EthernetInvention of EthernetEthernet StandardsEthernet System ElementsCSMA/CD ProtocolSlide 6CollisionSlide 8Slide 9Slide 10Slide 11Minimum LengthSlot TimeJam sequenceMaximum Network LengthMaximum lengthMAC AddressesMulticast and BroadcastSlide 19Slide 20Purpose of EthernetSlide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 3010 megabit/second100 megabit/secondSlide 331Ethernet``The diagram ... was drawn by Dr. Robert M. Metcalfe in 1976 to present Ethernet ... to the National Computer Conference in June of that year. On the drawing are the original terms for describing Ethernet. Since then other terms have come into usage among Ethernet enthusiasts.'' The Ethernet Sourcebook, ed. Robyn E. Shotwell (New York: North-Holland, 1985), title page. Source for this lecture: http://www.ethermanage.com/ethernet/ethernet.htmlRead Forouzan Pages 43-492Invention of Ethernet“In late 1972, Metcalfe and his Xerox PARC colleagues developed the first experimental Ethernet system to interconnect the Xerox Alto, a personal workstation with a graphical user interface. The experimental Ethernet was used to link Altos to one another, and to servers and laser printers. The signal clock for the experimental Ethernet interface 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 network 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--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 (i.e., a cable) carries bits to all stations, much the same way that the old "luminiferous ether" was once thought to propagate electromagnetic waves through space. Thus, Ethernet was born.”3Ethernet Standards•Formal specifications for Ethernet were published in 1980 by a multi-vendor consortium that created the DEC-Intel-Xerox (DIX) standard. This effort turned the experimental Ethernet into an open, production-quality Ethernet system that operates at 10-Mbps. Ethernet technology was then adopted for standardization by the LAN standards committee of the Institute of Electrical and Electronics Engineers (IEEE 802). •The IEEE standard was first published in 1985, with the formal title of "IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications." The IEEE standard has since been adopted by the International Organization for Standardization (ISO), which makes it a worldwide networking standard. •The IEEE standard provides an "Ethernet like" system based on the original DIX Ethernet technology. All Ethernet equipment since 1985 is built according to the IEEE 802.3 standard, which is pronounced "eight oh two dot three." To be absolutely accurate, then, we should refer to Ethernet equipment as "IEEE 802.3 CSMA/CD" technology. However, most of the world still knows it by the original name of Ethernet, and that's what we'll call it as well.4Ethernet System ElementsThe Ethernet system consists of three basic elements: 1. the physical medium used to carry Ethernet signals between computers, 2. a set of medium access control rules embedded in each Ethernet interface that allow multiple computers to fairly arbitrate access to the shared Ethernet channel, and 3. an Ethernet frame that consists of a standardized set of bits used to carry data over the system.5CSMA/CD Protocol•Access to the shared channel is determined by the medium access control (MAC) mechanism which is based on a system called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). •Each interface must wait until there is no signal on the channel, then it can begin transmitting. If some other interface is transmitting there will be a signal on the channel, which is called carrier. All other interfaces must wait until carrier ceases before trying to transmit, and this process is called Carrier Sense. •No one gets a higher priority than anyone else, and democracy reigns. This is what is meant by Multiple Access •Since signals take a finite time to travel from one end of an Ethernet system to the other, the first bits of a transmitted frame do not reach all parts of the network simultaneously. Therefore, it's possible for two interfaces to sense that the network is idle and to start transmitting their frames simultaneously. When this happens, the Ethernet system has a way to sense the "collision" of signals and to stop the transmission and resend the frames. This is called Collision Detect.6Carrier Sense Multiple Access with Collision Detection (CSMA/CD)Monitor to ensure wire free before transmittedMonitor during transmission to make sure not garbled by a collisionOnce collision detected stop transmission7Collision•If more than one station happens to transmit on the Ethernet channel at the same moment, then the signals are said to collide. The stations are notified of this event, and instantly reschedule their transmission using a specially designed backoff algorithm. As part of this algorithm the stations involved each choose a random time interval to schedule the retransmission of the frame, which keeps the stations from making transmission attempts in lock step. •Repeated collisions for a given packet transmission attempt indicate a busy network. The expanding backoff process, formally known as "truncated binary exponential backoff," is a clever feature of the Ethernet MAC that provides an automatic method for stations to adjust to traffic conditions on the network. Only after 16 consecutive collisions for a given transmission attempt will the interface finally discard the Ethernet packet.8Binary exponential backoff retransmission algorithmStation encounters first collision randomly select number from set {0,1}Other station(s) involved in collision select a random value from a set corresponding to number of collisions this other station has experienced.These randomly chosen values represent how many “slot times” a station must wait before attempting a retransmission.An Ethernet slot time is 512 bits (64 bytes). For 10 mbit/s Ethernet one bit takes 0.1 microseconds to transmit => slot time is 51.2 microseconds.Note: Minimum size Ethernet frame is 64