User`s guide

Appendix
User’s Guide 9-28 BreezeNET PRO.11 Series
These kinds of protocols are very effective when the medium is not heavily loaded
since it allows stations to transmit with minimum delay. But there is always a
chance of two or more stations simultaneously sensing the medium as being free
and transmitting at the same time, causing a collision.
These collision situations must be identified so the MAC layer can retransmit the
packet itself, not by the upper layers, to avoid significant delay. In the Ethernet
case, a collision is recognized by the transmitting stations which listen while
transmitting and go into a retransmission phase based on an exponential random
back-off algorithm.
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:
• Implementing a Collision Detection Mechanism would require the imple-
mentation of a Full-Duplex radio capable of transmitting and receiving at the
same time, an approach that would increase the price significantly.
• 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 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 it either receives
acknowledgment or is thrown away after a given number of retransmissions.