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Ethernet Analogy

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
  • Street Ethernet
  • Fritz with his car packet to be sent
  • traffic on the road channel is busy
  • checking the radio station channel sensing
  • Ambulance jamming signal
Now try and have fun!