Листовка для Cisco Aironet 2702i AIR-CAP2702I-E-K9
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AIR-CAP2702I-E-K9
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Second, before the device addressed by the replicated RTS responds with a CTS, the recipient device checks to
see if anyone is transmitting near itself, on its primary channel or on any other 20 MHz within the 80 MHz. If a
portion of the bandwidth is in use nearby, the recipient responds with a CTS only
on the available and “usable”
20-
MHz subchannels and also reports the bandwidth of the replicated CTS inside the CTS’s PPDU. Here “usable”
subchannels means the subchannels on which the initiating device is allowed to send something, such as a 20-,
Third, the CTS is sent, like the RTS, in an 802.11a PPDU format, replicated in 20-MHz chunks across the available
and useful bandwidth. Again, every nearby device receives a CTS that the device can understand on its primary
channel.
There are other variations on this protocol, for when the initiator is incapable of switching to a narrower bandwidth
on the fly and so forth, but the previous description captures the essence of the enhancement: the recipient can
say,
“These subchannels are busy - don’t use them.”
2.3.5 All A-MPDUs
802.11 defines that every 802.11 PPDU transmission is an A-MPDU, yet the A-MPDU might contain only a single
MPDU. Why? The short answer is that it’s complicated.
MPDU. Why? The short answer is that it’s complicated.
Here’s the long answer: There are three reasons: (1) In 802.11a/n, the duration of the transmission is set by the
number of octets and the data rate for the transmission. But a maximum-length 5.5-ms transmission at 6.93 Gbps
could contain over 4 million bytes, and this takes 23 bits to represent. These bits would be sent at the lowest
Modulation and Coding Scheme (MCS) rate at the start of every 802.11ac transmission and so practically would
add 4 microseconds each time. Instead, the length of an 802.11ac transmission is constrained to be a multiple of
the number of data bits per orthogonal frequency-division multiplexing (OFDM) symbol, and then only the number
of OFDM symbols needs to be signaled. Moreover, the number of (assumed to be) 4-microsecond-long OFDM
symbols is already implicitly available in the legacy portion of the preamble, so this signaling comes almost for
free.
2
Then we need a way to completely fill even the last OFDM symbol with data. A-MDPU makes this easy: send
the data as MDPUs within MDPU subframes in an A-MDPU, then pad the A-MDPU with enough null MDPU
subframes to fill up the last OFDM symbol. (2) This same padding mechanism will come in handy for the new
MU-MIMO feature. (3) A-MDPU is in general a good idea to increase reliability for long payloads.
2.3.6 Channelization and 80+80 MHz
802.11ac adopts a keep-it-simple approach to channelization. Adjacent 20-MHz subchannels are grouped into
pairs to make 40-MHz channels, adjacent 40-MHz subchannels are grouped into pairs to make 80-MHz channels,
and adjacent 80-MHz subchannels are grouped into pairs to make the optional 160-MHz channels, as shown in
. A BSS (that is, AP plus clients) uses the different bandwidths for different purposes, but the usage is
principally governed by the capabilities of the clients.
2
Just a single bit is needed to disambiguate the number of actual OFDM symbols present if the transmission instead uses the
short guard interval and the OFDM symbols are actually 3.6 microseconds long.