Ethernet was developed by Robert Metcalfe at Xerox PARC in the early 70s and is a packet switching technology that uses Carrier Sense Mulit-access with Collision Detection (CSMA/CD).

• Carrier Sense

The device (Network Inteface Card) determines if the media (wire) is available, no electrical signalling we can transmit.

• Multi-access

All devices can sense the media at the same time.  The wire is a shared connection versus a point to point with a central controller.

• Collision Detection

The detects electrical signalling that is higher than usual and defines that as collision.  Typically, this is jump in voltage outside the expected values.

A real world example of how human use CSMA/CD is when talking.  We have talking protocol just like Ethernet’s CSMA/CD. In face to face discussions, we use cues to determine if we speak, the media is air or device(s) are our mouth (transmit) and ears (receive).   When talking, we wait to see if the media is available, no one else talking.  If we are interrupted while talking by another, we stop and wait for them to stop to continue.  Sometimes they don’t stop and we wait till they do by sensing if the media (air) is free.

CSMA/CD was created when the media was shared, but with the introduction of switching technology the collision domains were not across 24 port hubs backplane, but with switches just the wire between its port and the attached end device.  Another, real world example is why we separate people into rooms so they don’t talk over each other.

Ethernet Frames carry the source and destination information of where the data is to be sent.  Frame formats have been updated as Ethernet has become the predominant method for moving data at Layer 2 of the OSI model.

• Ethernet II

• Jumbo 

• 802.1q

A variety of traffic is defined to be carried in Ethernet here is subset of the Ethertypes available

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