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Ethernet - Wikipedia, the Free Encyclopedia Página 1 De 12 Ethernet - Wikipedia, the free encyclopedia Página 1 de 12 Ethernet From Wikipedia, the free encyclopedia The five-layer TCP/IP model Ethernet is a family of frame-based computer 5. Application layer networking technologies for local area networks (LANs). The name comes from the physical concept of DHCP · DNS · FTP · Gopher · HTTP · the ether. It defines a number of wiring and signaling IMAP4 · IRC · NNTP · XMPP · POP3 · standards for the physical layer, through means of SIP · SMTP · SNMP · SSH · TELNET · network access at the Media Access Control RPC · RTCP · RTSP · TLS · SDP · (MAC)/Data Link Layer, and a common addressing SOAP · GTP · STUN · NTP · (more) format. 4. Transport layer Ethernet is standardized as IEEE 802.3. The TCP · UDP · DCCP · SCTP · RTP · combination of the twisted pair versions of Ethernet for RSVP · IGMP · (more) connecting end systems to the network, along with the 3. Network/Internet layer fiber optic versions for site backbones, is the most IP (IPv4 · IPv6) · OSPF · IS-IS · BGP · widespread wired LAN technology. It has been in use IPsec · ARP · RARP · RIP · ICMP · from the 1990s to the present, largely replacing ICMPv6 · (more) competing LAN standards such as token ring, FDDI, 2. Data link layer and ARCNET. In recent years, Wi-Fi, the wireless LAN 802.11 · 802.16 · Wi-Fi · WiMAX · standardized by IEEE 802.11, is prevalent in home and ATM · DTM · Token ring · Ethernet · small office networks and augmenting Ethernet in FDDI · Frame Relay · GPRS · EVDO · larger installations. HSPA · HDLC · PPP · PPTP · L2TP · ISDN · (more) Contents 1. Physical layer Ethernet physical layer · Modems · PLC 1 History · SONET/SDH · G.709 · Optical fiber · 2 General description Coaxial cable · Twisted pair · (more) 3 Dealing with multiple users 3.1 CSMA/CD shared medium Ethernet 3.1.1 Main procedure 3.1.2 Collision detected procedure 3.2 Ethernet repeaters and hubs 3.3 Bridging and switching 3.4 Dual speed hubs 3.5 More advanced networks 4 Autonegotiation and duplex mismatch 5 Ethernet frame types and the EtherType field 6 Physical layer 7 Related standards 8 Implementations 9 See also 10 References 11 External links History Ethernet was originally developed at Xerox PARC in 1973–1975.[1] Robert Metcalfe and David Boggs wrote and presented their "Draft Ethernet Overview" before March 1974. In March 1974, R.Z. Bachrach wrote a memo to Metcalfe and Boggs and their management, stating that "technically http://en.wikipedia.org/wiki/Ethernet 20/11/2007 Ethernet - Wikipedia, the free encyclopedia Página 2 de 12 or conceptually there is nothing new in your proposal" and that "analysis would show that your system would be a failure." This analysis was flawed in that it ignored the "channel capture effect", though this was not understood until 1994. In 1975, Xerox filed a patent application listing Metcalfe and Boggs, plus Chuck Thacker and Butler Lampson, as inventors (U.S. Patent 4,063,220: Multipoint data communication system with collision detection). In 1976, after the system was deployed at PARC, Metcalfe and Boggs published a seminal paper.[2] The experimental Ethernet described in that paper ran at 3 Mbit/s, and had 8-bit destination and source address fields, so Ethernet addresses were not the global addresses they are today. By software convention, the 16 bits after the destination and source address fields were a packet type field, but, as the paper says, "different protocols use disjoint sets of packet types", so those were packet types within a given protocol, rather than the packet type in current Ethernet which specifies the protocol being used. Metcalfe left Xerox in 1979 to promote the use of personal computers and local area networks (LANs), forming 3Com. He convinced DEC, Intel, and Xerox to work together to promote Ethernet as a standard, the so-called "DIX" standard, for "Digital/Intel/Xerox"; it standardized the 10 megabits/second Ethernet, with 48-bit destination and source addresses and a global 16-bit type field. The standard was first published on September 30, 1980. It competed with two largely proprietary systems, token ring and ARCNET, but those soon found themselves buried under a tidal wave of Ethernet products. In the process, 3Com became a major company. Twisted-pair Ethernet systems have been developed since the mid-80s, beginning with StarLAN, but becoming widely known with 10BASE-T. These systems replaced the coaxial cable on which early Ethernets were deployed with a system of hubs linked with unshielded twisted pair (UTP), ultimately replacing the CSMA/CD scheme in favor of a switched full duplex system offering higher performance. General description Ethernet was originally based on the idea of computers communicating over a shared coaxial cable acting as a broadcast transmission medium. The methods used show some similarities to radio systems, although there are fundamental differences, such as the fact that it is much easier to detect collisions in a cable broadcast system than a radio broadcast. The common cable providing the communication channel was likened to the ether and it was from this reference that the name "Ethernet" was derived. A 1990s Ethernet network From this early and comparatively simple concept, Ethernet interface card. This is a evolved into the complex networking technology that today combination card that supports underlies most LANs. The coaxial cable was replaced with point- both coaxial-based 10BASE2 (BNC connector, left) and to-point links connected by hubs and/or switches to reduce twisted pair-based 10BASE-T, installation costs, increase reliability, and enable point-to-point aka RJ45 (8P8C modular management and troubleshooting. StarLAN was the first step in the connector, right). evolution of Ethernet from a coaxial cable bus to a hub-managed, twisted-pair network. The advent of twisted-pair wiring dramatically lowered installation costs relative to competing technologies, including the older Ethernet technologies. Above the physical layer, 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 http://en.wikipedia.org/wiki/Ethernet 20/11/2007 Ethernet - Wikipedia, the free encyclopedia Página 3 de 12 is given a single 48-bit MAC address, which is used both to specify 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 generally come programmed with a globally unique address, but this can be overridden, either to avoid an address change when an adapter is replaced, or to use locally administered addresses. 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) share the same frame formats (and hence the same interface for higher layers), and can be readily interconnected. 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, obviating the need for installation of a separate network card. Dealing with multiple users CSMA/CD shared medium Ethernet Ethernet originally used a shared coaxial cable (the shared medium) winding around 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. When a computer wanted to send some information, it used the following algorithm: Main procedure 1. Frame ready for transmission. 2. Is medium idle? If not, wait until it becomes ready and wait the interframe gap period (9.6 µs in 10 Mbit/s Ethernet). 3. Start transmitting. 4. Does a collision occur? If so, go to collision detected procedure. 5. Reset retransmission counters and end frame transmission. Collision detected procedure 1. Continue transmission until minimum packet time is reached (jam signal) to ensure that all receivers detect the collision. 2. Increment retransmission counter 3. Is maximum number of transmission attempts reached? If so, abort transmission. 4. Calculate and wait random backoff period based on number of collisions. 5. Re-enter main procedure at stage 1. This can be likened to what happens at a dinner party, where all the guests talk to each other through a common medium (the air). Before speaking, each guest politely waits for the current speaker to finish. If two guests start speaking at the same time, both stop and wait for short, random periods of time (in Ethernet, this time is generally measured in microseconds). The hope is that by each choosing a random period of time, both guests will not choose the same time to try to speak again, thus avoiding another collision. Exponentially increasing back-off times (determined using the truncated binary exponential backoff algorithm) are used when there is more than one failed attempt to transmit. http://en.wikipedia.org/wiki/Ethernet 20/11/2007 Ethernet - Wikipedia, the free encyclopedia Página 4 de 12 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 adaptor). While a simple passive wire was highly reliable for small Ethernets, 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. Multipoint systems are also prone to very 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 of why); these could be much more painful to diagnose than a complete failure of the segment.
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