CSMA/CA attempts to avoid collisions by using
explicit packet acknowledgment
(ACK), which means an ACK packet is sent by the receiving station to confi rm
that the data packet arrived intact.
The CSMA/CA protocol is very effective when the medium is not heav-
ily loaded since it allows stations to transmit with minimum delay. But there is
always a chance of stations simultaneously sensing the medium as being free and
transmitting at the same time, causing a collision. These collisions must be identi-
fi ed so that the MAC layer can retransmit the packet by itself and not by the upper
layers, which would cause signifi cant delay. In the Ethernet with CSMA/CD the
collision is recognized by the transmitting station, which goes into a retransmis-
sion phase based on an exponential random backoff algorithm. While these colli-
sion detection mechanisms are a good idea on a wired LAN, they cannot be used
on a WLAN environment for two main reasons:
Implementing a collision detection mechanism would require the implemen-
tation of a full duplex radio capable of transmitting and receiving at the
same time, an approach that would increase the cost signifi cantly.
In a wireless environment we cannot assume that all stations hear each
other (which is the basic assumption of the collision detection scheme), and
the fact that a station wants to transmit and senses the medium as free does
not necessarily mean that the medium is free around the receiver area.
To overcome these problems, the 802.11 uses a CA mechanism together
with a positive ACK. The MAC layer of a station wishing to transmit senses the
medium. If the medium is free for a specifi ed time, called
distributed inter-frame
space
(DIFS), then the station is able to transmit the packet; if the medium is
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