Wireless Signal Congestion: Whatever Happened to Peaceful Coexistence?October 1, 2006 By: Wayne Manges, MS, Rob Conant, PhD, PhD , Dr. Peter Fuhr, Oak Ridge National Laboratory, MS , Jose Gutierrez, PhD, Hesh Kagan, Invensys Process Systems Sensors
Faced with an exhibit hall teeming with wireless devices, the authors decide to see how well all these gizmos coexist.
We all know that there are many flavors of wireless (Figure 1); different frequencies, different data rates, different power requirements, different prices. This shouldn't be a problem when there's a lot of room, so we decided to measure what happens in a confined area with lots of wireless devices operating. We chose the show floor at ISA Expo 2005 for our test.
The majority of the many devices with antennas—seen by individuals as they walked around the show floor—operate within the ISM frequency bands at 900 and 2450 MHz. In a quasi-scientific process, we walked around the exhibit hall performing spectral measurements to ascertain ISM-band channel congestion. The results presented here do not represent some definitive conclusion on technical issues, such as a functional and coexistence comparison of (seemingly) esoteric (but very important) topics such as frequency-hopping spread spectrum or direct-sequence spread spectrum systems. Rather, we present some of our findings in this setting; in essence, a mini site survey.
Figure 1. A snapshot of types of wireless, ca. 2005
With more than 90% of all equipment/instrument vendors displaying some components and systems with wireless capabilities, wireless was all over the exhibit floor. In many cases, the wireless devices weren't actually operating; they were there as show pieces. The same could not be said for the exhibit hall's Industrial Wireless Pavilion. As the name implies, the vendors and organizations located in that section were busy displaying functional devices. A quick unscientific query of the exhibitors revealed that the vast majority of the products were set to operate in the unlicensed Industrial, Scientific, and Medical (ISM) frequency bands. While there are many ISM frequency bands available for use in the U.S., (Figure 2), most of the wireless traffic was concentrated in the 902–928 MHz and (+/-)2450 MHz frequency bands because of bandwidth and operational restrictions.
Figure 2. License-free ISM frequency bands
In standards-compliant wireless operation, most devices have gravitated to using either an IEEE 802.15.4-compliant wireless channel or an IEEE 802.11b/g-compliant channel. Please note that not all of the exhibited devices operated under IEEE compliance. Some were running their own protocol and broadcasting in the ISM bands. Numerous sensors/instruments/transmitters all attempting to operate in the same 900 and 2400 MHz channels resulted in considerable congestion and coexistence issues.
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