Листовка для Cisco Aironet 2702i AIR-CAP2702I-E-K9

 

 

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Given the power of A-

MPDU and the 802.11n channel access mechanism, 802.11ac actually didn’t need to 

innovate much in the MAC. Indeed, extensions to the RTS/CTS mechanism are the only new mandatory MAC 

feature. 

802.11n does include many options with reduced value, and 802.11ac takes a very pragmatic approach to them. If 
a “useless” option is used and affects a third-party device, 802.11ac typically forbids an 802.11ac device (operating 
in 802.11ac mode) from using the option. If a “useless” option has not been used in 802.11n products or affects 

only 

the devices that activate the option, the feature is not updated for 802.11ac but is instead “left to die.” 

For instance, there is no 802.11ac version of the “802.11n greenfield” preamble format. 802.11ac defines only one 

preamble format, which, to legacy 802.11a/n devices, will look safely like an 802.11a preamble followed by a 

payload with a bad CRC. This means that legacy devices 

won’t try to transmit over the top of the 802.11ac 

transmission, nor will they attempt to send a bad payload up the stack. 

802.11n introduced reduced interframe spacing (RIFS), which reduces overheads between consecutive 

transmissions, but experience has shown that A-MDPU solves much the same problem even more efficiently. 

802.11ac devices operating in 802.11ac mode are not permitted to transmit RIFS (as of Draft 3.0). 

802.11n features that are not updated for 802.11ac (or are explicitly forbidden for 802.11ac devices operating in 

802.11ac mode) include all the 802.11n sounding options, including extension LTFs, the calibration procedure, 

antenna selection, PCO, L-SIG TXOP protection, unequal modulation, 4×3 and 3×2 STBC modes, MCS32, and 

dual CTS protection. 

Don’t worry if you don’t know these terms; you’ll almost certainly never need to understand 

them. 

Any device (with multiple antennas) can beamform to any other device at any time. What 802.11ac adds is the 

opportunity for the receiver to help the beamforming transmitter to do a better job of beamforming. This is called 
“sounding,” and it enables the beamformer to precisely steer its transmitted energy toward the receiver. 802.11ac 

defines a single, though optional, protocol for one 802.11ac device to sound other 802.11ac devices. The protocol 

selected closely follows the 802.11n explicit compressed feedback protocol, as follows. 

A device, typically an AP, sends a “Very High Throughput (VHT) Null Data Packet (NDP) Announcement” frame. Its 

only purpose is to contain the address of the AP and of the target recipients. The VHT NDP Announcement frame 
is immediately followed by a “VHT Null Data Packet” (VHT NDP) intended for those target recipients. Each 

intended recipient measures the RF channel from the AP to itself using the preamble of the VHT NDP and 

compresses the channel. The first intended recipient responds with the compressed channel information in a VHT 

Compressed Beamforming frame immediately, and other recipients respond when they are polled by the AP. 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.