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FCC Regulations Update

A new FCC Rule addressing Dynamic Frequency Selection (DFS) in the 5 GHz band will

go into effect for the US and Canada on July 20, 2007. This document outlines the

specifics of this rule and provides and overview of various WLAN product regulations for

the FCC regulatory domain.

Dynamic Frequency Selection for 5 GHz WLAN in the US and Canada

The 5 GHz band is divided into several sections referred to as Unlicensed National Information

Infrastructure (UNII) bands. The UNII-1 band is designated for indoor operations, the UNII-2 and

UNII-2 extended bands are for indoor and outdoor operations, and the UNII-3/ISM band is

intended for outdoor bridge products and may be used for indoor WLANs as well. In order to

operate in the 5 GHZ bands radios must comply with two features that are part of the 802.11h

specification-Dynamic Frequency Selection (DFS) and Transmitter Power Control (TPC).

DFS dynamically instructs a transmitter to switch to another channel whenever a particular

condition (such as the presence of a radar signal) is met. Prior to transmitting, a device’s DFS

mechanism monitors its available operating spectrum, listening for a radar signal. If a signal is

detected, the channel associated with the radar signal will be vacated or flagged as unavailable for

use by the transmitter. The transmitting device will continuously monitor the environment for the

presence of radar, both prior to and during operation. Portions of the 5 GHz band are allocated to

radar systems; this allows WLANs to avoid interference with incumbent radar users in instances

where they are colocated. Such features can simplify enterprise installations, because the devices

themselves can automatically optimize their channel reuse patterns.

FCC Rule # 15.407(h)(2) requires that products operating in the UNII-2 and UNII-2 extended

bands (5.25-5.35 GHz and 5.47-5.725 GHz) must support Dynamic Frequency Selection (DFS), to

detect and automatically adjusts channels to protect WLAN communications from interfering with

military or weather radar systems. All WLAN products that ship in Canada and the US on or after

July 20, 2007 must meet the DFS for FCC requirements.

Transmitter Power Control (TPC) technology has been used in the cellular telephone industry for

many years. Setting the transmit power of the access point and the client adapter can be useful to

allow for different coverage area sizes and, in the case of the client, to conserve battery life. In

devices that have the ability to set power levels, the settings are usually static and independent of

each other (access point and clients). For example, an access point can be set to a low 5mW

transmit power to minimize cell size, which is useful in areas with high user density. The clients

will, however, be transmitting at their previously assigned transmit power settings, which is likely

more transmit power than is required to maintain association with the access point. This results in

unnecessary RF energy transmitting from the clients, creating a higher level than is necessary of

RF energy outside the access point’s intended coverage area. With TPC, the client and access

point exchange information, then the client device dynamically adjusts its transmit power such that

it uses only enough energy to maintain association to the access point at a given data rate. The

end result is that the client contributes less to adjacent cell interference, allowing for more densely