Cisco Aironet 2702i AIR-CAP2702I-E-K9 Dépliant
Codici prodotto
AIR-CAP2702I-E-K9
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), 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.
2.3.4 RTS/CTS with Bandwidth Indication
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.