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180 Chapter 4 • Communication Security: Wireless

IEEE 802.11a

In spite of its nomenclature, IEEE 802.11a is a more recent standard than 802.11b.
This standard defines wireless networks that use the 5 GHz UNII bands. 802.11a
supports much higher rates of data transmission than 802.11b.These rates are 6, 9,
12, 16, 18, 24, 36, 48, and 54 Mbps, although higher rates are possible using propri-
etary technology and a technique known as rate doubling. Unlike 802.11b, 802.11a
does not use spread spectrum and Quadrature Phase Shift Keying (QPSK) as a
modulation technique at the physical layer. Instead it uses a modulation technique
known as Orthogonal Frequency Division Multiplexing (OFDM).To be 802.11a
compliant, devices are only required to support data rates of 6, 12, and 24 Mbps—
the standard does not require the use of other data rates.Although identical to
802.11b at the MAC layer, 802.11a is not backward compatible with 802.11b
because of the use of a different frequency band and the use of OFDM at the PHY
layer, although some vendors are providing solutions to bridge the two standards at
the AP. However, both 802.11a and 802.11b devices can be easily co-located
because their frequencies will not interfere with each other, providing a technically
easy but relatively expensive migration to a pure 802.11a network.At the time of
this writing, 802.11a-compliant devices are becoming more common, and the
prices for them are falling quickly. However, even if the prices for 802.11b and
802.11a devices were identical, 802.11a would require more APs and be more
expensive than an 802.11b network to achieve the highest possible rates of data
transmission, because the higher frequency 5 GHz waves attenuate more quickly
over distance.

IEEE 802.11g

To provide both higher data rates (up to 54 Mbps) in the ISM 2.4 GHz bands and
backward compatibility with 802.11b, the IEEE 802.11g Task Group members
along with wireless vendors are working on the 802.11g standard specifications.
802.11g has been approved as a standard, but the specifications for the standard are
still in draft form and are due for completion in late 2002.To achieve the higher
rates of transmission, 802.11g devices use OFDM in contrast to QPSK, which is
used by 802.11b devices as a modulation technique. However, 802.11g devices are
able to automatically switch to QPSK to communicate with 802.11b devices.At
the time of this writing, there are no 802.11g devices on the market, although
Cisco has announced that its 802.11g-compliant Aironet 1200 will be available in
2003. 802.11g appears to have advantages over 802.11a in terms of providing back-
ward compatibility with 802.11b; however, migrating to and co-existence with

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