Explaining 802.11a
 
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While there are strong similarities between 802.11a and 802.11b, there are also important differences. First, there's confusion about why 802.11b products came before 802.11a. Confounding the problem is that 802.11g is on the horizon. Like most oddities in computing, you need some history and a score card to keep the versions straight (with even more 802.11x versions coming soon).
The original purpose of the IEEE's 802.11 committee was to research metropolitan-area networking (MAN). While MANs have been around for years in various forms, the considerations of the committee turned toward wireless distribution methodologies. The high data rates needed to make wireless networks a viable option requires spectrum - an allocation of frequencies that can be used. The power needs to be low enough to prevent interference with other users of the frequencies, but strong enough to allow consistent data rates at speed.
When the frequencies are allocated, there must also be considerations made for users with different applications on the same frequencies. The 802.11b products arrived first because of several reasons, not the least of which was the near worldwide acceptance of the 2.4GHz range for low power, usually license-free operation. Cordless phones use this spectra, and so do microwave ovens, HomeRF and Bluetooth devices. It's a potentially dirty spectrum in some ways, but there are often configuration methods that let the 802.11b products be modified to find empty transmission space if and when interference is detected.
There are three nonoverlapping channels that can be used in 802.11b. This means placing transceivers/access points in ways where they won't have interference with one another. Eight such channels are available for 802.11a access points and the network geographies that they can serve. This means that the A Team has a better chance of covering geography than the current B Team. However, a heretofore-unused specification, 802.11g, proposes to add more channels and therefore more potential bandwidth and a potential reduction in overlapping geography. The 802.11g data rates are said to be the same as 802.11a. Products might ship using 802.11g this year.
After study and regulatory approval, the 5.25GHz to 5.45GHz and 5.725GHz to 5.825 GHz ranges were deemed usable, and 802.11a production began. This is a huge amount of spectrum - more than most broadcast television channel spectrum combined.
By design, there are eight possible channels in the band that can be used; broadcasting on these channels is unidirectional. The current crop of 802.11a de-vices are approved in North America, and limitations are imposed on them in Japan.
Soon, with regulatory approval, European versions and other areas will soon be approved for the frequency range that 802.11a needs. Receiving and transmitting on 802.11a (and b) is performed by contention in a similar way to Ethernet.
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Henderson and Ritchey are researchers at ExtremeLabs. They can be reached at thenderson@extremelabs.com and tritchey@extremelabs.com.
Henderson is also a member of the Network World Global Test Alliance, a cooperative of the premier reviewers in the network industry, each bringing to bear years of practical experience on every review. For more Test Alliance information, including what it takes to become a member, go to www.nwfusion.com/alliance.
