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|[Date:]||Source: Automated instrumentation by Zhou Yi Ling Zhihao Wu Qinqin||[Font: large, medium, and small]|
0 introduction the industrial field has improved labor productivity and reduced production costs by introducing various advanced technologies in the modernization process. Among these technologies, the most typical is digital technology and modern communication technology. On the basis of modern industrial digitalization, industrial production monitoring has already broken through the single-loop control and monitoring functions. With the continuous development of computer software and hardware technology, network technology and integrated industrial automation system integration level, it puts forward higher requirements on the openness of data interfaces, the real-time data transmission, and the security of data connections. Many large enterprises have scattered production regions and complicated division of labor. They often have one or more control centers and a large number of on-site data collection points. These collection points need to communicate with the central control unit through a certain amount of communication means to achieve data interaction, and thus realize the automation of the production process. Due to the limitations of traditional wired networks, network coverage and network support in many special environments are still a challenge. For example, in some industrial sites, some industrial environments prohibit or restrict the use of cables, in other industrial environments, cables must be completely shielded to prevent high interference from machines or other radio control equipment in most industrial facilities, some high-speed rotating devices cannot transmit data through cables. However, wireless WAN, wireless LAN, and non-personal network technologies can effectively provide solutions to these problems. With the development of existing wireless network technology, wireless standards increase flexibility and reduce the risk of Integrated patented wireless communication. In industrial control applications, short-distance wireless transmission is particularly eye-catching. Over the past few years, people have been constantly exploring and developed the dazzling wireless communication protocols and products. The most popular short-distance wireless data communication standards include Bluetooth, Wi-Fi (802.11), IRDA, extremely promising, and widely recognized ZigBee (IEEE 802.15.4. 1. Comparison of several wireless transmission technologies 1. 1 Bluetooth is a wireless module that Ericsson first studied in 1994 to enable communication between mobile phones and their accessories (such as headphones. In 1998, Ericsson, Nokia, IBM, and other companies jointly launched the bluetooth technology, which is mainly used for wireless connections between communication and information devices. It operates at 2.4 GHz and has an effective range of about 10 m. Bluetooth is included in IEEE 802.15.1, which specifies an integrated protocol stack, including phy, Mac, network, and application layer. To support voice and specific networks, the protocol stack needs to provide kb system overhead, which increases system costs and integration complexity. In addition, Bluetooth has a limit that only seven nodes can be configured for each "piconet" (micro-Network), which restricts its application in large-scale sensor network development. 1. 2 Wi-Fi (IEEE802. 11) Wi-Fi (Wireless fidelity, wireless high fidelity) is also a wireless communication protocol. The original specification of IEEE802.11 was proposed in 1997. The main purpose is to provide WLAN Access, which is also the main technical standard of WLAN, and its operating frequency is also 2.4 GHz. At present, the IEEE802.11 standard has not been widely accepted by the industry. IEEE802. 11 popular versions include "a" (54 mbps bandwidth in the 5.8 GHz band) and "B" (11 mbps bandwidth in the 2.4 GHz band) and "G" (bandwidth of 2.4 GHz is 22 Mbps ). This complexity makes it difficult for users to select a standard wireless platform. Wi-Fi specifies the Protocol's physical (PHY) layer and the Media Access Control (MAC) layer, and relies on TCP/IP as the network layer. Because its excellent bandwidth is at the cost of high power consumption, most portable Wi-Fi devices require conventional charging. These features limit its industrial application and promotion. 1. 3 IrDA (infrared dataassociation) was established in 1993. IRDA is a technology that uses infrared rays for point-to-point communication. The transmission rate of IrDA standard wireless devices has gradually increased from 115.2 kbps to 4 Mbps and 16 Mbps. Currently, software and hardware technologies supporting it are mature and widely used in small mobile devices (such as PDAs and mobile phones. It has the characteristics of small size, low power consumption, convenient connection, easy to use and low cost for mobile communication. The biggest problem with IrDA in industrial networks is that it can only be connected between two devices, and there are problems such as line of sight angle. ZigBee (802.15.4) is a recently developed short-range wireless communication technology with low power consumption. It is considered by the industry as the most likely wireless Method for industrial control applications. It also uses the GHz band, frequency hopping technology and spread spectrum technology. In addition, it can connect to 254 nodes. Nodes can include instruments and automated home application devices. Its own characteristics make it very promising in industrial monitoring, sensor networks, Home monitoring, security systems, and other fields. The main performance comparison of several common wireless transmission modes is shown in table 1.
2. the technical content and features of ZigBee are the latest commercial names. In the past, the initiators named "HomeRF lite", "Firefly", and "RF-EasyLink. To meet the wireless networking requirements of small and low-cost devices similar to sensors, IEEE set up the IEEE December 2000 Working Group to define a low-cost, fixed, portable, or mobile device, low-rate wireless connection technology with low complexity, cost, and power consumption. The ZigBee Alliance was established in August 2001. So far, in addition to internationally renowned companies such as megasys, Mitsubishi Electronics, Motorola, Samsung, and Philips, the Alliance has more than member companies and is growing rapidly. It covers Semiconductor manufacturers, IP service providers, consumer electronics manufacturers, and OEMs, such as Honeywell, Eaton, MeteringSystems, and other industrial control and Home Automation companies, there are even toys companies like Mattel. All these companies have participated in the IEEE 802.15.4 working group responsible for the development of ZigBee physical and media control layer technical standards. In the fields of industry, agriculture, on-board electronic systems, home networks, medical sensors, and servo actuators, the requirements for wireless networks are quite different from those for civil applications. It usually has low requirements on data throughput and low power consumption. In addition, a large number of wireless devices are emerging, which are simple, convenient, and can be used at will. A large number of wireless access points need to be arranged, and the low price will play a key role. Therefore, the ZigBee standard should solve the problem by designing a communication link that maintains the minimum traffic and a low-complexity wireless transceiver. The core issue to be considered is the design of low power consumption and low price, which requires the standard to provide applications with low bandwidth and low data transmission rate. 2. 1 ZigBee features ① Low Power Consumption: due to the low transmission rate of ZigBee, the transmit power is only 1 mW, and the hibernation mode is adopted, resulting in low power consumption. Therefore, the ZigBee device is very energy-saving. It is estimated that the ZigBee device can be used for about 6 months to 2 years by only two batteries, which is beyond the reach of other wireless devices. ② Low cost: the initial cost of the ZigBee module is about $6, and it is estimated that the initial cost will soon be reduced to 1.5 ~ $2.5, and the ZigBee protocol is free of patent fees. Low cost is also a key factor for ZigBee. ③ Short latency: both the communication latency and the activation latency from sleep state are very short. The latency of typical search devices is 30 ms, and that of sleep activation is 15 ms, the channel access latency of active devices is 15 ms. Therefore, ZigBee technology is suitable for wireless control applications that require high latency (such as industrial control scenarios. ④ Large Network Capacity: A star-structured ZigBee network can accommodate a maximum of 254 slave devices and a master device, and the network composition is flexible. ⑤ Reliability: A collision avoidance policy is adopted, and a special time slot is reserved for communications services requiring fixed bandwidth, avoiding competition and conflicts between sending data. The MAC layer adopts a fully validated data transmission mode. Each sent packet must wait for the recipient's confirmation information. If a problem occurs during transmission, you can resend it. ⑥ Security: ZigBee provides the packet integrity check function based on cyclic redundancy check (CRC), supports authentication and authentication, and uses the AES-128 encryption algorithm, each application can flexibly determine its security attributes. 2. 2 The connection between ZigBee and IEEE 802.15.4 often equate ZigBee with IEEE 802.15.4. In fact, there are still some differences between the two: ① ZigBee fully utilizes the advantages of the powerful physical features defined by 802.15.4; ② ZigBee adds Logical Networks and application software; ③ ZigBee is based on the IEEE 802.15.4 RF standard, meanwhile, the Zig-Bee Alliance works closely with IEEE to ensure an integrated and complete market solution. ④ the 802.15.4 working group is mainly responsible for developing the physical layer (PHY) and Media Access Control (MAC) layer standards, while ZigBee is responsible for the development of the network layer and application layer. Figure 1 illustrates the structure and division of labor of ZigBee.
802.15.4 protocol architecture and technical features 802.15.4 meet the requirements of the International Standard Organization (ISO) Open System Interconnection (OSI) Reference Model, defining a single MAC layer and a variety of physical layers. The ZigBee Alliance has developed Mac-layer and above protocols. The protocol suite consists of high-level application specifications, application aggregation layer, network layer, data link layer, and physical layer. 868/915. 1 physical layer 802.15.4 provides two physical layers (2.4 MHz and GHz) as shown in Figure 2. Collaboration between the physical layer and the MAC layer expands the scope of network applications. Both physical layers use Direct Sequence Spread Spectrum (DSSS) technology to reduce the cost of digital integrated circuits and use the same frame structure for low operation cycles and low power consumption. Figure 2 the data transmission rate of the 2.4g physical layer is 250 kbps, and the data transmission rate of the 868/915 MHz physical layer is 20 kbps and 40 kbps respectively. The higher speed of the GHz physical layer is mainly attributed to the quasi-orthogonal Modulation Technology Based on the DSSS method (16 States. Binary data from the physical layer convergence Protocol Data Unit (ppdu) is sequentially (in bytes from low to high) to form a four-bit binary data symbol, each data symbol (corresponding to a group in the 16-State group) it is mapped to a 32-bit pseudo-noise chip for transmission. Mski then modulated the continuous Pseudo Noise code sequence using the Minimum Shift Keying method, that is, the semi-sine pulse waveform offset four phase shift keying (o-PSK) method. The 868/915 MHz physical layer uses a simple DSSS method. Each ppdu Data Transmission Bit is extended by a code-piece sequence (m-sequence) with a maximum length of 15. Different data transmission rates are suitable for different scenarios, such as the low speed of the 868/915 MHz physical layer in exchange for better sensitivity (-85dbm/2.4g,-92dbm/868,915 MHz) and a large coverage area, thus reducing the number of nodes required to cover a given physical area; the higher speed of the 2.4g physical layer is suitable for scenarios with high data throughput, low latency, or low job cycle. 2. 3. 2. The design of the zigbeemac layer of the media access layer must take into account the requirements of cost reduction, easy implementation, reliable data transmission, short-range operations, and low power consumption, therefore, the following simple and flexible protocols are adopted: ① IEEE Standard 64-bit and 16-bit short addresses are used; ② the basic network capacity can reach 254 nodes; ③ you can configure a local simple network with more than 65,000 (216) nodes with little overhead. ④ Network Coordinator, full-featured device (FfD), and simplified functional device (RFD) (5) Simplified frame structure; (6) reliable data transmission; (7) Joint/separation; AES-128 Security Mechanism; and ⑨ CSMA-Ca channel; optional; Use the superframe structure of the beacon. The 802.15.4mac sub-layer defines four types of frames, including broadcast frames, data frames, validation frames, and Mac command frames. Only broadcast frames and data frames contain high-level control commands or data. Confirmation frames and Mac command frames are used to send and receive control information between MAC Sub-layers of ZigBee devices. The confirmation of broadcast frames and validation frames does not need to be performed by the receiver. The frame header of the data frame and Mac command frame contains the frame control domain, indicating whether the received frame needs to be confirmed. If so, after CRC verification is passed, the Receiver immediately sends a confirmation frame. If the sender fails to receive the confirmation frame within a certain period of time, the sender will automatically re-Send the frame. This is the basic process for reliable transmission of MAC Sub-layers. The common frame format of the MAC layer is 3. 2. 3. The network layer includes the Logical Link Control Sub-layer. The 802.2 standard defines LLC and is applicable to ZigBee wireless communication technologies such as 802.3, 802.11, and 802.15.1 and their applications, the MAC Sub-layer is closely related to the hardware and changes with the implementation of different physical layers. The network layer is responsible for the establishment and maintenance of the topology, naming and binding of services, which are indispensable tasks such as addressing, routing, and security. The IEEE 802.15.4 standard draft supports a variety of network topologies, including the new mesh network shown in Figure 4 ). Applications that require low-latency access such as computer peripherals generally use a star network structure, while other applications, such as peripheral security, may require coverage of a large network. The form of multiple access includes IEEE Standard 64-bit and short address 8-bit. 3 conclusion ZigBee is a short-distance wireless Technical Specification for sensor networks, building automation and other applications. ZigBee is a bidirectional wireless communication technology with short distance, low complexity, low power consumption, low data rate, and low cost. It is mainly applicable to automatic control and remote control, it is designed to achieve wireless networking and Control of small and low-cost devices. It is designed based on high power-saving requirements. Therefore, low power consumption and low data transmission rates mean that it will not compete with other wireless technologies such as Wi-Fi, but as a cost-effective solution for sensor network and other applications. The industry is confident in its application progress in the above fields.
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