1,Brief introduction
ZigBee whether one apply short distance range to in technology, low to transmit wireless communication skill between various electronic device under the data rate. The unreasonable person that ZigBee name comes from the bee colony to use is by way of communication surviving and developing, the honeybee notifies new food position, distance and direction of sources,etc. information found through dancing dance of ZigZag shape, regard this as the technical name of wireless communication of new generation. ZigBee was also called " HomeRF Lite " in the past , " RF-EasyLink " Or " FireFly " Radiotechnics, are called ZigBee technology in unison at present.
2,ZigBee technical characteristic
Since Marconi invented the radio, the wireless communication skill has been developed towards the direction of raising data rate and transmission distance constantly all the time. For example: Mobile communication network ( 3G) of the third generation that the long haul network range is inner The purpose lies in offering the radio service of the multimedia, the inner standard data rate from 1Mbit/s of IEEE802.11 to 54Mbit/s of IEEE802.11g of LAN range. And ZigBee technology of getting extensive research at present is devoted to offering a kind of low priced fixing, portable or extremely low complexity, the wireless communication skills of short rate of the cost and power consumption that the mobile equipment used. Such wireless communication skill has the following characteristics:
Low power dissipation: Under the work pattern situation, ZigBee technological transmission rate is low, it is very small to transmit the data bulk, so the time of receiving and dispatching of the signal is very short, secondly when it is not a work pattern, ZigBee node in the dormancy mode. It is generally 30ms that the apparatus searches for time delay, it is 15ms that the dormancy activates time delay, it is 15ms that the activity apparatus signal channel inserts time delay. Because operation time is shorter, the power consumption of receiving and send messages is lower and adopted the dormancy mode, the electric-saving of making ZigBee node extraordinary, ZigBee nodal battery operation time can be as long as about 6 months to 2 years. Meanwhile, because battery time depends on a lot of factors, for example: Battery kind, capacity and application, ZigBee technology uses the battery to optimize in the agreement. As to typical application, the alkaline battery can use for several years, as to some operation time and cumulative time (operation time + dormancy time) Than situation that is 1% less than,life-span of battery be by over 10 year even.
Data transmission is reliable: Media accessing control layer (MAC layer) of ZigBee The ones that adopted talk-when-ready were collided with and avoiding the mechanism. Under this kind of data transmission mechanism totally confirmed, convey at once when there are data transmission demandsing, each data packet sent must wait for the acknowledge information of the take over party, and reply acknowledge information, if show that has collided without getting the reply of acknowledge information, will spread once again, can improve the transmissive reliability of the system information while adopting this kind of method. At the same time in order to need the signal service of fixed bandwidth to reserve the specialized time slot, competitions and conflicts while avoiding the transmission data. ZigBee has been optimized to the responsive application of time delay at the same time, the time delay that communication time delay and dormant state activate is very short.
The network capacity is great: ZigBee short rate, low power consumption and transmissive characteristic of short distance make it very suitable for supporting the simple device. ZigBee has defined two kinds of devices: The whole function device (FFD) And simplify the function device (RFD) . To the whole function device, require it to support all 49 basic parameters. And simplify the function device correctly and only require it to support 38 basic parameters during minimum configuration. A complete function device can converse with simplifying function device and other the whole function devices, can work according to 3 way: A land network coordinator, coordinator or device. And simplifying the function device can only converse with the whole function device, is only used in very simple employing. The network of one ZigBee includes router node of 255 ZigBees at most, one of these is top management (Master) The apparatus, the others are subordinate to (Slave) Apparatus. If pass the coordinator of the network (Network Coordinator) ,Whole network at most can support over 64000 the intersection of ZigBee and the intersection of router and node, add, can interconnect by all Network Coordinator and then, the number of the network node of whole ZigBee will be very considerable.
Compatibility: ZigBee technology and standard seamless of Off-The-Shelf control network are integrated. Pass the coordinator of the network (Coordinator) Set up the network automatically, adopt carrier sense / collision detecting (CSMA-CA) The way inserts the signal channel. In order to transmit reliably, offer the agreement of shaking hands completely.
Safety: Zigbee has offered the check sum of the data integrity to authenticate the function, has offered the tertiary safety in data transmission. First step does not actually have safe way, as to a certain application, unimportant or the upper strata have already offered sufficient safe protection that safe, the device can choose this way to shift the data. As to the safe rank the second highest in rank, the device can use the access control list To prevent the illegal device from obtaining the data, does not take in this first grade and encrypt the measure. It is an advanced encryption standard (AES) that the third stage safe rank is adopted in the data are shifted Symmetric cipher. AES can be used for protecting the payload of the data and preventing the assailant from pretending to be the legal device.
Realize the low cost: The original cost estimate of the module can drop to 1.5-2.5 dollars soon about 6 dollars, and Zigbee agreement avoids the expenses of patent. The price of the baby UWB chip set of low speed is at least 20 dollars at present. And the price goal of ZigBee is only several cents.
3,ZigBee agreement chassis
The technical standards of the network connection, security and application software that Zigbee there is no line standard to develop in a series of 802.15.4 based on that IEEE approveses. Such as 802.11 - 802.16 different, Zigbee and bear the weight of definite the intersection of data and flowrate with the intersection of maximum and transmission rate of 250Kbps 802.15.4 with have line standard while being other. The network standard of ZigBee V1.0 edition is described and already together with the light control equipment put out at the end of 2004, the description of other application and relevant apparatus will be released in subsequent time successively. .
Fig. 1 ZigBee agreement chassis
In formulation of the standard specification, it is mainly IEEE 802.15.4 group and ZigBee Alliance two organizations, the two make hardware and software standard respectively, the role of the two divides the work like the relation between IEEE 802.11 groups and Wi-Fi. In IEEE 802.15.4, IEEE established the 802.15.4 group in December of 2000, responsible for making MAC and PHY (physical layer) The norm, through the 802.15.4 standard in May of 2003, the task group is being set about making b standard on 802.15.4 at present on 802.15.4, this standard is mainly to strengthen the standard on 802.15.4, comprising: Solve the standard in disputable place, reducing complexity, improving the adaptability and considering the distribution of the new frequency band,etc.. ZigBee was set up above the standard on 802.15.4, it confirmed the application outline that can share among different manufacturers. Only defined on 802.15.4 layer and mediumed visit layer in the entity, it is not enough to guarantee the all right dialogue between different apparatuses, then there are ZigBee alliances.
ZigBee compatible products work at PHY of IEEE802.15.4, frequency band its to open free, 2.4GHz (the whole world) respectively , 915MHz (U.S.A.) And 868MHz (Europe) . Can offer 250kbit/s (16 signal channels) on 2.4GHz while adopting ZigBee technical products , offer 40kbit/s (10 signal channel) in 915MHz And offer 20kbit/s (a signal channel) on 868MHz Transmission rate. The transmission range depends on output power and signal channel environment, is between 10m to 100m, it is generally about 30m. Because what ZigBee used is an open frequency band, have had technological use of many kinds of wireless communications already. So in order to avoid being interfered with, each frequency band adopts the spread spectrum technique of the direct sequence. Meanwhile, the direct sequence spread spectrum technique of PHY allows the apparatus not to need the closed loop to move ahead simultaneously.
In these 3 different frequency bands, all adopt the modulation technique of phase place, 2.4GHz relatively adopts QPSK modulation technique of the high-order in order to reach the speed of 250kbit/s, and reduce operation time, in order to reduce power consumption. But in 915MHz and 868MHz, adopt the modulation technique of BPSK. Compare with 2.4GHz frequency band, 900MHz frequency band is a frequency band of the low frequency, there are less wireless travelling losses, the transmission distance is longer, secondly this frequency band was mainly the frequency band that indoor cordless telephone was used in the past, now because the indoor cordless phone is transferred to 2.4GHz, it is less to interfere with instead.
On the MAC layer, mainly continue to use 802.11 serial canonial CSMA/CA ways in WLAN, in order to improve the system compatibility, the so-called CSMA/CA will check whether there is data transmission in the signal channel first before transmission, if there is not data transmission in the signal channel, start data transmission, if collide, retransmission of some time later.
