User's Manual

Appendix
BreezeNET PRO.11 Series 8-24 User’s Guide
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:
Figure 9.15: Transaction Between Stations A and B
The NAV State is combined with the physical carrier sense to indicate the
busy state of the medium.
8.5.3.3 MAC Level Acknowledgments
As mentioned earlier in this document, the MAC layer performs Collision
Detection by expecting the reception of an acknowledge to any transmitted
fragment (Packets that have more than one destination, such as Multicasts,
are not acknowledged.)