What is 802.11a?
802.11a is a high speed wireless LAN physical layer standard in the 5 GHz Unlicensed National Information Infrastructure (U-NII) band. 802.11a was almost simultaneously launched with 802.11b, in late 1999. It also provides higher data rates than 802.11b, upto 54 Mbps.

About the technology
The 802.11a standard uses Orthogonal Frequency Division Modulation (OFDM) at the PHY layer to provide higher data rates. OFDM has excellent resistance to multipath fading, while also providing high data speeds. Each channel has the same bandwidth as in the 802.11b standard, 20 MHz. The overall system bandwidth is 300 MHz, which is divided into three 100 MHz bands, each with its own permissible power levels. Each 20 MHz channel contains 52 subcarriers of 300 kHz each. The standard specifies variable data rates of 6, 9, 12, 18, 24, 36, 48 and 54 Mbps, of which 6, 12, and 24 Mbps are mandatory. To allow variable data rates, the modulation technique can also be varied among BPSK, QPSK, 16-QAM and 64-QAM, coupled with variable coding rates of 1/2, 2/3 and 3/4.
The MAC layer in this standard, however, is the same employed by 802.11b, Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) with RTS and CTS signals.

802.11a in the market
The 5 GHz U-NII band is relatively clear of other interfering sources, which is not the case in the 2.4 GHz band. The 2.4 GHz band has Bluetooth^TM, personal cordless phones and a host of other interfering sources which greatly reduce the environments and the range of operation of 802.11b networks. This however is not the case in 802.11a. Currently all those places which have not implemented wireless LAN (or 802.11b) can look to 802.11a as their solution as it provides higher speeds with less interference.