2. Noise, Speed And Range

802.11a and 802.11g operate in different frequency bands - 802.11g is in the 2.4 GHz band and 802.11a in is in the 5 GHz band. As result, they are not interoperable. A laptop equipped with only 802.11g, for example, can't associate directly with an 802.11a access point.

A problem with the 2.4 GHz band is that it's fairly noisy, with microwave ovens and cordless phones raising the RF noise level to a point that can harm performance. In comparison, the 5 GHz band doesn't occupy many sources of RF interference, and the overall noise level is lower than in the 2.4 GHz band. Therefore, 802.11a is a good choice if typical RF interference sources will critically impact the performance of the application.

There also is an ongoing debate about the transmission ranges of 802.11a and 802.11g. Because of physics, higher frequency RF signals (5 GHz) don't transmit as far as lower frequency ones (2.4 GHz). This assumes, however, that everything else is equal, which is not the case with 802.11 wireless LANs. 802.11a, for instance, operates at a higher transmit power and in the presence of less noise than 802.11g. In practice, this enables 802.11a solutions to often have greater range than 802.11g.

An 802.11b-equipped laptop can interface directly with an 802.11g access point. This makes an 802.11g wireless LAN interoperable with a plethora of existing mobile devices equipped with 802.11g and 802.11b adapters. Unfortunately, this isn't the case with 802.11a wireless LANs, which has never been as widely adopted. Consequently, lean toward implementing 802.11g for public networks and enterprise solutions where there's little control over adapter cards put in mobile user devices.

802.11a has much higher total capacity due to a greater number of non-overlapping channels. In fact, current 802.11a access points have eight completely separate channels. Also, FCC rule changes should enable many more channels in the near future. 802.11g, however, has limits of only three non-overlapping channels in the same area. So, choose 802.11a if high capacity is critical, such as the case with VoWLAN solutions. Mobile phones, such as the Motorola CN620, are now beginning to include 802.11a interfaces.

Something to keep in mind when planning the performance of a wireless LAN is that 802.11g users encounter substantial decreases in throughput if just one 802.11b device associates with the same access point. The reason is that the presence of both 802.11b and 802.11g users requires that protection mechanisms be in use so that they don't inadvertently transmit at the same time (neither can hear each other), which increases the transmission of control frames prior to sending each data frame. Thus, 802.11g's backward compatibility with 802.11b comes with the price of limiting performance.

How To Decide

Before making a decision on which technology to employ, have a firm understanding of solution requirements, especially performance and the potential for RF interference. Without solid requirements, you'll be making a choice on flimsy ground, which you will likely regret in the future when users complain about dropped VoWLAN calls and sluggish file downloads.

In some cases, solution requirements warrant implementation of both 802.11a and 802.11g. Data can generally travel okay over an 802.11g network, and VoWLAN connections will work better with a dedicated 802.11a network because of higher capacity and less interference. Most wireless LAN vendors have access points that include both 802.11a and 802.11g, and these technologies can easily coexist with overlapping coverage. So if you want to satisfy all situations, deploy both 802.11a and 802.11g.

Jim Geier is the principal of Wireless-Nets, Ltd., a consulting firm focusing on the implementation of wireless mobile solutions.