interference from a community of users. The latter putational complexity of decoding the convolutional causes devices to change their operating frequency to an code is about 3.5 times larger than the CCK code. To the unoccupied channel when possible. These changes in band author's knowledge, the PBCC variant has been imple- definitions and usage requirements improve consistency mented only in a few products from D-Link and US between U.S. and European (ETSI) requirements in the Robotics and has enjoyed very little commercial success. 5-GHz band and will presumably increase the market for Commercial politics may have played a significant role in compliant devices. The 802.11h standard, approved in this history, as Texas Instruments enjoyed certain in- 2003 to allow compliant devices that meet European tellectual-property rights in PBCC that may have made standards, adds several capabilities relevant to power other vendors reluctant to adopt it. It is also important to control and channel management. Clients inform access note that in most applications of a WLAN, performance points about their power and channel capabilities; quiet is secondary to price and convenience; the modest periods are added when no station transmits to enable advantage in link budget of PBCC was not enough to monitoring of the channel for interference, and access produce a commercial advantage in the marketplace. points can use the point coordination function interframe space to capture the medium to coordinate switching 5.1.3.6 802.11a PHY channels when interference is detected. Devices that are compliant to 802.11h should also The 802.11a PHY is a radical departure from the ap- meet the new FCC standards for UNII operation in the proaches above. The first major change is the use of the United States. Unlicensed National Information Infrastructure (UNII) The spectral mask limiting the bandwidth of the band at 5.15-5.825 GHz instead of the 2.4-GHz ISM transmitted signal is shown in Figure 5.1-16. Note that band. At the time the PHY was promu