If a cell phone is sometimes unreliable—as most of us know from personal experience—how could wireless technology possibly be reliable enough to work in a plant?

When it comes to WirelessHART, the answer lies in its design specifically for industrial applications, said Jose Gutierrez, corporate director of technology for Emerson Electric Co. in St. Louis. Gutierrez summarized the basics during a Wednesday morning Workshop, 12-486 “IEC 62591 (WirelessHART) Reliability…After 100 Million Hours of Operation.” (And that number today is closer to 200 million hours, Gutierrez added, as the title of his talk was set last February.)

The underlying radio standard for WirelessHART is IEEE 802.11.4, the same standard on which ZigBee is based. But while both share the same core technology, it’s unfair to compare the two, because they were designed differently for different industries, said Gutierrez— ZigBee for commercial and residential, and WirelessHART for industrial.

For example, as one of several ways to enhance reliability, WirelessHART sends the same signal three times through three different frequencies. ZigBee, on the other hand, doesn’t change frequencies—not because it can’t be done, but because the commercial and residential market requires a radio cost of less than $5, and frequency hopping adds cost, Gutierrez pointed out. That’s a restriction not placed on the industrial market, where a much higher-priced wireless component can still save money for a process plant, due to the high cost-per-foot of the conduit it replaces.

In all, Gutierrez noted, WirelessHART uses a combination of “diversity techniques” to maximize reliability in the industrial space. The result, he said, is a wireless technology that provides 99.99 percent-plus reliability. The list of diversity mechanisms includes:

• Path Diversity: Mesh Networking
• Frequency Diversity: Channel Hopping
• Time Diversity: Time Division Multiplexing
• Power Diversity: Power control over multiple communication links
• Coding Diversity: Use of advanced DSSS technology (Direct Sequence Spread Spectrum).

"These are the key secrets for the 100 million or 200 million hours of WirelessHART reliability,” Gutierrez told the Workshop audience. “And that’s the reason that we, Emerson, are able to put our name behind these products.”