Technology Reference Guide

	Technology Reference Guide 3. Protocols C. Datalink Protocols-Ethernet

What is it?
Ethernet is a datalink protocol originally developed by Xerox. It is used on a wide variety systems of all sizes, from PC s to minicomputers. Ethernet was originally designed to be transmitted over a thick coaxial cable, now called thickwire. Subsequent implementations allowed the use of a smaller coaxial cable, called thinnet, as well as unshielded twisted-pair cable (UTP). UTP is available in a variety of grades designated as Category 1 through 5, with Category 5 being the best, and Category 3 the minimum recommended grade. The UTP implementation is very popular, as it allows, in some cases, for the use of existing telephone wiring. Ethernet has traditionally been less expensive to implement than Token-ring. Common terms used in connection with Ethernet include IEEE 802.3 standards, 10BaseT (twisted pair), 10Base2 (thinnet) and 10Base5 (thicknet).

What does it do?
Ethernet uses an arbitration scheme based on carrier sensing and collision detection to decide which node gets to transmit at a given time. This is referred to as CSMA/CD. The way it works is that each station waits until it senses that the channel is clear (no carrier), and then transmits. If two stations transmit at once, a collision will be detected, and each station waits a random period of time, and then re-transmits. Standard Ethernet transmits at a data rate of 10 megabits/second. New versions of Fast Ethernet, with data rates of 100 megabits/second are becoming available, but at this point there is still some lack of standardization between fast Ethernet implementations.

How is it used?
Ethernet is a widely used datalink protocol for many types of computer networks, both PC-based and with larger systems. Coaxial Ethernet networks have traditionally employed a bus topology (see cabling), although they may appear as star configurations when using a hub. Twisted-pair Ethernet uses a star topology, and almost always employs the use of hubs.

Where do I get more information?
Contact your local CBV Office.