Cisco Cisco Aironet 1140 Access Point 백서
White Paper
© 2009 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information.
Page 3 of 6
ClientLink Technology
Cisco realized that for the current generation of 802.11n APs, while the second transmit path was being well utilized
for 802.11n clients (to implement spatial diversity), it was not being fully used for 802.11 a/g clients. In other words,
for 802.11 a/g clients, some of the capability of the extra transmit path was lying idle. In addition, Cisco realized that
for many networks, the performance of the installed 802.11 a/g client base would be a limiting factor on the network.
To take advantage of this fallow capacity and greatly enhance overall network capacity by bringing 802.11 a/g clients
up to a higher performance level, Cisco created an innovation in transmit beamforming technology, called ClientLink.
ClientLink uses advanced signal processing techniques and multiple transmit paths to optimize the signal received
by 802.11 a/g clients in the downlink direction without requiring feedback. And because it doesn’t require any
special feedback, Cisco ClientLink works with all existing 802.11a/g clients.
To understand how this works, consider a single transmitter 802.11a/g client sending an uplink packet to an 802.11n
access point with multiple transceivers. The access point receives the signal on each of its three receive antennas.
Each received signal has a different phase and amplitude based on the characteristics of the space between the
antenna and the client. The access point processes the three received signals into one reinforced signal by adjusting
their phases and amplitudes to form the best possible signal. The algorithm it uses, called maximal ratio combining
(MRC), is typically used on all 802.11n access points (see Figure 2). MRC only helps in the uplink direction, enabling
the access point to “hear” the client better.
Figure 2. Reinforcement of received signal via MRC algorithm
Cisco ClientLink technology takes this one step further to improve performance in the downlink direction, making the
client better able to hear the access point. The Wi-Fi channel is reciprocal, meaning that transmissions between
access points and clients happen on the same frequency and use the same antennas. Thus the access point can
use the adjustments calculated by MRC (referred to as “weights”) to optimize the reciprocal signal transmitted back
to that specific client using the APs two transmit antennas.
Enhancements to the Cisco Wi-Fi chipset that support ClientLink technology include hardware blocks that perform
four tasks: