Last month, the FCC officially opened up the use of the middle band of the Unlicensed National Information Infrastructure (UNII) spectrum (5.470 GHz to 5.725 GHz) to 54Mbps 802.11a Wi-Fi networks in the United States. The band adds another 255 MHz and 11 channels to the existing 325 MHz and 13 channels available for Wi-Fi in this band.

As of January 20, any products that apply for certification in the 5.470 GHz to 5.725 GHz band or in the lower end of the UNII band at 5.25 GHz to 5.35 GHz, were required to support dynamic frequency selection (DFS) and transmit power control (TPC) to minimize interference, per a February 2005 FCC order.

If you’ve already got equipment running in the lower band installed, don’t worry – you’re grandfathered.

DFS and TPC are part of 802.11h, the European “flavor” of 802.11a. DFS dynamically instructs a transmitter to switch to another channel under particular conditions, such as the presence of a radar signal.

Setting the transmit power of the access point and the client adapter can allow for different coverage area sizes and for the client to conserve battery life. For example, it helps to reduce transmit power in areas with high user density to avoid interference. It can also help with security by keeping transmit ranges confined so there is less chance of signals spilling out into the street or to neighboring offices.

Both the 5-GHz and 2.4-GHz spectrum ranges are likely to be component of future 802.11n networks. While the 5-GHz range has its issues with inconsistent global regulation (see Monday’s newsletter), the 2.4-GHz band supporting 802.11g and 802.11b faces its own hurdles. I’ll discuss those next time.

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