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Why we lost the weather radar channels

By Matthew Gast in · Experts · February 7, 2013

These days, I am doing a great deal of speaking and writing about 802.11ac, and one of the most frequent discussion topics is about radio spectrum and channel allocations. For maximum speed, 802.11ac requires big chunks of radio spectrum so that it can use 80 MHz or 160 MHz channels. Practically speaking, if you want 802.11ac to reach gigabit speeds for you in the first generation of products, you need good-size chunks of spectrum. 

One of the obstacles to having sufficient wide channels is that the FCC stopped allowing use of channels 120, 124, and 128. These three critically-positioned 20 MHz prevented use of wider channels that contain those frequencies, preventing use of two 40 MHz, two 80 MHz, and one 160 MHz channel.

Several years ago, the FCC began receiving reports of wireless equipment causing interference to Terminal-area Doppler Weather Radar (TDWR), an important safety measure taken at the 45 busiest airports in the U.S. Weather radar is the primary user of these channels, so Wi-Fi devices need to avoid using the frequencies if weather radar is present. Unfortunately, the problems of a few devices caused the FCC to take them away from all of us.

Just to make it clear where my priorities are, TDWR is more important than Wi-Fi. I hope it is clear from reading this blog that I love Wi-Fi. It is rare that you can work on network technologies that make people’s eyes light up, and Wi-Fi lets me do that. Even though Wi-Fi is pretty important to me, my life depends on TDWR. In the early 1980s, wind shear and microbursts caused several notable accidents. Delta 191 is perhaps the most famous.

My Aerohive colleague Jeff Haydel lived only a few miles from the crash of Pan Am 759 and remembers hearing the boom from the crash. The research effort that created TDWR was launched in response to these deadly crashes, and perhaps the best testament to its effectiveness is that it has been decades since an airliner has crashed in the U.S. due to wind shear. As a frequent flier, I appreciate arriving alive more than having Wi-Fi on the plane.

To bring the story back to Wi-Fi: before a device that emits radio waves can be sold in the U.S., it must be tested to ensure that it complies with FCC regulations. Testing Wi-Fi devices is demanding because Wi-Fi is carried on radio signals. An access point will emit radio waves by design, not as a mere side effect of being an electronic device. In the jargon of the regulations, Wi-Fi devices are “intentional radiators.”  Part of the FCC approval process is that a vendor demonstrates that we comply with relevant regulations. For Wi-Fi, those regulations are limits on power, operation only in certain approved frequency bands, and the like. The formal rules for this are sometimes called “Part 15”  after the FCC regulations governing secondary band usage.

In 2009, the Federal Aviation Administration (FAA) operators reported interference with a TDWR system to the FCC. In response, the FCC modified its rules for unlicensed devices to prevent operation in the TDWR band. You might notice this on your favorite Wi-Fi device as an inability to select channels 120, 124, and 128. Section 3 of the NTIA report on 5 GHz band sharing covered this familiar ground, but went on to list some of the root causes of weather radar interference: unlicensed devices were operating without DFS, with DFS disabled, outside of the authorized frequencies approved during the FCC certification, and with high-gain antennas that increased power beyond allowed limits.

Intrigued, I went and looked at the list of enforcement actions taken by the FCC against weather radar interference. The reading was highly repetitive, and followed the general form of “interference to TDWR was reported” followed by “we sent an enforcement agent to look for the interference” which resulted in a notice sent to the owner of the equipment, often with a fine of $25,000.

In twenty cases of reported interference, the following themes stood out (some of these overlap, so if you add them up you will get to more than 20):

  • DFS was disabled in three. If you want to use the DFS-enabled channels, the rules are clear. You must use DFS. As an example, HiveOS requires that DFS be enabled before you can access channels 100 through 144. Aerohive is not unique in this respect. Any vendor that takes its regulatory obligations seriously will ensure that DFS is used.
  • High-gain antennas exceeding allowed power were used in three cases. When a product vendor obtains certification from the FCC, we will submit a product with an antenna. Customers are not required to use our antennas, but they are responsible for selecting antennas that stay within the tested power limits. Nothing in the hardware prevents a user from installing higher gain antennas, but exceeding the allowed power is no laughing matter.
  • Oddly, exactly half of the cases came out of the FCC’s Puerto Rico office and involved reports of interference with the TDWR at San Juan Airport.  I don’t have a good explanation for this. My only guess is that Puerto Rico’s tropical weather spawns weather formations of the type TDWR is meant to detect, so interference is noticed more in a heavily used system.
  • TDWR interference is not just a Wi-Fi problem. Only half the cases of interference reported were Wi-Fi devices (all from a single vendor), with the remainder being WiMax-type equipment.

The moral of the story: if you are building an outdoor network, turn DFS on. Fliers will thank you, and you won’t need to pay fines.

Read other spectrum-related blogs by Matthew:

Matthew Gast (@MatthewSGast)

Matthew Gast is Director of Advanced Technology at Aerohive Networks. He currently serves as chair of both the Wi-Fi Alliance's security task groups, was the first chair of the Wireless Network Management Marketing task group, and is the past chair of the IEEE 802.11 revision task group. Matthew is also the author of 802.11 Wireless Networks: The Definitive Guide (O'Reilly), which is now in its second edition and has been translated into six languages. His companion book, 802.11n book, 802.11n: A Survival Guide (O’Reilly), was recently published and provides information on how 802.11n works and what it means for the WLAN planning process. Most recently, Matthew completed 802.11ac: A Survival Guide (O'Reilly).


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