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

 

 

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Also, to support the new MU-MIMO feature (see 

), the channel feedback can contain an extra level of 

detail. 

Explicit compressed feedback (ECFB) is known to provide the most precise estimate of the channel, taking into 

account all the imperfections at transmitter and receiver. 

However, ECFB comes with a lot of overhead: the VHT NDP Announcement frame, the VHT NDP itself, and the 

frame carrying the compressed feedback. For an AP 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 is 

expensive, since that same time could instead have been used to send an extra 13,000 bytes. 

And so technologies that solve the problem of sounding without depending on client assistance (such as Cisco 

ClientLink technology) continue to add genuine value. They (1) still help legacy 802.11a/n clients, (2) still help 

those 802.11ac clients that do not support 802.11ac sounding, (3) still help clients at 2.4 GHz, and (4) can avoid 

the overhead of standards-based explicit sounding when it is not actually necessary. 

An 802.11ac AP operating on 80 MHz (or 160 MHz and so on) should still be capable of allowing 802.11a or 

802.11n clients to associate. Thus, beacons are sent on one 20-MHz channel, known as the primary channel, 

within that 80 MHz. The AP and all clients associated with the AP receive and process every transmission that 

overlaps this primary channel and extract virtual carrier sense from the frames they can decode. 

However, the AP could be near other uncoordinated APs. Those APs could be preexisting 802.11a or 802.11n 

APs, and their primary channels could be any 20 MHz within the 80 MHz of the 802.11ac AP. The different APs 

and their associated clients then have a different virtual carrier sense and so can transmit at different times on the 

different subchannels, including overlapping times. With the wide 802.11ac channel bandwidths, this scenario 

becomes much more likely than with 802.11n.