Wireless Topology: You've Got Choices

It's Not Complicated, but Make Your Choices Carefully to Get the Network Best-Suited to Your Needs

By Ian Verhappen, Certified Automation Professional

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It should come as no surprise that many of the basic rules for selecting wireless networks are similar to those for wired or any other network. Just like our corporate networks are a combination of different wired and wireless installations with routers, domains and subnets, "the ideal wireless network should have a blend of mesh I/O nodes and some star-configured nodes when necessary," Dick Caro, CMC Associates reminds us.

Because wireless networks do follow the basic rules of network design, the contributors to this article all agreed that the following five steps suggested by Prabhu Soundarrajan, global director, RAE Systems, are similar to the ones they use, though, as we will see, there is much discussion on the degree of detail required for the various steps.

SEE ALSO: Wireless Topologies

1. Understand the application;

2. Do a thorough site survey assisted by mathematical modelling to understand the radio frequency (RF) environment and topology;

3. Select the right instrument protocol and standard for installation;

4. Engineer the network with redundancy to install a robust, reliable wireless network and system; and

5. Monitor the network topology post-installation using remote-monitoring techniques.

Though the last step is not actually part of the design stage, the information gathered ensures long-term network health and, of course, provides additional data points in the event of expansion. It's almost a sure thing that once a wireless system is in place, additional uses will be found for it. Now let's look at each of the above in a bit more depth.

Understanding the application is the necessary first step, since it's the basis or reason for installing or expanding the network. There are very different requirements for a field sensor network in a facility than for a pipeline/SCADA system, especially with regards to bandwidth requirements and, most importantly, distances. We will focus on facility or plant networks, meaning those using IEEE 802 standards-based radios.

The most common application for wireless is to capture some signal, previously uneconomical for a wired connection, which Jonas Berge, director, Applied Technology, Emerson Process Management, Singapore, has experienced. "Most wireless transmitters are used to measure points that were previously checked manually once per shift, once per day, week, month, year, turnaround or perhaps not at all," Berge says. "These applications don't need fast update periods. If, on the other hand, you plan to use wireless for closed-loop control, then the update rate will be determined by the process response time, which might require update rates of less than one second, and will require a reliable power supply other than, or in addition to, the battery to ensure long life.

Understanding what you're trying to do leads to the next item: determining where you will install the system, which requires that you have an understanding of the site conditions.

Site Conditions
Though not everyone agrees that a site survey is required. Mike Fahrion director of product management, B&B Electronics says, "RF background noise can come from sources such as solar activity, high-frequency digital products or competing forms of radio communications. The background noise establishes a noise floor that is a function of frequency at which the desired signals are lost in the background ruckus." So a site survey is the best method to determine your base level of background noise and for that reason is worth doing.

"Improve your receive sensitivity and, therefore, your range by reducing data rates over the air," Fahrion continues. "As baud rate goes down, the receive sensitivity goes up. However, the noise floor often will be lower than the radio-receive sensitivity of your radio, in which case, it wouldn't be a factor in your system design. But if you're in an environment where high degrees of RF noise exist in your frequency band, use the noise floor figures rather than the radio-receive sensitivity to make your calculations."

To aid an initial wireless network layout in cases where you have only very preliminary information, such as during a front-end engineering design or feasibility study, or when you might not have a site survey, or in the case of a greenfield facility, Fahrion has a path-loss rule of thumb. "Never exceed 50% of the manufacturer's rated line-of-sight distance," he says. "This alone yields a theoretical 6dB fade margin — a big step on the way to the required 10dB. De-rate more aggressively if you have obstacles between the two antennas, but not near the antennas, and de-rate to 10% of the manufacture's line-of-site ratings if you have multiple obstacles, obstacles located near the antennas, or if the antennas are located indoors."

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