The latest 802.11n topic to get its knickers in a twist revolves around a capability called dynamic frequency selection, or DFS. There are general performance issues associated with DFS as well as separate U.S. FCC regulatory compliance requirements to consider.

A few years back, the U.S. opened up additional channels in the 5GHz band to Wi-Fi traffic. The main idea was to make additional capacity available to Wi-Fi technology. Theoretically, the more channels you have over which to distribute traffic, the greater your aggregate available capacity becomes, so this step was considered a good thing.

The FCC blessing for Wi-Fi gear to use the Unlicensed National Information Infrastructure (UNII)-2 bands – specifically, channels in the 5.25GHz to 5.35GHz and 5.47GHz to 5.725GHz bands – at that time applied to 802.11a networks, given that 11n was still a gleam in the IEEE’s eye. And it included a caveat, because these bands are also used for military and weather radar: Products that operate in these bands must be specifically FCC-certified for DFS technology support.

DFS detects the existence of radar in UNII-2 channels and quickly moves any interfering transmission to other channels, clearing the path for the radar. Makes sense: Matters of national security should probably take precedence over bar code scans.

Vendors who do not wish to support DFS are allowed to block these particular channels in such a way that an integrator or end customer can’t override them, explains Terry Mahn, a managing principal in the regulatory practice at law firm Fish & Richardson in Washington, D.C.

In practice, the industry had barely gotten started with 802.11a network deployments, which could make use of these extra channels, when 802.11n became the talk of the town. 802.11n will rely heavily on the 5GHz band to deliver its enhanced throughput promise and many users are skipping 802.11a deployments, at least on a grand scale, in favor of 802.11n.

Now the question becomes: Does your 802.11n vendor support DFS? If no, is there a downside? If yes, might there also be a downside?

The answers relevant to you depend in part on your capacity needs and on the interference-avoidance approach and implementation of your chosen WLAN vendor. I’ll explain what I mean by that next time.

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