 N-TRON® Wireless Ethernet IEEE802.11a, b, g, n
Wireless Local Area Network (WLAN) Basics
IEEE 802.11 Wireless technology is being deployed rapidly in many industrial plants as the security and bandwidth of these products have improved. The need to gather data from and provide data to equipment and employees moving around a facility is one of the primary applications for these devices. Wireless communication may be used to link remote locations that require monitoring or data services without the need to install costly infrastructure. The wide use of new wireless equipment including wireless process control products, IP security cameras, voice over IP, and RFID equipment has also contributed to the growing use of wireless technology in the industrial environment.
Wireless network configurations require careful planning to insure that the network will function as required without causing problems with adjacent installations. This basic overview will provide a starting point for your wireless design.
IEEE 802.11 Network Standards
Figure 1 shows the current IEEE 802.11 wireless standards supported by the N-TRON 702-W products. The 802.11n is still in active development but has been defined to the point that many manufacturers are offering products with this standard. A detailed discussion of the standards is
not in the scope of this presentation but is readily available from the IEEE and many other sources.
The successful application of IEEE 802.11 requires the understanding of various configurations available. The frequency used can affect the performance and range of the wireless system. Network security must be considered when selecting equipment and the configuration of the installation. Antenna selection based on a survey of the actual site is a key element to the performance of the wireless network.
Stations and Access Points
Wireless Stations (clients) and Access Points (AP) are the building blocks of an 802.11 wireless network. Figure 2 shows a typical configuration of these components.
Basic Service Set (BSS)
A Basic Service Set is a single Access Point (AP) with a unique Service Set ID (SSID) and up to 253 Stations attached to the AP. Each Station is a client of the AP and will pass both Unicast and Multicasts generated by the equipment connected to the Station Ethernet interface.
Wireless Distribution System Protocol (WDS)
Wireless Distribution System Protocol allows network switches with multiple Media Access Control Addresses to be connected to a station or Access Point (see Figure 3). An N-TRON wireless network running WDS will become transparent to all layer two network devices connected to both the stations and AP Ethernet interfaces. This protocol allows all of the switches in the network to populate their MAC address tables with all of the MAC addresses in the network instead of the MAC address of the Wireless interface to which they are connected.
Wireless Distribution System Protocol allows the Ethernet packets to be forwarded more efficiently. The efficient delivery of process control heart beats between controller, I/O, drives and other process control equipment can be slowed or misdirected if WDS is not implemented. Any interruption of this heart beat process can lead to a communication failure in the process control system and production loss.
WDS is not a defined IEEE Standard and should only be used in networks where N-TRON 702-W Wireless equipment is deployed.
Signal Propagation 2.4GHz vs. 5.0GHz
The physics of radio transmission determine that given the same environmental and signal strength at the antenna a high frequency signal will diminish faster than that of a lower frequency. As the signal strength declines the data rate of the wireless network is reduced. Figure 6 shows the impact on throughput of frequency and distance. When comparing 2.4GHz and 5.0GHz , tests run under varying conditions including wall material, signal to noise ratio, and other environmental conditions can cause a 5.0MHz signal to diminish by between 45% to 75% when compared to the 2.4GHz signal. No one can predict with certainty what the signal strength will be at a given distance from the transmitter without a field test.
Site surveys can normally predict performance of the network using either frequency range. Surveys cannot predict what will happen when a new building is built in proximity to the wireless network or if other environment changes occur.
Factors affecting Wireless Networks
Many factors can affect the performance of both 2.4GHz and 5.0GHz Networks. Some of the most frequent culprits are:
- Distance - meters of separation between wireless units
- Antenna - type and gain,
- Transceiver - power and sensitivity
- Physical barriers - walls, chain link fences, trees, buildings and mountains
- Weather - temperature and humidity
- Interference - other Wireless Networks, Wireless Phones, Bluetooth Devices and other unlicensed wireless equipment in close proximity to the wireless network.
Environmental effects specific to 2.4GHz and 5.0 GHz signals are presented in Figure 9.
Channel and Bandwidth Selection
Channel selection and bandwidth use are important to the proper installation of wireless networks. When using multiple Access Points in an Extended Service Set configuration, proper channel and bandwidth selection becomes even more important.
Most residential, commercial and industrial Wireless Local Area Network (WLAN) devices are sold with a default channel selected. This frequency may already be in use in the area and should be changed to a channel not in use. If a wired backbone Wireless Extended Service Set is configured the adjacent Access Points should be on different channels and the channels selected should be chosen so that the bandwidth of one channel does not overlap the bandwidth of the adjacent channels (See Figures 10, 11, 12 and 13). By reviewing Figures 10, 11, 12 and 13 you can see that the 5.0GHz Channels offer more band width separation between channels and are therefore less likely to interfere with one another if proper channel and bandwidth selections are made.
Security
A variety of encryption algorithms are currently available on wireless network equipment. These algorithms include;
- WEP = Wired Equivalent Privacy was the original 802.11 security encryption algorithm. It can provide communication security for wireless networks but will not deter a determined cyber attacker. WEP security will only protect against unintentional use but will leave the network
vulnerable to a deliberate compromise. WEP security is the only security available for Extended Service Sets using wireless backbone (WDS Peers, see figure 5).
- WPA/WPA2 = Wi-Fi Protected Access, WPA, was introduced as an intermediate measure to address the weaknesses found in WEP while the IEEE802.11i specification was being developed. WPA will provide security that will protect communications from all but the most determined intruder. It was designed to be backwards compatible with legacy devices.
WPA2 is the full implementation of the IEEE802.11i standard. WPA2 provides a higher encryption level but requires more system resources and will reduce the communication rate when compared to WPA. The advanced protocol implemented in WPA2 is generally not backwards compatible with legacy networking hardware.
WPA/WPA2 cannot be used for Extended Service Sets using wireless backbone (WDS Peers, see figure 5), but may be used with wired backbone installations (see Figure 4).
Antennas
The two general categories into which most antennas can be classified are Omni Directional and Directional. Omni directional antennas are used to transmit signals in a 360 degree plane around the center point of the antenna (see figure 12). Directional Antennas are designed to direct most
of the signal to a specific area from the center point of the antenna (see figure 13). Selecting the correct antenna and the placement of the antenna will have a major impact on the performance of the wireless network. In most cases a site survey is the best way to determine the type and placement of the antennas used in a wireless network.
The antennas Effective Radiated Power (ERP) is used to measure the distance from the antenna that a usable signal is radiated or the specific pattern where the signal strength will support the desired data transfer rate. The general pattern that the antenna will radiate is normally available from the manufacturer. The actual pattern will be determined by the following factors;
- Transmitter Power, N-TRON 702W will supply 250mW (24dBm).
- The elevation of the antenna.
- Power Loss in the cable between the transmitter and the antenna.
- The gain of the antenna (see manufacturer's data sheet).
- The sensitivity of the receivers being used.
- The environmental factors.
Summary
The interest in industrial wireless applications is rising very quickly. The number of new installations will multiply as more industrial wireless devices become available. Adding to Plant wireless network infrastructure should include the following planning and installation steps;
- Site map of existing wireless access points
- Document wireless coverage required
- Site survey
- Frequency and channel selection for new installation based on site survey, site map of existing access points and coverage required.
- Select WLAN equipment and antennas based on site survey and frequencies selected.
- Field testing to prove installation plans.
- Document new wireless network.
The information in this white paper was first published by N-TRON. In an effort to bring out customers the best possible information, B&B Electronics has received permission to republish this document. B&B Electronics thanks N-TRON for allowing us to distribute this information to our customers. You can visit their site at http://www.n-tron.com.
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