Cisco Cisco Catalyst 6000 Multilayer Switch Feature Card MSFC2 White Paper

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Figure 7. 21 Channels on One 5 GHz Cell, and Resulting Capacity

With today’s radio designs, the radio could almost be placed on top of each another, but that would not serve the

high-density design well. It would result in the same coverage area of a single cell and likely would not cover the

required area, even in a relatively small lecture hall.

Data rates are a function of the received signal strength and the signal to noise ratio (SNR) at the receiver. It is not

practical or efficient to lock a radio down to a particular data rate since the radio makes efficiency decisions based

on available link conditions. Not every client will respond the same in an otherwise static environment. Variables

such as receiver sensitivity, antenna configuration, driver version, and even position within the cell in relation to

attenuating or reflecting objects will have a variable effect on the client. An environment that is conducive to good

radio efficiency can be helped by appropriate design. The higher the average received signal strength and the

better the SNR, the faster the data rate will be.

Cisco’s ClientLink technology can increase the signal selectively for legacy 802.11 a/g Orthogonal Frequency-

Division Multiplexing (OFDM) clients. Since legacy clients do not support the efficiency gains realized with 802.11n

clients, they represent the least efficient clients in our design. Using ClientLink in a high-density user design allows

the AP to improve SNR from 3-6 dB on a packet-by-packet basis for a client that is indicating the need to rate shift.

This has the overall effect of increasing the range and rate equation for the network and encourages legacy clients

to maintain higher data rates under adverse conditions. This is an excellent addition to a high-density design. The

document

Cisco ClientLink: Optimized Device Performance for 802.11n

provides a full discussion of the Cisco

ClientLink technology.

In a high-density deployment, channels will be aggregated to increase the total bandwidth. This means moving the

APs ever closer together in the design space. A key success factor is Co-Channel Interference (CCI). CCI effects

capacity of the cell by reducing the available bandwidth.

What Is Co-Channel Interference and Why Is It Important in High-Density WLANs?

CCI is a critical concept to understand when it comes to understanding the behavior and performance of 802.11

WLANs. It is a phenomenon where transmissions from one 802.11 device bleed into the receive range of other

802.11 devices on the same channel, causing interference and reducing the available spectrum and resulting

performance. CCI can cause channel access delays as well as collisions in transmissions that corrupt frames in

transit. Figure 8 illustrates how APs on the same channel interfere with each other.