Open Communications Principles and Best Practices
00.00.2007 I 9
3.0 Migrating to 802.11n
The path to 802.11n can be easy and economical, but poor planning can result in less than satisfactory
performance for both your new 802.11n WLAN and your legacy 802.11a/b/g WLAN. One of the tough-
est decisions facing the CIO is whether it is the right time to consider an 802.11n deployment.
3.1 Do You Need 802.11n?
Everyone wants more bandwidth, better reliability, more capacity and lower latency for their wireless
network. Sensitive applications like VoWLAN are extremely susceptible to low bandwidth and high
latency. However, the reality is that the vast majority of today s WLANs are under-utilized typical
Radio Frequency (RF) utilization is less than 20%. If you are considering using WLAN for simple data
applications, then perhaps you can wait. If however, you need to accommodate high-bandwidth wire-
less data applications such as real-time medical imagery, or mobile PLC for manufacturing; or if you are
currently in the process of evaluating and deploying an enterprise wide WLAN and do not want the
expense of another upgrade cycle, then moving to 802.11n may be the right move.
3.2 Plan your Radio Spectrum
One of the first issues to consider is radio spectrum and transmission frequency. The 2.4 GHz spectrum
is congested. It supports 802.11b/g Access Points, Bluetooth , cordless phones, microwave ovens,
industrial, medical, and scientific (ISM) equipment, and other licensed systems. The 5 GHz spectrum is
less congested, but it has issues of its own. First, there are licensing issues in some countries that have
to be considered. Another issue when selecting a spectrum is what type of legacy 802.11 network is
currently in place. Usually it is 802.11 b/g running at 2.4 GHz. When deploying your new WLAN, you
need to consider whether you want the 802.11n APs to support the legacy WLAN, or to operate inde-
pendently.
To properly deploy 802.11n and take advantage of the wide channels (40 MHz) for improved data
throughput, you need to channel bond. This could prove difficult for 2.4 GHz environments as there
are only three non-overlapping channels to begin with, and bonding two of them would leave only one
available non-overlapping channel. An 802.11n AP running 2.4 GHz and trying to use wide channels
will surely cause disruption with existing legacy devices. 802.11n Draft 2 provides mechanisms to
ensure backward compatibility and prevent co-channel interference. However, these mechanisms
would prevent the establishment of 40 MHz wide channels when operating at 2.4 GHz and only allow
20 MHz channels (e.g. 1, 6, 11 in North America). This would defeat the major purpose of deploying
802.11n by reducing its bandwidth significantly. The 5 GHz spectrum provides many non-overlapping
channels (23 in North America). An 802.11n AP running 5 GHz could dynamically change channels
until two adjacent channels are clear to allow channel bonding while allowing coexistence with
802.11a and 802.11b/g APs and clients alike.
3.3 Suggested Migration Planning
1. You should seriously consider deploying 802.11n now if you are in the process of evaluating and
deploying WLAN for the first time, upgrading to a controller based system from a fat AP solution,
or significantly expanding your WLAN coverage area. You should also consider deploying 802.11n
if you have reliability issues with your existing WLAN or your current or near future applications
require more bandwidth.
2. It is highly recommended to deploy concurrent 2.4GHz and 5 GHz 802.11n APs, where the Access
Point supports both 2.4 GHz and 5 GHz 802.11n simultaneously. Chances are that wireless clients
will not all be upgraded to 802.11n at the same time. This makes it likely that by the end of de-
ployment, the WLAN will need to support both legacy (802.11a/bg) and 802.11n clients for both
2.4 GHz and 5 GHz. This is especially true when you are overlaying 802.11n onto a legacy 2.4 GHz
network. For 802.11n client purchases, it is best to purchase dual mode clients (2.4 and 5 GHz), or
