Cisco Aironet 2702i AIR-CAP2702I-E-K9 Folheto

 

 

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Figure 3.    RTS/CTS Enhanced with Bandwidth Signaling 

 

For this reason, 802.11ac defines an enhanced RTS/CTS protocol. RTS/CTS can be used to determine when 

channel bandwidth is clear and how much, around both the initiator and the responder, as shown in 

. 

First, when an 802.11ac device sends an RTS, (1) this initiating device has to verify that the 80-MHz channel is 

clear in its vicinity, (2) the RTS is normally sent in an 802.11a Physical Protocol Data Unit (PPDU) format, and (3) 

the basic 802.11a transmission, which is 20 MHz wide, is replicated another three times to fill the 80 MHz (or 

another seven times to fill 160 MHz). Then every nearby device, regardless of whether it is an 802.11a/n/ac device, 

receives an RTS that the device can understand on its primary channel. And every device that hears the RTS has 

its virtual carrier sense set to busy (see 

). To make the protocol robust, the replication bandwidth of the 

RTS is reported inside the 802.11a PPDU.

1

 

                                                 

1

 

Since the 802.11a PPDU format doesn’t contain a bandwidth indication, 802.11ac has to play some tricks to maintain backward 

compatibility. The bandwidth indication is encoded in the scrambling sequence, and also the individual/group bit in the transmitter 
MAC address in the RTS frame is changed from “individual” to “group." This last change will be visible in sniffer traces.