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IEEE 802.11a Orthogonal Frequency Division Multiplexing (OFDM)

The IEEE 802.11a standard specifies a modulation that divides a high-speed serial information signal into multiple lower-speed "subsignals" that the system transmits simultaneously at different frequencies in parallel over a 20MHz channel located in the 5GHz frequency band. This parallel form of transmission over multiple subcarriers enables OFDM-based wireless LANs to operate at higher aggregate data rates. 802.11a has a wide variety of high-speed data rates available: 6, 9, 12, 18, 24, 36, 48, and 54Mbps. The 6, 12, and 24Mbps rates are mandatory for all products.

IEEE 802.11a OFDM provides the following advantages (in comparison to DSSS and FHSS):

  • Greater tolerance of multipath distortion because of OFDM operation

  • Less potential for RF interference because of operation in the less crowded 5GHz band

  • Highest potential data rates from individual physical layers

  • Highest aggregate throughput using collocated access points

IEEE 802.11's OFDM provides the following disadvantages (in comparison to DSSS and FHSS):

  • Less range because of higher operating frequency

  • Unavailability of radios and access points until late 2001

If you're implementing a wireless LAN today, 802.11b is the standard you should first consider for the solution. All primary wireless LAN vendors offer products based on 802.11b, and they satisfy most networking requirements similar to 10 Mbps Ethernet. If requirements for a wireless LAN exceed the performance limitations of 802.11b's 11 Mbps data rates, however, then you may want to consider waiting for the availability of the newer 22 Mbps DSSS products or the even higher data rate 802.11a models. The solution set for wireless LANs has certainly gotten a lot better since the days of 1 and 2 Mbps offerings.

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