Ethernet /ˈiːθərnɛt/ is a family of computer networking technologies for local area networks (LANs). Ethernet was commercially introduced in 1980 and standardized in 1985 as IEEE 802.3. Ethernet has largely replaced competing wired LAN technologies.
The Ethernet standards comprise several wiring and signaling variants of the OSI physical layer in use with Ethernet. The original 10BASE5 Ethernet used coaxial cable as a shared medium. Later the coaxial cables were replaced with twisted pair and fiber optic links in conjunction with hubs orswitches. Data rates were periodically increased from the original 10 megabits per second to 100 gigabits per second.
Systems communicating over Ethernet divide a stream of data into shorter pieces called frames. Each frame contains source and destination addresses and error-checking data so that damaged data can be detected and re-transmitted. As per the OSI model Ethernet provides services up to and including the data link layer.
Since its commercial release, Ethernet has retained a good degree of compatibility. Features such as the 48-bit MAC address and Ethernet frameformat have influenced other networking protocols.
History[edit]
Ethernet was developed at Xerox PARC between 1973 and 1974.[1][2] It was inspired by ALOHAnet, which Robert Metcalfe had studied as part of his PhD dissertation.[3] The idea was first documented in a memo that Metcalfe wrote on May 22, 1973, where he named it after the disproven luminiferous ether as an "omnipresent, completely-passive medium for the propagation of electromagnetic waves".[1][4][5] In 1975, Xerox filed a patent application listing Metcalfe, David Boggs, Chuck Thacker, and Butler Lampson as inventors.[6] In 1976, after the system was deployed at PARC, Metcalfe and Boggs published a seminal paper.[7][note 1]
Metcalfe left Xerox in June 1979 to form 3Com.[1][9] 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", specified 10 Mbit/s Ethernet, with 48-bit destination and source addresses and a global 16-bit Ethertype-type field. It was published on September 30, 1980 as "The Ethernet, A Local Area Network. Data Link Layer and Physical Layer Specifications".[10] Version 2 was published in November, 1982[11] and defines what has become known as Ethernet II. Formal standardization efforts proceeded at the same time.
Ethernet initially competed with two largely proprietary systems, Token Ring and Token Bus. Because Ethernet was able to adapt to market realities and shift to inexpensive and ubiquitoustwisted pair wiring, these proprietary protocols soon found themselves competing in a market inundated by Ethernet products, and, by the end of the 1980s, Ethernet was clearly the dominant network technology.[1] In the process, 3Com became a major company. 3Com shipped its first 10 Mbit/s Ethernet 3C100 transceiver in March 1981, and that year started selling adapters for PDP-11s and VAXes, as well as Multibus-based Intel and Sun Microsystems computers.[12]:9 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, making it one of the largest computer networks in the world at that time.[13] An Ethernet adapter card for the IBM PC was released in 1982, and, by 1985, 3Com had sold 100,000.[9]
Since then, Ethernet technology has evolved to meet new bandwidth and market requirements.[14] In addition to computers, Ethernet is now used to interconnect appliances and other personal devices.[1] It is used in industrial applications and is quickly replacing legacy data transmission systems in the world's telecommunications networks.[15] By 2010, the market for Ethernet equipment amounted to over $16 billion per year
[edit]
Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The methods used were similar to those used in radio systems,[note 3] with the common cable providing the communication channel likened to theLuminiferous aether in 19th century physics, and it was from this reference that the name "Ethernet" was derived.[22]
Original Ethernet's shared coaxial cable (the shared medium) traversed a building or campus to every attached machine. A scheme known as carrier sense multiple access with collision detection (CSMA/CD) governed the way the computers shared the channel. This scheme was simpler than the competing token ring or token bus technologies.[note 4] Computers were connected to an Attachment Unit Interface (AUI) transceiver, which was in turn connected to the cable (later with thin Ethernet the transceiver was integrated into the network adapter). While a simple passive wire was highly reliable for small networks, it was not reliable for large extended networks, where damage to the wire in a single place, or a single bad connector, could make the whole Ethernet segment unusable.[note 5]
Through the first half of the 1980s, Ethernet's 10BASE5 implementation used a coaxial cable 0.375 inches (9.5 mm) in diameter, later called "thick Ethernet" or "thicknet". Its successor, 10BASE2, called "thin Ethernet" or "thinnet", used a cable similar to cable television cable of the era. The emphasis was on making installation of the cable easier and less costly.
Since all communications happen on the same wire, any information sent by one computer is received by all, even if that information is intended for just one destination.[note 6] The network interface card interrupts the CPU only when applicable packets are received: The card ignores information not addressed to it.[note 7] Use of a single cable also means that the bandwidth is shared, such that, for example, available bandwidth to each device is halved when two stations are simultaneously active.
Collisions happen when two stations attempt to transmit at the same time. They corrupt transmitted data and require stations to retransmit. The lost data and retransmissions reduce throughput. In the worst case where multiple active hosts connected with maximum allowed cable length attempt to transmit many short frames, excessive collisions can reduce throughput dramatically. However, a Xerox report in 1980 studied performance of an existing Ethernet installation under both normal and artificially generated heavy load. The report claims that 98% throughput on the LAN was observed.[23] This is in contrast with token passing LANs (token ring, token bus), all of which suffer throughput degradation as each new node comes into the LAN, due to token waits. This report was controversial, as modeling showed that collision-based networks theoretically became unstable under loads as low as 37% of nominal capacity. Many early researchers failed to understand these results. Performance on real networks is significantly better.[24]
In a modern Ethernet, the stations do not all share one channel through a shared cable or a simple repeater hub; instead, each station communicates with a switch, which in turn forwards that traffic to the destination station. In this topology, collisions are only possible if station and switch attempt to communicate with each other at the same time, and collisions are limited to this link. Furthermore, the 10BASE-T standard introduced a full duplex mode of operation which has become extremely common. In full duplex, switch and station can communicate with each other simultaneously, and therefore modern Ethernets are completely collision-free.
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