Wired vs. Wireless: Cost and Reliability

What's Wrong With Wires?

What's all this fuss about wireless communications? What's wrong with using wires, just as we've always done?

Signed, Jack Plugg

Dear Jack:
There's nothing wrong with wired communications, such as Ethernet, ModBus, or the perennial favorite, RS-232. And there will always be situations where a wired link makes more sense than a wireless link. It's just that wireless systems are continually becoming less expensive and more dependable, making them the better choice in more and more applications.

 Wise Guy
Wise Guy

Here's why. The cost of wired communications—the cost of the wire, the connectors, and the labor needed to connect point A to point B—has remained constant or has increased slightly over the years. By contrast, the cost of wireless communications has dropped dramatically. High-speed wireless radio chips that cost $27 each just three years ago are now available for around $4. This is largely due to the phenomenon called "Moore's Law", which can be roughly paraphrased as "Anything built using silicon chips will become exponentially less expensive over time." And this trend will continue.

At the same time, wireless modems are becoming more reliable. Advanced signal-processing techniques and new network architectures that were merely an academic curiosity a few years ago are now practical to implement in wireless systems. In turn, these advances are finding their way into new standards and new products.

Wireless More Reliable Than Wired?

A friend of mine claims that wireless communications are sometimes more reliable than wired communications. He's crazy, right?
Signed, Knott Krazie

Dear Knott:
I wouldn't commit your friend to the loony bin just yet: wireless systems can be more reliable than wired systems in specific applications. Here are just a few examples:

"Chainsaw-resistant" connections. Before some evildoer can cut a connection, they have to know it is there. Unlike wired connections, wireless connections are invisible, which offers significant protection against malicious attacks. An intruder could launch an attack with a radio-frequency jammer, but advanced radios, such as spread spectrum (SS) and ultra-wideband (UWB), coupled with advanced network designs—such as redundant mesh architectures—work together to create communications networks that are extremely difficult to disrupt, whether deliberately or by accident.

Fire-and backhoe-proof connections. Wired connections can be broken by fires and backhoes; wireless connections can't. Consider an office building with smoke detectors that use "home-run" wiring to a central control panel. In addition to being expensive, home-run wiring is prone to massive failure because a fire can burn through all the connections in one place. By contrast, wireless mesh networking can provide redundant, fail-safe monitoring of all of the smoke detectors in a building.

Avoiding slip rings. Any time you need to make a measurement on a rotating object, you should consider a wireless link. Short-range, low-cost radio transmitters are being put into commercial use for measuring tire pressure from inside the tires of light vehicles manufactured in the U.S. For example, in addition to satisfying the mandates of the National Highway Traffic Safety Administration for tire pressure monitoring systems, this approach avoids the cost and complexity of connections made using mechanical slip rings.

Avoiding a tether. A colleague told me about a demonstration of an unmanned forklift in an automated warehouse that went horribly wrong because of a wired connection. After the forklift was put into motion, the president of the forklift company accidentally tripped and broke the wire connecting the forklift to its control system. With no signal to stop it, the forklift broke through the wall of the warehouse (luckily, nobody was hurt). Of course, a good engineer would have implemented a fail-safe system to stop the forklift once communication was lost, but communication would not have been lost if a wireless link had been used instead.

Electrical isolation. Some applications, such as electric utility substations, need to monitor equipment in very-high–voltage systems. Running wires, even with optoisolators, is impractical, because induced currents can be high. Radio links, with the proper choice of modulation and error correction, can provide economical and reliable communication links between the high-voltage gear and the operator control panel.

Wise Guy is the problem-solving persona of WINA, the acronym for the Wireless Industrial Networking Alliance (www.wina.org ). Send your questions to [email protected].