Regardless of what information you need to send, or where you need to send it, if your data is going “over the air”, you’ll need to choose a frequency in one of three ranges that do not compete with the FCC licensed bands for radio transmissions.
These three ISM (industrial, scientific, medical) bands are 900 MHz, 2.4 GHz, and 5 GHz. They’ve often been described as the industrial equivalent to Citizens’ Band radios, specified so they don’t interfere with broadcast radio signals.
History with technology would lead you to believe that the more Hertz you have, the better your radios will perform. But what you need to understand is that there’s a tradeoff. Each band has its strengths and weaknesses, and there’s a best use for each.
Distance vs. Throughput
Generally speaking, when you compare these three bands, you’re looking at a tradeoff between the distance you can send your data and the amount of data you can send (throughput).
The 900 MHz band is the slowest of the three, and transmits less data, but it can send data a significantly farther distance than the two GHz bands. So you get fewer bits of data, but they can go long distances, indoors or out.
900 MHz band radio systems have an effective usable distance measured in miles. It is the least affected by line-of-sight obstructions (stuff that interrupts visibility between radio and receiver antennae). The 900 MHz band is primarily used for transferring serial data and analog or digital I/O over long distances.
The 900 MHz band is ideal for sending process data over wireless Ethernet (process variables, status, and alarms), but it’s not sufficient for WiFi applications. It just can’t offer the kind of bandwidth necessary to support the streaming data requirements of wireless PCs or handheld computers.
The 2.4 GHz band is growing in popularity, as it’s the main band used in WiFi (wireless Ethernet), ISA100 and WirelessHART systems. This band can transfer more data, faster, but at a distance measured in hundreds or thousands of feet, not in miles. It’s more affected by line-of-sight obstructions and noise. Generally, the 2.4 GHz band is used for indoor installations.
The newest addition, the 5.8 GHz band, is used primarily for wireless Ethernet networks (IEEE 802.11 systems). It has a higher data capacity and speed, but at the cost of distance. Its useful range is measured in hundreds of feet. The 5.8 GHz band is even more susceptible to noise and is weakened by obstacles. In real-world industrial situations, you’ll likely only consider this band under two situations: (1) Your IT department says you can’t use the 2.4 GHz band, or (2) the 2.4 GHz band is already overwhelmed with data traffic.
Watch for More Wireless 101
As budgets get tighter, and the need for more widely-available data grows, our customers are looking toward using industrial wireless technology as a solution. Over the next few weeks, I’ll try to shed some light on the basics of industrial wireless, to help demystify the language and terminology of this truly flexible and powerful technology.
Do you have a question about wireless? What confuses you most? What concerns do you have about putting in a wireless system? Ask me.
The Lesman team recently released our latest Industrial Wireless Solutions Guide, a 48-page catalog that will walk you through the different wireless technologies, instrument options for each, and applications where wireless has proven successful. It’s free, and available now at Lesman.com. (4.9MB PDF… please be patient as it loads.)