Alvarion BreezeNET PRO.11 Series User Manual page 185

9.5.3.2 Virtual Carrier Sense
In order to reduce the probability of two stations colliding because they cannot hear
each other, the standard defines a Virtual Carrier Sense mechanism:
A station wanting to transmit a packet first transmits a short control packet called
RTS (Request To Send), which includes the source, destination, and the duration of
the following transaction (i.e. the packet and the respective ACK), the destination
station responds (if the medium is free) with a response control Packet called CTS
(Clear to Send), which includes the same duration information.
All stations receiving either the RTS or the CTS, set their Virtual Carrier Sense
indicator (called NAV, for Network Allocation Vector), for the given duration, and
use this information together with the Physical Carrier Sense when sensing the
medium.
This mechanism reduces the probability of a collision on the receiver area by a
station that is "hidden" from the transmitter to the short duration of the RTS
transmission because the station hears the CTS and "reserves" the medium as busy
until the end of the transmission. The duration information on the RTS also protects
the transmitter area from collisions during the ACK (from stations that are out of
range of the acknowledging station).
It should also be noted that, due to the fact that the RTS and CTS are short frames,
the mechanism also reduces the overhead of collisions, since these are recognized
faster than if the whole packet was to be transmitted. (This is true if the packet is
significantly bigger than the RTS, so the standard allows for short packets to be
transmitted without the RTS/CTS transmission. This is controlled per station by a
parameter called RTS Threshold).
The following diagrams show an exchange between stations A and B, and the NAV
setting of their neighbors:
BreezeNET PRO.11
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