Many networks still support a
mix of 802.11n and 802.11ac clients. Because 802.11a/g clients (legacy clients)
operate at lower data rates, the older clients can reduce the capacity of the
entire network. Cisco’s
ClientLink technology can help solve problems related to
adoption of 802.11n in mixed-client networks by ensuring that 802.11a/g clients
operate at the best possible rates, especially when they are near cell
Advanced signal processing
has been added to the Wi-Fi chipset. Multiple transmit antennas are used to
focus transmissions in the direction of the 802.11a/g client, increasing the
downlink signal-to-noise ratio and the data rate over range, thereby reducing
coverage holes and enhancing the overall system performance. This technology
learns the optimum way to combine the signal received from a client and then
uses this information to send packets in an optimum way back to the client.
This technique is also referred to as MIMO (multiple-input multiple-output)
beamforming, transmit beamforming, or cophasing, and it is the only
enterprise-class and service provider-class solution in the market that does
not require expensive antenna arrays.
The 802.11n systems take
advantage of multipath by sending multiple radio signals simultaneously. Each
of these signals, called a spatial stream, is sent from its own antenna using
its own transmitter. Because there is some space between these antennas, each
signal follows a slightly different path to the receiver, a situation called
spatial diversity. The receiver has multiple antennas as well, each with its
own radio that independently decodes the arriving signals, and each signal is
combined with signals from the other receiver radios. This results in multiple
data streams receiving at the same time. This enables a higher throughput than
previous 802.11a/g systems, but requires an 802.11n capable client to decipher
the signal. Therefore, both AP and client need to support this capability. Due
to the complexity of issues, in the first generation of mainstream 802.11n
chipsets, neither the AP nor client chipsets implemented 802.11n transmit
beamforming. Therefore, the 802.11n standard transmit beamforming will be
available eventually, but not until the next generation of chipsets take hold
in the market. We intend to lead in this area going forward.
We 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 802.11a/g clients. In other words, for 802.11 a/g clients,
some of the capabilities of the extra transmit path was lying idle. In
addition, we 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, we 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 802.11a/g clients in the downlink direction without requiring
feedback. Because no special feedback is required, Cisco ClientLink works with
all existing 802.11a/g clients.
Cisco ClientLink technology
effectively enables the access point to optimize the SNR exactly at the
position where the client is placed. ClientLink provides a gain of almost 4 dB
in the downlink direction. Improved SNR yields many benefits, such as a reduced
number of retries and higher data rates. For example, a client at the edge of
the cell that might previously have been capable of receiving packets at 12
Mbps could now receive them at 36 Mbps. Typical measurements of downlink
performance with ClientLink show as much as 65 percent greater throughput for
802.11a/g clients. By allowing the Wi-Fi system to operate at higher data rates
and with fewer retries, ClientLink increases the overall capacity of the
system, which means an efficient use of spectrum resources.
ClientLink in the 1552 access
points is based on ClientLink capability available in AP3500s. Therefore, the
access point has the ability to beamform well to nearby clients and to update
beamforming information on 802.11ACKs. Therefore, even if there is no dedicated
uplink traffic, the ClientLink works well, which is beneficial to both TCP and
UDP traffic streams. There are no RSSI watermarks, which the client has to
cross to take advantage of this Beamforming with Cisco 802.11n access points.
ClientLink can beamform to 15
clients at a time. Therefore, the host must select the best 15 if the number of
legacy clients exceeds 15 per radio. AP1552 has two radios, which means that up
to 30 clients can be beamformed in time domain.
Although ClientLink is
applied to legacy OFDM portions of packets, which refers to 11a/g rates (not
11b) for both indoor and outdoor 802.11n access points, there is one difference
between ClientLink for indoor 11n and ClientLink for outdoor 11n. For indoor
11n access points, SW limits the affected rates to 24, 36, 48, and 54 Mbps.
This is done to avoid clients sticking to a far away AP in an indoor
environment. SW also does not allow ClientLink to work for those rates for 11n
clients because the throughput gain is so minimal. However, there is a
demonstrable gain for pure legacy clients. For outdoor 11n access points, we do
need more coverage. Thus, three more additional legacy data rates lower than 24
Mbps have been added. ClientLink for outdoors is applicable to legacy data
rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.
enabled by default.