Wireless technology tips from Control Engineering wireless webcast
More wireless technology tips and answers follow, resulting from feedback and questions submitted during a Control Engineering wireless webcast.
Editor’s note: Control Engineering Wireless Webcast included Stephen Muenstermann, RoviSys Building Technologies, DC market manager, who presented his information as scheduled on March 14 (archived thereafter) despite suffering from flu. The article linked below in Adobe PDF format corresponds to the webcast presentation and provides additional information, with apologies from Muenstermann, who was severely under the weather for the webcast but went “on with the show” anyway. The linked PDF includes expanded presentation and application advice beneath each wireless webcast slide. Additionally, questions from webcast registrants and his answers to the questions are provided below.
Muenstermann offered the following answers to these questions to his portion of the Control Engineering wireless webcast.
Q&A Control Engineering wireless webcast
What are important performance metrics of wireless networks?
Top 5 performance metrics for wireless networks are radio signal strength indication, packet delivery, signal-to-noise ratio, receivers’ sensitivity, and line of sight. More information on each follows.
Wireless network metrics:
- RSSI – Radio Signal Strength Indication – How strong is the receive signal
- Packet Delivery – This indicates that that your message was sent and received. Typically greater than 50% is solid as the data is spread out across a spectrum.
- SNR – Signal to Noise Ratio – which is the power ratio between the signal (meaningful information) and the background noise (unwanted signal)
- Receivers’ sensitivity – a negative number -83dBm or greater is common. The more negative, the better sensitivity. Every time your receiver sensitivity drops by a factor of -3, it makes its ability to hear the incoming signal two times greater.
- LOS (line of site) – what this means is the distance the two radio frequency (RF) transceivers can visibly see each other without obstruction. This is more important when trying to shoot through walls, trees, canyons of steel, etc.
- Diversity – In many of these technologies they have what they call multipathing. That is where the same signal bounces off a metal wall and comes in slightly later. Most systems have great diversity.
- Determinism – Is the signal/network deterministic? This assures that you only get the data you want from each location.
- Encryption and authentication – These are security measures. The first refers to the RF in the cloud, the second refers to how it becomes recognized into the network.
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After you become more familiar with RF you will find that it will be easy to pick and choose. About 70% of the problems I have seen in the field have been related to mounting practices of the radios or antennas. Fortunately, this can be easily fixed by moving either item around.
What is an IT wireless network?
The real important part is the network side of it. That is what we are forming connections with wireless. We can create multiple layers with multiple stacks, while exchanging copious amounts of information. Information technology (IT) is used as we are exchanging, storing, manipulating, retrieving, and transmitting data.
Would multiple signals go through a single transmitter?
All sensor networks have unique points. Most of the “standard” sensor networks are designed to send multiple signals through a single transmitter. This mesh style network is to prevent a single point of failure and to be self-healing. So the short answer is: yes!
Did anything ever come of OCARI wireless?
OCARI (Optimization of Communication for Ad hoc Reliable Industrial network) has not yet been used in the United States, so I am only familiar with it by what I have studied since the standard started in 2010.
By its design it looks fairly solid. It’s late to the party on standards and probably hasn’t grabbed acceptance yet. It was developed mostly by the French, so it may take a while for it to grab broader acceptance simply do to the momentum of the other standards and that they are not in the heart of heavy industrial applications. Only time will tell.
What about designing RF links for facilities overseas, what advice do you have?
Many of the challenges overseas are the limitations of what is accepted by the country in which you are working. The 2.4 GHz band has been the most widely accepted because of the microwave oven. A typical microwave oven leaks about 1 W or more of RF energy. As a result, most countries have opened that frequency spectrum to allow people the ability to cook food (and popcorn) quickly.
In doing so it opened up a vast number of technologies in that spectrum from cordless phones, Wi-Fi, to WirelessHART, ZigBee, ISA100.11a, and others.
So my advice is stick with 2.4 GHz but be cautious of your power output and check the local limits. The last network I did in Finland only allowed for 280 mW radio output and 350 mW of ERP (effective radiated power). In the USA we can generate as much as 1 W from the radio and 4 W of ERP. (The ERP is related to what the antenna adds to the radio output power.)
Testing for wireless network health
You mentioned many wireless technologies. Since we are dealing with heterogeneous sensor networks deployment, have you thought of how to address remote testing of the performance and health of these sensors and sensor networks?
This is where the standards protect you most. The IEEE is pretty hard on what they will accept. And most manufacturers do not want to create radio networks that will fail. So these technologies are built with what the call anti-collision technology. So if they sense someone climbing into their specific spot in the frequency, they will divert the other direction. They keep a list of multiple nodes in their memory so they can jump to another node if the signals become too interruptive.
In understanding how a radio works, look at a cell phone. We have millions of people talking here in Chicago simultaneously with little or no interruption. So what the radios do is locate the cell that has the cleanest signal-to-noise ratio and links with that cell. It avoids the other cells even though the signal strength may be better.
From a heterogeneous standpoint: That is why you want to manage those networks, antennas, RF propagation, radio placement, and power output.
The radio receiver would be the most likely to fail if blasted with RF over numerous years. But, then again, the cost of a radio replacement is very cheap.
See additional industrial wireless Q&A, next page.
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