802.11 Collision Avoidance
A further problem is that most wireless cards are unable to both transmit
and receive at the same time, on the same frequency -
they employ half-duplex transmissions.
This means that while collisions do occur, they generally cannot be
detected (while transmitting).
Unlike their 802.3 wired counterparts,
802.11 wireless LANs do not employ the
Carrier Sense Multiple Access with Collision Detection protocol
(CSMA/CD).
(pedantically, it is incorrect to call 802.11 as being 'wireless
Ethernet', but everyone does).
Instead, 802.11 employs collision avoidance to reduce (but not
eliminate) the likelihood of collisions occurring.
The algorithm is termed Multiple Access with Collision Avoidance (MACA),
or CSMA/CA,
in which both physical channel sensing
and virtual channel sensing are employed.
The basic idea is that before transmitting data frames,
the sender and receiver must first exchange additional control frames
before the 'true' data frames.
The succcess or failure of this initial exchange either
reserves the medium for communication between A and B,
or directs how A, B, and all other listening nodes should act.
CITS3002 Computer Networks, Lecture 4, Local Area Networks (LANs and WLANs), p19, 20th March 2024.
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CITS3002