Interference and the Law

Reader explains how interference between 802.11b and microwave networks can result in the shutdown of 802.11b service.

[Response to But How Much Does It Cost? [Part 3] from April 16, 2002.]

[April 22, 2002]

Dear Editors:

I read the information about setting up a wireless ISP by Gerry Blackwell.

In general it is pretty accurate from my experience, but what is not mentioned is the FCC rules regarding unlicensed Part 15 operations in the 2.5 GHz band.

Gerry did mention Amateur Radio but he did not mention Part 74 Broadcast Auxiliary Service. What is commonly known as "Channel 8" and "Channel 9" in Part 74 BAS (TV microwave) overlaps the Part 15 band with much higher power and, by FCC Rules, pre-emption rights.

The Society of Broadcast Engineers coordinates the use of Part 74 frequencies, and at the SBE Frequency Coordinator's meeting at the NAB convention in Las Vegas a couple of weeks ago the increasing use of Part 15 unlicensed microwave devices was discussed at length.

Several areas reported experience with 802.11b devices in a sports facility. Most of the time there was no interference to Part 74 TV microwave but the use of the microwave transmitters usually completely shut down the 802.11b systems.

This is a good reason why it may not be a good idea to colocate with any site that uses Part 74 2.5 GHz frequencies. The licensed broadcaster can—and if there is interference will—shut down any unlicensed devices. Either the FCC can do it or the landlord can do it, but it will be shut down. If that particular site was a key location in your WISP then you are no longer operating. Period.

One important piece of advice, therefore, is that before you start investing in unlicensed 802.11b systems contact the local SBE frequency coordinator to find out what locations might be able to operate without interference. A list of frequency coordinators can be found on www.sbe.org.

In the San Francisco Bay Area I am the chair of the Northern California Frequency Coordinating Committee (this area started organized Part 74 coordination before the SBE) and can be reached at "ncfcc@lns.com".

Beyond Part 74, these frequencies are also used by Part 90 licensed users (such as stadium video cameras) and in many places, law enforcement agencies. If you think that a TV broadcaster gets nasty when his Part 74 link gets interfered with, think about causing interference to an organization that has badges, guns, and jails!

Bill Ruck,
Principal Engineer, CSI Telecom
Chair, Northern California Frequency Coordinating Committee

P.S. I'd just like to restate an important point. For information about coordinating use of 2 GHz frequencies throughout the United States, look at http://www.sbe.org/ and click on Frequency Coordinators. There is a frequency coodinator for every state; and in many cases for specific parts of the more populated areas.