Cisco Cisco Catalyst 6000 Multilayer Switch Feature Card MSFC2 White Paper
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Figure 3. Seating and Interference
What is ultimately going to effect the client devices more than any other factor is the degradation of signal-to-noise
ratio (SNR) through both co-channel and adjacent channel interference driven by co-located devices. Proper
system engineering can minimize the impact by maximizing proper spatial reuse but it cannot be eliminated in
highly dense environments entirely. Operating margins become more critical as space is condensed and a bad
radio or behavior in the mix can have a large impact within a cell. Client behavior under these conditions will vary
widely and trends based on environment and event type have also been reported. There is not much that can be
done about the particular client mix or behavior. The design goal is to engineer the network side as robustly as
possible and to control and understand all variables.
Within environments that qualify as high-density, there are also submodels built by use case. For example, in a
high-density environment such as a public venue or stadium, capacity is planned based on what percentage of
users are likely to be active on the network at any one time. In higher education there is a different model, where
casual WLAN activity is one use case while activity when a professor is lecturing may increase dramatically, up to
100 percent.
Planning
The WLAN design process can begin in many ways but generally it begins with an expressed desire to provide
connections to a specific area where a number of users will participate in a focused activity. To evaluate what is
possible, it is first necessary to understand what is required as well as what is possible. There is generally a
primary application that is driving the need for connectivity. Understanding the throughput requirements for this
application and for other activities that will take place on the network will provide the designer with a per-user
bandwidth goal. Multiplying this number by the number of expected connections yields the aggregate bandwidth
that will be required.
The required per connection bandwidth will be used to drive subsequent design decisions.