By Jim Geier

There`s no doubt that the most widely available and implemented wireless LANs today comply with the 802.11b standard. The recent availability of 802.11a radio cards and access points, however, introduces what is becoming another mainstream wireless LAN solution. If you`re planning the deployment of a wireless LAN, you now face making a decision on which one to use: 802.11a or 802.11b.

802.11a vs. 802.11b

As you probably know, 802.11a and 802.11b each define a different physical layer. 802.11b radios transmit at 2.4 GHz and send data up to 11 Mbps using direct sequence spread spectrum modulation; whereas, 802.11a radios transmit at 5 GHz and send data up to 54 Mbps using OFDM (Orthogonal Frequency Division Multiplexing).

Of course the superior performance of 802.11a offers excellent support for bandwidth hungry applications, but the higher operating frequency equates to relatively shorter range. I`ve seen demonstrations of 802.11a radios delivering 54 Mbps with distances of about 60 feet, which is far less than the 300 feet or so that you`ll have with 802.11b systems. As compared to 802.11b, you`ll need a much larger number of 802.11a access points to cover a facility, especially large ones.

The different radio frequency and modulation types of 802.11a and 802.11b causes them to not interoperate. For example, an end user equipped with an 802.11a radio card 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.

Decision guidelines

When making the decision of whether to go with 802.11a or 802.11b, think about the performance, range, and interoperability issues that we discussed above. Also, here are some general guidelines that will help you make the right decision:

Consider using 802.11b if:

Range requirements are significant. For larger facilities, such as a warehouse or department store, 802.11b will provide the least costly solution because of fewer access points.

You already have a large investment in 802.11b devices. The relatively high costs associated with migrating from a large-scale 802.11b system to 802.11a will be difficult to sell to the company`s financial decision makers.

End users are sparsely populated. If there are relatively few end users that need to roam throughout the entire facility, then 802.11b will likely meet performance requirements because there are fewer end users competing for each access point`s total throughput. Unless there are significant needs for very high performance per end user, then 802.11a would probably be overkill in this situation.

Consider using 802.11a if:

There`s need for much higher performance. By far the top driver for choosing 802.11a is the need to support higher end applications involving video, voice, and the transmission of large images and files. For these applications, 802.11b probably won`t be able to keep up.

Significant RF interference is present within the 2.4 GHz band. The growing use of 2.4 GHz wireless phones and Bluetooth devices could crowd the radio spectrum within your facility and significantly decrease the performance of 802.11b wireless LANs. The use of 802.11a operating in the 5 GHz band will avoid this interference.

End users are densely populated. Places such as computer labs, airports, and convention centers need to support lots of end users in a common area competing for the same access point, with each user sharing the total throughput. The use of 802.11a will handle a higher concentration of end users by offering greater total throughput.

Potential interoperability improvements

Something to keep in mind is that the interoperability among 802.11a and 802.11b will considerably improve over the next year. For example, Synad, an engineering company based in London, recently announced their development of a dual 802.11a/b chipset. This will enable product developers to deliver wireless LAN radios that talk both 802.11a and 802.11b.

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 the dual 802.11a/b solution, enabling interoperability with end user devices equipped with either an 802.11a or 802.11b radio. With this in mind, maybe your decision will be to deploy both 802.11a and 802.11b!