frequency-division multiplexing (OFDM) modulation for 802.11g data rates and complementary code keying (CCK) modulation for 802.11b data rates. Because of the backward compatibility of 802.11g, it is likely that both 802.11b and 802.11g clients associate to an 802.11g access point. The 802.11g protection mechanism allows the coexistence of 802.11b and 802.11g clients in an 802.11g wireless cell. The protection mechanism has the following characteristics: The 802.11g standard uses the same frequencies as 802.11b depending on regulatory domains. (Japan has not approved OFDM for channel 14). The 802.11g standard combines the modulations of 802.11b with the modulation of OFDM for 802.11g data rates. The 802.11g specification supports the data rates of 1, 2, 5.5 and 11 Mbps for 802.11b, and adds the data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps for 802.11g. The access points transact with 802.11b clients at their highest capable data rate given their configuration and position in the coverage cell, and does the same for 802.11g clients. This means that an 802.11b client can receive packets at the 11-Mbps data rate while an 802.11g client right next to the 802.11b client can receive packets at the 54-Mbps date rate. When 802.11b and 802.11g clients are in the same cell, the 802.11g specification requires a protection mechanism that involves the 802.11 Request to Send/Clear to Send (RTS/CTS) protocol. CTS packets can be sent without RTS packets from the access points. It is the protection mechanism of 802.11g that slows the throughput of 802.11g clients when there are 802.11b clients in the coverage cell. The protection mechanism is not active when the cell has only 802.11g clients. With the protection mechanism active, the access point still transmits to the clients at rates up to their capabilities. The protection mechanism slows 802.11g throughput but provides for the fewest collisions of packets. When RTS/CTS is in use, most stations hear the RTS, and all stations hear the CTS. In either case, each node receives information indicating the length of the subsequent OFDM packet and ACK transmission. Every station has an internal timer referred to as the network allocation vector (NAV). The NAV is set to have the same duration as the OFDM packet exchange. The NAV acts in parallel with conventional carrier sensing and is referred to as a virtual carrier sense mechanism. The channel is not considered idle unless no active signal is detected and the NAV timer has expired. After both criteria are met, stations can once again begin to contend for channel access. In this manner, 802.11b and 802.11g radios can operate in a mixed environment with 802.11g access points. It should also be noted that every 802.11g client and access point must be capable of falling back and operating exactly like a legacy 802.11b device. Therefore, migration to 802.11g technology can be smooth and painless. As new 802.11g access points are brought online, legacy 802.11b access points can remain in service and be fully interoperable with newer 802.11g clients.
Content 6.3 Explaining Wireless LAN Technology Standards 6.3.6 802.11 Comparison The 802.11b standard, the most widely deployed wireless standard, operates in the 2.4-GHz unlicensed radio band and delivers a maximum data rate of 11 Mbps. The 802.11b standard has been widely adopted by vendors and customers who find its 11-Mbps data rate more than adequate for most applications. Interoperability between many of the products on the market is ensured through the Wi-Fi Alliance certification program. Therefore, if your network requirements include supporting a wide variety of devices from different vendors, 802.11b is probably your best choice. The 802.11g standard was ratified in June 2003. The 802.11g standard delivers the same 54 Mbps maximum data rate as the 802.11a standard, yet it offers an additional and compelling advantage: backward compatibility with 802.11b equipment. This compatibility means that 802.11b client cards work with 802.11g access points, and that 802.11g client cards work with 802.11b access points. Because 802.11g and 802.11b operate in the same 2.4-GHz unlicensed band, migrating to 802.11g is an affordable choice for organizations with existing 802.11b wireless infrastructures. Note that 802.11b products cannot be “software upgraded” to 802.11g. This limitation is because 802.11g radios use a different chipset to deliver the higher data rate. However, much like Ethernet and Fast Ethernet, 802.11g products can be combined with 802.11b products in the same network. Both 802.11g and 802.11b operate in the same unlicensed band. As a result, they share the same three channels, which can limit wireless capacity and scalability. The IEEE ratified the 802.11a standard in 1999, but the first 802.11a-compliant products did not begin appearing on the market until December 2001. The 802.11a standard delivers a maximum data rate of 54 Mbps and 12 non-overlapping frequency channels. This provision results in increased network capacity, improved scalability, and the ability to create microcellular deployments without interference from adjacent cells. Operating in the unlicensed portion of the 5-GHz radio band, 802.11a is also immune to interference from devices that operate in the 2.4-GHz band, such as microwave ovens, cordless phones, and Bluetooth devices (a short-range, low-speed, point-to-point, PAN wireless standard). The 802.11a standard is not, however, compatible with existing 802.11b-compliant wireless devices. Organizations with 802.11b equipment that want the extra channels and network speed supported by 802.11a technology must upgrade to products that support the technology. Some products support dual-band operation, and it is important to note that 2.4-GHz and 5 GHz equipment can operate in the same physical environment without interference. Figure summarizes the features of the 802.11 WLAN standards, including frequency band, data rates, and throughput. The 802.11 b and 802.11 g ranges are based on default power settings with 2.2 dBi 2.4-GHz antennas on the access points and 0-dBi antennas on the clients. The 802.11a ranges are based on default power settings with a 5-dBi omnidirectional antenna on the access point and a 6-dBi omnidirectional antenna on the client. Figure compares the range of the different data rates and the different WLAN standards in an open-office environment. Actual distances can be different due to absorption and reflection. The size of a wireless cell depends on the data rate. It is possible to limit the range by disabling lower data rates. To limit the range to 150 feet, data rates of 5.5, 2, and 1 Mbps (802.1b/g) and 6, 9, 12, and 18 Mbps (802.11g) could be disabled. Figure shows the relative range of the different wireless standards and data rates. The absolute range depends on the environment, the equipment used, the access point configuration, the antenna, and the wireless client. The 802.11a, 802.11b, and 802.11g specifications all relate to WLAN physical layer standards. Cisco Aironet Access Points support the 802.11d standard for world mode. World mode enables the access point to inform an 802.11d client device which radio setting the device should use to conform to local regulations. The IEEE 802.11e standard, ratified in October 2005, was developed to enhance the current 802.11 MAC standard. The 802.11e standard expands support for applications with QoS requirements and improves the capabilities and efficiency of the WLAN datalink layer. This standard assists with voice, video, and other time-sensitive applications. The IEEE 802.11f standard is a recommended practice guideline that defines a protocol for intercommunication between access points. It assists in roaming and handoff of traffic. Most vendors have implemented their own proprietary Inter-Access Point Protocol (IAPP) for use with