band), but there is minimal interference because each access point can detect transmissions in progress. If the same three access points operate on interfering overlapping channels, such as 5, 6, and 7, then the throughput of the access points is greatly reduced by the interference and can drop below 1 Mbps. In this case, the access points do not operate on the same center frequency and thus cannot always detect transmissions outside their frequency range. Therefore, two simultaneous transmissions may create interference. Remember that WLANs are half-duplex, and only one device is allowed to transmit at any time.Figure illustrates judicious usage of the three non-overlapping channels available with the 802.11b and 802.11g standards.The goal of access point and cell placement is to reduce the overlapping of cells that are on the same channel. You can correlate this concept to the placement of FM radio stations throughout the country. You never see two radio stations in the same geographic area on the same channel. The same concept holds true for WLAN cells and channels. WLAN clients have the ability to shift data rates while moving. This technique allows the same client operating at 11 Mbps to shift to 5.5 Mbps, 2 Mbps, and finally still communicate in the outside ring at 1 Mbps. This rate shifting happens without losing the connection and without any interaction from the user. Rate shifting also happens on a transmission-by-transmission basis; therefore, the access point has the ability to support multiple clients at multiple speeds depending on the location of each client. Considerations for rate shifting include the following: Web Links Wireless LAN Compliance Status
Content 6.3 Explaining Wireless LAN Technology Standards 6.3.4 IEEE 802.11a Standard The 802.11a standard was ratified at the same time as 802.11b. However, because of limited supplies of silicon and other components, products did not start to appear in the market until late 2000. The technology provides up to a 54-Mbps data rate and, in most countries, provides eight channels of indoor WLAN use. However, the regulations vary widely across countries and are subject to change. More channels are expected to become available in many countries. To use the 11 new channels allocated by the FCC, 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 prior to 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. The cellular telephone industry has used TPC technology 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 (such as with access points 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, however, are 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. 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 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 be capable of accessing the additional channels. The 5-GHz band is divided into several sections. The lower eight channels cover 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, as well as 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, the radiated power is as follows: Figure illustrates the channel deployment of 802.11a products throughout a given area. The cells are easier to deploy because there are 12 different channels to work with. It is recommended that neighboring cells not be placed on neighboring frequencies. 802.11h DFS replaces manual channel assignment. Only frequency bands can be selected. This means that you would choose UNII-2 and then DFS would assign channels dynamically within UNII-2. DFS changes the channel if other transmissions, such as radar or satellite communication, are detected on the current channel. With 802.11h, up to 23 channels are available in the United States and 19 channels in Europe (if 5 GHz is allowed).
Content 6.3 Explaining Wireless LAN Technology Standards 6.3.5 : IEEE 802.11g Standard The 802.11g WLAN standard was ratified in June 2003. The aim was to provide higher data rates than the 802.11b standard. By using the 2.4-GHz band, backward compatibility was possible with existing 802.11b WLANs. The 802.11g standard uses the same three non-overlapping channels: 1, 6, and 11. There are 11 channels for North America, 13 channels for ETSI, and 14 channels for Japan. The 802.11g standard provides full backward compatibility with 802.11b. 802.11g uses orthogonal