The 802.11 Extensions - 802.11a

The 802.11a standard was approved in December 1999, right around the same time as 802.11b was approved. 802.11a is an extension to 802.11, which operates at speeds of up to 54−Mbps transmission rate (with a fallback to 48, 36, 24, 18, 12, and 6 Mbps) in the more recently allocated 5−GHz Unlicensed National Information Infrastructure (UNII) band.

802.11a uses an Orthogonal Frequency Division Multiplexing (OFDM) encoding scheme as its spread spectrum technology. 802.11a is to 802.11b networking what 100 Mbps was to the 10−Mbps Ethernet.

The acceptance of the 802.11a standard lagged behind the 802.11b because of the relative complexity of the standard and the cost of equipment that it incurs.

In addition, 802.11a networks are incompatible with the 802.11b networks due to the difference in the radio frequency band used by 802.11a (802.11b uses 2.4 GHz whereas 802.11a uses 5 GHz), and the speeds they operate at (802.11b has a maximum operating speed of 11 Mbps whereas 802.11a operates at up to 54 Mbps).

Enhancements Offered by 802.11a over 802.11

In the United States, 802.11a operates in three unlicensed radio frequencies in the 5−GHz radio band, instead of the 2.4−GHz frequency used by 802.11.

At the 2.4−GHz frequency, only three channels can be used simultaneously; 802.11a supports eight simultaneous channels, and full bandwidth is available within each channel. The additional channels mean that more users can share the same frequency.

  • 802.11a and Orthogonal Frequency Division Multiplexing (OFDM)

OFDM technique distributes the data to be transmitted into smaller pieces, which are simultaneously transmitted over multiple frequency channels that are spaced apart. This spacing provides the orthogonality that prevents the demodulators from seeing frequencies other than their own.

When transmitting data using OFDM, the data is first divided into frames and a mathematical algorithm known as Fast Fourier Transformation (FFT) is applied to the frame, then OFDM parameters (for example, timing) are added.

An Inverse Fast Fourier Transformation (IFFT) is then applied on each frame. The resulting frames are then transmitted over the designated frequencies.

A receiver performs the inverse operations to get the transmitted data by performing FFT on the frames. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multipath distortion.

  • 802.11a Applications

Currently, not many devices are available in the markets that comply with the 802.11a standard. With growing usage of 802.11b, 802.11a is slow to gain the market share that it deserves because implementation choices and vendor support were limited until this year.

Still, 802.11a is gaining acceptance in the enterprise market. Several large equipment vendors have announced 802.11a implementations. 802.11a is being compared with 802.11a like fast Ethernet is compared with Ethernet.

Because 802.11a operates in 5 GHz, it can coexist with 802.11b networks without causing any interference. 802.11a is being used to connect network backbones in small enterprise environments and the applications that require high bandwidth.

Enterprise users normally desire a higher level of reliability and speed than SoHo or home users do. 802.11 a is well suited for such scenarios. 802.11a operates at speeds up to 54 Mbps and is less vulnerable to the interference caused by devices competing for the bandwidth in the 2.4−GHz band.

  • 802.11a Interoperability and Compatibility with 802.11

The 802.11a−compliant devices are not directly compatible with the original 802.11 standard or the 802.11b extension. The primary reason is the RF band in which 802.11a operates.

The original 802.11 specification calls for devices that would operate in the 2.4−GHz ISM band, whereas 802.11a devices operate in the 5−GHz UNII band. This gives the 802.11a devices the freedom of operating in an RF band with a smaller number of devices.

In addition, 802.11a devices use OFDM as their spread spectrum technology versus FHSS or DSSS, which 802.11 originally mandated.

However, 802.11a uses the same MAC layer (CSMA/CA) as the original 802.11 specification recommended. The usage of the same MAC−level protocol makes 802.11a devices interoperable at the MAC layer with other 802.11 devices.