Ethernet

-> Ethernet was developed by the Xerox Corporation's Palo Alto Research Centre (known colloquially as Xerox PARC) in 1972 and was probably the first true LAN to be introduced.

-> In 1985, the Institute of Electrical and Electronic Engineers (IEEE) in the United States of America, produced a series of standards for Local Area Networks (LANs) called the IEEE 802 standards. These have found widespread acceptability and now form the core of most LANs. One of the IEEE 802 standards, IEEE 802.3, is a standard known as "Ethernet". This is the most widely used LAN technology in the world today. 

-> The IEEE standards have been adopted by the International Standards Organisation (ISO), and is standardised in a series of standards known as ISO 8802-3. ISO was created in 1947 to construct world-wide standards for a wide variety of Engineering tasks. Adoption of ISO standards allows manufacturers to produce equipment which is guaranteed to operate anywhere it is finally used. ISO standards tend to be based on other standards (such as those produced by the IEEE), the only problem is that the ISO standards tend to be issued later, and are therefore less up to date.

-> The simplest form of Ethernet uses a passive bus operated at 10 Mbps. The bus is formed from a 50 Ohm co-axial cable which connects all the computers in the LAN. 

-> A single LAN may have up to 1024 attached systems, although in practice most LANs have far fewer. 

-> One or more pieces of coaxial cable are joined end to end to create the bus, known as an "Ethernet Cable Segment". Each segment is terminated at both ends by 50 Ohm resistors (to prevent reflections from the discontinuity at the end of the cable) and is also normally earthed at one end (for electrical safety). Computers may attach to the cable using transceivers and network interface cards.

                                                                    An Ethernet LAN consisting of three computers joined by a shared coaxial cable

Frames of data are formed using a protocol called Medium Access Control (MAC), and encoded using Manchester line encoding. Ethernet uses a simple Carrier-Sense Multiple Access protocol with Collision Detection (CSMA/CD) to prevent two computers trying to transmit at the same time (or more correctly to ensure both computers retransmit any frames which are corrupted by simultaneous transmission).

100 Mbps networks may operate full duplex (using a Fast Ethernet Switch) or half duplex (using a Fast Ethernet Hub). 1 Gbps networks usually operate between a pair of Ethernet Switches. (N.B. It is not possible to have a dual-speed hub, since a hub does not store and forward frames, however a number of manufacturers sell products they call "dual-speed hubs". In fact, such devices contain both a 10 Mbps and a 100 Mbps hubs, interconnected by a store-and-forward bridge.)