In the process of using Bluetooth technology, people find that although bluetooth technology has many advantages, it still has many defects. For industry, home automation control and industrial telemetry and remote control fields, Bluetooth technology is too complex, power consumption is high, close range, network size is too small, etc.. And industrial automation, the demand for wireless data communication is increasingly strong, and, for the industrial site, this wireless data transmission must be highly reliable, and can resist all kinds of electromagnetic interference in the industrial site. Therefore, after a long period of efforts, the Zigbee protocol was adopted in the middle of 2003 and officially came into being in 2004.

 

1、 History of origin

What technology does ZigBee originate from?

In the process of using Bluetooth technology, people find that although Bluetooth technology has many advantages, it still has many defects. In the field of industry, home automation control and industrial telemetry and remote control, Bluetooth technology appears to be too complex, large power consumption, close distance, and too small scale of networking. However, industrial automation requires more and more wireless data communication. Moreover, for industrial field, this kind of wireless data transmission must be highly reliable and can resist various electromagnetic interference in industrial field. Therefore, after people's long-term efforts, ZigBee agreement was officially launched in mid-2003. In addition, ZigBee uses home RF Lite, a communication protocol for home network that has been studied before.

What is ZigBee?

Firstly, ZigBee MAC layer and PHY layer are IEEE802.15.4 protocol. According to this protocol, the technology is a short distance, low complexity, low power consumption, low data rate, low cost two-way wireless communication technology, mainly suitable for automatic control and remote control field, can be embedded in a variety of devices, while supporting geographic positioning function. Because bee transmits pollen location and distance information with its peers by flying and "zigging" the "dance" of its wings, that is to say, the bees constitute the communication "network" in the group. Therefore, the inventors of ZigBee vividly use this behavior of bees to describe the wireless information transmission technology vividly.

3 what are the characteristics of ZigBee technology?

ZigBee has the following characteristics:

(1) Low power consumption: in the low power consumption standby mode, two No. 5 dry batteries can support one node to work for 6-24 months, or even longer. This is a prominent advantage of ZigBee. In contrast, Bluetooth can work for weeks and WiFi can work for hours;

(2) Low cost: by greatly simplifying the protocol, the cost is very low (less than 1 / 10 of Bluetooth), and the requirements for communication controller are reduced. According to the prediction analysis, based on the calculation of 8051 8-bit microcontroller, 32KB code is required for full-function master node, and 4KB code is less for sub function node, and ZigBee protocol patent is free;

(3) Low rate: ZigBee works at the communication rate of 250kbps to meet the application requirements of low-speed data transmission;

(4) Short distance: the transmission range is generally between 10-100m, and can be increased to 1-3km after increasing RF transmission power. This refers to the distance between adjacent nodes. If routing and relay of communication between nodes, the transmission distance will be further;

(5) Short time delay: the response speed of ZigBee is fast. Generally, it only takes 15ms to switch from sleep to working state, and it only takes 30ms for node connection to enter the network, which further saves power. Compared with Bluetooth, it takes 3-10s for Bluetooth and 3S for WiFi;

(6) High capacity: ZigBee can adopt star, slice and mesh network structure, with one master node managing several sub nodes, and at most one master node can manage 254 sub nodes; at the same time, the master node can also be managed by the upper network node, which can form a large network of 65000 nodes at most;

(7) High security: ZigBee provides a three-level security mode, including no security settings, the use of access control list (ACL) to prevent illegal access to data, and the use of Advanced Encryption Standard (aes128) symmetric password to determine its security attributes flexibly;

(8) Unlicensed frequency band: Industrial Science and medical (ISM) band, 915MHz (USA), 868mhz (Europe), 2.4GHz (global).

Because the physical layers of these three bands are not the same, their respective channel bandwidths are also different, which are 0.6mhz, 2MHz and 5MHz respectively. There are 1, 10 and 16 channels respectively. The spread spectrum and modulation modes of these three bands are also different. Direct sequence spread spectrum (DSSS) is used in spread spectrum, but the transformation from bit to chip is quite different. However, BPSK is used in 868mhz and 915MHz bands, and OQPSK is used in 2.4GHz frequency band.

When the transmitting power is 0 DBM, Bluetooth usually has a range of 10 meters. However, ZigBee can reach an operating distance of 30-50 meters indoors and even 400 meters in open outdoor areas (Ti CC2530 without power amplification).

Therefore, ZigBee can be classified as low rate short-range wireless communication technology.

Why does ZigBee meet the growing demand of industrial automation for reliable wireless data transmission?

ZigBee technology itself is because blue tooth technology can not meet the needs of low data volume, low cost, low power consumption and high reliability wireless data communication in industrial automation. Because, for the industrial field, this kind of wireless data transmission must be highly reliable, and can resist all kinds of electromagnetic interference in the industrial field. ZigBee technology uses mesh network topology, automatic routing, dynamic networking, direct sequence spread spectrum. Is to meet the needs of industrial automation control field.

2. Why is the 2.4G band of ZigBee wireless communication free of charge?

In China and most other countries in the world, the general use of radio equipment is to pay the frequency use fee, including mobile phone communication, but the mobile operator has already paid this fee to the country, and charged the user this fee through the number occupation fee and other ways. When using other wireless devices, you should first apply for a frequency license from the relevant national departments, and then charge fees according to the frequency, power and quantity of your wireless devices. This is not a small cost, general equipment a year often have to pay thousands of yuan. The free frequency band refers to a frequency band divided by each country according to its actual situation and considering the consistency with the provisions of other countries in the world as far as possible. It is specially used for industrial, medical and scientific research (ISM band), and can be used free of charge without application. China's 2.4G band is such a frequency band. However, in order to ensure that everyone can use it reasonably, the state has made corresponding restrictions on the power of wireless transceiver equipment in different environments. For example, in the urban environment, the transmission power cannot exceed 100MW.

3. Is ZigBee only suitable for short distance communication?

ZigBee LAN can not only expand the network by increasing the transmitting power and receiving sensitivity of each node module, but also monitor the remote ZigBee control network through the traditional Internet. However, in the expansion, we should pay attention to the following:

(1) With the increase of transmitting power, the power consumption will naturally increase. As long as the power consumption is large, ZigBee will lose its advantage of low power consumption;

(2) Although 2.4GHz is a free band, it can not exceed the maximum power limit in radio wave method.

What are the advantages of using direct sequence spread spectrum in ZigBee?

For example, ask, FSK, FHSS, DSSS and so on have different anti-jamming ability, communication security and reliability. ZigBee system, like CDMA system, adopts direct sequence spread spectrum (DSSS), which is a communication mode with strong anti-jamming ability, high security and reliability. If you've used the communications products of these two technologies, you'll find that the reliability of the two technologies is remarkable.

Because the signal-to-noise ratio required by spread spectrum technology in normal communication can be very low, that is to say, it can still work normally in the environment with strong interference. According to the calculation and experiment, this is equivalent to an increase of 7 DBM in the receiving sensitivity. It's not easy to interfere with others. In other words, it can transmit longer distances with lower power. Some information about the advantages of spread spectrum technology are cited for reference

CDMA is a new type of digital cellular technology introduced in 1995 after the introduction of digital communication technology in the world. It uses digital transmission method and spread spectrum communication technology to greatly improve the frequency utilization rate. It has the outstanding advantages of large capacity, wide coverage, low power consumption of mobile phone and high voice quality, and pushes the mobile communication technology to a new stage of development. Compared with FDMA and TDMA systems, CDMA system has the following outstanding advantages:

(1) CDMA has good anti-jamming performance. It can coexist with narrowband in the same frequency band without affecting its normal operation.

(2) Multipath fading is a prominent problem that affects the quality of mobile communication. It must be overcome by spatial diversity, adaptive equalization and other technologies. CDMA system can use multipath signal to provide path diversity, which not only eases Rayleigh fading, but also eases the slow fading caused by physical occlusion, thus greatly improving the communication quality.

(3) For FDMA and TDMA, if the frequency points or time slots of the cell are allocated completely, the cell can not receive new calls, and the capacity is limited. CDMA is an interference limited system. Even if the number of users has reached the limited number, it is allowed to increase individual users under the specified interference level. Its disadvantage is that the voice quality is reduced. Service providers can balance capacity and voice quality. CDMA precise power control and soft handoff technology greatly reduce the intensity of interference signal and the required signal-to-noise ratio requirements, and effectively use such as voice activation or variable rate voice coding, diversity reception, power control. It is reported that the signal-to-noise ratio of CDMA is 3.7 times that of damps and TDMA, 11.2 times of TACs, 13.6 times of amps and 20 times of FM / FDMA.

(4) The communication quality is good. CDMA system adopts direct sequence spread spectrum technology, and uses time diversity, frequency diversity, space diversity, path diversity and other diversity technologies to overcome the multipath effect, which can obtain strong anti-interference ability. In addition, it uses soft switch which is established first and then interrupts during handoff Switching technology ensures the communication quality of CDMA, especially in handoff without ping-pong effect. The system is broadband and low noise ratio. The waveform allows the use of high redundancy error correction coding and efficient digital modulation technology to ensure high quality voice and data transmission.

(5) In this way, FDMA and CDMA can be reused at the same cell frequency (FDMA and CDMA) with high repetition rate.

(6) The multiple access capability of CDMA system determines the magnitude of multiple access interference between spread spectrum codes. It is related to the spread spectrum coding scheme used and the multiple access interference among users who send signals at the same time (i.e. the correlation characteristics of spread spectrum coding) Therefore, the lower the multiple access interference (MAI) between users working at the same time, the lower the allowable reception quality, the stronger the multiple access capability of CDMA technology.

(7) If the security of CDMA communication system can be greatly improved, the security of this communication system can not be greatly improved. Analyzing the spread spectrum system, it is almost impossible to intercept other people's communication content. For example, as long as the lithium battery in the machine is not discharged completely, it changes the real-time status of its sequence at the clock frequency of 512khz. Even if it works continuously, the period of its spread spectrum address sequence is as long as seven years. It can also conveniently set and change the master key, secondary key, spread spectrum code table and standard encryption algorithm in CDMA system to make the communication more secure.

(8) The minimum power consumption of FDMA is 5MW, the average transmitting power is 794mw and the peak power is 3W, while the minimum power, average transmit power and peak power of CDMA are 2.3mw, 5MW and 100MW respectively. It can be seen that the average transmit power and the maximum transmit power of CDMA are lower than those of FDMA, which increases the system capacity, reduces the number of cells and reduces the cost of equipment.

5. What are the characteristics of ZigBee compared with the existing digital radio stations?

Its advantages are as follows:

(1) High reliability: because the integration of ZigBee module is much higher than that of general data transmission radio station, the separated components are less, so the reliability is higher;

(2) Compared with CDMA, it is not only easy to use, but also easy to use, because it is small in size and easy to use when it is installed in the mobile phone;

(3) Strong anti-interference, good security, low bit error rate: ZigBee transceiver module uses 2.4G direct sequence spread spectrum technology, which has better anti-interference ability and longer transmission distance than general FSK, ask and frequency hopping data transmission radio;

(4) Free frequency band: ZigBee uses the free frequency band, while many data transmission stations need to apply for the frequency band, and they need to pay the corresponding frequency use fee to the national non committee every year;

(5) Low price: the price of ZigBee data transmission module is only a fraction of that of a digital radio station with similar functions.

Why is ZigBee better than Wi Fi and Bluetooth?

After recent years of research on ZigBee technology and related testing practice, MORLAB summed up the following points, which are also the main reason for the strong anti-interference ability of ZigBee products.

The anti-jamming characteristic of ZigBee is mainly anti-interference of the same frequency, that is, the interference from other technologies sharing the same frequency band. The main characteristics are as follows:

1. Clear channel assessment (CCA)

Determine whether the channel is idle. IEEE 802.15.4 physical layer provides CCA capability in collision avoidance mechanism, that is, if the channel is occupied by other devices, transmission can be allowed to exit without considering the communication protocol of the channel.

There are three main idle channel evaluation modes

(1) If the signal energy is lower than a certain threshold, the channel is considered to be idle;

(2) The characteristics of wireless channel include two aspects: spread spectrum signal and carrier frequency;

(3) Combining the first two modes, the signal strength and signal characteristics are detected at the same time, and the channel idle judgment is given.

2. Dynamic channel selection:

Coordinator in ZigBee personal area network (Pan) (the central node of the network, responsible for the organization and maintenance of the network). Firstly, scan all channels, and then confirm and add a suitable pan (it is not necessary to create a new pan) to reduce the number of pan in the same frequency band and reduce the potential interference. If the interference source appears on the overlapping channel, the software on the upper layer of the coordinator should apply the channel algorithm to select a new channel.

3. Channel algorithm:

During network initialization or response to interruption, ZigBee devices will scan a series of channels listed in channel table parameters for dynamic channel selection, and set channel table parameters according to the above idle channels, so as to enhance the coexistence performance of the network.

During severe interference, ZigBee does not change the channel, but relies on its low duty cycle, conflict free algorithm (each device listens to the channel before sending data) to reduce the data loss caused by transmission conflict.

4. Direct sequence spread spectrum (DSSS) and fast changing frequency fa:

Direct sequence spread spectrum (DSSS) technology has certain anti-interference effect (as shown in the figure below). Under the same other conditions, the transmission distance is larger than that of frequency hopping technology (when the transmission power is 0dbm, Bluetooth has 10m range, while ZigBee can reach 30-50m indoor range). The so-called frequency fast change is to change the frequency to avoid the influence of a known interference source or signal source.

It can be seen from the above figure that the BER of ZigBee can reach 10-9 when the signal-to-noise ratio is 4dB. To achieve the same bit error rate, Wi Fi should reach 10dB and Bluetooth can reach 16dB. ZigBee's anti-interference performance is obviously better than Wi Fi and Bluetooth technology.

5. Reply retransmission and frame caching:

The MAC layer and application layer (APS part) of ZigBee have the function of automatic request retransmission ARQ and frame buffer. When transmitting a data frame to a device, if the receiving device is busy or dormant and cannot receive the frame, the main coordinating device of the network will temporarily cache the frame until the receiving end receives the frame.

The modulation mode is very simple, but ZigBee has good anti-interference performance in 2.4GHz ISM band, which is very suitable for low power consumption and low data transmission applications. At present, the main test standard of zibee products in CE certification is en300328, while the test of part 15C is mainly used in FCC.