Cisco Cisco Catalyst 6000 Multilayer Switch Feature Card MSFC2 White Paper

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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.

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.

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.

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.

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.

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.

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.

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.