Tuesday, February 2, 2010


Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name came from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model as well as a common addressing format and Media Access Control at the Data Link Layer.
Ethernet is standardized as IEEE 802.3. The combination of the twisted pair versions of Ethernet for connecting end systems to the network, along with the fiber optic versions for site backbones, is the most widespread wired LAN technology. It has been used from around 1980 to the present, largely replacing competing LAN standards such as token ring, FDDI, and ARCNET.

Ethernet was developed at Xerox PARC between 1973 and 1975. It was inspired by ALOHAnet, which Robert Metcalfe had studied as part of his Ph.D. dissertation. In 1975, Xerox filed a patent application listing Metcalfe, David Boggs, Chuck Thacker and Butler Lampson as inventors. In 1976, after the system was deployed at PARC, Metcalfe and Boggs published a seminal paper.
Metcalfe left Xerox in 1979 to promote the use of personal computers and local area networks (LANs), forming 3Com. He convinced Digital Equipment Corporation (DEC), Intel, and Xerox to work together to promote Ethernet as a standard, the so-called "DIX" standard, for "Digital/Intel/Xerox"; it specified the 10 megabits/second Ethernet, with 48-bit destination and source addresses and a global 16-bit Ethertype-type field. The first standard draft was first published on September 30, 1980 by the Institute of Electrical and Electronics Engineers (IEEE).[citation needed] Support of Ethernet's carrier sense multiple access with collision detection (CSMA/CD) in other standardization bodies (i.e., ECMA, IEC, and ISO) was instrumental in getting past delays of the finalization of the Ethernet standard due to the difficult decision processes in the IEEE, and due to the competitive Token Ring proposal strongly supported by IBM.[citation needed] Ethernet initially competed with two largely proprietary systems, Token Ring and Token Bus. These proprietary systems soon found themselves inundated by Ethernet products. In the process, 3Com became a major company. 3Com built the first 10 Mbit/s Ethernet adapter (1981). This was followed quickly by DEC's Unibus to Ethernet adapter, which DEC sold and used internally to build its own corporate network, which reached over 10,000 nodes by 1986; far and away the largest extant computer network in the world at that time.
Through the first half of the 1980s, DEC's Ethernet implementation utilized a coaxial cable about the diameter of a US nickel, which became known as Thick Ethernet when its successor, Thinnet Ethernet was introduced. Thinnet uses a cable similar to cable television cable of the era. The emphasis was on making installation of the cable easier and less costly.
The observation that there was plenty of excess capacity in unused unshielded twisted pair (UTP) telephone wiring already installed in commercial buildings provided another opportunity to expand the installed base, and, thus, twisted-pair Ethernet was the next logical development in the mid-1980s, beginning with StarLAN. UTP-based Ethernet became widely known with 10BASE-T standard. This system replaced the coaxial cable systems with a system of hubs linked via UTP.
In 1990, Kalpana introduced the first Ethernet switch, which replaced the CSMA/CD scheme in favor of a switched full duplex system offering higher performance and at a lower cost than using routers.

Notwithstanding its technical merits, timely standardization was instrumental to the success of Ethernet. It required well-coordinated and partly competitive activities in several standardization bodies such as the IEEE, ECMA, IEC, and finally ISO.
In February 1980, IEEE started a project, IEEE 802, for the standardization of local area networks (LAN).
The "DIX-group" with Gary Robinson (DEC), Phil Arst (Intel), and Bob Printis (Xerox) submitted the so-called "Blue Book" CSMA/CD specification as a candidate for the LAN specification. Since IEEE membership is open to all professionals, including students, the group received countless comments on this brand-new technology.
In addition to CSMA/CD, Token Ring (supported by IBM) and Token Bus (selected and henceforward supported by General Motors) were also considered as candidates for a LAN standard. Due to the goal of IEEE 802 to forward only one standard and due to the strong company support for all three designs, the necessary agreement on a LAN standard was significantly delayed.
In the Ethernet camp, it put at risk the market introduction of the Xerox Star workstation and 3Com's Ethernet LAN products. With such business implications in mind, David Liddle (General Manager, Xerox Office Systems) and Metcalfe (3Com) strongly supported a proposal of Fritz Röscheisen (Siemens Private Networks) for an alliance in the emerging office communication market, including Siemens' support for the international standardization of Ethernet (April 10, 1981). Ingrid Fromm, Siemens representative to IEEE 802 quickly achieved broader support for Ethernet beyond IEEE by the establishment of a competing Task Group "Local Networks" within the European standards body ECMA TC24. As early as March 1982 ECMA TC24 with its corporate members reached agreement on a standard for CSMA/CD based on the IEEE 802 draft. The speedy action taken by ECMA decisively contributed to the conciliation of opinions within IEEE and approval of IEEE 802.3 CSMA/CD by the end of 1982.
Approval of Ethernet on the international level was achieved by a similar, cross-partisan action with Fromm as liaison officer working to integrate IEC TC83 and ISO TC97SC6, and the ISO/IEEE 802/3 standard was approved in 1984.

Ethernet is an evolving technology. Evolutions have included higher bandwidth, improved media access control methods, and changes to the physical medium. Ethernet evolved into the complex networking technology that today underlies most LANs. The coaxial cable was replaced with point-to-point links connected by Ethernet repeaters or switches to reduce installation costs, increase reliability, and enable point-to-point management and troubleshooting. There are many variants of Ethernet in common use.
Ethernet stations communicate by sending each other data packets, blocks of data that are individually sent and delivered. As with other IEEE 802 LANs, each Ethernet station is given a 48-bit MAC address. The MAC addresses are used to specify both the destination and the source of each data packet. Network interface cards (NICs) or chips normally do not accept packets addressed to other Ethernet stations. Adapters come programmed with a globally unique address. Despite the significant changes in Ethernet from a thick coaxial cable bus running at 10 Mbit/s to point-to-point links running at 1 Gbit/s and beyond, all generations of Ethernet (excluding early experimental versions) use the same frame formats (and hence the same interface for higher layers), and can be readily interconnected through bridging.
Due to the ubiquity of Ethernet, the ever-decreasing cost of the hardware needed to support it, and the reduced panel space needed by twisted pair Ethernet, most manufacturers now build the functionality of an Ethernet card directly into PC motherboards, eliminating the need for installation of a separate network card.