802154及ZigBee协议工作原理及特点分析

1、zigbee及802.15.4协议分析大学牛.创新计划阶段总结报告信息科学与技术学院电子信息工程专业2012ieee 802.15.4 （介绍丿802.15.4标准定义了一个osi模型中的第二层通信协议，位于物理层之上。他的主要目的 在于建立两个器件间的通信，而如何通信则由一些其它的上层协议控制。他由ieee即电气电子 工程师协会制定。zigbee （介绍丿这个协议定义了 osi参考模型中口第三层往上的内容。他的主要冃标是：创建具冇层次结 构的网络拓扑，实现一定数量的设备进行相互间通信，并且具有身份认证、数据加密、设备绑定并且提供了一个开发上层应用的框架。他有zigbee联盟的诸多公司一同制定

2、。1理解802.15.4正如上面所说的，802.15.4是一个位于osi参考模型屮第二层上的协议，一般成为数据链路层。 这一层中数据比特被汇总组织成为下一层即物理层中的电磁脉冲（波形）。这一层的功能类似于 一些较为常用的协议如：802.11,即通常说的wifi ,或者是802.3,即以太网。他的工作频率分 布与三个主频段屮的27个不同的通道上。& 868.0 868.6mhz -> 1 通道（欧洲）人 902.0-928.0mhz> 10 通道（eeuu）龙2.40-2.48ghz-> 16通道（世界范圉内通用）数据比特率:a 868.0 868.6mhza 902.

3、0-928.0mhz龙 2.40-2.48ghz-> 20/100/250 kb/s-> 40/250 kb/s-> 250 kb/s1-1为什么802.15.4抗噪声性能好？802.15.4在将信号送往物理层发送出去前，使用了直接频谱扩展（dsss）技术来调制信息。实际上, 每比特信息都被映射成了另一个4比特的编码（使用另一种编码方案），这一过程使得每一比特 所占据的带宽显著增大，但同时信号在频谱范围内的功率密度下降。这样做使得所使用的频带宽带内的干扰所带来的不利影响显著减小，提高了接受端信噪比 （snr）,因为更容易侦测解码发送端传输的消息。根据电路物理层硬件的限制以及在

4、给定时间内能同时处理的码元数量的限制，通常有不同种类的 dsss直接扩频方式。采用二进制相移键控(bpsk)、差分正交相移键控(o-qpsk)以及psss 可以获得20kb/s到250kb/s的数据传输速率。1-2为什么802.15.4协议抗干扰性能好？802.15.4使用了两种技术来避免所冇节点同时发送信息：载波侦听多路复用冲突避免以及时隙 分配。最为常见的技术就是载波侦听多路复用-冲突避免。这种方法是这样的：每个节点在发送前都要 先监听媒质上的信号。如果监听到的信号强度高于某一特定值，那么则认为当前通道被占用，等 待一段时间，具体等待的时间长度是随机的，然后再次尝试发送。通常在协议栈的实现

5、中有一个 参数可以设定等待时间的长度变化。另一种技术就是：保证性时隙分配。这种系统中使用一个中央节点即局域网协调器来分配各个节 点的吋隙，从而是的每个节点都知道自己应该在何时传输数据，从而避免冲突发生。总共可以分 配的时隙有16个。对于一个节点来说，首先要做的就是向局域网协调器发送一个时隙请求，作 为回应，协调器会发送一个beacon消息，其中包含了新开辟的时隙以及己开辟的时隙数量。在802.15.4协议中实现了一个称为信道能量扫描的功能。这个功能设立的目的在于获得一种可以 在启用一个新信道前获取该信道中当前能量强度的方法，这个能量强度包括有效信号、噪声、干 扰等。通过选择一个相对空闲信道，可

6、以使新建立的网络使用较低的传输功率来通信，从而节省 电源。一般有三种不同的扫描方式：瓦能量扫描：扫描信道中的信号强度，然后报告。不管这信号是由zigbee节点还是由其 他类型的网络产生的或者是噪声z类的。总之如果信道上有超过一定强度的信号在传 输，则认为有信号在传输，否则则可以启用该信道进行通信。人载波侦听：扫描并且当信道屮有802.15.4类型的通信时，则认为忙，只有当空闲时在使 用其进行传输。龙 载波侦听+能量扫描：扫描信道，当信道中有基于802.15.4的通信并且其信号能量高于 一个阈值时，认为信道忙，否则则使用该信道。1-3为什么802.15.4协议是一个低功耗的协议？因为802.15

7、.4协议是一个可以工作在低占空比的协议。这意味着收发器可以在绝大多数时间里处 于休眠状态(最多可达99%),从而有效降低能量消耗。这个工作比例取决于所采用的通信模型。 如果采用beacon模式，那么用于收发beacon数据包的时间将会使收发器总的工作时间延长。1- 4 transmission power and reception sensibility for the 802.15.4 transceivers802.15.4 sets the minimum amount of energy needed to transmit in 3dbm, (0*5 mw) and the min

8、imum sensibility in the receiver is 92dbm (6*3 * e-10 mw). waspmote radio module has a variable transmission power strength of odbm (lmw) in xbee 802.15.4 oem and 20dbm (loomw) in the xbee-pro 802.15.4 oem, both values are higher than the minimum set in the 802.15.4 standard. regarding the reception

9、 sensibility , the xbee shows -92bm (6*3 * e-10 mw) and -loodbm (1 * le-lomw) the xbee-pro flavorthis means we will detect any packet which reach us with a energy of as low as 0'000()000()0()63mw for the xbee and owoooooooo1 mw for the xbee-pro.1-5 what is there on top of 802.15.4?there are seve

10、ral protocols which use 802.15.4 as its mac layer. the most known is zigbee, although there are a lot of them:£ wireless hart: it is the wireless version of the hart protocol which is the most used in the automation and industrial applications which require real time. it uses time synchronized

11、mesh protocol (tsmp). a "time coordinator0 node is required in order to assign the time slot to all the motes.人 isa - sp100: it also centers in the process and factory automation. it is being developed by the systems and automation society (isa) and tries to be an standard for this kind of proj

12、ects.人 ietf ipv6 - lowpan: as the same name points it is the implementation of the ipv6 stack on top of 802.15.4 to let any device be accessible and access from and to internet入 new mesh protocols: a lot of different mesh protocols are being implemented by companies over the 802.15.4 mac layer. one

13、example is the digi's own mesh protocol (digimesh). which is though to be a completely distributed mesh protocol where all the nodes can sleep and route their brothers packets. waspinote support also the digimesh protocol when the xbee 802.15.4 oem module is chosen . paper about the digimesh imp

14、lementation: digimcsh-whifcpape匚pdf.2- understaning zigbeezigbee offers basically four kinds of different services:a extra encription services (application and network keys implement extra 128b aes encryption)a association and authentication (only valid nodes can join to the network).人 routing proto

15、col: aodv, a reactive ad hoc protocol has been implemented to perform the data routing and forwarding process to any node in the network£ application services: an abstract concept called ''cluster" is introduced each node belongs to a predefined cluster and can take a predefined nu

16、mber of actions example: the "house light system clusterm can perform two actions: hturn the lights onm, and ntum the lights off*.zigbee is a layer thought to organize the network the first thing a node (route or end device) which want to join the network has to do is to ask to the coordinator

17、for a network address (16b), as part of the association process. all the information in the network is routed using this address and not the 64b mac address .in this step authentication and encryption procedures are performed once a node has joined to the network can send information to its brothers

18、 through the routers which are always awake waiting for the packets when the router gets the packet and the destination is in its radio of signal, the router first looks if the destination end device is awake or slept. in the first case the router sends the packet to the end device, however if it is

19、 sleeping, the router will bufferize the packet untill the end device node gets awake and ask for news to the route匚2-1 is zigbee a mesh protocol?to answer this question we have to clarify what the mesh concept means. let's see first how azigbee network works.there are three kinds of nodes in a

20、zigbee network:£ coordinator: is the nmasterm device, it governs all the networ£ routers: they route the information which sent by the end devices入 end device: (the motes): they are the sensor nodes, the ones which take the information from the environmentcoordinator and routes can not be

21、battery powered, motes can. zigbee creates star topologies. there are some basic rules:£ the end devices connect to a router or a coordinator.入 the routers can connect among them and with the coordinator.人 the routers and coordinators can not sleep they have to save in their buffer the packets

22、which go to the end devices人 the end devices can sleepthe concept mmesh network11 relays in the ad hoc communications, also called peer to peer (p2p). this means all the devices in the network can communicate with each other directly. they have to be able to discover each other and send broadcast me

23、ssages to all the brothers ("hello!，is there anybody out there?*1). they have to be able to create networks like the one represented in the image below.zigbee creates start network topologies, not mesh ones. to create a completely mesh network such as the one showed in the image below all the n

24、odes have to have the same role, all of them have to be "end devices + routers" so that they can route their brothers information and sleep when no action is required (saving energy) the digimesh protocol (over 802.15.4) sets a completely distributed network where all the nodes talk among

25、them using p2p (equal to equal) datagrams.2-2 what is the difference between zigbee and zigbee-pro?read the zigbee vs zhbee-pro article.2- 3 what is the difference between zigbee certified and zigbee compliance?read the zigbee complianee vs zigbee certified article.3 802.15.4 versus zigbee, summariz

26、ing.£ 802.15.4 is thought to be a protocol to get point to point and energy efficient communications.人 zigbee definces extra services (start topology routing, encryption, application services) over 802.15.4.a zigbee creates semi-centralized networks where just the end devices can sleep人 differe

27、nt completely distributed mesh algorithms are being used over 802j5.4 is the protocol used to createboth waspmolc benefit from all the 802.15.4, zigbee and digimesh protocols and support all the frequency bands 869mhz, 900mhz and 2.4ghz.3- 1 802j5.4 / zigbee frequently answers and questions (faq)a can the 802.15.4 sensor motes talk each other directly? yes. they can send packets directly to any of their brothers which are in range£ can the zigbee sensor motes talk each other directly? no. the se