Белая книга для Cisco Cisco Content Switching Module with SSL
Wireless LAN Design Guide for High Density
Client Environments in Higher Education
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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.
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:
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”)
• 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.
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.
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
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
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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.
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.
At a sporting event, for example, there are certain areas that will require ubiquitous and instant access during the entire event.
Ticketing, vendor sales, staff, and press areas will generally require the highest amount of access. Of these, the press area is the
only one that requires a high level of capacity in the arena itself. For the fans attending the event, only a percentage will be active
on the WLAN at any one time From experience we see a 20 to 30 percent take rate with some well defined peaks occurring
during period breaks. During play, very few fans are accessing the WLAN. However, this is changing as applications such as
video replay, instant stats, and concession orders from the seat become more commonplace.
Ticketing, vendor sales, staff, and press areas will generally require the highest amount of access. Of these, the press area is the
only one that requires a high level of capacity in the arena itself. For the fans attending the event, only a percentage will be active
on the WLAN at any one time From experience we see a 20 to 30 percent take rate with some well defined peaks occurring
during period breaks. During play, very few fans are accessing the WLAN. However, this is changing as applications such as
video replay, instant stats, and concession orders from the seat become more commonplace.