Most wired networks today use the Ethernet data link. Ethernet protocols were first developed at Xerox??™s
Palo Alto Research Center in the 1970s. Today the descendent Ethernet data link is standardized in the IEEE
802.3 standard. The Ethernet protocol is interesting in its simplicity and its similarity to human speech interaction
in groups.
Ethernet uses a CSMA/CD protocol. The full description is carrier sense, multiple access/collision detection.
Each computer on an Ethernet network attaches to the same wire. This is referred to as a ???bus??? architecture;
each computer can listen to all signals on the wire; that??™s the multiple access part. When a computer wants to
send information, the protocol requires the computer to ???listen??? to what??™s on the wire, waiting for a quiet time,
before broadcasting; that??™s the carrier sense part.
When a computer sends its message, it also listens to its own message. If the sender hears a jumbled mess
instead of a clear message, it knows that another computer must have started broadcasting at exactly the same time;
that??™s the collision detection part. When a sender detects a collision, the sender stops, and then waits a randomly
chosen interval of time before trying again.
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