User's Manual

Appendix
BreezeNET PRO.11 Series 8-23 User’s Guide
While these Collision Detection Mechanisms are a good idea on a wired
LAN, they cannot be used on a wireless LAN environment for two main
reasons:
1. Implementing a Collision Detection Mechanism would require the
implementation of a Full-Duplex radio capable of transmitting and
receiving at the same time, an approach that would increase the price
significantly.
2. In a wireless environment we cannot assume that all stations can hear
each other (a basic assumption of the Collision Detection scheme), and
the fact that a station wants to transmit and senses the medium as free
doesn’t necessarily mean that the medium is free around the receiver’s
area.
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 Distrib-
uted 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 indi-
cates to the transmitter that no collision occurred. If the sender does not
receive the acknowledgment, then it retransmits the fragment until it
either receives acknowledgment or is thrown away after a given number
of retransmissions.
8.5.3.2 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