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 802.11a: An Excellent Long Term Solution By Jim Geier
July 31, 2002
As 802.11a products began shipping months ago, more and more companies have been taking advantage of 802.11a's superior performance. 802.11a radios transmit at 5GHz and send data up to 54Mbps using OFDM (orthogonal frequency division multiplexing). The results have been very good. 802.11a products deliver excellent performance. Inside 802.11aBefore discussing benefits and implications of 802.11a, let's take a look at how 802.11a devices operate. 802.11a defines one of several different 802.11 Physical Layers (PHYs). The actual name of 802.11a is the "High Speed Physical Layer in the 5GHz band," commonly referred to as the "OFDM PHY." Another popular PHY of course is 802.11b, which most companies have been installing for the past couple years. Others include 802.11 FHSS (frequency hopping spread spectrum) and 802.11 IR (infrared). No matter which 802.11 PHY you deploy, the MAC (medium access control) Layer is the same. The MAC Layer manages and maintains communications between 802.11 radio NICs and access points by coordinating access to a shared radio channel. The MAC Layer is actually a program that runs on a processor; whereas, the PHY involves digital communications circuitry and an RF (radio frequency) modulator to prepare data for transmission over the air medium. The 802.11a PHY is quite different than 802.11b, which uses direct sequence spread spectrum (DSSS). 802.11a specifies the use of OFDM to support higher data rates. OFDM divides the data signal across 48 separate sub-carriers to provide transmissions of 6, 9, 12, 18, 24, 36, 48, or 54Mbps of which 6, 12, and 24Mbps are mandatory for all products. For each of the sub-carriers, OFDM uses PSK (phase shift keying) or QAM (quadrature amplitude modulation) to modulate the digital signal depending on the selected data rate of transmission. In addition, four pilot sub-carriers provide a reference to minimize frequency and phase shifts of the signal during transmission. This form of transmission enables OFDM to operate extremely efficiently, which leads to the higher data rates, and minimize the affects of multi-path propagation. The operating frequencies of 802.11a in the U.S. fall into the national information structure (U-NII) bands: 5.15-5.25GHz, 5.25-5.35GHz, and 5.725-5.825GHz. Within this spectrum, there are twelve, 20MHz channels, and each band has different output power limits. The Code of Federal Regulations, Title 47, Section 15.407, regulates these frequencies in the U.S. OFDM is becoming very popular for high speed transmission. In addition to being selected as the basis for the 802.11g PHY, OFDM is the basis for the European-based HiperLAN/2 wireless LAN standards. In fact the 802.11a PHY is very similar to the HiperLAN/2 PHY. In addition, OFDM has also been around for a while supporting the global standard for asymmetric digital subscriber line (ADSL).
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