802.11a Benefits and Implications - Page 2

By Jim Geier

July 31, 2002

802.11a Benefits and Implications

The following are benefits of 802.11a:

  • Higher performance. By far the top reason for choosing 802.11a is the need to support higher end applications involving video, voice, and the transmission of large images and files. In addition, 802.11a does a superior job of supporting densely populated areas of users having lower bandwidth needs, such as surfing the Internet. 802.11a can deliver data rates up to 54Mbps and there's enough room in the 5GHz spectrum to support up to 12 access points operating in the same area without causing interference between access points. This equates to 432Mbps (12 X 54Mbps) total data rate performance. Even the upcoming 802.11g standard, which will deliver 54Mbps data rates in the 2.4GHz band doesn't come close to the performance of 802.11a. With 802.11g, the same problem exists as with 802.11b: You have only three non-overlapping channels for setting access point frequencies, which severely limits capacity.
  • Less RF interference. The growing use of 2.4GHz cordless phones and Bluetooth devices is crowding the radio spectrum within many facilities. This significantly decreases the performance of 802.11b wireless LANs. Cordless phones wreak enough havoc to cause companies to either ban the use of the phones or not install wireless LANs. The use of 802.11a operating in the relatively un-crowded 5GHz band avoids this interference. Of course non-802.11 devices will eventually occupy the 5GHz band as well; however, there's much more room with 12 non-overlapping channels to limit interference with the other devices.

The following are drawbacks of 802.11a:

  • Less range. The superior performance of 802.11a offers excellent support for bandwidth hungry applications, but the higher operating frequency equates to relatively shorter range. Even with this limitation, however, 802.11a can sometimes deliver better performance than 802.11b at similar ranges from the access point. For example at ranges of 100 feet, 802.11a may deliver 24Mbps, but 802.11b devices at the same range are operating at 5.5Mbps.

    If you're planning to deploy 802.11b networks for 11Mbps throughout the facility, it's very likely that you can install 802.11 access points at the same locations and still achieve 6 to 12Mbps data rates. As a result, you can install approximately the same number of 802.11a access points as 802.11b and likely have similar performance. When needs for higher performance occur in the future, you can add more access points to increase the coverage to 54Mbps throughout the facility. This approach enables you to grow into a longer term, higher performing solution while spreading the costs over time.

  • Limited interoperability. 802.11a doesn't talk to 802.11b. For example, an end user equipped with an 802.11a NIC will not be able to connect with an 802.11b access point. The 802.11 standard offers no provisions for interoperability between the different physical layers. The solution to this problem is multimode radio cards that support multiple 802.11 PHYs, such as 802.11a/b, 802.11a/g, etc. These cards should be available on the market by the end of 2002. As a result, an 802.11a/b radio within an end user device will automatically sense whether the access point is 802.11a or 802.11b and then communicate accordingly.

    Likewise, an access point can also deploy a dual 802.11a/b solution, enabling interoperability with end user devices equipped with either an 802.11a or 802.11b radio. In the meantime, however, you can still install 802.11a wireless LANs. This assumes, however, that you're able to implement 802.11a radios in the user devices. Some devices today, such as bar code scanners, come equipped with 802.11b cards that you can't easily change.

  • Higher prices. The current list prices of 802.11a products are approximately 30 percent higher than 802.11b, but the price gap should close over the next couple years. The higher price today, nevertheless, causes some companies to install 802.11b in order to lower initial costs. The problem is that the primary migration path for these companies to deliver higher data rates in the future will be to upgrade their 802.11b access points to 802.11g. It's not clear when 802.11g products will be available, though, because the standard still requires major work before IEEE ratification and FCC approval takes place. 802.11a is available today and operates in a much less crowded part of the spectrum that includes higher capacity. 802.11a is clearly a better long term solution, especially when future performance needs are not very well known. It's better to pay a little more now for a better solution rather than a lot more later to replace hardware.

Of course the your decision on which 802.11 PHY to support depends on requirements of your specific wireless LAN application. Based on the benefits, I highly recommend using 802.11a unless requirements dictate otherwise. It's always better to have too much performance rather than not enough, especially for large numbers of users and higher end applications.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs.

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