Cisco Cisco Catalyst 6000 Multilayer Switch Feature Card MSFC2 White Paper

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If DFS channels will not be used, with 15 channels needed, use the nine non-DFS channels and reuse six of

these. The cells must all be isolated from one another to prevent CCI from robbing available bandwidth.

Non Wi-Fi Interference and the High-Density Network

The important role of non Wi-Fi interference in the high-density network should now be clear. The success of a

high-density WLAN will be compromised if any non Wi-Fi interference is operating within the same environment.

Non Wi-Fi interference has a much larger impact on throughput in a high-density environment than unmanaged

Wi-Fi energy. This is because 802.11 utilizes contention-based access mechanisms to coordinate station access

to the channel. All 802.11 devices operate this way. Non Wi-Fi devices operating in the same band do not share

these rules and do very well breaking the queuing and back-off mechanisms, forcing all stations within range to

wait until the air is free.

When the 802.11 standards were being drafted, there was great concern that proliferation of Wi-Fi networks would

create interference for licensed users operating in the same frequency bands. As a result of this concern, Wi-Fi

was designed to be very “polite”, yielding the band to almost anything else found operating there. Twenty years

later, there are many consumer devices sharing the industrial, scientific, and medical (ISM) bands with Wi-Fi.

The challenge is that while these devices operate under the same power restrictions of Wi-Fi devices, they are in

no way obligated to yield the band for Wi-Fi traffic, and most do not. This creates a problem for normal Wi-Fi

operations, since a Wi-Fi modem can only classify energy as:

●

Wi-Fi (the energy detected can be demodulated)

●

Noise (all remaining energy is considered to be “noise”)

The impact of non Wi-Fi interference is logarithmic in its impact on Wi-Fi network operations. The higher the

utilization of the Wi-Fi network, the more destructive non Wi-Fi energy will be. This means that if there is

interference present and the network is only slightly utilized (e.g., there is ample duty cycle available within the

spectrum), the presence of non Wi-Fi energy may not even be noticeable. There is space for both to share the

spectrum. However, if the Wi-Fi network is highly utilized, then even a small amount of non Wi-Fi interference can

have a large and noticeable effect.

This is why Cisco created CleanAir, a system-level, proactive monitoring, reporting, and mitigation mechanism as

a core system resource for the CUWN. A high-density WLAN is designed to take advantage of every bit of the

available spectrum. Any spectrum being consumed by even a relatively benign non Wi-Fi interference source will

have a large impact in a dense environment. Interference needs to be identified, managed, and eliminated to

provide the required bandwidth for a high-density network to work properly.

See

Cisco CleanAir

for more details.

Design Point #5: Account for and Manage All Energy Within the Operating Spectrum to
Ensure All of It Is Available for Use

Client Duty Cycle

The discussion until now has centered on a use case where every client in the room will be competing for

bandwidth simultaneously. This is the case when the users in the room simultaneously access a resource on

queue. However there are many instances where the design requirement is to offer access to resources or the

Internet for casual use at an event or within a venue such as a sports arena. Planning and sizing for these types of

events can be quite different and will be based on expected Client Duty Cycle.