I. Introduction to Wireless LAN
Generally speaking, any LAN that uses wireless transmission media can be called wireless LAN. The wireless media here can be radio waves, infrared or lasers. Wireless LAN (Wireless LAN) technology can be very convenient to connect network devices wirelessly, people can access network resources at any time, anywhere, at will, is an important direction of the development of modern data communication systems. Wireless LAN can provide network interconnection function without using network cable wires.
1. Introduction to wireless protocols
The wireless local area network protocol standards have been established for a long time now, but due to the low speed of wireless LANs, the protocol standards are not uniform, expensive, and the users are reluctant to use wireless networks in order to protect their investments, so wireless LANs have not been widely used. In recent years, with the introduction of higher rate wireless communication protocols, wireless LANs have been developing rapidly.
IEEE802.11 was the first international standard for wireless LANs adopted by the IEEE802 Standards Committee in 1997, and in September 1999, the committee promulgated the IEEE802.11b standard, which incorporates the physical and media access control (MAC) layers of the ISO/OSI model. The standard operates at 2.4 GHz and has a transmission rate of up to 11 Mbps. The IEEE802.11b standard divides the node equipment into base stations and client stations, and each client station can communicate with each other directly or under the unified management of the base station. The connection between a base station and a group of client stations is called Basic Service Set (BSS), and two or more BSSs constitute an extended service set.The IEEE802.11b standard specifies three realization methods for the physical layer, namely, Frequency Hopping Spectrum Spreading Method (FHSS), Direct Sequence Spectrum Spreading Method (DSSS), and Infrared Technology (IR).The CSMA/CA (Carrier Addressing System) method is used in the MAC layer. CSMA/CA (carrier-sniffing multiple access/collision avoidance) technique for communication medium access. In order to minimize the conflicts. 802.11b has designed a unique MAC sub-layer as shown in Figure 1. The following layer is called the Distributed Coordination Function DCF (Distributed Coordination Function) sublayer which enables individual nodes to use the channel in a competitive manner to provide contention services upwards. This type of channel access may lead to conflicts, but the utilization of the channel is high. The layer above is called the Point Coordination Function (PCF)
Figure 1 MAC sublayer of IEEE802.11
Sublayer, which uses centrally controlled access algorithms, where the base station polls to give the communication rights to the individual client stations in turn, thus avoiding conflicts. However, the base station needs to poll all client stations periodically, which takes up a lot of time, and is therefore suitable for medium and small networks. The technology of wireless LAN is still evolving. Radia-ta and Atheros have each announced the release of an IEEE802.11a chipset. 802.11a has a data rate of 54 Mbps, and Atheros claims that their chipset can be used in "Turbomode" (Enhanced Mode). （For 802.11a, it's not just the transfer rate that will increase, it will operate in the 5 GHz frequency, avoiding the crowded 2.4 GHz band. on November 15, 2001, the IEEE gave experimental approval to a new technology, 802.11g, which could increase the speed of wireless Internet access in homes, businesses, and public **** places. wireless Internet access speeds in homes, companies and public **** places. The technology enables wireless networks to transmit up to 54 Mbps per second as well, which is five times faster than the now common 802.11b, and is compatible with 802.11b. The technology standards introduced above can be compared by Table 1 below.
Table 1 Technical Standards, Frequency Allocations, and Transmission Rates
Technical Standards
Year of Establishment
Frequency Occupancy
Maximum Rate
Modulation Technique
802.11
1997
2.4 GHz
2 Mbps
FHSS
802.11b
1999
2.4GHz
11Mbps
DSSS
802.11a
1999
5GHz
54Mbps
OFDM
802.11g
2000
2.4GHz
54Mbps
DSSS
Description:
1. 802.11, 802.11b, 802.11g are all operating in the 2.4GHz ISM (Industrial, Scientific, Medical) public*** ** band, which does not require an application from the No Commission; while 802.11a works in the 5GHz band, which is currently closed and requires an application.
2. 802.11a and 802.11g physical layer rate of up to 54Mbps, the transmission layer rate of up to 25Mbps, but the stability to be further improved, and the cost is also higher. The 802.11b maximum rate of up to 11Mbps, because of the earlier start, more mature technology, the cost is not high, will be the future of the most promising wireless LAN standards, the following focus on the 802.11b standard.
The IEEE 802.11b wireless network standard
1. The physical layer of wireless LAN
The difference between wireless LAN and traditional wired LAN is that the physical layer is generally used as a transmission medium for wireless LAN, instead of the traditional cable. For IEEE 802.11b WLANs, there are three optional physical layers: the Frequency Hopping Spread Spectrum (FHSS) physical layer, the Direct Sequence Spread Spectrum (DSSS) physical layer, and the Infrared (IR) physical layer. The choice of physical layer depends on the requirements of the actual application. Frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) are two commonly used spectrum expansion techniques in communication technology to improve the utilization of wireless channels and the security of data communication. Currently, most IEEE 802.11b-based WLAN products operate in the 2.4000-2.4835GHz radio frequency band (ISM band), and utilize direct sequence spread spectrum technology to provide data transfer rates up to 11Mbps.
2. MAC Protocol for Wireless LANs
In principle, there is no essential difference between the MAC protocol for wireless LANs and the MAC protocol for wired LANs. However, due to the inherent characteristics of wireless transmission media and the impact of mobility, the MAC protocol for wireless LANs cannot follow the original LAN protocol. For example, the MAC layer of IEEE 802.3 uses CSMA/CD to enable different stations to *** enjoy the same physical channel. An important prerequisite for the implementation of CSMA/CD is that the sites can very easily implement a conflict detection function. In the case of wired LANs (e.g., Ethernet), conflict detection can be easily implemented based on detecting changes in the DC component on the cable line. However, when using wireless transmission media, conflict detection is difficult to implement for the following reasons.
1) The ability to detect conflicts requires that each station be able to transmit (send its own signals) and receive (decide whether the transmissions of other stations are interfering with its own transmissions) at the same time, which increases the channel expense.
2) More importantly, due to the hidden terminal problem, even if a station has conflict detection capability and has detected a conflict while transmitting, a conflict can still occur at the receiving end.
For these reasons, the wireless LAN protocol standard IEEE 802.11b employs a carrier-surveillance multiple access (CSMA/CA) protocol with conflict avoidance to realize wireless channel **** enjoyment.
A simple CSMA/CA can be implemented as follows: before a packet is transmitted, a wireless device will listen to see if another wireless device is transmitting. If transmission is in progress, the device will wait for a randomly determined period of time, then listen again, and if no other device is using the medium, the device starts transmitting the data; because it is very likely that while one device is transmitting the data, another device starts transmitting the data, in order to avoid data loss caused by such conflicts, the receiving device detects the CRC of the packet it receives, and if it is correct, transmits an acknowledgement message (acknowledgement message) to the transmitting device. If it is correct, it transmits an acknowledgement to the transmitting device to indicate that no conflict has occurred. Otherwise, the sending device repeats the CSMA/CA process.
In order to minimize the possibility of two wireless devices transmitting at the same time, which would result in a conflict, the 802.11 designers use a mechanism called Request to Send/Clear to Send (RTS/CTS). For example, if data arrives at a wireless access point (AP) specified by a wireless node, that AP will send an RTS frame to that wireless node requesting a certain amount of time to transmit data to it, and the wireless node will respond with a CTS frame indicating that it will block any other communication until the AP has finished sending data. Other wireless nodes can also hear the data transmission taking place and delay their transmission until after that time. In this way, data is transmitted between nodes with minimal possibility of creating conflicts on the medium caused by the device. This transmission mechanism also solves the problem of hidden terminals in wireless LANs.
In order to ensure that data is not lost in transmission, CSMA/CA also introduces an acknowledgement (ACK) mechanism, where the receiver sends an acknowledgement notification ACK to the transmitting unit upon receipt of the data. if the sender does not receive the ACK, it indicates that the data is lost and the data will be transmitted again.
3. Real-time performance analysis of wireless LANs
The IEEE 802.11b wireless LAN standard uses the CSMA/CA protocol at the media access control layer to achieve wireless channel **** enjoyment. In the case of a lighter network load, there is little chance of conflict, plus some wireless network products take some additional measures, or even avoid conflict altogether. Such as Wi-LAN's wireless product AWE 120-24 wireless network bridge utilizes dynamic time allocation polling: when there are multiple wireless remote devices to communicate with the base station, the base station will be based on the remote station's ID to ask each remote station in turn whether there is data to be sent, and if there is data to be sent, it will be given the allocation of the time slice, and if there is none, it will continue to ask downward, and the cycle will begin again and again. The so-called dynamic polling here means that the user can set the polling method of the base station to reduce the number of times the inactive station is polled, so as to ensure that the time slice is not wasted. The dynamic time allocation polling technique completely avoids conflicts and allows for better real-time performance than CSMA/CA. This makes it possible to use wireless technology in industrial control networks.
Three, based on wireless technology, networked smart sensors
Computer network technology, wireless technology, and smart sensor technology combination, resulting in a "wireless technology-based networked smart sensors," a new concept. This intelligent sensor integrates data acquisition, data processing and wireless network interface module, wireless network interface module bottom network interface (hardware interface) using IEEE 802.11b-based network interface chip, high-level network interface (software interface) using the TCP/IP protocol, the TCP/IP protocol as an embedded application, i.e., solidify the TCP/IP protocol into the intelligent sensor ROM. The TCP/IP protocol is used as an embedded application, i.e., the TCP/IP protocol is solidified into the ROM of the smart sensor, so that the sending and receiving of the field data are carried out by the TCP/IP protocol. This networked smart sensor based on wireless technology enables industrial field data to be transmitted, distributed and ****enjoyed directly on the network through a wireless link.
Wireless LANs can be realized on the basis of ordinary LANs through wireless Hubs, wireless access stations (APs), wireless bridges, wireless Modems, and wireless NICs.
In the field of industrial automation, there are thousands of sensors, detectors, computers, PLCs, card readers, and other devices that need to be connected to each other to form a control network, and usually the communication interface provided by these devices is RS-232 or RS-485. Wireless LAN devices use isolated signal converters to convert the RS-232 serial signals of the industrial devices with the signals of wireless LAN and Ethernet, which are compatible with wireless LAN and Ethernet signals. Ethernet signals to each other, in line with the wireless LAN IEEE802.11b and Ethernet IEEE 802.3 standards, support for standard TCP / IP network communication protocols, effectively expanding the networking communication capabilities of industrial equipment.
Four, wireless LAN in the industrial control network
The industrial control system networked for the application of wireless technology in the industrial control system provides the basis and possible. In recent years, many researchers have also launched a research work in this area. Zeng Peng et al. from Shenyang Institute of Automation, Chinese Academy of Sciences, constructed a field-level wireless communication protocol stack based on the FFHSE (High Speed Ethernet) issued by FF (Fieldbus Foundation), combined with the wireless Ethernet standard IEEE802.11b. The protocol stack maintains the communication model of the Foundation's fieldbus and is capable of accomplishing time synchronization and real-time communication between wireless devices. Kyung Chang Lee et al. from Pusan National University, South Korea, designed a protocol conversion model to interconnect Profibus-DP networks and IEEE802.11 wireless LANs.Mario Alves et al. estimated the delay time of hybrid network messaging based on the broadcast method of fieldbus/wireless networks. C. Koulamas et al. investigated the performance of Profibus fieldbus in combination with the IEEE802.11b-based DSSS physical layer.
In addition to the theoretical research work, wireless communication technology has gained applications in some industrial control networks. For example, the United States Rockwell company in the DeviceNet, Control-net, Ethernet / IP based on the three-layer control network system, adding a wireless Ethernet part of the wireless communication can be realized. Germany Siemens in Profibus-DP, Profinet-based control network combined with wireless Ethernet technology, so that the control network has a wireless communication function. Due to the incomparable superiority of wireless network, it can eliminate a large number of line connections, save the construction cost and maintenance cost of the system, but also meet the needs of some special occasions, at the same time, greatly enhance the flexibility of the system composition. Coupled with the continuous improvement of wireless communication technology itself, wireless communication technology in the field of industrial control will have a broad space for development and application prospects.
V. Wireless technology in the industrial control network in the application of the program and the use of equipment
1. Wireless industrial control
By using wireless technology based on networked intelligent sensors, combined with a variety of current market emergence of the IEEE 802.11b based on the wireless LAN bridge, you can realize the wireless LAN technology in the industrial control network An application program. The wireless LAN bridge is used as a wireless access point (AP), and the networked intelligent sensor based on wireless technology collects the field data, processes it, and packages the data with TCP/IP protocol, and sends it to the AP through the wireless link, because the wireless link and the wired Ethernet high-level are using TCP/IP protocol, and the low-level protocol is transparent to the high-level protocol, which realizes the seamless connection between the wireless network and the wired network. 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 the wireless network, you need to choose the equipment is generally two kinds. One is a wireless LAN bridge, which can connect multiple wireless sites into an existing LAN; the other is a wireless communication device, such as wireless LAN cards, wireless Modem, etc. The following is an introduction to Advantech's wireless devices. Advantech's wireless devices are described below.
A. WLAN-9200 Series 11Mbps Industrial Wireless LAN Access Point
The WLAN-9200 is an enhanced 11Mbps wireless LAN bridge for outdoor use. It is capable of connecting multiple remote stations to the LAN without any physical wiring.
Features:
-Supports IEEE 802.1lb standard 2.4GHz ISM band
-Supports Advanced User Authentication for ruggedized security WEP128, MAC address control
-With IP 66/NEMA 4x compliant waterproof and rustproof enclosure protects the system from damage
- Provides cooling fan and heater. Provides cooling fans and heaters to prevent the system from overheating and overcooling
- Provides push buttons and LED displays for easy temperature setting
- Protects the power supply, LAN, and wireless interfaces with IP66 waterproof connectors
- Provides a variety of antennas for increased transmission range
The WLAN-9200 is an enhanced 11Mbps wireless LAN network for outdoor use. 11Mbps wireless LAN bridge. It is capable of connecting multiple remote stations to a LAN without any physical cabling. This saves a great deal of cost in maintaining and setting up a corresponding cable network.The WLAN-9200 comes with a ruggedized enclosure that protects the system from damage caused by water, acid, lightning, low and high temperatures. Due to these features, the WLAN-9200 is extremely stable and reliable, making it ideal for outdoor applications. As a result, the WLAN-9200 is ideally suited for use in harsh locations where wiring is difficult, such as reservoirs and buildings.Compatible with the IEEE 802.1lb standard, the WLAN-9200 comes with a variety of powerful features. It is capable of delivering high transmission speeds of 11Mbps while providing high security protection (WEP: 128-bit), DHCP clients, SNMP agents, etc. In addition, in order to meet the requirements for use in harsh outdoor environments, the WLAN-9200 provides advanced system protection features: glow-in-the-dark protection, cooling fans, heaters, waterproof interfaces, industrial equipment cases, power supply/LAN coaxial cables, and more.
Low Cost, Easy Installation
The WLAN-9200 can connect different distributed sites together to form a wider range wireless network. It saves the cost of wiring to remote locations. the WLAN-9200 features a specialized design that allows users to attach and detach it easily and quickly. In addition, the WLAN-9200 provides push buttons and LED displays for showing and setting high/low temperatures. Users can use it to quickly set up their own wireless network. To be able to use it in a longer range, the WLAN-9200 also provides various antennas for longer transmission distance.
Robust Design for Reliability and Stability
The WLAN-9200 features an advanced design with a rust-free, waterproof enclosure that provides effective protection for the system. It meets IP 66/NEMA 4x standards for corrosion resistance, UV protection, security and automatic fire suppression. To prevent the WLAN-9200 from overheating or overcooling internally, Advantech also designed a cooling fan and a heater inside it, and users can set the high/low temperature settings. The cooling fan or heater will start working when the operating temperature is above or below the user-specified temperature. In addition, the WLAN-9200 provides a waterproof interface and anti-flash protection for power, LAN and antenna interfaces.
Fast Data Transmission Between Remote Sites
The WLAN-9200 is fully compatible with the high-speed wireless LAN standard, IEEE 802.1 lb, and it offers 11 Mbps (over the air) for faster data transmission.The WLAN-9200 utilizes the DSSS technology in the 2.4 GHz ISM band, which is not interrupted by noise. interference, making data transmission safer and more reliable.
Keeping Communications Private
The WLAN-9200 employs a variety of security features to protect your wireless network (WEP128 encryption, MAC address control and password security). By employing advanced WEP128 encryption, you have the option of a WEP key to protect your data from being viewed by unauthorized wireless users, only the accessibility of the access point and wireless adapter, and multiple security mechanisms working in tandem, can effectively prevent unauthorized access to your wired and wireless network.
B.ADAM-4550 Series 2.4GHz Wireless Modem (RS-232/485 Interface)
The ADAM-4550 is a direct-sequence spread spectrum wireless modem. It operates on the 2.4GHz ISM band, which is available worldwide without application. Through the RS-232 or RS-485 serial port, the ADAM- 4550 can communicate with computers or other devices at speeds up to 115.2Kbps.
The ADAM-4550 operates in half-duplex and transmits data wirelessly at 1Mbps. It has an output power of 100mW and it can transmit up to 150 meters if using its own small antenna, or more than 20 kilometers (line-of-sight) if using Advantech's high-gain outdoor antenna.
The RS-485 standard supports half-duplex communication. This means that data can be sent and received using a pair of twisted pairs. The direction of the data flow is normally controlled by the handshake signal RTS (Request to Send). However, there is a dedicated I/O circuit in the ADAM-4550 that can be used to detect the direction of data flow and automatically switch the transmission direction without the need for a handshake signal.
The ADAM-4550 wireless modem provides reliable wireless connectivity for point-to-point or point-to-multipoint networks. A typical application is to connect one ADAM-4550 module to a host computer via RS-232 and place other ADAM-4550 modules at a remote site. Each ADAM-4550 module can be connected to a remote device via an RS- 4550 network. The remote ADAM-4550 module transmits remote data to the master ADAM-4550 module, which in turn sends control commands to the remote ADAM-4550 module via wireless transmission.
Specifications
-RS-232/RS-485 Transmission Rate (bps): 1200, 2400, 4800, 9600, 19.2K, 38.4K, 57.6K, 115.2K
-RS-232 Interface Connector: Hole DB-9
-RS-485 Interface Connector: Plug-in screw terminal Support AWG1-#12 or 2-#14-#22 (0.5 to 2.5mm2 wire diameter) cables
-Wireless Transmission Rate: 1Mbps
-Wireless Transmission Frequency: 2.45GHz (Nominal)
-Wireless Transmission Power: 100mW (Nominal)
-Wireless Modulation: Straight Order Spread Spectrum PSK
-Wireless Transceiver Address: Software configurable to 254 different addresses
-Communication Range: 550 ft. effective distance (with 2dBi omni-directional antenna in open field), actual distance depends on ambient conditions, antenna type, and location
-Operating Temperature: -10? to 70°C (14? to 158°F) <
-Power Requirement: +10~+30VDC
-Power Consumption: 4W
-Dimensions: 60mm x 120mm (2.36" x 4.41")
Features
-Software configurable RS-232 or RS-485. Data rates up to 115.2Kbps
-Transmission radius of more than 20km with external antenna and amplifier
-Built-in watchdog timer and automatic RS-485 flow control
-Spread spectrum radio modulation
-Operates in the worldwide, application-free band (2.4GHz)
-1Mbps wireless data transfer rate between modules
-Software-configurable wireless transceiver address
-Convenient DIN-rail, panel, or stack mounting
-EEPROM with storage for communication settings
-Supports point-to-point or point-to-multipoint applications
-Transparent IEEE802.1 protocol and 10K cache for data integrity
-Power and data flow indicators for troubleshooting
-Diagnostic software with wireless connectivity test
-FCC Part 15 and ETSI 3000.683/300.328 compliant
VI.CONCLUSION
Control of industrial equipment via wireless LAN is simple and easy. Control of industrial equipment is simple and easy, but slightly more expensive. At present, the vast majority of wireless control, as mentioned earlier, is the use of IEEE802.11 series of protocols, which can be seamlessly connected with most of our LAN Ethernet, so the user layer of the measurement and control program does not have any impact on the physical layer of the original program only need to make a simple configuration of the device can be. For example, use the Advantech wireless products mentioned above to replace the original wired communication devices, other hardware and software configuration are not affected.