The common view of wireless networks is that they keep your laptop or your PDA connected to the corporate network or the Internet even when you're in a conference room or at your local Starbucks. Office workers and mobile professionals love the convenience.
But wireless networks are also flourishing in the down-and-dirty real world. From NASCAR racecars, they collect sensor data for performance analysis. In police departments, they keep cruisers connected to law enforcement databases. In factories, they control overhead cranes. In hospital emergency rooms, they speed medical treatment. And that's just the tip of the iceberg.
Wireless networks, in fact, are going just about everywhere. IEEE wireless standard 802.11b, approved in 1999, opened the gate to a flood of products for which shipments in 2002, according to market-research firm Forward Concepts (Tempe, AZ), increased 100%. Between 2002 and 2007, predicts Allied Business Intelligence (Oyster Bay, NY), shipments of wireless-network ICs will increase six fold, from roughly 25 million to about 150 million. And after about 2004, according to ABI, most of those chips will go into embedded devices and systems.
What wireless networks provide is a liberating mobility. To a user, an 802.11b wireless network looks just like wired Ethernet, but with untethered operation up to several hundred feet from a network access point. Data rates for 802.11b (also often referred to as Wi-Fi or just 11b) are a nominal 11 Mbps versus 10 Mbps or 100 Mbps for wired Ethernet. Other 802.11 variations—802.11a and 802.11g—are starting to extend the data rate to as high as 54 Mbps.
Not surprisingly, wireless networks are finding use in industrial settings. "A trend on the plant floor is to have fewer operators," says Ken Hall, Rockwell Automation's (Milwaukee, WI) vice president of architecture and system design for advanced technology. "One obvious cost savings is for the operator to walk around with a wireless display, which would reduce the number of operator interfaces needed," notes Hall. Also, says Hall, wireless networks are good for controlling overhead cranes. "Wireless is much better than having a flexible cable harness," he says, "because those tend to break over time." On the factory floor itself, wireless extensions to wired networks save the cost of installing cable conduit, which can require digging up a concrete floor and even shutting down production.
The industrial environment is not friendly to wireless operation, however. "Heavy equipment generates a lot of noise in the RF spectrum," says Tim Cutler, marketing vice president at Cirronet (Norcross, GA), a supplier of industrial wireless Ethernet products. "Arc welders and some lighting systems and heavy equipment can all generate RF energy in the 2.4-GHz band where 802.11b operates." Also, says Cutler, the large amounts of metal in industrial settings can block wireless signals or create performance-degrading reflections. Sometimes, he says, the distances in large facilities exceed 802.11b's range. Because of these limitations, some companies, including Cirronet, have implemented wireless Ethernet using proprietary technologies. Their systems are still Ethernet compatible, but the wireless connections are not 802.11b compatible.
But many companies have implemented 802.11b technology for industrial use despite the difficulties it faces. Industrial automation companies Rockwell Automation, Siemens Energy and Automation (Alpharetta, GA), and GE Fanuc Automation (Charlottesville, VA), for example, have all taken the 802.11b path. They see value in using standard technology and suggest that 802.11b performs adequately when appropriately implemented. Also, many manufacturing giants—including General Motors, Ford, DaimlerChrysler, and BMW—have incorporated 802.11b technology into their factories.
And 802.11b does offer some protection against industrial environment perils. To guard against RF interference, for example, it uses direct-sequence spread spectrum (DSSS) transmissions to spread signals across a wider range of frequencies than would be necessary to represent the signal data itself. Consequently, a narrowband interference source, such as a high-frequency harmonic of an industrial motor, isn't as likely to clobber the signal. Even if interference occurs at a frequency within the signal's spread spectrum, other signal frequencies can get through, and error-correction techniques in the wireless receiver can help reconstruct affected data packets.
Many industrial wireless providers argue, however, that DSSS is less effective at preventing interference than another technique—frequency-hopping spread spectrum (FHSS). FHSS was part of 802.11b's predecessor technology, 802.11, and is still part of many proprietary industrial wireless products. Unlike a DSSS transmitter that uses a relatively broad frequency range, an FHSS transmitter uses one narrow range at a time, switching its signal frequency many times a second in coordination with receivers (thus the term frequency hopping).
Because an FHSS signal changes frequency so often, it's never on the same frequency for long and thus avoids most interference from narrowband noise sources. If a narrowband noise source affects a particular frequency, says Cirronet's Cutler, "We might be trying to transmit at that frequency every 50 or 75 times, but we'll be transmitting on clear frequencies the rest of the way." Also, says Cutler, "Frequency hopping, because it's concentrating the RF energy into a narrower bandwidth, just has a little higher energy density to punch through some of the noise."
Another problem in industrial applications is multipath fading, a situation that results when signals reflect from metal surfaces such as machinery, metal roofs, warehouse racks, and even moving fork lifts. In multipath fading, a signal and an out-of-phase reflected signal cancel each other out, resulting in a reception dead spot, or "hole." Often, a hole can be only inches away from an area of good reception, because the out-of-phase signals that produce dead spots are out of phase for a simple reason: Their travel distances from transmitter to receiver differ by some multiple of half a signal wavelength—about 2.5 inches in the 2.4-GHz band.
Not Fade Away
Linking the Plant Floor: Wireless access modes, suc as the SEM2411 from Cirronet, can connect wireless industrial devices to a wired network.
Fortunately, wireless technologies have ways of minimizing reflected signals' harmful effects. For example, 802.11b transceivers often have two so-called diversity antennas that are mounted a few inches apart, and a transceiver in receive mode dynamically switches to the antenna with the stronger signal. Although it's possible for each antenna to be in a dead spot, two antennas reduce the likelihood. Also, because DSSS and FHSS transmissions cover a range of frequencies, holes that would result from a single frequency are less pronounced. Of the two spread-spectrum schemes, many wireless experts say, FHSS is the more effective.
Industrial applications also present another challenge—range. Nominal range for 802.11b systems is about 300 ft, but range in normal operating conditions is often only 100 or 150 ft. For large factories, and for long-range outdoor applications like police networks, those distances are puny.
One solution to the distance problem is to use equipment that isn't 802.11b compatible, but has greater range capabilities. Cirronet, for example, sells wireless Ethernet access points with ranges from 450 to 900 ft. Other companies, such as Industrial Networking Solutions (Dallas, TX), Microwave Data Systems (Rochester, NY), and Synetcom (Torrance, CA), also provide extended-range products. Longer range usually means a lower data rate, however—typically 200 kbps to 1 Mbps versus the 802.11b nominal rate of 11 Mbps (usually less than 5.5 Mbps in practice). For industrial applications, however, those rates are usually adequate. As Cirronet's Cutler notes, "You're not sending a 300-kbyte Word document to a printer. You're sending 10-kbyte batches or 50-byte messages."
For really long distances, or for moderate distances at high data rates, a "bridge" is usually necessary. These devices, as their name implies, connect two or more networks, usually, but not always, in different buildings. For example, the Aironet 350 bridge from Cisco Systems (San Jose, CA) works at ranges up to 25 miles and at the full 802.11b 11-Mbps data rate. Bridges don't always strictly adhere to 802.11b specifications, however, even when they're from a provider of 802.11b technology, as Cisco is. Because they don't involve direct communication with network users, they can—and often must—take other approaches to provide the range and data rates needed.
Another issue to deal with in wireless networks is that of data security. Although security can seem unimportant for industrial data such as sensor readings, a factory's wireless access points can link through a corporate network to other, more sensitive, data. Unfortunately, security features in 802.11b are notoriously ineffective against hacker attacks, although they're currently being revised both by IEEE standards bodies and by the Wireless Ethernet Compatibility Alliance (WECA). In the meantime, some wireless vendors have improved product security independently.
Choosing a wireless network technology also involves the consideration of upgrading to new technologies. For example, products conforming to IEEE standard 802.11a are now becoming available, offering 54 Mbps data rates in the 5-GHz Unlicensed National Information Infrastructure (UNII) band. Products based on 11a are not compatible with 11b products, however, and the UNII band is only available in North America. A not-yet-finalized 11g standard will also offer 54 Mbps rates and does provide 11b compatibility, but it will not gain formal approval until sometime later this year.
Just how well 802.11a and 802.11g will fare commercially remains to be seen. Chris New-some, director of information technology for NASCAR competitor Hendrick Motorsports (Charlotte, NC), welcomes the 5-GHz 11a as a way to escape the increasingly crowded 2.4-GHz band of 11b. "When 5 GHz is available and stable," Newsome says, "we'll be there." For Ochsner Hospital and Clinic in New Orleans, however, the incompatibility of 11a with 11b diminishes its appeal. "We don't want to rip out our entire 11b infrastructure," says Kurt Induni, Ochsner manager of network services. "We'll probably wait instead for 11g."
Network provider Cisco Systems expects both 802.11a and 802.11g to be popular—11a for its presence in the less-crowded 5-GHz band and 11g for its compatibility with 11b. Eventually, says Chris Bolinger, product manager for Cisco's wireless networking business unit, combination products supporting multiple technologies will prevail. Bolinger says, "There will be much less concern about these individual letters, this alphabet soup."
Prognosticators agree. According to a recently released market research report from Forward Concepts (Tempe, AZ), "The 5-GHz 802.11a market will have a very short life, giving way to combo 802.11ab and 802.11ag devices." The market will stay hot, the report predicts, growing from 2002 to 2006 at an annual compound growth rate of 43%. Something is obviously in the air.