How to: Choose the Right Spectrum for 802.11n Deployments
by Jim Geier

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Most wireless LANs today operate in 2.4 GHz spectrum. Deployments started over a decade ago with the initial 802.11 standard and continued with 802.11b and then 802.11g as the most popular versions. This has made the choice of spectrum fairly easy when designing a wireless network because the majority of the versions already deployed operate in the 2.4 GHz band. 802.11a networks, which utilize 5 GHz frequencies, have been available for a number of years, but their deployments are fairly uncommon because they don’t interoperate with the more preferred and much larger installed base of 2.4 GHz client devices. This has left 2.4 GHz as the primary spectrum to deploy.

Today, with 802.11n beginning to proliferate, the decision to deploy 2.4 GHz or 5 GHz is much tougher. 802.11n supports both 2.4 GHz and 5 GHz clients. Also, in order to realize top-end performance with 802.11n, you must seriously consider deploying 5 GHz devices. As a result, 2.4 GHz is no longer the lone contender. 

When assessing the pros and cons of 2.4 GHz and 5 GHz systems, be sure to first define wireless network requirements. This provides a solid basis for defining all design elements. Without firm requirements, you’ll be making the choice on flimsy ground. 

Consider the following to choose the right spectrum for 802.11n deployments:

Geographical location. Before getting to far with an 802.11n deployment, think about the geographical location of the wireless LAN. 2.4 GHz spectrum has regulatory acceptance throughout most of the World; however, the use of 5 GHz for wireless LANs is somewhat limited. Thus, your location may require you to use only the 2.4 GHz band with 802.11n networks. This makes the choice of spectrum easier, but the 802.11n network will have limited performance. 

Performance. Compared to 2.4 GHz, the 5 GHz band has much greater spectrum available, which leads to significantly better performance as compared to the 2.4 GHz band. In fact, the use of 5 GHz devices is really the only way to achieve the highest performance from 802.11n networks, mainly because of the need to provide adequate bandwidth for 802.11n’s optional 40 MHz (rather than 20 MHz) channels. If the highest performance is an important requirement, then certainly lean toward the 5 GHz band. 

Existing client devices. In most scenarios, client devices with 802.11b/g radios will already exist, and it won’t likely be practical to replace all of those with 5 GHz radios. In fact, many client devices with embedded wireless interfaces won’t even have 5 GHz versions available yet. As a result, you’ll probably need to continue supporting 2.4 GHz operation, at least until it’s feasible to roll the legacy client devices over to 802.11n (which would be a good time to consider 5 GHz versions). 

Facility size. As frequency increases, range generally decreases. As a result, 5 GHz systems, based only on frequency, may have less range than ones operating in the 2.4 GHz band. This means that the selection of 5 GHz spectrum could require a greater number of access points, which results in higher costs. As a result, you may achieve cost benefits by deploying 2.4 GHz systems in larger facilities (unless high performance is critical). Keep in mind, however, that 5 GHz systems may have equal or even better range in some situations. The construction and shape of the facility may attenuate 2.4 GHz signals more than 5 GHz signals, which can give 5 GHz signals an edge over 2.4 GHz signals. Consequently, it’s best to conduct a wireless site survey to fully understand the behavior of radio frequency (RF) signals throughout the facility before choosing which spectrum to use. 

RF Interference. 2.4 GHz wireless LANs can experience RF interference from cordless phones, microwaves, and other existing wireless LANs. The interfering signals degrade the performance of a wireless network by periodically blocking users and access points from accessing the shared air medium. If it’s not possible to reduce potential interference in the 2.4 GHz band to an acceptable level, consider deploying a 5 GHz system. The noise floor in the 5 GHz band is generally lower compared to the 2.4 GHz band, which allows 802.11n to function at higher data rates. 

Hopefully the above tips will point you in the right direction. 2.4 GHz may be necessary at first to support existing 802.11b/g client devices, but also look toward possibly implementing 5 GHz initially or migrating to it in the future for higher performance applications. 

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