Cisco Cisco Catalyst 6000 Multilayer Switch Feature Card MSFC2 White Paper

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802.11n - 20 MHz or 40 MHz Channels?

802.11n can operate in a 40 MHz channel by bonding two 20 MHz channels together and this significantly

increases throughput. However, this is reserved for burst mode transfers only. It is only practical to do this in 5

GHz because 2.4 GHz is already limited by the number of channels available. If there are enough 5 GHz channels

to achieve the WLAN goals using a bonded channel plan (9 in the U.S. if using available DFS channels) to meet

throughput goals, consider it. If forced to reuse 5 GHz channels, more consistent results will be delivered using

strictly 20 MHz channels and avoiding loss of efficiency due to CCI.

Evaluating Requirements for 2.4 GHz and 5 GHz Connection Support

The essential question for a high-density design is how many channels for each band will be needed to match the

client base? This can be a tricky question since even dual band capable clients do not always select the faster

5 GHz band. Since bandwidth in 2.4 GHz is going to be limited, 5 GHz must be relied on to reach the goal.

Dual band adapters have been shipping with most laptops for some time. This does not mean that every laptop is

a dual band client, but many are. Simply having a dual band client does not guarantee that it will choose 5 GHz

over 2.4 GHz. The Microsoft Windows operating system defaults to a Wi-Fi channel search that starts with the 5

GHz channel 36 and continues searching through all of the 5 GHz channels that the client is capable of. If no 5

GHz AP is found then it will continue the search in 2.4 GHz starting at channel 1. Unless the Windows default is

changed or the user has chosen a third party Wi-Fi utility to set spectrum preference to 2.4 GHz, the client radio

will first try to associate to a 5 GHz AP. Apple Computer’s latest release for Atheros and Broadcom chipsets also

searches 5 GHz first.

The Cisco BandSelect feature enables the infrastructure to optimize these types of client connection choices.

Where possible, it helps make sure that devices are attaching to the 5 GHz spectrum channels where interference

sources tend to be significantly lighter. A much greater channel selection leads to the alleviation of bandwidth

challenges.

Tablet computers and smartphones have begun entering the market at a staggering rate. The vast majority of

smartphones shipping today operate in 2.4 GHz only. While many of them are 802.11n clients, of these most have

implemented a single input single output (SISO) rather than Multiple Input, Multiple Output (MIMO). A SISO device

is only capable of supporting MCS7 data rates, or 54 Mbps.

Design Point #4: 5 GHz Support Will Be Critical for High-Density, So Determine the Channel
Plan That You Will Support and How It Will Be Administered

Evaluating the particular client mix for the WLAN can be done easily on Cisco wireless networks by utilizing the

reporting features in the Cisco Wireless Control System or by reviewing the WLAN controller’s connection logs.

Determine the Number of Channels and Cells Needed

A sample high-density WLAN project may include a design that yields 300 Mbps consistently to support 300

concurrent users. Under optimal conditions, 802.11g and 802.11a data rates yield 25 Mbps throughput. However,

high-density environment will be less than optimal from a SNR standpoint. A better number to use is 20 Mbps

throughput. Table 4 provides a quick reference using 20 Mbps per cell and per channel as the throughput value.

Looking strictly at 5 GHz and assuming no channel reuse at this point, it is clear that 1 Mbps per user with

15 channels and 15 cells can be easily supported.