The initial 802.11 standard defines two forms of spread spectrum modulation for the physical layer: frequency hopping (802.11 FHSS) and direct sequence (802.11 DSSS). These two standards specify a 2.4GHz operating frequency with data rates of 1 and 2Mbps. Another initial physical layer utilizes infrared passive reflection techniques for transmission of data at 1 and 2Mbps; however, this standard has not been implemented in products.
In late 1999, the IEEE published two supplements to this 802.11 standard: 802.11a and 802.11b. The 802.11a standard defines operation at up to 54Mbps using orthogonal frequency division multiplexing (OFDM) modulation in the 5.8GHz frequency band. The IEEE 802.11b version of the standard is a data rate extension of the initial 802.11 DSSS, providing operation in the 2.4GHz band with additional data rates of 5.5 and 11Mbps.
Most companies implementing wireless LANs today are installing 802.11b-based systems. The 802.11 DSSS radios interoperate with 802.11b access points; however, the 802.11 FHSS radios do not.