IEEE 802.11a

The second flavor of Wi-Fi is the wireless network known officially as IEEE 802.11a. 802.11a (also referred to as Wireless-A) uses the 5 GHz frequency band, which allows for much higher speeds (up to 54 Mb/s) and helps avoid interference from devices that cause interference with lower-frequency 802.11b networks. Although real-world 802.11a hardware seldom, if ever, reaches that speed (almost five times that of 802.11b), 802.11a relatively maintains its speeds at both short and long distances.

For example, in a typical office floor layout, the real-world throughput (always slower than the rated speed due to security and signaling overhead) of a typical 802.11b device at 100 feet might drop to about 5 Mb/s, whereas a typical 802.11a device at the same distance could have a throughput of around 15 Mb/s. At a distance of about 50 feet, 802.11a real-world throughput can be four times faster than 802.11b. 802.11a has a shorter maximum distance than 802.11b (approximately 75 feet indoors), but you get your data much more quickly.

Given the difference in throughput (especially at long distances), and if we take the existence of 802.11g out of the equation for a moment, why not skip 802.11b altogether? In a single word: frequency. By using the 5 GHz frequency instead of the 2.4 GHz frequency used by 802.11b/g, standard 802.11a hardware cuts itself off from the already vast 802.11b/g universe, including the growing number of public and semipublic 802.11b/g wireless Internet connections (called hot spots) showing up in cafes, airports, hotels, and business campuses.

The current solution for maximum flexibility is to use dual-band hardware. Dual-band hardware can work with either 802.11a or 802.11b/g networks, enabling you to move from an 802.11b/g wireless network at home or at Starbucks to a faster 802.11a office network.

IEEE 802.11b

IEEE 802.11b (Wi-Fi, 2.4 GHz band–compliant, also known as Wireless-B) wireless networks run at a maximum speed of 11 Mb/s, about the same as 10BASE-T Ethernet (the original version of IEEE 802.11 supported data rates up to 2 Mb/s only). 802.11b networks can connect to conventional Ethernet networks or be used as independent networks, similar to other wireless networks. Wireless networks running 802.11b hardware use the same 2.4 GHz spectrum that many portable phones, wireless speakers, security devices, microwave ovens, and the Bluetooth short-range networking products use. Although the increasing use of these products is a potential source of interference, the short range of wireless networks (indoor ranges up to approximately 150 feet and outdoor ranges up to about 300 feet, varying by product) minimizes the practical risks. Many devices use a spread-spectrum method of connecting with other products to minimize potential interference.

Although 802.11b supports a maximum speed of 11 Mb/s, that top speed is seldom reached in practice, and speed varies by distance. Most 802.11b hardware is designed to run at four speeds, using one of four data-encoding methods, depending on the speed range:

• 11 Mb/s—Uses quaternary phase-shift keying/complementary code keying (QPSK/CCK)
• 5.5 Mb/s—Also uses quaternary phase-shift keying/complementary code keying (QPSK/CCK)
• 2 Mb/s—Uses differential quaternary phase-shift keying (DQPSK)
• 1 Mb/s—Uses differential binary phase-shift keying (DBPSK)

As distances change and signal strength increases or decreases, 802.11b hardware switches to the most suitable data-encoding method. The overhead required to track and change signaling methods, along with the additional overhead required when security features are enabled, helps explain why wireless hardware throughput is consistently lower than the rated speed. The figure below is a simplified diagram showing how speed is reduced with distance. Figures given are for best-case situations; building design and antenna positioning can also reduce speed and signal strength, even at relatively short distances.

**Source by wikipedia **

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