Virtual Carrier Sense - Black Box LW0050A Manual

PRO 11 SERIES WIRELESS ETHERNET
In order to overcome these problems, 802.11 uses a Collision Avoidance (CA)
mechanism together with a Positive Acknowledge scheme, as follows:
1. A station wanting to transmit senses the medium. If the medium is busy, then it
delays. If the medium is free for a specified time (called Distributed Inter-
Frame Space [DIFS] in the standard), then the station is allowed to transmit.
2. The receiving station checks the CRC of the received packet and sends an
acknowledgment packet (ACK). Receipt of the acknowledgment indicates to
the transmitter that no collision occurred. If the sender does not receive the
acknowledgment, then it retransmits the fragment until either it receives
acknowledgment or the fragment is thrown away after a given number of
retransmissions.
E.3.2 V C
IRTUAL ARRIER
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, because 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).
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