Voice over Wireless LAN Solution Guide v1.0 December 2005
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WLAN Site Survey
Technical support for VoWLAN is contingent on customers performing a prior site survey of the
WLAN. Currently Nortel recommends the use of the Ekahau Site Survey tool to verify the network
deployment, though other site survey tools are acceptable as well. The Ekahau product runs on a
PC and uses your WLAN NIC to collect data for analysis. The output of the tool is a number of
robust visualizations of the network. Aside from verifying the basic coverage of the network, the
software provides a number of visualizations that are particularly useful to VoWLAN deployments.
Perform capacity planning using the data rate analysis view, which shows a color coded view of
the maximum data rate across all APs in the network. With this view, you can see where your
handsets will be able to utilize the 11 Mbps data rate as opposed to rate scaling down to lower
rates. As discussed earlier, planning based on data rate can have a big impact on voice call
capacity planning.
Predict AP selection and roaming using the strongest AP view. This view shows the AP with the
strongest signal for each location in the building, using color codes for each AP. This allows you
to predict the APs that will be the primary choice of voice devices to use given their location and
allows you to predict where the handoff to another AP (and which AP) will likely occur given a
moving user.
Perform resiliency planning through the AP reachability view. This view presents a color coded
visualization of the number of reachable APs from each point in the network. Locations where the
tool detects one AP, locations where the tool detects two APs, locations where the tool detects
three APs, and so on, are marked in distinct colors. With this visualization, you can see where the
network is vulnerable to a single point of failure. It is preferable to have at least two APs that are
capable of offering coverage to every point in your building. Note that if the APs are AP 2330s
and the WSS 2300 is using auto-tune power, the resiliency is in the power adjustment, meaning
that the site survey may show only one AP reachable from a given point. In reality, the hole
covering capability of the WSS 2300 may be able to cover the location with two APs though
current power levels do not indicate this. To plan for resiliency, turn off auto-tune power in order
to see what the true potential coverage is in the event of AP failure.
You can also use the AP reachability view to perform location service planning. A minimum of
three APs must be reachable for triangulation to be effective. So use the AP reachability feature
to verify a consistent 3+ AP coverage across the building. Location capabilities have a number of
client dependencies, so verifying triangulation coverage is more complex than it appears. There
are two main location solution types: those that use the client to collect information about the APs
in the network (client-based location), and those that use the APs to collect information about the
client (network-based location). Both use a form of triangulation to compute the location of the
device. Depending on the power level of an AP, it may be able to hear devices it cannot transmit
to. These factors combined create the following two scenarios. 1) It is hard to calibrate network-
based solutions using a laptop running the site survey because APs may be able to hear clients
that cannot hear the AP. If AP transmission power levels are not at a maximum, then they will be
able to hear clients over a farther distance than its own transmissions will travel. This can cause
the site survey application to underestimate the number of APs that can participate in
triangulation. 2) Client-based solutions cannot triangulate APs that are not detectable because
their power is lower. But the site survey application will accurately reflect the number of APs that
can be used for triangulation.
What this means is that to plan triangulation for network-based solutions, turn off auto-tune power
to more accurately reflect the range of the APs. Yet to verify the triangulation coverage of client-
based solutions, you must turn on auto-tune power if auto-tune power is enabled in the normal
running state of the network.