802.11n: Lessons Learned - Part 1

Since early 2008, Xirrus has deployed over 1,000 802.11n Wi-Fi networks for universities, K-12
school districts, enterprises, hospitals, conventions centers, and other customers. During the
design and implementation of these networks, Xirrus has gained a wealth of experience and
knowledge in 802.11n technology and what it takes to successfully install and operate high
performance, resilient 802.11n networks.
This White Paper outlines key lessons Xirrus has learned from our 802.11n deployment
experience. Every network is different, but as our experience has shown, following several key
guidelines will help optimize new 802.11n networks to achieve maximum performance and
robustness towards the ultimate goal of using wireless to replace wired networks.
Network Design
Proper planning is crucial prior to deploying 802.11n networks. Appropriate planning is a good
idea with any 802.11 network, but it is especially important with new 802.11n networks because:
1. 802.11n will be used more often as the primary network connection compared with
2. 802.11n has more flexibility and configuration options than legacy Wi-Fi, so network
designs must take into account end-user needs in order to optimize performance and
There are five key parameters to look at for a proper 802.11n network design, as reviewed in the
sections below.
Site Survey
Executing an active site survey prior to deploying Wi-Fi equipment (whether a Xirrus Array or
other AP) is important because it lets the network administrator know exactly where equipment
needs to be placed prior to deploying. With an active site survey, real equipment is taken on site
and used to determine the best placement prior to pulling cables and drilling walls for the actual
installation. Rather than doing an active site survey some Wi-Fi vendors and network designers
merely guess at the location of APs by looking at floor plans and assuming that the RF
propagation of all buildings is the same. Active site surveys are always important with Wi-Fi, but
they are especially important with 802.11n networks for the following reasons.
D-Link DIR-655 Extreme N Wireless RouterMIMO
To improve throughput, 802.11n networks use Multiple Input Multiple Output (MIMO). MIMO
increases overall throughput by using multiple antennas and signals to send the traffic. With
MIMO, rather than having a single high-powered stream carrying the data traffic, there are 2 or 3
lower powered streams carrying the data traffic. In general, MIMO increases network and station
performance, however because of MIMO 802.11n penetration, characteristics are more
dependent on the environment; MIMO can be affected by wall and other objects differently than
standard transmission techniques. Before deploying an 802.11n network, it is imperative to see
how 802.11n radios will behave in the current environment and not assume that the RF
propagation will be the same as existing 802.11abg networks.

 Lesson Learned: Because of MIMO, 802.11n RF propagation can be
significantly different than with traditional 802.11abg networks. An active site
survey should always be done to tests the RF characteristics of the
environment prior to deployment.
Dual-band Coverage
A discussed in the RF Design section later in this White Paper, operating 802.11n in the 5GHz
band is a key requirement to fully realizing the benefits of 802.11n. When doing a site survey, it is
imperative that readings be taken for both 2.4GHz and 5GHz. In most environments, 2.4GHz will
propagate further than 5GHz. If 5GHz is to be used in 802.11n networks (as is highly
recommended), the survey should take readings in both bands and both should be seen from all
areas to be covered.
The diagrams below show the results of a standard survey for 802.11n. As can be seen, the
coverage characteristics of 2.4GHz and 5GHz are different, with 2.4GHz providing coverage
where 5GHz does not. Adjusting equipment location or adding additional equipment may be
necessary to provide full 5GHz band coverage, and hence realize the benefits of 802.11n’s high

2.4GHz Coverage 5GHz Coverage

 Lesson Learned: When doing site surveys, look at both 5GHz and 2.4GHz
bands. 802.11n can operate in both bands and to fully realize its benefits, both
bands should be supported throughout the entire network.

Multiple Radios
Another consideration when doing a site survey is ensuring that multiple radios are available at a
sufficient RSSI level (Xirrus recommends -72dBm or greater for most applications) from every
area to be covered by the Wi-Fi network. For a resilient, dynamic connection there should be
multiple radios from which a station can choose in case one of the radios is heavily utilized or if
one of the radios goes down. The table below shows the signal readings from a sight survey of
an actual Xirrus customer survey. As one can see, multiple 2GHz and 5GHz channels can be
seen with strong signal coverage (-72dBm or higher).

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