Cisco Cisco Aironet 3700i Access Point 白書
© 2014 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information.
Page 9 of 10
A device, typically an access point (more on this to follow), sends a Very High Throughput (VHT) Null Data Packet
(NDP) Announcement frame whose only purpose is to contain the address of the access point (the beamformer)
and of the target client recipients (the beamformees). The VHT NDP Announcement frame is immediately followed
by a VHT NDP intended for those target recipients. Each intended recipient measures the RF channel from the
access point to itself using the preamble of the VHT NDP and compresses the channel. The first intended recipient
responds immediately with the compressed channel information in a VHT Compressed Beamforming frame. Other
recipients respond subsequently when they are polled by the access point. The VHT NDP Announcement frame,
the VHT NDP, and the VHT Compressed Beamforming frame are all similar to features in 802.11n. However,
because of some subtle differences, the 802.11ac sounding is not backward compatible with 802.11n devices.
ECBF is known to provide the most precise estimate of the channel that takes into account all the imperfections at
transmitter and receiver.
However, ECBF comes with a lot of overhead: the VHT NDP Announcement frame, the VHT NDP itself, and the
frames carrying the compressed feedback from each recipient. For an access point with four antennas, the
compressed feedback varies from 180 to 1800 bytes, depending on the number of client antennas and level of
compression. Sounding just one single-antenna 80-MHz client takes about 250 microseconds. When devices can
transmit at 433 Mbps, this has an impact, since that same time could have instead been used to send an extra
13,000 bytes.
Also, although any device with multiple antennas can beamform to any other device at any time, the practical
limitation on the effectiveness of ECBF is that the target recipient ideally has fewer antennas than the device
intending to beamform. Since many client devices have only a single antenna, they cannot beamform to the access
point, and those clients that have multiple antennas are at a disadvantage, since the access point typically has
more antennas.
Therefore, technologies that solve the problem of sounding without being limited by client implementation and/or
depend on client assistance (such as Cisco's ClientLink technologies) continue to add genuine value. They help (1)
legacy 802.11a/n clients, (2) those 802.11ac clients that do not support 802.11ac sounding, and (3) clients at 2.4
GHz. They also avoid the overhead of standards-based explicit sounding.
Introducing Cisco ClientLink 3.0 Beamforming
802.11a/g clients cannot support standards-based beamforming, either explicit or implicit, and many 802.11n and
802.11ac clients do not support standards-based beamforming either. For this reason, it is vital that vendors aiming
at a comprehensive solution provide beamforming modes that work for any client. As with Cisco ClientLink and
ClientLink 2.0, ClientLink 3.0 beamforming does just that.
ClientLink was designed for single-antenna clients or clients using one spatial stream. The access point can
measure the wireless channel any time the client transmits, even if
it’s just one packet, and the access point then
uses that information to maximally reinforce the data send-back to the client. For multiple-antenna clients, vendors
must use innovative technology such as ClientLink 2.0. and 3.0. Single-antenna clients also benefit from ClientLink
2.0 and 3.0.