This higher rate extension to the original 802.11 standard has enabled it to support a raw bit rate of 54 Mbps. To achieve this much higher speed, 802.11a differs markedly at the Physical Layer from 802.11 and 802.11b. It does, however, adhere to the same MAC Layer definitions for all 802.11 options.
802.11a uses neither infrared nor spread spectrum for encoding and transmitting data, but has introduced a totally different scheme to the standard, called orthogonal frequency division multiplexing (OFDM). OFDM accomplishes its high speed by dividing its transmission channel into 52 subcarriers. Each transmission channel is 20 MHz wide, and each subcarrier is 312.5 kHz wide. The subcarriers are transmitted in parallel resulting in the full transmission data rate. Since each subcarrier operates at a different frequency, a level of protection is provided against any noise or interference occurring in the channel. Each subcarrier utilizes a BPSK, QPSK, or QAM depending on the data rate. A 64 level QAM is used for the peak 64 Mbps rate and a 16 level QAM is used for other rates above 24 Mbps.
802.11a uses the 5.8 GHz Unlicensed National Information Infrastructure (U-NII) band. In the U.S., the FCC has allocated 300 MHz of bandwidth to this U-NII band versus the 83 MHz of bandwidth allocated to the 2.4 GHz band used by 802.11 and 802.11b. This increase in spectrum means that 802.11a has more channels available to it than does 802.11b. A total of 12 separate channels are available, although 8 are typically used, versus the 3 channels available to 802.11b. This increase in channels means that 802.11a is more scalable than 802.11b in that it can support more active devices. 802.11a has met with resistance in Europe where the 5 GHz band is used for HiperLAN. Another feature of the 802.11a Physical Layer is the use of FEC to guard against data loss, and minimize the need for retransmissions.
As with 802.11b, the distance of the station from the access point also affects the data rate. The raw data rate of 54 Mbps is valid for distances up to 25 feet. As the distance increases, the data rate decreases. Beyond 25 feet, 802.11a has fallback rates of 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12 Mbps, 9 Mbps, and 6 Mbps to a range of about 300 feet.
Although the raw data rate of transmission is 54 Mbps, the throughput is often less due to several issues. One is the CSMA/CA access control, which requires stations to wait a random amount of time before determining that the medium is free to transmit. The second is the Request to Send/Clear to Send (RTS/ CTS) sequence that is used to overcome the hidden node syndrome. A third impact results from the transmission of a preamble and start of frame delimiter, followed by a PLCP header indicating signal, service, length, and header error check. These are all transmitted at 6 Mbps regardless of the actual transmission rate for the payload. This PLCP header and preamble mean that for a 1500 octet frame it is at best 85 percent efficient and even less efficient for smaller frames.
IEEE 802.11a was slow to come to market, although it was approved at the same time as IEEE 802.11b. The introduction of IEEE 802.11g has cast this specification in doubt and there is relatively little support in the market for it.
|| IEEE 802.11a|