To use the 11 new channels, radios must comply with two features that are part of the 802.11h specification: Transmit Power Control (TPC) and Dynamic Frequency Selection (DFS). 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, the DFS mechanism of a device monitors its available operating spectrum, listening for a radar signal. If a signal is detected, the channel associated with the radar signal is vacated or flagged as unavailable for use by the transmitter. The transmitting device continuously monitors the environment for the presence of radar, both before and during operation.
Portions of the 5-GHz band are allocated to radar systems. This allocation allows WLANs to avoid interference with incumbent radar users in instances where they are collocated. Such features can simplify enterprise installations because the devices themselves can (theoretically) automatically optimize their channel reuse patterns.
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 5-mW 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 probably more transmit power than is required to maintain association with the access point. This approach results in unnecessary RF energy transmitting from the clients, creating a higher-than-necessary level of RF energy outside the intended coverage area of the access point.
6-46 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.
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 deployed high-performance WLANs. As a secondary benefit, the lower power on the client provides longer battery life; less power is used by the radio.
The Cisco Aironet RM21A and RM22A 5-GHz radio modules for Cisco Aironet 1200 and 1230 Series and the 1130AG and 1240AG Series access points support the 12 channels made up of the UNII-1, UNII-2, and UNII-3 bands. These devices have the hardware capability to support the 11 new channels. However, until the FCC releases a test program, the firmware will not provide the availability to access the additional channels.
The 5-GHz band is divided into several sections. The lower eight channels cover two of the sections known as UNII-1 and UNII-2. Each of these includes 100 MHz of spectrum in which there are four channels. The UNII-1 band has limitations in the United States (and some other countries) that require it to be for indoor use. UNII-2 is permitted for both indoor and outdoor use, and it also permits external antennas. UNII-3 was designated for outdoor use and was primarily set aside for bridging.
Rule changes are underway and, with the adoption of 802.11h, will provide up to an additional 12 channels in many countries, in addition to using the UNII-3 band for WLANs. The number of WLAN channels will then increase from 8 to as many as 24.
If a 6-dBi antenna is used, then the radiated power is as follows:
■ UNII-1: 50 mW in the United States and Japan, 200 mW in Europe, 4 channels (5.15 GHz to 5.25 GHz), indoor access, flexible antenna
■ UNII-2: 250 mW in the United States, 4 channels (5.25 GHz to 5.35 GHz), indoor and outdoor use, flexible antenna
■ HiperLAN: 200 mW in Europe, 8 channels (5.15 GHz to 5.35 GHz), indoor use only
■ HiperLAN II: 1 W in Europe, 11 channels (5.470 GHz to 5.725 GHz), indoor and outdoor use, flexible antenna
■ UNII- 3: 1 W in the United States, 4 channels (5.725 GHz to 5.825 GHz), indoor and outdoor use, flexible antenna
© 2006 Cisco Systems, Inc. Wireless LANs 6-47
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