Ethernet is based on the CSMA/CD (Carrier Sense Multiple Access with Collision
Detection) algorithm. This analogy tries to make the main steps of CSMA/CD
clear by a scenario where the packets are sent by a packet service between
the university (FH Offenburg) and the railway station. At a time there
can be only one car on the road.
Let us have a look on the main steps of the Ethernet protocol:
Listen before talk: each node has to sense the channel
and is only allowed to transmit a packet when the channel is idle.
Otherwise it has to wait for an idle channel.
Listen while talk: during transmission the sending
host performs collision detection and as soon as it detects a collision,
it has to stop its transmission.
Retransmission after collision: when a collision
is detected by a host, it sends a short jamming signal and afterwards
enters the backoff phase. It chooses a random value and waits a time
period corresponding to this value before it tries to transmit the
packet again (collision avoidance).
The analogy relates as flows to CSMA/CD:
FH Offenburg, Railway station – hosts connected to an Ethernet