IEEE 802.11a, b and g

As of this writing, the IEEE 802.11 standard has evolved into 3 complementary recommendations, called A, B and G.

802.11A
IEEE 802.11a devices use a different radio technology from 802.11b and operate in the 5 GHz bands. IEEE 802.11a therefore is a supplement to the basic IEEE 802.11 standard.
Although the IEEE 802.11a standard operates in a different unlicensed radio band, it shares the same proven Medium Access Controller (MAC) protocol as Wi-Fi. In more technical terms, IEEE 802.11a standardizes a different physical layer (PHY). Since products conforming to the IEEE 802.11a standard will operate in different radio bands, they will not be interoperable with Wi-Fi radios, which follow the b-recommendation (see below).

802.11B
802.11b contains some further definitions of the physical layer, and provides for interoperability of Wi-Fi™ WLAN products. Wi-Fi products operate in the worldwide 2.4 GHz Industry, Science, and Medicine (ISM) band.


802.11G
As of this writing, the IEEE 802.11g recommendation has been accepted, but not implemented. An example, using Intersilดs idea OFDM (Orthogonal Frequency Division Multiplexing) is a compulsory part of IEEE 802.11g and provides for transmission speeds up to 54 Mbit/sec. It would be compatible with WiFi. It also supports CCK (Complementary Code Keying) in order to be compatible with existing radio units that adhere to IEEE 802.11b.
The CCK transmission mode, also used by WiFi, uses one single carrier, while OFDM is a new technique, just entering the WLAN-market. It can be used both at 2.4 and 5 GHz carrier frequencies.
OFDM is quite interesting. Different blocks of the same data transmission is divided between sub-carriers, thus enhancing receptivity also in environment having strong signal distorsion. It also has greater transmission capacity than CCK.