发送的信息，假设重发后继续有隐藏终端冲突，那么网络的通讯会堕入

22、一种恶性循环22University of Electronic Science and Technology of ChinaWireless Sensor NetworksExposed Terminal Problem？ABCD23University of Electronic Science and Technology of ChinaWireless Sensor NetworksDFWMAC-DCF CSMA with RTS/CTSDIFSNext FrameCTSRTSOtherSourceDestinationDIFS SIFSContention WindowDefer

23、 AccessBackoff After DeferSIFSDataSIFSACKTime24University of Electronic Science and Technology of ChinaWireless Sensor NetworksHidden Terminal Problem A sends RTS B sends CTS C overhears CTS C inhibits its own transmitter A successfully sends DATA to BABCRTSCTSDATACTS25University of Electronic Scien

24、ce and Technology of ChinaWireless Sensor NetworksTx notinhibitedExposed Terminal Problem B sends RTS to A (overheard by C) A sends CTS to B C cannot hear As CTS C assumes A is either down or out of range C does not inhibit its transmissions to DABCDRTSRTSCTSCannot hear CTS26University of Electronic

25、 Science and Technology of ChinaWireless Sensor NetworksMAC Protocols for WSNsContention (RANDOM/CSMA)-Based MAC ProtocolsSleep-MACBMACCCMAC27University of Electronic Science and Technology of ChinaWireless Sensor NetworksS-MAC: SLEEP MACW. Ye, et. al., “Medium Access Control with Coordinated Adapti

26、ve Sleeping for Wireless Sensor Networks, IEEE/ACM Trans. on Networking, June 2004. Problem: “Idle Listening consumes significant energy Solution: Periodic listen and sleepDuring sleeping, radio is turned offReduce duty cycle to 10% (Listen for 200ms and sleep for 1.8s)LatencyEnergysleeplistenlisten

27、sleeptime28University of Electronic Science and Technology of ChinaWireless Sensor NetworksS-MACEach node goes into periodic sleep mode during which it switches the radio off and sets a timer to awake laterWhen the timer expires it wakes up and listens to see if any other node wants to talk to it29U

28、niversity of Electronic Science and Technology of ChinaWireless Sensor NetworksS-MACThe duration of the sleep and listen cycles are application dependent and they are set the same for all nodesRequires a periodic synchronization among nodes to take care of any type of clock drift各节点时钟必需同步，节点间需求同步唤醒！

29、？30University of Electronic Science and Technology of ChinaWireless Sensor NetworksPeriodic Sleep and ListenAll nodes are free to choose their own listen/ sleep schedules, Yes or no? Why?To reduce control overhead, neighboring nodes are synchronized together.They listen at the same time and go to sl

30、eep at the same time (synchronized sleep).31University of Electronic Science and Technology of ChinaWireless Sensor NetworksSynchronizationSYNC packets are exchanged periodically to maintain schedule synchronization.SYNCHRONIZATION PERIOD: Period for a node to send a SYNC packet.Receivers will adjus

31、t their timer counters immediately after they receive the SYNC packetSender Node ID Next Sleep TimeSYNC PACKET32University of Electronic Science and Technology of ChinaWireless Sensor NetworksMaintaining Synchronization Listen interval is divided into two parts: one for receiving SYNC packets and ot

32、her for transmitting/receiving RTS (Request To Send)33University of Electronic Science and Technology of ChinaWireless Sensor NetworksPERIODIC LISTEN AND SLEEP34University of Electronic Science and Technology of ChinaWireless Sensor NetworksChoosing and Maintaining SchedulesEach node maintains a sch

33、edule table that stores schedules of all its known neighborsFor initial schedule, DO:A node first listens to the medium for a certain amount of time (at least the synchronization period)35University of Electronic Science and Technology of ChinaWireless Sensor NetworksChoosing and Maintaining Schedul

34、esIf it does not hear a schedule (SYNC packet) from another node, it randomly chooses a schedule and broadcasts its schedule with a SYNC packet immediatelyThis node is called a Synchronizer36University of Electronic Science and Technology of ChinaWireless Sensor NetworksChoosing and Maintaining Sche

35、dulesIf a node receives a schedule from a neighbor before choosing its own schedule, Follows this neighbors scheduleBecomes a Follower Waits for a random delay and broadcasts its schedule37University of Electronic Science and Technology of ChinaWireless Sensor NetworksCoordinated SleepingSchedule 2S

36、chedule 1In a large network, we cannot guarantee that all nodes follow the same schedule. The node on the border will follow both schedules. When it broadcasts a packet, it needs to do it twice, first for nodes on schedule 1 and then for those on schedule 2.38University of Electronic Science and Tec

37、hnology of ChinaWireless Sensor NetworksBorder NodesBorder nodes have less time to sleep and consume more energy than others.OPTION: Let border node adopt only one schedule (received first).39University of Electronic Science and Technology of ChinaWireless Sensor NetworksCollision AvoidanceS-MAC is

38、based on contention, i.e., if multiple neighbors want to talk to a node at the same time, they will try to send when the node starts listening.Similar to IEEE802.11, i.e. use RTS/CTS mechanism to address the hidden terminal problemPerform carrier sense before initiating a transmissionWhats the mecha

39、nism?40University of Electronic Science and Technology of ChinaWireless Sensor NetworksCollision AvoidanceIf a node fails to get the medium, it goes to sleep and wakes up when the receiver is free and listening again Broadcast packets are sent without using RTS/CTS.Unicast(单播) data packets follow th

40、e sequence of RTS/CTS/DATA/ACK between the sender and receiver41University of Electronic Science and Technology of ChinaWireless Sensor NetworksCollision AvoidanceDuration field in each transmitted packet indicates how long the remaining transmission will be so if a node receives a packet destined t

41、o another node, it knows how long it has to keep silentThe node records this value in network allocation vector(NAV) and sets a timer for it42University of Electronic Science and Technology of ChinaWireless Sensor NetworksCollision AvoidanceWhen a node has data to send, it first looks at NAV. If thi

42、s value is not zero, then medium is busy (virtual carrier sense)43University of Electronic Science and Technology of ChinaWireless Sensor NetworksCollision AvoidanceThe medium is determined as free if both virtual and physical carrier sense indicate the medium is freeAll immediate neighbors of both

43、the sender and receiver should sleep after they hear RTS or CTS packet until the current transmission is over44University of Electronic Science and Technology of ChinaWireless Sensor NetworksAdaptive Listening FeatureReduce multi-hop latency due to periodic sleepBASIC IDEA: Let the node which overhe

44、ars its neighbors transmission stay awake45University of Electronic Science and Technology of ChinaWireless Sensor NetworksAdaptive Listening Feature4123CTSRTSCTS Both neighbors will learn about how long the transmission is from the duration field in the RTS and CTS packets. They are able to adaptiv

45、ely wake up when the transmission is over. Reduce latency by at least half (e.g., CTS of 2 is heard by 3 also. 3 remains awake!)listenlistenlistent1t2t046University of Electronic Science and Technology of ChinaWireless Sensor NetworksMessage Passing FeatureLong messages are broken down in to smaller

46、 packets and sent continuously once the channel is acquired by RTS/CTS handshake.Increases the sleep time, but leads to fairness problems.47University of Electronic Science and Technology of ChinaWireless Sensor NetworksS-MAC - EXAMPLETopologyTwo-hop network with two sources and two sinksSources per

47、iodically generate a sensing message which is divided into fragmentsTraffic load is changed by varying the inter-arrival period of the messages: (for inter-arrival period of 5s, message is generated every 5s by each source. Here it varies between 1-10s).48University of Electronic Science and Technol

48、ogy of ChinaWireless Sensor NetworksS-MAC - EXAMPLE49University of Electronic Science and Technology of ChinaWireless Sensor NetworksS-MAC - EXAMPLEIn each test, there are 10 messages generated on each source node. Each message has 10 fragments, and each fragment has 40 bytes (200 data packets to be

49、 passed from their sources to their sinks). The total energy consumption of each node is measured for sending this fixed amount of data.50University of Electronic Science and Technology of ChinaWireless Sensor NetworksExperiments024681020040060080010001200140016001800Average energy consumption in th

50、e source nodes A&BMessage inter-arrival period (traffic load) (second)(small value heavy traffic load)Energy consumption (mJ)802.11-like protocolwithout sleep S-MAC with Periodic SleepS-MAC without periodic sleep51University of Electronic Science and Technology of ChinaWireless Sensor NetworksExperi

51、mentsS-MAC consumes much less energy than 802.11-like protocol without sleepingAt heavy load, idle listening rarely happens, energy savings from sleeping is very limited. SMAC achieves energy savings by avoiding overhearing and efficiently transmitting long messages.At light load, periodic sleeping

52、plays the key role52University of Electronic Science and Technology of ChinaWireless Sensor NetworksEnergy Consumption over Multi-HopsTen-hop (11 nodes; 1m apart) linear (tandem) network at different traffic load; inter-arrival time 0-10s; source node sends 20 messages with each 100 bytes long.3 con

53、figurations for S-MAC: No sleep cycles10% duty cycle without adaptive listening 10% duty cycle with adaptive listening-Periodic listen interval: 115ms; 10% duty cycle means a frame length for 1.15sec.53University of Electronic Science and Technology of ChinaWireless Sensor Networks024681005101520253

54、0Message inter-arrival period (S)Energy consumption (J)10% duty cycle without adaptive listenNo sleep cycles10% duty cycle with adaptive listenAggregate energy consumption at different traffic load in the entire networkEnergy Consumption over Multi-HopsAt light traffic load, periodic sleeping has si

55、gnificant energy savings over fully active modeAdaptive listen saves more at heavy load by reducing latency54University of Electronic Science and Technology of ChinaWireless Sensor Networks0246810024681012Latency as Hops IncreaseAdaptive listen significantly reduces latency causes by periodic sleepi

56、ng0246810024681012Latency under highest traffic loadNumber of hopsAverage message latency (S)10% duty cycle withoutadaptive listen 10% duty cycle with adaptive listen No sleep cycles Latency under lowest traffic loadNumber of hopsAverage message latency (S)10% duty cycle withoutadaptive listen 10% d

57、uty cycle withadaptive listen No sleep cycles 55University of Electronic Science and Technology of ChinaWireless Sensor NetworksThroughput as Hops IncreaseAdaptive listen significantly increases throughputUses less time to pass the same amount of data0246810020406080100120140160180200220Effective da

58、ta throughput under highest traffic loadNumber of hopsEffective data throughput (Byte/S)No sleep cycles 10% duty cycle with adaptive listen 10% duty cycle without adaptive listen 56University of Electronic Science and Technology of ChinaWireless Sensor NetworksS-MAC - CONCLUSIONSA mainly static netw

59、ork is assumedTrades off latency for reduced energy consumptionRedundant data is still sent with increased latencyIncreased collision rate due to sleep schedules 57University of Electronic Science and Technology of ChinaWireless Sensor NetworksSMACDuty cycle operation requires sleepwakeup schedulest

60、wo drawbacks in terms of energy inefficiencyFirstly, nodes need to send periodic messages, such as the SYNC packets used by S-MAC in each frame.Secondly, all the nodes need to be active during the listen period to wait for a possible incoming packet.As a result, even when there is no traffic, nodes