How Ethernet Works

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.

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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 support research on the office of the future, which included one of the world’s first personal workstations, the Xerox Alto. The first Ethernet system ran at approximately 3-Mbps and was known as experimental Ethernet. 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. The 802.3 standard is periodically updated to include new technology. Since 1985 the standard has grown to include new media systems for 10-Mbps Ethernet (e.g. twisted-pair media), as well as the latest set of specifications for 100-Mbps Fast Ethernet. Elements of the Ethernet System The 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. The following chapters describe the configuration rules for the first element, the physical media segments. Next we’ll take a quick look at the second and third elements; the set of medium access control rules in Ethernet, and the Ethernet frame Operation of Ethernet Each Ethernet-equipped computer, also known as a station, operates independently of all other stations on the network: there is no central controller. All stations attached to an Ethernet are connected to a shared signaling system, also called the medium. Ethernet signals are transmitted serially, one bit at a time, over the shared signal channel to every attached station.

To send data a station first listens to the channel, and when the channel is idle the station transmits its data in the form of an Ethernet frame, or packet. After each frame transmission, all stations on the network must contend equally for the next frame transmission opportunity. This ensures that access to the network channel is fair, and that no single station can lock out the other stations. Access to the shared channel is determined by the medium access control (MAC) mechanism embedded in the Ethernet interface located in each station. The medium access control mechanism is based on a system called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

I. The CSMA/CD Protocol The CSMA/CD protocol functions somewhat like a dinner party in a dark room. Everyone around the table must listen for a period of quiet before speaking (Carrier Sense). Once a space occurs everyone has an equal chance to say something (Multiple Access). If two people start talking at the same instant they detect that fact, and quit speaking (Collision Detection.) To translate this into Ethernet terms, 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. All Ethernet interfaces are equal in their ability to send frames onto the network. 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. The CSMA/CD protocol is designed to provide fair access to the shared channel so that all stations get a chance to use the network. After every packet transmission all stations use the CSMA/CD protocol to determine which station gets to use the Ethernet channel next. II. Collisions 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. It’s unfortunate that the original Ethernet design used the word collision for this aspect of the Ethernet medium access control mechanism. If it had been called something else, such as stochastic arbitration event (SAE), then no one would worry about the occurrence of SAEs on an Ethernet. However, collision sounds like something bad has happened, leading many people to think that collisions are an indication of network failure. The truth of the matter is that collisions are absolutely normal and expected events on an Ethernet, and simply indicate that the CSMA/CD protocol is functioning as designed. As more computers are added to a given Ethernet, and as the traffic level increases, more collisions will occur as part of the normal operation of an Ethernet.

The design of the system ensures that the majority of collisions on an Ethernet that is not overloaded will be resolved in microseconds, or millionths of a second. A normal collision does not result in lost data. In the event of a collision the Ethernet interface backs off (waits) for some number of microseconds, and then automatically retransmits the data. On a network with heavy traffic loads it may happen that there are multiple collisions for a given frame transmission attempt. This is also normal behavior.

If repeated collisions occur for a given transmission attempt, then the stations involved begin expanding the set of potential backoff times from which they chose their random retransmission time. 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. This can happen only if the Ethernet channel is overloaded for a fairly long period of time, or is broken in some way.

III. Best Effort Data Delivery This brings up an interesting point, which is that the Ethernet system, in common with other LAN technologies, operates as a best effort data delivery system. To keep the complexity and cost of a LAN to a reasonable level, no guarantee of reliable data delivery is made. While the bit error rate of a LAN channel is carefully engineered to produce a system that normally delivers data extremely …