An Ethernet LAN consisting of three computers joined by a shared coaxial cable.
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. Although originally specified at 2 Mbps using a 75 Ohm Coax passive bus, Ethernet continued to be developed and the final form used 50 Ohm Coax. It supported 10 Mbps. The first full Ethernet specification was known as the DIX specification - named after the rthree companies that worked on the specification: Digital, Intel and Xerox. The name proved unpopular, and it became collogually known as Blue Book Ethernet. This specification actually included Pascal source code to implement the MAC layer!
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. Although IEEE 802.3 differs somewhat from the original standard (the "blue book " ). it is very similar, and both sets of standards may be used with the same LAN.
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 guarented to operate anywhere it is finally used. ISO standards tend to be based on other standards (such as those produced by the IEEE).
The IEEE Specification desctibes a LAN formed from a 50 Ohm co-axial cable that is used to connect all the computers in the LAN. A single LAN may have up to 1024 attached systems. One or more pieces of coaxial cable are joined end to end to create a 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 earthed at one end (for electrical safety). Computers attach to the cable using transceivers and network interfaces.
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). Before transmission on the coaxial cable, these are encoded by the Network Interface using Manchester line encoding.
Transmission on the cable is regulated by a simple Carrier-Sense Multiple Access protocol with Collision Detection (CSMA/CD). This seeks to avoid two or more computers transmitting at the same time (or more correctly it ensures that when a colision occurs, the network interface retransmits any frames that are corrupted by simultaneous transmission).
CSMA/CD is used for copper and fibre cables. It is not possible to detect a collision over a radio link, so WiFi uses a different method, known as CSMA/CA.
100 Mbps networks typically operate full duplex (using a Fast Ethernet Switch). Although a half duplex method was secified (using a Fast Ethernet Hub), this was seldom implemented and is not commonly found. (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 did sell products they called "dual-speed hubs". In fact, such devices contain both a 10 Mbps and a 100 Mbps hubs, interconnected by a store-and-forward bridge). 1 Gbps and higher speed networks operate between a network interface or between a pair of Ethernet Switches.
Ethernet LANs may be implemented using a variety of media (not just the coaxial cable described above). The types of cabled media segments supported by Ethernet are:
>Wireless Ethernet uses a different physcial layer, but has many common link layer design features (e.g., )Wireless Medium access (CSMA/CA))
The network design rules for 10 Mbps using these types of media are summarised below:
| Segment type | Max Number of systems per cable segment | Max Distance of a cable segment |
| 10B5 (Thick Coax) | 100 | 500 m |
| 10B2 (Thin Coax) | 30 | 185 m |
| 10BT (Twisted Pair) | 2 | 100 m |
| 10BFL (Fibre Optic) | 2 | 2000 m |
Ethernet can operate fibre optic links at 40 Gbps and at 100 Gbps. Many LANs combine the various speeds of operation using dual-speed switches which allow a switch to connect some ports to one speed of network, and other ports at another speed. The higher speed ports are usually used to connect switches to one another.
There is also a page with brief information about various types of LAN products.
Ethernet has also evolved to provide network Operations and Management (OAM) functions. These standards allow operators to manage the network (Connectivity Fault Management, CFM) and to validate the performance of the service. Standards such as 802.3ah manages a single-hop link, other standards extend this to multi-segment networks (Y.1731) - assuring that Alarm signals can reach the approriate endpoints.
10 Gigabit interfaces are available as uplink ports on higher-end Ethernet switches, and are supported on some high-specification server NICs. The primary use is interconnection between core switches in a LAN. Currently this interface is supported only for fibre and special short-distance copper cables. It is not supported over UTP cabling.
40 Gbps interfaces are available for specialist networks.
The original work on Ethernet at Xerox PARC used 75 Ohm coaxial cable, and operated at 3 Mbps. The main reason for developing Ethernet was to share very expensive printers. Robert Metcalf (who went on to found 3COM Corp) was working at Xerox PARC, and is often considered the "father" of Ethernet.
1972 - Ethernet used at Xerox PARC
1980 - Consortium of DEC, Intel and Xerox announced the Blue Book
1982 - Version 2 of the Blue Book issued.
1982 - ISOC RFC 826 definition of the address resolution protocol for Ethernet
1984 - ISOC RFC 894 definition of IP network using Ethernet links
1985 - IEEE 802.3 (slightly incompatible with v2)
1988 - IEEE published a collection of supplements.
1988 - ISOC RFC 1042 definition of IP network using IEEE 802.3/LLC links.
1989 - ISO 802.3a Ethernet for thin coaxial cable (10Base2).
1990 - IEEE 802.3i Ethernet over CAT-5 Unshielded Twisted Pair (10BaseT).
1990 - IEEE 802.1D Ethernet Bridging (and managed switches)
1993 - 10BT Hubs and Bridges have become a common component in LANs, and start replacing 10Base2/10Base5.
1993 - IEEE 802.3j defines Ethernet over Fibre (10BaseF).
1993- IEEE 802.1D MAC Layer Bridges (ISO 10038).
1995 - IEEE 802.3u defines Fast Ethernet (100BaseTX, 100BaseT4, 100BaseFX).
1998 - 100BT Fast Ethernet has become a common component in LANs (100BaseT4 was not widely adopted).
1998 - Full-duplex mode supported in Fast Ethernet.
1998 - IEEE 802.3z defines Gigabit Ethernet over Fibre (some years later in 802.3 ab over UTP).
2001 - IEEE 802.11 (wireless) and Gigabit Ethernet have become common LAN components.
2005 - IEEE 802.3ah Link-Layer OAM
2006 - 10 Gbps Ethernet over Category-6 (10000BT) UTP is available in commercial products.
2009 - Power over Ethernet (PoE)
2009 - 40 Gbps Ethernet (extendable to 100 Gbps) IEEE P802.3ba
See also