Installation guide

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Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Deployment Best Practices
Once again, keep in mind that location readiness inspection is a distance-based predictive tool. As is the
case with most predictive tools, it can be expected that some degree of variance naturally occurs between
predicted and actual results. Cisco recommends that this location readiness should be used in
conjunction with other best-practice techniques outlined in this document, including the new capability
introduced with release 4.0 referred to as the location quality inspector (described in Inspecting Location
Quality, page 76).
Avoiding Excessive Co-Channel Interference
A concern that sometimes arises whenever the number of access points is increased beyond that required
to adequately service wireless voice or data needs is the potential for excessive co-channel interference.
Co-channel interference occurs when two access points and their associated clients are transmitting on
the same channel, and each cell is in close enough physical proximity that each is capable of receiving
the transmissions of the other.
Normally, the addition of access points increases the overall wireless bandwidth available to users. This
is most certainly the case when access points are added using non-overlapping frequencies, and can also
be seen to a lesser degree when adding access points on overlapping frequencies. Because of the inherent
nature of the 802.11 protocols, collision-avoidance and backoff mechanisms are used to minimize the
number of collisions that occur. In some cases, however, especially with heavier traffic loads on
overlapping frequencies, these anti-collision mechanisms exact a price in the form of lost transmission
opportunities. This penalty is most often seen as a reduction in the amount of performance recognized
per added access point. Although such lost transmission opportunities can be annoying in wireless data
applications, the effects can be more serious with latency-sensitive wireless voice applications.
To minimize the degree of co-channel interference, traditional best practice recommends deploying
access points on alternating non-overlapping channels, using directional antennas where possible and
judiciously limiting access point power levels as necessary. Before the advent of dynamic automated
mechanisms such as Cisco Radio Resource Management (RRM), the management of channel
assignments and power levels was typically a manual and static process. Whether done dynamically or
statically with only three non-overlapping 2.4 GHz channels available, both of these measures are able
to manage co-channel interference but not always completely eliminate it.
In many cases, location-based services are added or retrofitted to an existing wireless design, some of
which encompass wireless voice handheld devices such as the Cisco 7920. When designing a
location-aware solution to be used in conjunction with voice, special care needs to be exercised to ensure
that excessive co-channel interference is not introduced into the environment. The need for additional
access points to support optimum location accuracy and precision (as recommended in Access Point
Placement Considerations, page 54) must be counterbalanced against the design requirements for an
acceptable wireless voice infrastructure. A fair and equitable compromise between the two goals must
be reached by the network designer. In some cases, automated tools such as RRM can be helpful in
achieving this goal quickly and effectively. In other more challenging and often times unique
environments, manual channel intervention and power assignment may be required.
For further details regarding best practice voice design and deployment recommendations specifically
for the Cisco 7920, see the Cisco Wireless IP Phone 7920 Design and Deployment Guide at the following
URL:
http://www.cisco.com/en/US/products/hw/phones/ps379/products_implementation_design_guide_boo
k09186a00802a029a.html.
For further details regarding best practice voice design and deployment recommendations specifically
for Spectralink wireless telephony products, see the SpectraLink Phone Design and Deployment Guide
located at the following URL:
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a00806d11cb.shtml.