industrial wireless book special edition - Networking ...

I n d u s t r i a l W i r e l e s s
IEEE802.11n WLAN – lessons
learned from 1000 installations
The introduction of the wireless IEEE802.11n standard is an opportunity to provide Ethernet capability
without costly and disruptive installation of Cat5 cable. However, 802.11n offers its own challenges – by
following the guidelines in this article, taken from a Xirrus white paper, 802.11n networks can achieve
similar performance to 100baseT networks, and even Gigabit networks. While the emphasis of the paper is
about enterprise usage, the newest LAN standard brings significant benefits to industrial applications.
Although quoted wireless client numbers are unlikely to be met in factory and plant use, many of the
thoughts offered here may be usefully applied for a significant operational advantage.
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SINCE EARLY 2008, wireless infrastructure
company Xirrus has deployed over 1000
IEEE802.11n Wi-Fi networks for universities,
schools, enterprises, hospitals, convention
centres and others. During the design and implementation
of these networks, the company
gained much experience and knowledge in
802.11n technology and what it takes to successfully
install and operate high performance,
resilient 802.11n networks. A white paper
outlines key lessons learned from these 802.11n
deployments, and this article summarises it.
Proper planning is crucial with any network,
but it is especially important with 802.11n as
this has more flexibility and configuration
options than legacy Wi-Fi. Network designs can
take into account end-user needs and environmental
issues to optimise performance and
robustness. A site survey is important because
it lets the network administrator know exactly
where equipment needs to be placed for best
performance and RF propagation. Real
equipment should be used to determine the
best placement.
802.11n networks use Multiple Input Multiple
Output (MIMO) – traffic is carried on two or
three lower powered streams, increasing
network throughput while ending reflection
nulls through active phase management of the
carriers. Because of MIMO, 802.11n RF propagation
can be significantly different from
802.11abg networks. A site survey should
always be carried out to test the RF characteristics
of the environment before deployment.
To realise the benefits of 802.11n, the network
2.4GHz Coverage
should operate in the 5GHz band. During the site
survey, readings should be taken for both 2.4GHz
and 5GHz to ensure that both bands can be seen
from all areas to be covered. In most environments,
2.4GHz will propagate further than 5GHz
– changing equipment location or adding
additional equipment may be necessary to
provide full 5GHz band coverage (Fig. 1).
The site survey should establish that multiple
radios operate at an RSSI level of -72dBm or
more from every area to be covered by the
network. For a resilient connection, there
should be multiple radios from which a station
can choose, should one of them be too heavily
used or go down.
5GHz Coverage
Fig. 1 . Note that 2.4GHz (left) and 5GHz coverage differs significantly as shown in this example. Lesson Learned:
When doing site surveys, look at both 5GHz and 2.4GHz bands. 802.11n can operate in both and to fully realise its
benefits, both bands should be supported throughout the entire network.
industrial ethernet book
10
Setting cell size
Setting the appropriate cell size is important
when planning a wireless network. Choosing a
cell size as large as possible for best equipment
utilization may seem to be the way forward.
However, if a single radio covers too large an
area, overall performance will drop because too
many users are forced to use a single radio,
and stations at the edge of the cell will have
slower connection speeds. One or two slow
stations will affect the overall throughput of
the network. If the goal is to do wire-switch
replacement, care should be given to set the
cell size for optimum performance.
Many network administrators assume they can
use fewer 802.11n access points (AP) than with
802.11abg. However, if there are legacy
802.11abg stations in the network, APs will need
to give adequate coverage for 802.11n and
802.11abg stations. If the existing 802.11abg
network was providing adequate coverage, it
can be assumed that the new 802.11n APs can
be placed in the same locations providing similar
coverage – this should be confirmed by the site
survey. (Fig. 2).
The recommended security configuration is
802.1x for authentication and WPA2/AES for
encryption. Other authentication options are
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ARTWORK: FRANK OGDEN