Dynamic frequency selection, or DFS, is moving onto WLAN users' radars (pun intended) as 802.11n materializes and promises to greatly increase Wi-Fi usage in the 5GHz band. The DFS channel-changing capability applies, from a U.S. regulatory standpoint, to particular 5GHz bands used occasionally by military and weather radar that have also been sanctioned by the FCC to accommodate Wi-Fi traffic when radar data isn't present. What does this mean from an implementation and performance perspective?

What’s relevant to you depends both on your overall capacity needs and on the interference-avoidance implementation of your WLAN vendor. First, do you need access to every one of the 12 available non-overlapping U.S. channels in the three Unlicensed National Information Infrastructure (UNII) 5GHz bands?

If the answer is yes, your vendor will have to do more than just support DFS technology. Its products will have to have been FCC certified specifically for use in the 5.25GHz to 5.35GHz and 5.47GHz to 5.725GHz (check your spec sheet), which means that an FCC lab has checked for DFS capabilities.

If a 5GHz Wi-Fi product isn’t FCC-certified for use in the channels, one of two things is happening: 1) the vendor has blocked the channels so you can’t use them, which is in compliance with FCC regulation, but means you give up the potential to use some spectrum; or 2) the vendor is selling you use of those channels illegally, either out of naivete or on purpose.

If the FCC discovers your vendor has left the channels open without FCC DFS certification, the agency “will be all over your vendor,” says Terry Mahn, managing principal in the regulatory practice at law firm Fish & Richardson in Washington, D.C.

He’s quick to add that, as the enterprise customer, you wouldn’t be liable to pay fines or suffer other penalties – but you would have to shut down your network until blocking or compliance is achieved, he warns.

Gartner, which last week issued a short report about DFS issues, recommends avoiding running mission-critical applications in the UNII-2 bands, since those channels are most prone to channel-changing and associated delays.

In another scenario, your vendor might legitimately support DFS and have been FCC-certified for it. That vendor, however, might have chosen to support DFS across all channels, not just the required UNII-2 band, as its foundational interference-avoidance technique. Such products would bump any traffic encountering interference in any channel to another channel.

Joanie Wexler is an independent networking technology writer/editor in Silicon Valley.