How Ethernet Works Essays - Ethernet, IEEE 802.3,

How Ethernet Works What Is Ethernet and How Is It Used? ITSK2511 Cecil Jackson Invention of Ethernet A gentlemen by the name of Bob Metcalfe realized that he could improve on a system called the Aloha System which arbitrated access to a shared communications channel. He developed a new system that included a mechanism that detects when a collision occurs (collision detect). The system also includes listen before talk, in which stations listen for activity (carrier sense) before transmitting, and supports access to a shared channel by multiple stations (multiple access). Put all these components together, and you can see why the Ethernet channel access protocol is called Carrier Sense Multiple Access with Collision Detect (CSMA/CD). Metcalfe also developed a much more sophisticated backoff algorithm, which, in combination with the CSMA/CD protocol, allows the Ethernet system to function all the way up to 100 percent load. In late 1972, Metcalfe and his Xerox PARC colleagues developed the first experimental Ethernet system to interconnect the Xerox Alto. The Alto was a personal workstation with a graphical user interface, and experimental Ethernet was used to link Altos to one another, and to servers and laser printers. The signal clock for the experimental Ethernet interfaces was derived from the Alto's system clock, which resulted in a data transmission rate on the experimental Ethernet of 2.94 Mbps. Metcalfe's first experimental net was called the Alto Aloha Network. In 1973 Metcalfe changed the name to Ethernet, to make it clear that the system could support any computer, and not just Altos, and to point out that his new network mechanisms had evolved well beyond the Aloha system. He chose to base the name on the word ether as a way of describing an essential feature of the system: the physical medium (cable) carries bits to all stations, much the same way that the old luminiferous ether was once thought to propagate electromagnetic waves through space. Physicists Michelson and Morley disproved the existence of the ether in 1887, but Metcalfe decided that it was a good name for his new network system that carried signals to all computers. Thus, Ethernet was born. The Ethernet System This is a brief tutorial on the Ethernet system. We'll begin with the origins of Ethernet and the Ethernet standards, and then describe the essential features of Ethernet operation. Ethernet is a local area network (LAN technology that transmits information between computers at speeds of 10 and 100 million bits per second (Mbps). Currently the most widely used version of Ethernet technology is the 10-Mbps twisted-pair variety. The 10-Mbps Ethernet media varieties include the original thick coaxial system, as well as thin coaxial, twisted-pair, and fiber optic systems. The most recent Ethernet standard defines the new 100-Mbps Fast Ethernet system which operates over twisted-pair and fiber optic media. Ethernet is a Vendor-Neutral Network Technology There are several LAN technologies in use today, but Ethernet is by far the most popular. Industry estimates indicate that as of 1994 over 40 million Ethernet nodes had been installed worldwide. The widespread popularity of Ethernet ensures that there is a large market for Ethernet equipment, which also helps keep the technology competitively priced. From the time of the first Ethernet standard, the specifications and the rights to build Ethernet technology have been made easily available to anyone. This openness, combined with the ease of use and robustness of the Ethernet system, resulted in a large Ethernet market and is another reason Ethernet is so widely implemented in the computer industry. The vast majority of computer vendors today equip their products with 10-Mbps Ethernet attachments, making it possible to link all manner of computers with an Ethernet LAN. As the 100-Mbps standard becomes more widely adopted, computers are being equipped with an Ethernet interface that operates at both 10-Mbps and 100-Mbps. The ability to link a wide range of computers using a vendor-neutral network technology is an essential feature for today's LAN managers. Most LANs must support a wide variety of computers purchased from different vendors, which requires a high degree of network interoperability of the sort that Ethernet provides. Development of Ethernet Standards Ethernet was invented at the Xerox Palo Alto Research Center in the 1970s by Dr. Robert M. Metcalfe. It was designed to