无线传感器网络S-MAC协议研究

1、无线传感器网络无线传感器网络 S-MACS-MAC 协议研究协议研究江雪(Nanjing University of Posts and Telecommunications, Nanjing 210003) 基金项目：国家自然科学基金(60372107)摘要：摘要：传感器节点能量受限，节能是传感器网络中媒体访问控制(MAC)协议设计的首要问题。采用周期性睡眠机制、自适应侦听机制、串音避免机制和消息传递机制可使得传感器媒体访问控制(S-MAC)协议在网络能耗和时延方面得到改进。对 S-MAC 协议的改进主要有两种方式：动态调整、区别控制包与数据包的发送条件进行发送。对无线传感器网络，要想设计出

2、一种满足各方面要求的 MAC 协议是不现实的，可针对不同应用的要求，灵活采用不同的方式，设计出相应的协议。无线传感器网络通常包含大量自组织的分布式节点。由于其组网快捷、灵活，且具有不受有线网络约束的优点，可广泛应用于紧急搜索、灾难救助、军事应用等特殊环境，因而具有广泛的应用前景。由于传感器节点能量受限，节能成为传感器网络媒体访问控制(MAC)协议设计首要的问题。由文献1可以看到传感器媒体访问控制(S-MAC)协议就是针对传感器网络的节能需求而提出的。周期性睡眠机制、自适应侦听机制、串音避免机制和消息传递机制使得 S-MAC 协议在网络能耗和时延方面的性能很优越，但距离实际应用的要求还有一段差距

3、。1 1 S-MACS-MAC 协议介绍协议介绍S-MAC 协议是在 802.11 协议的基础上提出的，设计的主要目标是减少能量消耗，提供良好的扩展性。其主要实现机制包括周期性侦听和睡眠、串音避免、消息传递和流量自适应侦听。1.11.1 S-MACS-MAC 协议实现的关键技术协议实现的关键技术(1)数据包的嵌套结构MAC层数据包类型序号负载校验物理层数据包目地地址源地址长度应用层MAC层物理层应用层数据包包头MAC：媒体访问控制图 1 S-MAC 协议中数据包格式载波侦听控制退避重传控制握手机制控制消息传递机制控制无线收发装置开关控制CRC校验编码/解码无线收发装置控制载波侦听机制控制前导码

4、侦听MAC层物理层CRC：循环冗余校验MAC：媒体访问控制图 2 S-MAC 协议的堆栈的网络模型S-MAC 协议数据包的嵌套结构如图 1 所示。在 S-MAC 协议中，上一层数据包包含了下一层数据包的内容。数据包传送到哪一层，那一层只需要处理属于它的部分。(2)堆栈结构和功能在 S-MAC 协议堆栈内，当 MAC 层接收到上层传送过来的数据包后，它就开始载波侦听。如果结果显示MAC 层空闲，它就会把数据传到物理层；如果 MAC 层忙，它将会进入睡眠状态，直到下个可用时间的到来，再重新发送。当 MAC 层在收到物理层传送过来的数据包后，先通过循环冗余校验(CRC)表示没有错误，MAC 层就会将

5、数据包传向上层。具体网络模型如图 2 所示。(3)选择和维护调度表在开始周期性侦听和睡眠之前，每个节点都需要选择睡眠调度机制并与邻居节点一致。如何选择和保持调度机制分为以下 3 种情况：(a)节点在侦听时间内，如果它没有侦听到其他节点的睡眠调度机制，则立即选择一个睡眠调度机制。(b)当节点在选择和宣布自己的调度机制之前，它收到了邻居节点广播的睡眠调度机制，它将采用邻居节点的睡眠调度机制。(c)当节点在选择和广播自己的睡眠调度机制之后，收到几种不同的睡眠调度机制时，就要分以下两种情况考虑：当节点没有邻居节点的时候，它会舍弃自己当前的睡眠调度机制，采用刚接收到的睡眠调度机制；当节点有一个或更多邻居

6、节点的时候，它将同时采用几种不同的调度机制。(4)时间同步在 S-MAC 协议中，节点与邻居节点需要保持时间同步来同时侦听和睡眠。S-MAC 协议采用的是相对而不是绝对的时间戳，同时使侦听时间远大于时钟误差和漂移，来减少同步误差，并且节点会根据收到的邻居节点的数据包来更新自己的时间，从而与邻居节点保持时间同步。(5)带冲突避免的载波侦听多路访问带冲突避免的载波侦听多路访问(CSMA/CA)的基本机制是在接收者和发送者之间建立一个握手机制来传输数据，这种握手机制是：由发送端发送一个请求发送(RTS)包给它的接收者，接收者在收到以后就回复一个准备接收(CTS)包，发送端在收到 CTS 包后，开始发

7、送数据包，RTS 与 CTS 之间的握手是为了使发送端和接收端的邻居节点知道它们正在进行数据传输，从而减少传输碰撞。(6)网络分配矢量在 S-MAC 协议中，每个节点都保持了一个网络分配矢量(NAV)来表示邻居节点的活动时间，S-MAC 协议中在每个数据包中都包含了一个持续时间指示值，持续时间指示值表示目前这个通信需要持续的时间。邻居节点不论是收到发送者或接收者发往其他节点的数据包的时候，它就可以知道它需要睡眠多久，即用数据包中的持续时间更新 NVA 的值，当 NVA 的值不为零的时候，节点应该进入睡眠状态来避免串音。当 NVA 变为零的时候，它就马上醒来，准备进行通信。1.21.2 S-MA

8、CS-MAC 协议的网络性能分析协议的网络性能分析(1)能量在无线传感器网络中造成能源消耗的主要因素包括下列几方面：空闲侦听、竞争冲突、串扰和控制开销。S-MAC 协议针对这些能耗的主要因素，制订了相应的机制。节点采用周期性的睡眠和侦听机制，使节点周期性地进入睡眠状态，减少空闲侦听的能耗；采用串音避免机制使会干扰正在进行通信的节点进入睡眠状态，减少串扰的能耗；采用消息传递机制减少发送 RTS/CTS 控制包来减少控制开销的能耗。(2)延时在 S-MAC 协议中，引入周期性睡眠机制的同时也增加了不少延时，自适应侦听机制可以改善其延时特性。自适应侦听机制是指节点在通信结束后，不是马上进入睡眠状态，

9、而是侦听一段时间，如果在这段时间内，节点收到发往它的 RTS 或 CTS 控制包，节点马上准备通信，而无需等到下个侦听时间到来。下面分析比较一下没有睡眠机制又没有自适应侦听机制的 S-MAC 协议、有睡眠机制没有自适应侦听的 S-MAC 协议和既有睡眠机制又有自适应侦听的 S-MAC 协议的网络时延差别。先做如下定义：Tnothing 表示没有睡眠机制也没有自适应侦听机制的 S-MAC 协议的网络平均时延；Tsleep 表示有睡眠机制没有自适应侦听机制的 S-MAC 协议的网络平均时延；Tall 表示既有睡眠机制又有自适应侦听的 S-MAC 协议的网络平均时延。由文献2中分析我们可以得到：Tn

10、othing=N(tcs+ttx) (1)Tsleep=NTf-Tf/2+tcs+ttx (2)Tall=NTf/2+2tcs+2ttx-Tf/2 (3)其中：Tcs 为竞争窗的大小，ttx 为传输时延，Tf 为一帧时间；N 为数据包传输总跳数。在占空比不大的条件下，Tf 远远大于 tcs 和 ttx，由公式(1)、(2)和(3)，可以看到 Tsleep 与 Tall 随跳数的增加速度比 Tnother 大很多，而 Tsleep 变化大小约为 Tall 的一半。这表明，在有睡眠机制 S-MAC 协议中，采用自适应侦听机制的延时增加速度比没有自适应侦听机制 S-MAC 协议的网络会减半，但它们比

11、没有睡眠机制 S-MAC 协议的网络延时还是增加了很多，延时性能不够理想。2 2 S-MACS-MAC 协议的研究现状协议的研究现状目前在 S-MAC 协议的基础上，对 S-MAC 协议的改进主要有两种方式：动态调整、区别控制包与数据包的发送条件进行发送。2.12.1 基于动态调整的改进协议基于动态调整的改进协议在 S-MAC 协议中，动态调整是固定的，即周期性的睡眠和侦听固定长度的时间，不够灵活。下面几种协议就是针对这个缺点，动态调整，节约能量。唤醒 MAC 协议(T-MAC)3协议主要根据 5 种事件和一个记时器 TA(Time Active)动态调整，5 种事件分别为：*帧长度超时*节点

12、接收到数据*数据传输发生冲突*节点数据确认发送成功*邻居节点完成数据交换如果在 TA 时间内，射频模块没有侦听到这 5 种事件中任何一种，则认为信道进入空闲状态，节点关闭射频模块，进入睡眠状态。样式 Mac 协议(P-MAC 协议)4中节点及其邻居节点对自己将来几个时隙内的睡/醒的计划用一个比特串表示出来，在该协议中，称这种比特串为“样本” ，节点依靠自身以及邻居节点的“样本”便可适应性调节，形成自己的调度表。流量感知节能的 Mac 协议(TEEM)5中发送的同步包 SYNC 划分为两种，一种是 SYNCdata(表示节点中有数据包在排队准备发送)，另外一种是 SYNCnodata(表示节点没

13、有数据包需要发送)。当节点收到SYNCnodata 的时候，它会在原来发送数据包的时间内睡眠。当节点收到 SYNCdata 的同步包的时候，它会直接回复 CTS 数据包，开始通信。参数 U MAC 协议(U-MAC)6中，定义了一个变量 U 并且有：U=Trx+Ttx/Trx+Ttx+Tidle (1)其中：Trx 表示接收所用的总时间；Ttx 表示发送所用的总时间；Tidle 空闲状态的总时间。根据实际计算的 U 的大小来动态调整。如果 U 的值大于流量上限(Uhigh)，就表示目前通信量大过目前调度机制可以承受的范围，就增加动态调整的值：如果 U 的值小于流量下限(Ulow)，就表示相对目

14、前的动态调整通信量很小，需要减小动态调整的值，使睡眠时间延长节约能量。自适应 MAC 协议(AC-MAC)7的基本思想是：根据在 MAC 层中排队的数据包数量表示通信量的大小，记为 Ni；通过针对应用函数 Ri=f(Ni)和公式 Tframe=TRTS/CTS+Tsleep/Ri 得到 Tframe 这个小周期来划分原来的大周期，这里 Tframe 表示帧时间大小，Ri 为 Ni 通过针对应用函数的映象，f 为针对应用函数。增加了原来动态调整中发送数据的机会。2.22.2 区分数据包与控制包发送条件的改进区分数据包与控制包发送条件的改进在 S-MAC 协议中，节点发送控制包 RTS/CTS/A

15、CK 与数据包 DATA 均采用相同的条件，如发送功率、信道条件等等，但实际中，控制包的长度一般比数据包小很多，而且控制包中内容不重要。功率控制 MAC协议(PCSMAC)与双信道 MAC 协议(DCMA/AP)就是针对 S-MAC 协议的这个缺点，区分控制包与数据包的发送条件，节约能量。PCSMAC 协议中，节点发送控制包 RTS/CTS/ACK 与数据包 DATA 的发送功率不同。节点的调度表中增加了节点与各个邻居节点通信所需要的最小传输功率表。这样节点在传送 RTS/CTS/ACK 时，选取功率表中最大的，在传送 DATA 的时候，则选取到达目的节点所需功率即可。DCMA/AP 协议中，

16、节点发送控制包 RTS/CTS/ACK 与数据包 DATA 的发送信道不同，即在协议中使用双信道传送，传送 RTS/CTS/ACK 控制包的信道为控制信道，传送 DATA 数据包的信道为数据信道。将上述协议与 S-MAC 协议的性能比较总结，可得表 1。表 1 协议性能比较协议名称能量消耗延时吞吐量T-MAC减少增加减少P-MAC减少(负载轻)增加(负载高)TEEM减少U-MAC减少降低AC-MAC降低增加PCSMAC减少DCMA/AP减少注：“”位置表示性能相当通过以上分析比较可以看到，基于竞争的 MAC 协议今后应在以下几个方面展开研究：(1)在保证一定的节能性的前提下，在各种性能指标之间

17、进行折中。因为在达到节能的同时必然牺牲延时或吞吐量性能，因此应在保证延时和吞吐量的条件下，实现节能。(2)增强协议对服务质量(QoS)保障的支持，随着各种应用的发展，能为不同业务提供不同的服务质量保障显得越来越重要。对于具有 QoS 支持能力的无线传感器网络的 MAC 协议有待进一步研究。(3)与其他层的结合，跨层的协议实现起来相对复杂，但跨层的设计协议可以使各个方面的网络性能都达到最好。3 3 结束语结束语传感器网络是目前通信界最新研究热点，具有广阔的应用前景。通过近几年的研究，人们对传感器网络MAC 协议设计要求的认识已经逐渐明确8-9。当然，就无线传感器网络而言，设计出一种可以满足各方面

18、要求的 MAC 协议是不现实的，应该针对不同应用的要求，灵活采用不同的方式，设计出合理的 MAC协议。4 4 参考文献参考文献1 于海斌, 曾鹏, 梁韡. 智能无线传感器网络系统 M. 北京:科学出版社, 2006:68-102.2 YE Wei, HEIDEMANN J, ESTRIN D. Medium access control with coordinated adaptive sleeping for wireless sensor networks J. IEEE/ACM Transactions on Networking, 2004,12(3):493-506. 3 ZHEN

19、G Tao, RADHAKRISHNAN S, SARANGAN V. PMAC: an adaptive energy-efficient MAC protocol for wireless sensor networks C/Proceedings of 19th IEEE International Symposium on Parallel and Distributed, Apr 4-8, 2005,Denver,CO,USA.Piscataway, NJ,USA: IEEE Computer Society, 2005: 8. 4 SUH Changsu, KO Youngbae.

20、 A traffic aware, energy efficient MAC protocol for wireless sensor networks C/Proceedings of International Symposium on Circuits and Systems:Vol3, May 23-26,2005,Kobe,Japan. New York,NY,USA:IEEE, 2005:2975-2978. 5 YANG Shihhsien, TSENG Hungwei, WU E H K, et al. Utilization based duty cycle tuning M

21、AC protocol for wireless sensor networks C/Proceedings of IEEE Global Telecommunications Conference:Vol6,Nov 28-Dec 2,2005,St Louis,MO,USA. Piscataway,NJ,USA:IEEE, 2005:5. 6 AI Jin, KONG Jingfei, TURGUT D. An adaptive coordinated medium access control for wireless sensor networks C/Proceedings of Ni

22、nth International Symposium on Computers and Communications: Vol1, Jun 28 -Jul 1, 2004, Alexandria,Eqypt. Piscataway,NJ,USA:IEEE Computer Society, 2004:214-219. 7 NAR P C, CAYIRCI E. PCSMAC: a power controlled sensor MAC protocol for wireless sensor networks C/ Proceeedings of the Second European Wo

23、rkshop on Wireless Sensor Networks,Jan 31-Feb 2,2005,Istanbul,Turkey. Piscataway,NJ,USA:IEEE Computer Society, 2005:81-92 . 8 纪阳, 张平. 无线传感器网络的体系结构 J. 中兴通讯技术, 2005,11(4):32-36. 9 苗付友, 熊焰, 卫国. 一种无线传感器网络简单按需路由协议 J. 中兴通讯技术, 2005,11(4):42-46. 收稿日期：2006-10-12作者简介：江雪，南京邮电大学在读硕士研究生，主要研究方向为无线传感器网络。A Research

24、 on S-MAC Protocol of Wireless Sensor NetworkJiang Xue(Nanjing University of Posts and Telecommunications, Nanjing 210003) Fund Project: National Natural Science Fund (60372107)Abstract:As sensor node energy is limited, so energy efficient is the primary issue for Sensor network Medium Access Contro

25、l (MAC) protocol design. Adopting Periodical Sleep mechanism, Adaptive Snooping mechanism, Crosstalk Avoidance mechanism and Message Transfer mechanism is able to improve network energy-consuming and latency of Sensor Medium Access Control (S-MAC) protocol. There are mainly two methods to improve S-

26、MAC protocol: dynamic adjustment and distinguish the packet-sending conditions for control packet and data packet. For wireless sensor network, its not realistic to design one MAC protocol that meets all requirements of every aspect; however, it is possible to design corresponding protocol in accord

27、ance with different requirement by flexibly adopting different methods. Wireless Sensor Network (WSN) usually contains a large quantity of self-organizing distributed nodes. Since its internetworking is simple, flexible and not constrained like the wired network, it can be widely applied in emergenc

28、y searching, disaster salvation and military applications etc. Therefore, Wireless Sensor Network (WSN) has a promising future. As sensor node energy is limited, so energy efficient is the primary issue for Sensor network Medium Access Control (MAC) protocol design. It can be seen in document 1 that

29、 Sensor Medium Access Control (S-MAC) protocol is designed to fulfill the energy efficient demand of sensor network. Periodical Sleep mechanism, Adaptive Snooping mechanism, Crosstalk Avoidance mechanism and Message Transmission mechanism make the S-MAC pretty excellent in network energy-consuming a

30、nd latency performances, but theres still a long way to go to satisfy all requirements of practical applications.1. Introduction of S-MAC Protocol S-MAC is initiated based upon 802.11 protocols, and its major design target is to reduce energy-consuming so as to provide good expansibility. Main mecha

31、nisms to realize it include periodical snooping and sleep, crosstalk avoidance, message transfer and traffic adaptive snooping.1.1 Critical Technology for Realizing S-MAC Protocol(1) Nesting Structure of Data PacketMAC层数据包类型序号负载校验物理层数据包目地地址源地址长度应用层MAC层物理层应用层数据包包头Application Layer MAC Layer Data Pack

32、et Type S.N. Load CRCMAC Layer Physical Layer Data Packet Destination Address Source AddressPhysical Layer LengthMAC：Medium Access ControlFigure1. Data Packet Format of S-MAC Protocol载波侦听控制退避重传控制握手机制控制消息传递机制控制无线收发装置开关控制CRC校验编码/解码无线收发装置控制载波侦听机制控制前导码侦听MAC层物理层MAC Layer - - - - - - - - - - - - - - - - -

33、 Carrier Sense Control Backoff Retransmit Control Handshake Mechanism Control Message Transfer Mechanism Control Radio Transceiver Device Switch ControlPhysical Layer CRC Check Coding/decoding Radio transceiver Device Control Carrier Sense Mechanism Control Lead Code Snooping CRC: Cyclic Redundancy

34、CheckMAC: Medium Access ControlFigure2. Network Model of S-MAC Protocol StackThe nesting structure of S-MAC protocol packet is shown as Figure1. In S-MAC protocol, the upper layer packet contains the information of lower layer packet. Each layer is only assumed responsibility to deal with the inform

35、ation belong to it when a packet is received.(2) Structure and Function of S-MAC StackIn S-MAC protocol stack, when MAC Layer receives a packet from upper layer, it will start carrier sensing. If the result shows the MAC layer is idle, it will send data to Physical Layer; if MAC layer is busy, it wi

36、ll enter sleeping state, waiting until the next idle time, and the data will be resent by then. When MAC Layer receives a packet from Physical Layer and finds there isnt any error after Cyclic Redundancy Check (CRC), MAC Layer will send this packet to upper layer. The network model is shown in Figur

37、e2.(3) Select and Maintain Scheduling TableBefore starting periodical snooping and sleeping, each node needs to select sleep scheduling mechanism and must be consistent with that of neighbor nodes. How to select and keep the consistence? There are three conditions, as follows:(a) In snooping time, i

38、f a node does not discover the sleep scheduling mechanisms of other nodes, it will select a sleep scheduling mechanism immediately;(b) If a node receives a sleep mechanism the neighbor nodes broadcast before selecting and declaring its own scheduling mechanism, it will apply the sleep scheduling mec

39、hanism of neighbors; (c) After selecting and broadcasting its own sleep scheduling mechanism, if a node receives several different sleep mechanisms, this case should be considered in two ways: if one node does not have a neighbor node, it will discard current sleep scheduling mechanism of its own an

40、d adopt the latest received one; if this node have one or more neighbor nodes, it will adopt several different sleep scheduling mechanisms simultaneously. (4) Clock SynchronizationIn S-MAC protocol, node and neighbor nodes should keep clock synchronization for snooping and sleeping simultaneously. S

41、-MAC protocol adopts relative but not absolute timestamp, and meanwhile let snooping time much greater than clock error and floating so as to reduce synchronizing errors. Besides, node will update its own clock in accordance with the packet received from neighbor nodes to keep clock synchronization

42、with them. (5) Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA)Basic mechanism of Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) is establishing a handshake mechanism to transfer data, this handshake mechanism is: Firstly a sender sends a request-to-send (RTS) packet to its r

43、eceiver, receiver reply a clear-to-send (CTS) packet to sender. After receiving this CTS packet, sender begins to send packets to receiver. The handshake between RTS and CTS is to ensure the neighbor nodes know that sender and receiver are transmitting data, and then the transmission collision can b

44、e greatly reduced.(6) Network Allocation VectorIn S-MAC protocol, each node maintains a Network Allocation Vector (NAV) to indicate the active time of neighbor nodes, and each packet of S-MAC protocol contains a continuous time duration indicator, which indicates the time duration for current commun

45、ication. When a neighbor node receives a packet, no matter this packet is sent by sender or the receiver sent to other nodes, the node will know how long it should sleep, that is, to update NAV value according to the time duration in the packet. When NAV value is not zero, node should enter sleep st

46、ate to avoid crosstalk. As soon as the NAV value is zero, it will wake up and get ready for communication. 1.2 Network Performance Analysis of S-MAC Protocol(1) EnergyThe factors of energy-consuming in Wireless Sensor Network include following aspects: idle snooping, competition collision, crosstalk

47、 and cost control. S-MAC protocol has formulated corresponding mechanisms based upon these energy-consuming factors. Node adopts periodical sleep and snooping mechanism to make node enter sleep state periodically in order to reduce the energy-consuming in idle snooping; it adopts crosstalk avoidance

48、 mechanism make the nodes that might interfere the communication enter sleep state to reduce crosstalk energy-consuming; it also adopts message transfer mechanism to reduce the energy for control cost for sending RTS/CTS control packet.(2) LatencyIn S-MAC protocol, introducing periodical sleep mecha

49、nism brings extra latency as well; however, adaptive snooping mechanism is able to improve its latency feature. Adaptive snooping mechanism refers to that node does not enter sleep state immediately after the communication ends, instead, it will keep on snooping for a period of time, and if node rec

50、eives RTS or CTS control packet sent to it within this period, it will get ready for communication immediately, without waiting for the next snooping time. In the following, there are analysis and comparison of network latency among three types of S-MAC protocols, namely, S-MAC protocol with neither

51、 sleep mechanism nor adaptive snooping mechanism, S-MAC protocol with sleep mechanism but without adaptive snooping mechanism and S-MAC protocol with both sleep mechanism and adaptive snooping mechanism.Firstly, we give following definitions:Tnothing refers to average network latency for S-MAC proto

52、col with neither sleep mechanism nor adaptive snooping mechanism;Tsleep Refers to average network latency for S-MAC protocol with sleep mechanism but without adaptive snooping mechanism;Tall Refers to average network latency for S-MAC protocol with both sleep mechanism and adaptive snooping mechanis

53、mWe can conclude from the analysis of document 2:Tnothing=N(tcs+ttx) (1)Tsleep=NTf-Tf/2+tcs+ttx (2)Tall=NTf/2+2tcs+2ttx-Tf/2 (3)For above parameters thereinto, Tcs is the size of competition window (CW), ttx is transmission latency, Tf is time of one frame and N is the sum of packet hop count.In con

54、dition that duty cycle is not large, Tf is much greater than tcs and ttx, from formula (1), (2) and (3), we can see the rates of Tsleep and Tall increasing with hop count is much greater than Tnother, and the rate changes of Tsleep is approximately half of that of Tall. It indicates that the latency

55、 increasing rate of S-MAC protocol with sleep mechanism and adopts adaptive snooping mechanism is reduced to half of that without adaptive snooping mechanism. Anyhow, both of them have more latency than the S-MAC protocol without sleep mechanism, so it is not ideal in latency performance either.2. C

56、urrent Research Status of S-MAC ProtocolBased upon original S-MAC protocol, there are mainly two methods for improvement about S-MAC protocol: dynamic adjustment and send by distinguishing packet-sending conditions of control packets and data packets.2.1 The Revised Protocol based upon Dynamic Adjus

57、tmentIn S-MAC protocol, dynamic adjustment is fixed, that is, the time duration for periodical sleep and snooping are fixed, so this is not flexible enough. Following several protocols are aiming at this disadvantage to dynamically adjust and save energy.Timeout MAC (T-MAC) 3 protocol conducts dynam

58、ic adjustment in accordance with five events and a Time Active (TA), the five events are:* Frame-length timeout* Node receives data* Data Transmission collision occurs* Node data acknowledges data sending is successful * Neighbor nodes completes the data exchangesIf Radio Frequency (RF) module doesn

59、t detect any of those five events within TA time duration via snooping, it will consider the signal channel enters idle state, node then will shutdown Radio Frequency (RF) module and enter sleep state.In Pattern MAC protocol (P-MAC protocol) 4, node and its neighbor nodes indicate their sleep/wake p

60、lan within future several slots using a bit-string, which is called “sample” in this protocol. Node is able to adaptively adjust relying on its own “samples” as well as that of neighbor nodes and finally form its own scheduling table. In Traffic aware Energy Efficient MAC protocol (TEEM) 5, there ar