802.11a, referred to in full as IEEE 802.11a, is the first wireless networking standard ever created. It was developed by the Institute of Electrical and Electronics Engineers LAN/MAN Standards Committee (IEEE 802) and ratified in 1999. 802.11a operates in 5 GHz band, which protects it from interference from external sources. It features a maximum raw data rate of 54 Mbps. If required, the data rate may be reduced to 48, 36, 24, 18, 12, 9 then 6 Mbps.
A 52-subcarrier coded orthogonal frequency-division multiplexing (COFDM) is used, providing a total bandwidth of 20 MHz and an occupied bandwidth of 16.6 MHz. COFDM is able to reduce multipath effects in reception and increase spectral efficiency. The COFDM used by 802.11a has a symbol duration of 4 microseconds and a guard interval (also known as a "cyclic prefix") of 0.8 microseconds.
Of the 52 COFDM sub-carriers, 48 are used for data transmission and the remaining 4 are pilot sub-carriers with a carrier separation of 0.3125 MHz. The sub-carriers may employ the following digital modulation schemes: binary phase-shift keying (BPSK) for 6 and 9 Mbps, quadrature phase-shift keying (QPSK) for 12 and 18 Mbps, 16-quadrature amplitude modulation (16-QAM) for 24 and 36 Mbps, or 64-QAM for 48 and 54 Mbps.
Orthogonal components are generated and decoded in baseband using digital signal processing (DSP). At the transmitter, the components are then upconverted to 5 GHz. Generation of the time domain signal is done by taking an Inverse Fast Fourier Transform (IFFT). The receiver will then downconvert, sample at 20 MHz before doing an FFT to retrieve the original coefficients.
The original version of 802.11a had 12 or 13 non-overlapping channels. Of these channels, 12 could be used indoor, and 4 or 5 may be used in outdoor point to point configurations. In order to add another 12 to 13 channels, a 5.47 to 5.725 GHz band may be a secondary user through a sharing method derived in 802.11h.
In comparison with two more popular 802.11 amendments, 802.11b and 802.11g, 802.11a held an advantage on bandwidth as it used a 5 GHz band while 802.11b and 802.11g used 2.4 GHz bands, which were susceptible to interference from microwave ovens, baby monitors and cordless telephones. It had more usable channels, about 4 to 8 times as many as that of 802.11b or 802.11g.
However, the high carrier frequency also has a disadvantage, since the effective overall ranges of 802.11b and 802.11g exceeded that of 802.11a, and their signals could penetrate further, since 802.11a signals were easily absorbed by walls and solid objects. There was also a disadvantage in marketing: since the 5 GHz components needed were harder to manufacture, 802.11a products experienced a delay in shipping, and they were also more expensive than 802.11b.
This shipping lag, as well as the cost comparison between the two amendments, led to 802.11b being more widely accepted among consumers, although Enterprise network environments sometimes make use of 802.11a.
Since they operate on separate bands, 802.11a and 802.11b are not interoperable. However, dual-band or dual-mode Access Points and Network Interface Cards (NIC), which are now widely available in markets, will allow interoperation between the 802.11a, b, and g.