5GHz (802.11a)
The IEEE ratified the 802.11a standard in 1999, but the first 802.11a products didn’t begin
appearing on the market until late 2001—and, boy, were they pricey! The 802.11a standard
delivers a maximum data rate of 54Mbps with 12 nonoverlapping frequency channels.
Figure 9.4 shows the UNII bands.
FIGURE 9 . 4
The UNII 5GHz band has 12 nonoverlapping channels (in the United States).
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2.483GHz
Channels
2.402GHz 22MHz
161
5.15
Lower band
5.15–5.25
indoor
Upper band
5.725–5.825
outdoor
Middle band
5.25–5.35
indoor and outdoor 5.825
Channel center
frequencies
5.180 5.200 5.220 5.240 5.260 5.280 5.300 5.320 5.745 5.765 5.785 5.805
Operating
channels
36 40 44 48 52 56 60 64 149 153 157
Operating in the 5GHz radio band, 802.11a is also immune to interference from devices
that operate in the 2.4GHz band, such as microwave ovens, cordless phones, and Bluetooth
devices. 802.11a isn’t backward compatible with 802.11b because they are different frequencies,
so you don’t get to just “upgrade” part of your network and expect everything to work
together in perfect harmony. But no worries—plenty of dual-radio devices will work in both
types of networks. A definite plus for 802.11a is that it can work in the same physical environment
without interference from 802.11b users.
Similar to the 802.11b radios, all 802.11a products also have the ability to data-rate shift
while moving. The 802.11a products allow the person operating at 54Mbps to shift to
48Mbps, 36Mbps, 24Mbps, 18Mbps, 12Mbps, and 9Mbps and finally still communicate farthest
from the AP at 6Mbps. Roaming capabilities are also supported as long as all access
points are configured with the same SSID.