What is wireless?
Wireless communication, unlike wired, relies on an unbounded medium to transfer information. Wireless local area networks (WLANs) use electromagnetic radiation to transmit information between nodes without the use of a bounded, physical medium. As you would guess, this presents issues and problems unique to the medium. The air around us is alive with electromagnetic radiation. The challenge is to reliably and securely transfer information between two network nodes without significant errors or corruption.
In many environments, particularly industrial, the amount of electromagnetic interference, or EMI, is so significant that even the use of shielded cables is insufficient to guarantee error-free communication. In otherwise "clean" environments, interference generated by ordinary household items can render a radio channel useless-microwave ovens have taken down many a WLAN. Radio frequency (RF) technology can be very tricky to properly implement; even with vastly improved technology, problems can and do happen.
Before we get into specific technical detail, however, I want to review the different flavors of wireless technology in current use. Each iteration was developed to suit a specific need, and each is well suited to a specific function or data type. It should be noted that the technologies discussed here are in a broad category of low-power, wide-bandwidth technologies; these differ from commercial RF technology, such as that used in broadcasting, which is typically high-power, narrow-band transmission.
Aside from Wi-Fi, or IEEE 802.11, which will be covered in much greater detail, there are other familiar wireless technologies, which are defined by IEEE 802.15, Personal Area Networks (PANs). Briefly, Bluetooth, a short-range variant, was originally standardized by IEEE 802.15.1. Bluetooth uses a master-slave structure and a form of modulation called frequency hopping spread spectrum (FHSS).
Essentially, the carrier "hops" between forty 1 Mhz channels. Bluetooth is used for short-range communication in the 2.4 GHz ISM band and chiefly for audio delivery and simple control function, as with a mobile phone. (Bluetooth was named after 10thcentury Danish King Harald Bluetooth.) Another common protocol is Wi-HART, or WirelessHART. It conforms to the IEEE 802.15.4 standard and is used chiefly for short range communication among wireless capable HART devices. WirelessHART forms a mesh network between devices and the network gateway. WirelessHART uses both channel hopping and other forms of spread spectrum in the 2.4 GHz ISM band. (HART stands for Highway Addressable Remote Transducer. Learn more at www.hart.org.)
Z-Wave is another short range wireless technology used extensively in home automation and operates in the 900 MHz ISM band. Z-Wave also uses FHSS, and allows for the formation of a source routed mesh network.
ZigBee is another short range technology that has had limited acceptance. It also adheres to the IEEE 802.15.4 standard and forms an ad hoc wireless mesh network in the 2.4 GHz ISM band. ZigBee provides a low data rate, but operates at low power over long distances through mesh connections. The name refers to the behavior of honeybees.
Wi-Max (Worldwide Interoperability for Microwave Access) is another wireless technology designed to provide widespread and robust wireless connectivity. Wi-Max is defined by the IEEE 802.16 standard and is similar to Wi-Fi but operates over a much greater coverage area; the proposed system is something of a hybrid of Wi-Fi and cellular principles. Wi-Max operates in the spectrum surrounding 2.4 GHz, 2.5GHz, and 3.5GHz, with throughputs of up to 1 Gbps.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, email@example.com.
www.controleng.com/blogs has other wireless tutorials from Capano.
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.