Ethernet
uses CSMA/CD (carrier sense multiple access with collision detection). An Ethernet device
can send and listen to the wire at the same time, detecting the pattern that shows a collision
is taking place. When a radio attempts to send and listen on the same channel at the same
time, its own transmission drowns out all other signals. Collision detection is impossible.
Because they cannot be reliably detected, collisions must be avoided. 802.11 WLANs use
CSMA/CA (carrier sense multiple access with collision avoidance).
802.11 WLAN standards provide two basic methods for gaining access to the radio
medium: the mandatory Distributed Coordination Function (DCF) and the optional Point
Coordination Function (PCF).
Under DCF, stations listen to make sure the medium is clear, wait for a specified
length of time, wait an additional random backoff interval, then attempt to send. The period
during which stations are waiting their respective random backoff intervals is known as the
contention period. Data and management packets also contain a Duration/ID field. Stations
within range use this to determine how long the current transaction will take, deferring
contention until it is complete.
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