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are fast. But clearly, in a noisy environment data rates can slow markedly – effectively to zero in a pathological case. On the other hand small packets increase transmission overhead, so frame aggregation was added in 802.11n. This combines multiple packets, and is most effective in low-noise environments where few retransmissions are needed. However, frame aggregation was not supported in earlier versions of the standard, so 802.11b/g routers cannot process such data. Quality of service Video and voice transmissions cannot tolerate even momentary interruptions in service, yet 802.11 originally did not provide any quality of service (QoS) guarantees. Starting with the 802.11n variant extensive (and complex) features were added to minimize packet latency, jitter, and loss. These QoS features, coupled with its 150 Mbit per second data rate, make the 802.11n variant a good choice for networks requiring real-time guarantees. One of the biggest obstacles to using 802.11 in an industrial environment is multipath interference, with signals reflecting off walls and other equipment create. Echoes can destructively interfere with the signal, causing nulls where reception is non-existent or limited; in the latter case the equipment will downgrade transmission rates in an attempt to maintain a connection. 802.11n added multiple in and multiple out (MIMO) technology, which utilizes advanced signal processing and multiple antennas to boost gain from the multiple reflections and maintain high data rates. Another way to maintain high data rates is the use of channel bonding, which pairs multiple 20 MHz-wide channels. Also first supported by 802.11n, the wider bandwidth only works when operating at 5 GHz (and above for later versions of the standard). All 802.11 versions specify a guard interval, a time between transmissions used to reduce interference. In a home or office this is often on the order of 400 ns. However, in an industrial environment where distances are large and lots of metal can cause erratic reflections 800 ns is recommended. The downside, of course, is a somewhat reduced transmission rate due to the transmitter’s inactive time. Another approach entails boosting power levels. Microchip’s SST11CP16 is an RF amplifier (see Figure 1) that delivers 25 dBm of output power over the 5.1 to 5.9 GHz band. Security Security is of increasing concern, and wireless networks are particularly vulnerable to attacks and undesired monitoring. Earlier versions of the 802.11 standard included wire equivalent privacy (WEP), but that proved to be insecure. Wi-Fi Protected Access 2 (WPA2) was described by 802.11i and incorporated into the general 802.11 standard in 2007. Older 802.11b and 802.11g devices may or may not support it. WPA2 uses the AES encryption standard rather than the less robust RC4 stream cipher and adds two protocols to protect keys and establish trust in the connection. Unfortunately, WPA2 is still vulnerable to password cracking, packet spoofing, and various man-in-the-middle and denial of service attacks. Two trains of thought exist about these issues: some feel that inside a warehouse or other large facility users can be trusted; others point out the most industrial facilities have a continual stream of vendors, not all of whom may be well-intentioned. Wireless networks are inherently less secure than wired ones, so in situations where security is an issue it would pay to take a layered, system-level approach to the problem. The case for modules All of the IEEE 802.11 standards are very complex (typically over 1,200 pages), and the equipment is subject to governmental regulations, which are not entirely consistent around the world. Semiconductor vendors provide extensive design support, but in situations where volumes are not very high it usually makes sense to design an 802.11 module into a system rather than start at the IC level. Many exist; an example is Digi International’s XB2B-WFWR-001 (see Figure 2). Instead of mastering the intricacies of the standards, one merely sends the same AT commands long used to control modems over an SPI or UART link. Figure 2: Digi’s XB2B-WFWR-001 802.11 module (Courtesy of Digi International). Other challenges include the harsh operating conditions in industrial sites. Shock and vibration, oil, dust, and temperature all may have to be considered. Extended-temperature range modules are available such as Digi’s XB2B-WFPS-001, which can operate from -300C to +850C. Figure 3: Digi’s XB2B-WFPS-001 802.11 module (Courtesy of Digi International). Long-Term Viability It seems like new data communication standards appear weekly, and it is hard to access the viability of some. Industrial applications may have 20 to 30 year lifetimes. The fifteen-year old IEEE 802.11 standard has grown in robustness, capability, and data rates, while (mostly) maintaining backward compatibility with earlier versions. The emerging 802.11ac/ad variants show that this wireless protocol will continue to thrive for many years to come. To learn more about RF Wireless and other applications visit mouser.com/applications. Or for newsletters, please visit mouser.com/subscriptions and manage your subscription preferences. Jack Ganssle has written over 600 articles and six books about embedded systems, as well as one about his sailing fiascos. He has started and sold three electronics companies and now lectures and consults about the industry. He also works as an expert witness from time to time. www.electronics-eetimes.com Electronic Engineering Times Europe October 2013 11


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