Congestion of wireless signals

InTech, Jun 2006 by Fuhr, Peter, Kagan, Hesh, Conant, Rob, Gutierrez, Jose A

Editor's note: There are all types of wireless technologies; different frequencies, different data rates, different power requirements, different prices. In a quasi-scientific test at ISA EXPO 2005, the authors measured what happens in a confined area with wireless devices operating at the same time.

With over 90% of all equipment/instrument vendors displaying components and systems with wireless capabilities at ISA EXPO 2005, wireless was on display all over the show floor.

In the exhibit hall's Industrial Wireless Pavilion, wireless devices were up and running. A quick unscientific query of the exhibitors revealed 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., due to bandwidth and operational restrictions, most of the wireless traffic was concentrated in the 902-928 MHz and 2450 ( /-) 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, not all of the exhibited devices operated under IEEE-compliance, rather they could run their own protocol and broadcast in the ISM bands. The result is easy to predict, namely numerous sensors/instruments/transmitters all attempting to operate in the same 900 and 2400 MHz channels, resulting in considerable congestion and coexistence issues.

There is no requirement for devices operating in either of these bands to comply with either of these standards-plenty of wireless devices are simply sensors/devices onto which has been slapped a "conventional" wireless transmitter. As long as their radiated power doesn't exceed the maximum value, the device is "compliant" with operating in an ISM band.

After obtaining a background noise floor spectral snapshot, we walked onto the exhibit floor at 7 a.m. on the second day of the event, turned on the portable spectrum analyzer, and tuned it into the 900-930 MHz range. With very few people, it set the stage for a "quiet" moment to take measurements. The amplitude level was flat at -82dBm.

A different electromagnetic "scene" occurred at 12:30 p.m. the same day. The EXPO floor was buzzing with vendor displays operating and hundreds of people walking around. We picked up the spectrum analyzer and returned to the same physical location on the exhibit floor where we first took the noise floor measurement. With the analyzer set to the same sampling/measurement settings, we measured significant congestion. Peak amplitudes top out around 5dBm, with local minima in the -25dBm range-a 67dB increase (in regular numbers a factor of over 6,000,000).

Later in the day, we grabbed the analyzer and repeated the 900 MHz measurements, but this time performing a time average to ascertain the average channel activity.

It is difficult to present definitive information on the impact of such RF congestion on system performance. We obtained anecdotal "evidence" by walking around and talking to exhibitors and asking how their demos were functioning. The "study" results verified what the measurements showed: significant channel congestion and systems that didn't always function as hoped for.

We reiterate this was a quasi- (at best) scientific study conducted in a somewhat unnatural setting of an exhibit hall with literally hundreds of vendors' wireless systems operational. On the flip side, if the analysts' forecasts are right with thousands of wireless sensors and networks deployed throughout industrial settings, it is probably worthwhile to give a bit of attention to congestion in ISM band traffic and the associated negative effects of co-channel interference.

Two IEEE standards groups are working toward addressing the coexistence issue, namely P802.15.2 - Recommended Practice for Coexistence in Unlicensed Bands and 802.19 - Coexistence of Wireless Data Transport.

Two organizations are attempting to bridge the chasms between IT, wireless, and industrial bus systems with a focus on a standard for coexistence and interoperability of networked systems in industrial settings: WINA (www.wina.org) and ISA's SP100, Wireless Systems for Automation committee (www.isa.org/community/sp100).

ABOUT THE AUTHORS

Peter Fuhr is with Apprion, Hesh Kagan is with Invensys, Rob Conant is with Dust Networks, and Jose A. Gutierrez is with Eaton. In addition to their duties at their respective companies, all four are on the executive teams of the Wireless Industrial Networking Alliance (WINA) and ISA-SP100.