Choose the Right Spectrum for 802.11n Deployments

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, consider 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 device

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 perform
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 (pun intended). 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.

Jim
Geier
provides independent consulting services and training to companies developing
and deploying wireless networks for enterprises and municipalities. He is the author of a
dozen books on wireless
topics.

Article courtesy of Wi-Fi Planet

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