Friday 22 February 2013

Ethernet


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. AlthoughIEEE 802.3 differs somewhat from the original standard (the "blue book" defined in September 1980) 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 only problem is that the ISO standards tend to be issued later, and are therefore less up to date.
The simplest form of Ethernet was defined for a passive bus operated at 10 Mbps. The bus was 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 andnetwork 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.)
Ethernet LANs may be implemented using a variety of media (not just the coaxial cable described above). The types of media segments supported by Ethernet are:
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)100500 m
 10B2 (Thin Coax)30185 m
 10BT (Twisted Pair)2100 m
 10BFL (Fibre Optic)22000 m
Network Design Rules for Different types of Cable Technology
There is also a version of Ethernet which operates fibre optic links at 40 Gbps and at 100 Gbps. Many LANs combine the various speeds of operation using dual-speed switches which allow the same 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.

Did you know?

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.

A Chronology 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

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