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Selecting RF protocols for ISM applications

Posted: 01 Jul 2013 ?? ?Print Version ?Bookmark and Share

Keywords:RF protocols? ISM? Bluetooth? ZigBee? DECT?

The RF protocols that developers utilise for industrial, scientific, and medical (ISM) applications must abide by exacting requirements for robustness, reliability, and connectivity. Today, developers can choose among several good options.

This article gives designers the pros and cons of each of the five leading RF protocols ? Wi-Fi, Bluetooth, Bluetooth Low Energy, ZigBee, and DECT C used for ISM applications. The article also explains which protocols are most often chosen for which types of applications. In this way, designers can make informed decisions as they develop devices for ISM applications. The expertise for this article comes from Symmetry Electronics' years of advising designers as a distributor specialising in RF.

Perhaps the most ubiquitous of RF technologies, Wi-Fi enables electronic devices to exchange data wirelessly over Wireless Local Area Networks (WLANs) using high-speed Internet connections. A set of IEEE 802.11a/b/g/n standards plus 802.11ad and soon 802.11ac define various Wi-Fi communication protocols, and provide further enhancements to the standard for security, quality of service (QoS), and improved mobility.

The Wi-Fi Alliance has certified more than 12,500 products, each of which has passed stringent interoperability certification testing to ensure that it will work with a myriad of other vendors' Wi-Fi certified products.

The fact that Wi-Fi is an off-the-shelf (OTS) solution is one of its biggest selling points, substantially reducing time-to-market and development costs. Designers do not have to develop a radio or a transceiver from scratch, and because there are so many suppliers, there is a wealth of readily available parts from second or third sources.

Wi-Fi most commonly operates on the unlicensed 2.4GHz ISM band and now the 5GHz band. Most countries reserve these bands for uses other than communications. The technology employs Direct Sequence Spread Spectrum (DSSS), which in the US spreads transmissions across an available 13 channels in which two are reserved for low power use. Non-Wi-Fi devices such as ZigBee and Bluetooth devices that operate in this band can cause interference. When Wi-Fi senses interference, it will cease transmissions and force a retransmission, slowing both throughput and performance. There are methods designers can use to avoid interference with other devices operating in the 2.4GHz band, including changing channels, slowing transmissions, or moving to the 5GHz band for industrial or medical applications. Network administrators can also construct a careful frequency plan so that devices can co-exist on the ISM band using available channels.

Wi-Fi is highly suitable for certain types of ISM applications that require network connectivity and high data throughput. Wi-Fi power has decreased significantly with 802.11n and opened avenues for low-power applications, which is a big advantage since Wi-Fi already exists in so many places as the local network. For medical applications, this includes remote patient monitoring, tele-health and tele-presence because the applications are mobile and highly collaborative, requiring high data throughput. Some industrial applications include production planning, data acquisition and network interaction from the factory floor.

Bluetooth/Bluetooth Smart
Bluetooth is a short-range communications technology best known for its robustness, high levels of security, and low power consumption (range and power consumption are class dependent, see table). Bluetooth devices create wireless personal area networks (WPANs) that operate in the ISM band. Low-cost transceiver microchips control each Bluetooth device.

Bluetooth Smart is a feature of Bluetooth Version 4.0, which is the most recent version of the standard. This feature enables wireless devices to operate within a short range (up to 50m) on coin cell batteries, making it ideal for portable devices. Bluetooth Smart features include ultra-low peak-, average-, and idle-mode power consumption, low cost, and multi-vendor interoperability. With Bluetooth Smart, developers and manufacturers can develop smaller form factor devices for markets such as healthcare, sports and fitness, security, and home entertainment.

Bluetooth devices connect by pairing, creating ad hoc networks called piconets. Each device in a piconet can connect to up to seven other devices and can also belong to several other piconets simultaneously, which means the connectivity options are vast. Bluetooth is easy to install and commission, ensuring personal safety (a technician does not need to be physically next to a device to configure it).

Bluetooth uses Frequency Hopping Spread Spectrum (FHSS) to "hop" between 79 different 1MHz-wide channels in this band to avoid channel conflicts with other wireless technologies sharing the 2.4GHz spectrum. Each pair of devices has its own frequency-hopping schema to help avoid conflicts with other technologies, including Wi-Fi or ZigBee devices operating on the band. Frequency planning also can help ensure that devices coexist on the same network.

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