802.11a
Even though the 802.11a standard was available in 1999, 802.11a access points and radio cards
did not become commercially available until several years later. The primary reasons for the delay
to market were the difficulties in developing 5-GHz 802.11 hardware and the weak market
802.11 Physical Layer Standards
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potential for wireless LAN components that do not interoperate with existing DSSS wireless
LANs. 802.11a products are available now, but their use is somewhat limited to specialized
applications, especially where high performance is necessary.
A significant advantage of 802.11a is that it offers very high capacity as compared to the other
physical layers. The reason is that the 802.11a 5-GHz spectrum defines 12 RF channels that do not
overlap in frequency. As a result, it is possible to have up to 12 802.11a access points set to
different channels and operating within the same room. This produces up to 12 separate radio cells,
each of which can support its own group of wireless users. Most indoor 802.11a access points
implement only eight of these RF channels, but that still provides a large potential capacity as
compared to 802.11b and 802.11g. If your wireless application needs very high performance, then
802.11a may be the best way to go.
Another advantage of 802.11a is that it operates in the 5-GHz band, which is mostly free from
sources of RF interference. Microwave ovens, Bluetooth devices, most cordless phones, and the
majority of neighboring wireless LANs operate in the 2.4-GHz band of frequencies. The lower
noise floor in the 5-GHz band affords lower retransmission rates and higher resulting throughput
as compared to 802.11b and 802.11g systems.
On the other hand, however, 802.11a has limited regulatory acceptance around the world. In fact,
some countries do not yet allow 802.11a networks. This could certainly impact the selection of the
802.11 physical layer (a, b, or g) for products that you want to use worldwide.
Also, relatively few user devices currently incorporate 802.11a interfaces. Most new laptops
today, for example, implement 802.11b or 802.11g, which do not interoperate with 802.11a access
points. 802.11a is not common. If it is not practical to use 802.11a radio cards in user devices, then
802.11a is probably not a good alternative for the wireless LAN.