11A WAS RATIFIED IN 1999 AND OPERATES IN THE 5-GHZ FREQUENCY RANGE...

802.11a was ratified in 1999 and operates in the 5-GHz frequency range. This makes it in-

compatible with 802.11, 802.11b, and 802.11g, while avoiding interference from these de-

vices in addition to microwaves, Bluetooth devices, and cordless phones. 802.11a had

late-market adoption, so it is not as widely deployed as the 802.11b and g protocols.

Another difference is that 802.11a supports anywhere from 12 to 23 nonoverlapping chan-

nels as opposed to the 3 nonoverlapping channels in 802.11b/g. Because OFDM is used,

subchannels can overlap. 802.11a requires that the data rates of 6, 12, and 24 Mbps be

supported but allows for data rates up to 54 Mbps.

Table 6-5 shows some details on the 802.11a standard.

Table 6-5 The 802.11a Protocol

Key Topic

Ratified 1999

RF Technology OFDM

Frequency Spectrum 5.0 GHz

Coding Convolution Coding

Modulation BPSK, QPSK, 16-QAM, 64-QAM depending on the subcarrier.

Data Rates 6, 9, 12, 18, 24, 36, 48, 54 Mbps with OFDM

Nonoverlapping Channels Each band has a 4; the middle 8 are used with 52 subcarriers on

each channel.

*Convolution coding is a form of error correction in which redundant information analogous

to a parity bit in a file system is added to the data. The error correction is calculated across all

the subcarriers, so if narrowband interference corrupts data on one subcarrier, the receiver

can reconstruct that data using the convolution coding on another subcarrier.

The rules under ETSI specifications are a little different. ETSI allows 19 channels and re-

quires that dynamic frequency control (DFC) and transmit power control (TPC) be used.

What makes 802.11a unique is the way the 5-GHz frequency band is divided into multiple

parts. These parts, the Unlicensed National Information Infrastructure (UNII), were de-

signed for different uses. UNII-1 was designed for indoor use with a permanent antenna.

UNII-2 was designed for indoor or outdoor use with an external antenna, and UNII-3 was

designed for outdoor bridges and external antennas.

The FCC revised the use of the frequency in 2004 by adding channels and requiring com-

pliance of DFC and TPC to avoid radar. The revision also allows all three parts of the UNII

to be used indoors. This is not the case with ETSI, however, because it does not allow un-

licensed use of UNII-3.

Table 6-6 shows the frequency ranges of each of the UNII bands.

Table 6-6 The UNII Frequency Bands

Band Frequency Use

UNII-1 5.15–5.25 GHz (UNII Indoor) FCC allows indoor and outdoor use.

UNII-2 5.25–5.35 GHz (UNII Low) Outdoor/indoor with DFC and TPC

UNII-3 5.725–5.825 GHz (U-NII/ISM) FCC allows indoor and outdoor use.

ETSI does not allow unlicensed use.

In the 802.11a spectrum, the higher-band channels are 30 MHz apart. This includes UNII-

2 and above. The lower bands are 20 MHz apart.