1.802. 1 1, 802. 1 1b, 802. 1 1g all work in 2.4GHz ISM (industry, science and medical care). However, 802. 1 1a works in the 5GHz band and is temporarily closed, so an application is required.
2.802. 1 1a and 802. 1 1g both have the highest physical layer rate of 54Mbps and the highest transport layer rate of 25Mbps, but the stability needs to be further improved and the cost is high. 802. 1 1b has a maximum rate of 1 1Mbps, which will be the most promising wireless LAN standard in the future due to its early start, mature technology and low cost. The following focuses on the 802. 1 1b standard.
Ii. IEEE 802. 1 1b wireless network standard
1. Physical layer of WLAN
The difference between wireless LAN and traditional wired LAN is that wireless LAN generally uses radio as transmission medium instead of traditional cable. For IEEE 802. 1 1b WLAN, there are three optional physical layers: frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS) and infrared (IR). The choice of physical layer depends on the requirements of practical application. Frequency hopping spread spectrum and direct sequence spread spectrum are two commonly used spread spectrum technologies in communication technology, which are used to improve the utilization rate of wireless channels and the security of data communication. At present, the physical layer media of most WLAN products based on IEEE 802. 1 1b work in the radio frequency band (ISM band) of 2.4000~2.4835GHz, and direct sequence spread spectrum technology is adopted to provide data transmission rate as high as 1 1Mbps.
2. MAC protocol of WLAN
In principle, there is no essential difference between the MAC protocol of wireless LAN and the MAC protocol of wired LAN. However, due to the inherent characteristics of wireless transmission media and the influence of mobility, the MAC protocol of WLAN cannot follow the original LAN protocol. For example, the MAC layer of IEEE 802.3 uses CSMA/CD to make different sites * * * share the same physical channel. An important prerequisite to realize CSMA/CD is that each site can easily realize the conflict detection function. In the case of wired LAN (such as Ethernet), collision detection can be easily realized according to the change of DC component on the tested cable. However, when wireless transmission media is used, it is difficult to achieve conflict detection for the following reasons.
1) collision detection requires each station to send (send its own signal) and receive (decide whether the transmission of other stations interferes with its own transmission) at the same time, which will increase the cost of the channel.
2) More importantly, because of the hidden terminal problem, even if a station has the ability to detect conflicts and has detected conflicts during transmission, there will still be conflicts at the receiving end.
In view of the above reasons, the wireless LAN protocol standard IEEE 802. 1 1b adopts the carrier sense multiple access (CSMA/CA) protocol with collision avoidance to realize wireless channel sharing.
A simple CSMA/CA can be implemented as follows: Before sending a data packet, a wireless device will listen to see if other wireless devices are sending. If the transmission is in progress, the device will wait for a randomly determined time and then listen. If no other device is using the medium, the device will start transmitting data. Because it is likely that one device is sending data while another device is sending data, in order to avoid data loss caused by this conflict, the receiving device will detect the CRC of the received packet, and if it is correct, it will send an acknowledgement to the sending device, indicating that there is no conflict. Otherwise, the transmitting device will repeat the above CSMA/CA process.
In order to minimize the possibility of collision caused by simultaneous transmission of two wireless devices, 802. 1 1 The designer used a mechanism called send request/clear send (RTS/CTS). For example, if data arrives at a wireless access point (AP) designated by a wireless node, the AP will send an RTS frame to the wireless node, requesting a certain amount of time to send data to it, and the wireless node will respond with a CTS frame, indicating that it will block any other communication until the AP sends the data. Other wireless nodes can also hear the ongoing data transmission and delay its transmission after this time. In this way, when transmitting data between nodes, the possibility of collision on the medium caused by equipment is the least. This transmission mechanism also solves the problem of hiding terminals in WLAN.
In order to ensure that data will not be lost in transmission, CSMA/CA also introduces an acknowledgement (ACK) mechanism. After receiving the data, the receiver sends a confirmation notification ACK to the sending unit. If the sender does not receive an ACK, it indicates that the data is lost and the data will be transmitted again.
3. Real-time performance analysis of WLAN.
IEEE 802. 1 1b wireless LAN standard adopts CSMA/CA protocol in the media access control layer to realize the enjoyment of wireless channels. In the case of light network load, the probability of conflict is very small, and some wireless network products take some additional measures, or even completely avoid conflicts. For example, AWE 120-24, a wireless product of Wi-LAN, adopts the way of dynamic time allocation polling: when multiple wireless remote devices communicate with the base station, the base station will ask each remote station in turn whether there is any data to send according to the ID of the remote station, and if there is any data to send, it will be allocated a time slice. If not, it will continue to ask, and so on. The so-called dynamic polling here means that users can set the polling mode of base stations to reduce the number of inquiries to inactive sites to ensure that time slices will not be wasted. Dynamic time allocation polling technology completely avoids conflicts and can obtain better real-time performance than CSMA/CA. This makes the application of wireless technology in industrial control network possible.
3. Introduction of networked intelligent sensor based on wireless technology.
The combination of computer network technology, wireless technology and intelligent sensor technology has produced a brand-new concept "networked intelligent sensor based on wireless technology". The intelligent sensor integrates data acquisition, data processing and wireless network interface module. The bottom network interface (hardware interface) of wireless network interface module adopts the network interface chip based on IEEE 802. 1 1b, and the high-level network interface (software interface) adopts TCP/IP protocol, which is an embedded application, that is, the TCP/IP protocol is solidified into the ROM of intelligent sensors to make the field data. This networked intelligent sensor based on wireless technology enables industrial field data to be directly transmitted, published and appreciated on the network through wireless links.
Wireless local area network can be realized by wireless hub, wireless access point (AP), wireless bridge, wireless modem and wireless network card on the basis of ordinary local area network.
In the field of industrial automation, there are thousands of sensors, detectors, computers, PLC, card readers and other devices, which need to be interconnected to form a control network. Usually, the communication interface provided by these devices is RS- 232 or RS-485. Wireless LAN equipment uses isolated signal converter to convert RS-232 serial signal of industrial equipment into signals of wireless LAN and Ethernet, which conforms to IEEE802. 1 1b of wireless LAN and IEEE 802.3 of Ethernet, supports standard TCP/IP network communication protocol, and effectively expands networking communication capability of industrial equipment.
Fourthly, the application of WLAN in industrial control network.
The networking of industrial control system provides the foundation and possibility for the application of wireless technology in industrial control system. In recent years, many researchers have also carried out research work in this field. Ceng Peng and others of Shenyang Institute of Automation, Chinese Academy of Sciences built a field-level wireless communication protocol stack based on FFHSE (High Speed Ethernet) promulgated by FF (Fieldbus Foundation) and the wireless Ethernet standard IEEE 802.11b. This protocol stack maintains the communication model of Foundation Fieldbus, and can complete the time synchronization and real-time communication between wireless devices. Kyung Chang Lee of Busan National University in South Korea and others designed a protocol conversion model to realize the interconnection between PROFIBUS-DP network and IEEE802. 1 1 WLAN. Mario Alves et al. estimated the message transmission delay time of hybrid network based on broadcast fieldbus/wireless network. C.Koulamas and others studied the performance of combining Profibus with DSSS physical layer based on IEEE 802.11b.
In addition to theoretical research, wireless communication technology has been applied in some industrial control networks. For example, in the three-layer control network system based on DeviceNet, Control-net and Ethernet/IP, Rockwell Company of America added wireless Ethernet to realize wireless communication. Siemens Germany combined wireless Ethernet technology into the control network based on PROFIBUS-DP and Profinet, which made the control network have wireless communication function. Due to the unparalleled advantages of wireless network, it can avoid a large number of line connections, save the construction cost and maintenance cost of the system, and meet the needs of some special occasions. At the same time, the flexibility of system composition is greatly enhanced. Coupled with the continuous improvement of wireless communication technology itself, wireless communication technology will have broad development space and application prospects in the field of industrial control.
Verb (abbreviation of verb) Application scheme and equipment of wireless technology in industrial control network
1. Wireless industrial control method
By using networked intelligent sensors based on wireless technology and combining various wireless LAN bridges based on IEEE 802. 1 1b, the application scheme of wireless LAN technology in industrial control network can be realized. A wireless LAN bridge is used as a wireless access point (AP). The networked intelligent sensor based on wireless technology collects and processes the field data, encapsulates the data with TCP/IP protocol, and sends it to AP through wireless link. Because the high-level wireless link and wired Ethernet adopt TCP/IP protocol, and the low-level protocol is transparent to the high-level protocol, the seamless connection between wireless network and wired network is realized. Through the Internet, remote monitoring can be realized.
2. Selection of wireless devices
To realize wireless network, there are generally two devices to choose from. One is wireless LAN bridge, which can connect multiple wireless stations to the existing LAN; The other is wireless communication equipment, such as wireless network card, wireless modem, etc. Next, we will introduce Advantech's wireless devices.
A.WLAN-9200 series 1 1Mbps industrial wireless LAN access equipment.
WLAN-9200 is an enhanced 1 1Mbps wireless LAN bridge for outdoor use. It can connect multiple remote workstations to the LAN without any physical wiring.