Factors Affecting Reliability
I want to understand what makes some wireless systems more reliable than others.
Signed, Seeking Reliability
Dear Seeking:
Three environmental factors affect how reliably one radio can receive signals from another (see Figure 1). These concepts are easy to understand if you imagine that you are listening to a friend talking to you: Your friend is the transmitter and you are the receiver, and "reliability" is simply how well you can understand what your friend is saying.
 Wise Guy |
Attenuation indicates how a signal fades between transmitter and receiver. Barring obstructions, each time the distance between the transmitter and the receiver doubles, a signal is attenuated by a factor of four. If you are standing far away from your friend, or if there is a wall between you, it will be difficult to understand what your friend is saying.
Noise is interference in the radio environment—coming from other radios, such as WiFi LANs, cordless phones, and garage door openers, or from "incidental radiators" such as microwave ovens, arc welders, and electric motors. As noise increases reliability decreases. If your friend is speaking to you in a quiet environment, you can hear easily, but if there are many other people talking, or if there is much ambient noise, you will likely have a hard time understanding your friend.
Multipath is an engineer's term for "electronic echoing," which happens whenever a signal from a transmitter reaches a receiver over multiple routes of different lengths. From the receiver's point of view, the signal becomes "smeared" or repeated. For instance, if your friend is speaking rapidly in a highly reverberant gymnasium, you will have a difficult time understanding the words.
To overcome the effects of attenuation and noise you can increase transmitter power, but this doesn't always work. One radio's signal is another radio's noise, and upping transmitter power decreases the reliability of other, nearby radios—just the way shouting at a restaurant makes it harder for the other diners to hold conversations. What's more, increasing transmitter power does not overcome the effects of multipath: the echoes simply become louder. And incidentally, increasing transmitter power increases energy use.
So what's a design engineer to do? An entire alphabet soup of new radio designs, including Direct Sequence Spread Spectrum (DSSS), Frequency Hopped Spread Spectrum (FHSS), Ultra-Wideband (UWB), and Multiple-In-Multiple-Out (MIMO) that can increase reliability in many environments. And networking architectures such as mesh and cluster tree help by creating redundant communications paths and putting radios closer together.
 Figure 1: Three factors can interrupt a communications signal. |
Effects of Structural Change Over Time
The scary scenario for wireless is when everything works as installed, but a subsequent installation of air ducting, motors, wires, and so on knocks out the wireless links. The processes aren't in place at many locations to coordinate with the "new kid on the block," and yet fixing unexpected problems will incur new costs. Please help!
Signed, Anticipating Trouble
Dear Anticipating:
You raise three important points. First, the causes of wireless connection failure differs from the causes of wired failure. Second, most organizations haven't yet developed the skills—perhaps 'intuition' is a better word—to create and maintain wireless connections. And third, while companies tout the reduced installation cost of wireless systems, there is some cost associated with developing those intuitions.
But there is hope. For instance, WINA (the Wireless Industrial Networking Alliance, www.wina.org brings together end users, technology providers, and standards bodies to pool knowledge and develop best practices.
Also, just as there are purpose-built continuity testers for various wired systems (e.g., Ethernet, telephone), we are starting to see special-purpose "wireless continuity testers," which help installers and maintainers monitor their networks. These will become as commonplace as "tone tracer/generator" kits used by telephone companies.
If you're a fan of standards bodies, check into IEEE 802.19, the Coexistence Technical Advisory Group (http://grouper.ieee.org/groups/802/19), which is working to assure that wireless systems built on IEEE standards (WiFi, Bluetooth, ZigBee, etc.), as well as emerging standards, "play nice" with one another or at least understand when and how they might cause trouble for each other.
Finally, it pays to anticipate challenges with wireless communications and "overengineer" an installation.
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].