User's Manual

Appendix
BreezeNET PRO.11 Series 8-22 User’s Guide
• Frequency Hopping Spread Spectrum (FHSS) in the 2.4 GHz Band
• Direct Sequence Spread Spectrum (DSSS) in the 2.4 GHz Band, and
• InfraRed
802.2
802.11 MAC
Data Link
Layer
FH DS IR PHY Layer
Beyond the standard functionality usually performed by MAC Layers, the
802.11 MAC performs other functions that are typically related to upper
layer protocols, such as Fragmentation, Packet Retransmissions, and
Acknowledges.
8.5.3. The MAC Layer
The MAC Layer defines two different access methods, the Distributed
Coordination Function and the Point Coordination Function:
8.5.3.1 The Basic Access Method: CSMA/CA
The basic access mechanism, called the Distributed Coordination
Function, is basically a Carrier Sense Multiple Access with Collision
Avoidance mechanism (known as CSMA/CA). CSMA protocols are well-
known in the industry, the most popular being Ethernet, which is a CSMA/
CD protocol (CD standing for Collision Detection).
A CSMA protocol works as follows: A station desiring to transmit senses
the medium. If the medium is busy (i.e. some other station is transmitting)
then the station defers its transmission to a later time. If the medium seems
free then the station is allowed to transmit.
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 backoff algorithm.