Cisco Cisco MGX Service Resource Module Enhanced [SRM-E] Weißbuch
Wireless LAN Design Guide for High Density
Client Environments in Higher Education
16 © 2011 Cisco and/or its affiliates. All rights reserved.
Additionally, if the WLAN is located in a regulatory domain where the bandwidth to deploy four channels is available
(e.g., availability of channel 13 and 14) unless the WLAN is sufficiently isolated from every other network it is likely that someone
will deploy using the standard 1, 6, 11 model and drastically increase the interference to the WLAN.
(e.g., availability of channel 13 and 14) unless the WLAN is sufficiently isolated from every other network it is likely that someone
will deploy using the standard 1, 6, 11 model and drastically increase the interference to the WLAN.
If it is necessary to maximize a 2.4 GHz connection, it is possible to increase the bandwidth and efficiency of cells by physically
limiting the propagation through the use of antennas and creative placement options. This will require site specific engineering
and careful measurement and design. Cisco Advanced Services and experienced Cisco partners can help with this type of
design and have achieved amazing results in extremely large and complex environments. This, however, is not always an option
for budgetary or aesthetics reasons. We will discuss this in much more detail in the section on AP placement.
limiting the propagation through the use of antennas and creative placement options. This will require site specific engineering
and careful measurement and design. Cisco Advanced Services and experienced Cisco partners can help with this type of
design and have achieved amazing results in extremely large and complex environments. This, however, is not always an option
for budgetary or aesthetics reasons. We will discuss this in much more detail in the section on AP placement.
5 GHz Channel Reuse in a High-Density Design
In contrast to 2.4 GHz, 5 GHz has many more channels with which to work. As many as 20 channels can be received in the
United States and between five and 21 in the rest of the world. Most regions have between 19 and 21 channels. But all 5
GHz channels are not created equally. Limitations on maximum power for parts of the band are not of concern, but Dynamic
Frequency Selection (DFS) channels represent some challenges that must be addressed.
United States and between five and 21 in the rest of the world. Most regions have between 19 and 21 channels. But all 5
GHz channels are not created equally. Limitations on maximum power for parts of the band are not of concern, but Dynamic
Frequency Selection (DFS) channels represent some challenges that must be addressed.
Dynamic Frequency Selection and High-Density Design
DFS was implemented so that APs and clients can share the band with radar devices. DFS details how radar is detected and
what should be done in the event of detection. APs operating on DFS channels must first listen to a channel for 60 seconds to
determine if there is a radar present before transmitting any energy. If an AP is operating on a DFS channel and detects a radar
(real or false) it must shut down operations on that channel and abandon it for 30 minutes before that channel can be evaluated
again for use.
what should be done in the event of detection. APs operating on DFS channels must first listen to a channel for 60 seconds to
determine if there is a radar present before transmitting any energy. If an AP is operating on a DFS channel and detects a radar
(real or false) it must shut down operations on that channel and abandon it for 30 minutes before that channel can be evaluated
again for use.
Cisco APs were some of the first in the industry to support DFS channels. Client support for DFS channels has been inconsistent,
however. Client devices do not have the ability to detect radar and rely on the infrastructure established by a DFS certified AP.
Most clients today support channels 52-64. Client support for channels 100-140 has been slow in coming. Often it is a matter
of not only the hardware but the version of the driver for the client that determines its operating channel range.
however. Client devices do not have the ability to detect radar and rely on the infrastructure established by a DFS certified AP.
Most clients today support channels 52-64. Client support for channels 100-140 has been slow in coming. Often it is a matter
of not only the hardware but the version of the driver for the client that determines its operating channel range.
Client support has been steadily increasing and to-date Intel 5100 a/g/n, 5300 a/g/n, and 6300 a/g/n all operate on channels
52-64 and 100-140. The Cisco Cius and the Apple iPad and the Cisco 7925 IP phone also support the full range of DFS
channels.
52-64 and 100-140. The Cisco Cius and the Apple iPad and the Cisco 7925 IP phone also support the full range of DFS
channels.
The effect of using channels that are not supported by all clients can result in coverage holes for those clients. Channels
100-140 are disabled by default on a Cisco Unified Wireless Network but can be enabled easily in the DCA channel selections
by choosing the extended UNII-2 channels. Before doing so, it is highly advisable to inventory the clients and drivers that must
be supported.
100-140 are disabled by default on a Cisco Unified Wireless Network but can be enabled easily in the DCA channel selections
by choosing the extended UNII-2 channels. Before doing so, it is highly advisable to inventory the clients and drivers that must
be supported.
If DFS channels have been used in a WLAN installation, their suitability within the WLAN will be established. If they have not
been enabled previously, it is advisable that the DFS channels are surveyed using Cisco equipment and that monitoring for
radar detection is done before enabling the channels. In public and other venues within higher education environments, it is
often recommended to avoid using these extended UNII-2 channels due to their current lack of client support. The base UNII-2
channel availability in clients is more pervasive and these are channels that could be considered but ongoing monitoring of client
capabilities should not be overlooked.
been enabled previously, it is advisable that the DFS channels are surveyed using Cisco equipment and that monitoring for
radar detection is done before enabling the channels. In public and other venues within higher education environments, it is
often recommended to avoid using these extended UNII-2 channels due to their current lack of client support. The base UNII-2
channel availability in clients is more pervasive and these are channels that could be considered but ongoing monitoring of client
capabilities should not be overlooked.