Editing Ethernet (section)

=== Shared medium ===
[[File:10Base5transcievers.jpg|thumb|Older Ethernet equipment. Clockwise from top-left: An Ethernet transceiver with an in-line [[10BASE2]] adapter, a similar model transceiver with a [[10BASE5]] adapter, an [[Attachment Unit Interface|AUI]] cable, a different style of transceiver with 10BASE2 [[BNC connector|BNC]] T-connector, two 10BASE5 end fittings ([[N connector]]s), an orange ''[[vampire tap]]'' installation tool (which includes a specialized drill bit at one end and a socket wrench at the other), and an early model 10BASE5 transceiver (h4000) manufactured by DEC. The short length of yellow 10BASE5 cable has one end fitted with an N connector and the other end prepared to have an N connector shell installed; the half-black, half-grey rectangular object through which the cable passes is an installed vampire tap.]]

Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The method used was similar to those used in radio systems,{{Efn|There are fundamental differences between wireless and wired shared-medium communication, such as the fact that it is much easier to detect collisions in a wired system than a wireless system.}} with the common cable providing the communication channel likened to the ''Luminiferous aether'' in 19th-century physics, and it was from this reference that the name ''Ethernet'' was derived.<ref name="Spurgeon 2000">{{cite book |title=Ethernet: The Definitive Guide |url=https://archive.org/details/ethernetdefiniti0000spur |url-access=registration |author=Charles E. Spurgeon |publisher=O'Reilly |isbn=978-1-56592-660-8 |year=2000}}</ref>

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 competing Token Ring or [[Token Bus]] technologies.{{Efn|In a CSMA/CD system packets must be large enough to guarantee that the leading edge of the propagating wave of a message gets to all parts of the medium and back again before the transmitter stops transmitting, guaranteeing that [[collisions]] (two or more packets initiated within a window of time that forced them to overlap) are discovered. As a result, the minimum packet size and the physical medium's total length are closely linked.}} Computers are connected to an [[Attachment Unit Interface]] (AUI) [[transceiver]], which is in turn connected to the cable (with [[thin Ethernet]] the transceiver is usually integrated into the network adapter). While a simple passive wire is highly reliable for small networks, it is not reliable for large extended networks, where damage to the wire in a single place, or a single bad connector, can make the whole Ethernet segment unusable.{{Efn|Multipoint systems are also prone to strange failure modes when an electrical discontinuity reflects the signal in such a manner that some nodes would work properly, while others work slowly because of excessive retries or not at all. See [[standing wave]] for an explanation. These could be much more difficult to diagnose than a complete failure of the segment.}}

Through the first half of the 1980s, Ethernet's [[10BASE5]] implementation used a coaxial cable {{convert|0.375|in}} in diameter, later called ''thick Ethernet'' or ''thicknet''. Its successor, [[10BASE2]], called ''thin Ethernet'' or ''thinnet'', used the [[RG-58]] coaxial cable. The emphasis was on making installation of the cable easier and less costly.<ref name=Hegering>{{cite book |author1=Heinz-Gerd Hegering |author2=Alfred Lapple |title=Ethernet: Building a Communications Infrastructure |publisher=Addison-Wesley |date=1993 |isbn=0-201-62405-2 |url-access=registration |url=https://archive.org/details/ethernetbuilding0000hege }}</ref>{{rp|57}}

Since all communication happens on the same wire, any information sent by one computer is received by all, even if that information is intended for just one destination.{{Efn|This ''one speaks, all listen'' property is a security weakness of shared-medium Ethernet, since a node on an Ethernet network can eavesdrop on all traffic on the wire if it so chooses.}} The network interface card interrupts the [[CPU]] only when applicable packets are received: the card ignores information not addressed to it.{{Efn|name=promiscuous}} Use of a single cable also means that the data bandwidth is shared, such that, for example, available data bandwidth to each device is halved when two stations are simultaneously active.<ref>{{citation |url=http://www.lantronix.com/resources/networking-tutorials/ethernet-tutorial-networking-basics/ |title=Ethernet Tutorial – Part I: Networking Basics |date=December 9, 2014 |publisher=Lantronix |access-date=January 1, 2016 |archive-date=February 13, 2016 |archive-url=https://web.archive.org/web/20160213014814/http://www.lantronix.com/resources/networking-tutorials/ethernet-tutorial-networking-basics/ |url-status=live }}</ref>

A collision happens when two stations attempt to transmit at the same time. They corrupt transmitted data and require stations to re-transmit. The lost data and re-transmission reduces 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 claimed that 98% throughput on the LAN was observed.<ref>{{cite journal| author1=Shoch, John F. |author2=Hupp, Jon A. | title = Measured performance of an Ethernet local network| journal=Communications of the ACM| volume = 23| issue = 12| pages = 711–721| publisher=ACM Press| date=December 1980| issn = 0001-0782
| doi = 10.1145/359038.359044|s2cid=1002624 | doi-access = free}}</ref> 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.<ref>{{cite journal |author1=Boggs, D.R. |author2=Mogul, J.C. |author3=Kent, C.A. |name-list-style=amp |title=Measured capacity of an Ethernet: myths and reality |date=September 1988 |publisher=DEC WRL |url=http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-88-4.pdf |journal= |access-date=December 20, 2012 |archive-date=March 2, 2012 |archive-url=https://web.archive.org/web/20120302125906/http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-88-4.pdf |url-status=live }}</ref>

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 became common with [[Fast Ethernet]] and the de facto standard with [[Gigabit Ethernet]]. In full duplex, switch and station can send and receive simultaneously, and therefore modern Ethernets are completely collision-free.

<gallery class="center" caption="Comparison between original Ethernet and modern Ethernet" widths="250">
File:Bustopologie.png|The original Ethernet implementation: shared medium, collision-prone. All computers trying to communicate share the same cable, and so compete with each other.
File:HUB SWITCH 6.jpg|Modern Ethernet implementation: switched connection, collision-free. Each computer communicates only with its own switch, without competition for the cable with others.
</gallery>