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Ethernet
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Ethernet
This section is a short review of Ethernet technology.
An Ethernet frame contains the destination address, source address, type field, and data.
An Ethernet address is 6 bytes. Every device has its own Ethernet address and listens for Ethernet frames with that destination address. All devices also listen for Ethernet frames with a wild- card destination address of "FF-FF-FF-FF-FF-FF" (in hexadecimal), called a "broadcast" address.
Ethernet uses CSMA/CD (Carrier Sense and Multiple Access with Collision Detection). CSMA/CD means that all devices communicate on a single medium, that only one can transmit at a time, and that they can all receive simultaneously. If 2 devices try to transmit at the same instant, the transmit collision is detected, and both devices wait a random (but short) period before trying to transmit again.
A good analogy of Ethernet technology is a group of people talking in a small, completely dark room. In this analogy, the physical network medium is sound waves on air in the room instead of electrical signals on a coaxial cable.
Each person can hear the words when another is talking (Carrier Sense). Everyone in the room has equal capability to talk (Multiple Access), but none of them give lengthy speeches because they are polite. If a person is impolite, he is asked to leave the room (i.e., thrown off the net).
No one talks while another is speaking. But if two people start speaking at the same instant, each of them know this because each hears something they haven't said (Collision Detection). When these two people notice this condition, they wait for a moment, then one begins talking. The other hears the talking and waits for the first to finish before beginning his own speech.
Each person has an unique name (unique Ethernet address) to avoid confusion. Every time one of them talks, he prefaces the message with the name of the person he is talking to and with his own name (Ethernet destination and source address, respectively), i.e., "Hello Jane, this is Jack, ..blah blah blah...". If the sender wants to talk to everyone he might say "everyone" (broadcast address), i.e., "Hello Everyone, this is Jack, ..blah blah blah...".