Communication Networks/Ethernet

What is Ethernet?

Ethernet was invented in 1973 at Xerox Corporation's Palo Alto Research Center(PARC) by a researcher named Bob Metcalfe. Bob Metcalfe was asked to build a networking system for the computers at PARC. Xerox wanted such a network because they were designing the world's first laser printer and wanted all of the computers there to be able to print using it. There were two challenges he faced, he had to make it fast enough for the laser printer as well as be able to connect hundreds of computers in the same building together.

CSMA/CD

Ethernet uses the carrier sense multiple access with collision detection (CSMA/CD) algorithm for arbitration. CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this:

a. A device with a frame to send listens until Ethernet is not busy.

b. When the Ethernet is not busy, the sender begins sending the frame.

c. The sender listens to make sure that no collision occurred.

d. Once the sender hears the collision, they each send a jamming signal, to ensure that all stations recognize the collision.

e. After the jamming is complete, each sender randomizes a timer and waits that long.

f. When each timer expires, the process starts over with Step 1.

So, all devices on the Ethernet need to use CSMA/CD to avoid collisions and to recover when inadvertent collisions occur.

Additional Specifications

http://inventors.about.com/library/weekly/aa111598.htm