If you want to know more about how all these wireless systems really work be prepared to spend some time on it. It is a very complicated area and one that a lot of people know just a little about.
But if you really want to know industrial wireless then the radio is the place to start. It’s the guts of everything and for a large part determines how well your end user application is going to work. And where it is going to work.
If the device you make has a 900 Mhz radio for example, it’s going to blast through a lot of the walls in your customers building. Great, you say? Well, not so fast. 900 is pretty much a standard around here, in North and South America. If you want to sell your device in Europe, well now you’re out of luck. They have a different frequency spectrum and they prohibit 900 Mhz. They use 868 and 2.4 Ghz. And if you replace your 900 with a 2.4 Ghz radio, well then you can’t go very far through the walls of a building.
There are lots of factors that go into industrial radio development and they all influence what applications you can do, where you can deploy it and how well it is going to work in those applications.
Before we get to those we should make a comparison. In wired systems, we have…ready for this…wire. There are different kinds of wire, there’s twisted pair for RS232 and RS485 and there’s CAT5 (and CAT6) for Ethernet. There are other specific types of wire for Profibus, CAN and ControlNet. Each one of those has a specific MAC (Media Access Controller) and PHY (Electrical Interface).
In RS232, the UART is the MAC part of the physical communications while the RS232 driver is the Physical Interface (PHY). In CAN and Ethernet, there is a MAC usually embedded in a microprocessor and a PHY that does the conversion of microprocessor data bit to data bit on the wire. For RS232 the data bit on the wire is 15 volts. For RS485, it’s 5 volts and for CAN it can be up to 33 Volts.
Now in wireless, we have a single transmission medium, air, but different radios, or transceivers (transmitter/receiver), with different MACs and PHYs for sending messages through that medium. There are transceivers for cellular, satellite, mesh, Bluetooth and many other types of radios. Each type is usually described by a set of key features:
Licensing – Does the radio use a licensed or unlicensed radio band? All radios are licensed. You really can’t transmit much of anything with out an authorization from the federal government of whatever country you’re in. Now most of what we transmit is in the unlicensed band. That means that the person transmitting doesn’t have to hold a license but the manufacturer has one. Opposite of unlicensed transmission is licensed band transmission. That’s for those applications where every transmitter site must be licensed. Think of your radio and over the air TV. Those stations have a license for every transmitter and are strictly regulated as to power output and frequency.
Frequency – this is how fast a transmitter sends a complete cycle usually measured in Hertz. In the US, you’ll typically find 2.4 Giga Hertz and 900 Mega Hertz. Europe also uses 2.4 but also had an 868 MHz standard.
Power Output – This is the amount of power output generated by the radio. The power output is directly related to the range of the radio and somewhat related to how well the radio can penetrate obstacles. A typical industrial radio might put out 30 milliwatts while your cell phone emits a whopping 3 watts.
Spread Spectrum – To avoid narrow band interference (interference from point sources on a single channel) many industrial radios incorporate spread spectrum. This means that they continuously change frequencies. If there is interference on one frequency, it’s not a problem because the next message is sent on another frequency. There are two types of spread spectrum systems, direct and random. In direct, the radios use a table of frequencies to sequence through. In random spread spectrum, a random algorithm is used to determine the next frequency.
Data Rate – This is the number of bits that can be transferred over the air by the radio. It is usually expressed in kbps (thousand bits per second). For the time being, radio is a fairly slow medium. Some radios are only 9600 baud, less than what a lot of RS232 interfaces use. Others use 250 Kbps, still slow compared to the 10 or 100 Mbps of wired Ethernet systems.