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

 

 

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Figure 5.    Example of Parallel Transmissions with Two BSSs on the Same 80 MHz but with Different Primary 20-MHz 

Subchannels 

 

The ability to have overlapped APs but different primary channels is made possible by: 

● 

The enhanced secondary CCA thresholds mandated by 802.11ac, which are up to 13 dB more stringent 

than the secondary CCA thresholds defined by 802.11n 

● 

The addition of a bandwidth indication to the RTS/CTS exchange (see 

) 

Over time, clients will transition from 802.11n to 802.11ac, so that 80 MHz is used more and more. In this 

environment APs should change to align their primary 20-MHz channels. 

The capability of 80-MHz channels is markedly increased over narrower bandwidths. This offers a lot of value in 

many typical scenarios: a few clients transferring a lot of traffic associated with a 40-MHz AP are limited to 
802.11n’s 300 or 450 Mbps. This is true even if the APs on the adjacent 40 MHz are all lightly loaded. With the 

wider channel, more clients get to transfer their data more quickly and can complete their transmissions that much 

sooner. Ov

erall, less battery energy is consumed, and other clients don’t have as long to wait (for better quality of 

service [QoS]

). This discussion comes under the umbrella of “statistical multiplexing,” in which more multiplexing is 

more efficient for bursty traffic. 

Since the number of 160-MHz channels is tiny, 160 MHz is unsuited to typical enterprise use. In the home, every 

160-MHz channel is subject to difficult radar detection regulatory requirements. Thus, 802.11ac also introduces a 

noncontiguous 80+80 MHz mode. As can easily be imagined from the name, it is the 160-MHz waveform but is 

transmitted in two separate 80-MHz segments, each of which can lie on any allowed 80-MHz channel. To make 

this feasible, it is still a time-division-duplex system, in that APs and clients only ever transmit on 80+80 or receive 

on 80+80; they are never expected to transmit on one 80-MHz segment and receive on the second 80-MHz 

segment. 

In lightly and moderately used spectrum, this appears to provide vastly more flexibility to avoid interference, as 

shown in 

. 80+80 MHz has 13 options versus the 2 options for 160 MHz (ignoring regulatory issues). 

Unfortunately, an 80+80 MHz device is much more complicated than a 160-MHz device, since the 80+80 MHz 

device needs twice as many RF chains. A device might operate either as a two-spatial-stream 80-MHz device or as 

a one-spatial-stream 80+80 MHz device. In this case, 80+80 MHz allows the use of more spectrum but uses that 

spectrum only half as efficiently.