Many MAC protocols are derived from the basic Carrier Sense Multiple Access (CSMA)
mechanism (Bertsekas and Gallager 1991). CSMA is a pure distributed protocol without
centralized control, which operates as follows. The node that wants to transmit a packet
first performs the clear channel assessment procedure, i.e., it listens to the medium, for a
prescribed time. If the medium is found to be clear (or idle) during that time, the node can
transmit its packet. Otherwise, i.e., if another transmission is in progress, the node backs
off – i.e., waits for a certain time before undertaking the same procedure again.
Different MAC algorithms use different ways to calculate the time they need to listen
to the channel during the clear channel assessment procedure and to calculate the time to
wait (i.e., the duration of the backoff period) before the next transmission attempt.
It is possible that the transmissions from two or more nodes overlap in time, which
results in a collision and loss of all packets involved. If lossless communication is desired,
collisions must be detected so that the lost packets can be retransmitted. Since a collision
can be detected only at the receiver side, some form of acknowledgment from the receiver
may be needed; some MAC protocols provide this facility, while others leave it to some of
the upper layers – most likely, the transport layer. The former approach is more efficient
in terms of reaction time, whereas the latter allows for much simpler implementation of
the MAC protocol used.
In the basic CSMA protocol, carrier sensing is performed only at the sending node.
Therefore, the hidden terminal problem is still present. Moreover, the exposed terminal
problem leads to deferred transmissions and thus reduces bandwidth utilization.
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