Chapter 1: The Evolution ofEthernet reached its 25th birthday in 1998, and has seen many changes as computer technology evolved over the years. Ethernet has been constantly reinvented, evolving new capabilities and in the process growing to become the most popular network technology in the world.
This chapter describes the invention of Ethernet, and the development and organization of the Ethernet standard. Along the way we provide a brief tour of the entire set of Ethernet media systems.
History of Ethernet
On May 22, 1973, Bob Metcalfe (then at the Xerox Palo Alto Research Center, PARC, in California) wrote a memo describing the Ethernet network system he had invented for interconnecting advanced computer workstations, making it possible to send data to one another and to high-speed laser printers. Probably the bestknown invention at Xerox PARC was the first personal computer workstation with graphical user interfaces and mouse pointing device, called the Xerox Alto. The PARC inventions also included the first laser printers for personal computers, and, with the creation of Ethernet, the first high-speed LAN technology to link everything together.
This was a remarkable computing environment for the time, since the early 1970s were an era in which computing was dominated by large and very expensive mainframe computers. Few places could afford to buy and support mainframes, and few people knew how to use them. The inventions at Xerox PARC helped bring about a revolutionary change in the world of computing.
A major part of this revolutionary change in the use of computers has been the use of Ethernet LANs to enable communication among computers. Combined with an explosive increase in the use of information sharing applications such as the World Wide Web, this new model of computing has brought an entire new world of communications technology into existence. These days, sharing information is most often done over an Ethernet-, from the smallest office to the largest corporation, from the single schoolroom to the largest university campus, Ethernet is clearly the networking technology of choice.
The Aloha Network
Bob Metcalfe's 1973 Ethernet memo describes a networking system based on an earlier experiment in networking called the Aloha network. The Aloha network began at the University of Hawaii in the late 1960s when Norman Abramson and his colleagues developed a radio network for communication among the Hawaiian Islands. This system was an early experiment in the development of mechanisms for sharing a common communications channel-in this case, a common radio channel.
The Aloha protocol was very simple: an Aloha station could send whenever it liked, and then waited for an acknowledgment. If an acknowledgment wasn't received within a short amount of time, the station assumed that another station had also transmitted simultaneously, causing a collision in which the combined transmissions were garbled so that the receiving station did not hear them and did not return an acknowledgment. Upon detecting a collision, both transmitting stations would choose a random backoff time and then retransmit their packets with a good probability of success. However, as traffic increased on the Aloha channel, the collision rate would rapidly increase as well.
Abramson calculated that this system, known as pure Aloha, could achieve a maximum channel utilization of about 18 percent due to the rapidly increasing rate of collisions under increasing load. Another system, called slotted Aloha, was developed that assigned transmission slots and used a master clock to synchronize transmissions, which increased the maximum utilization of the channel to about 37 percent. In 1995, Abramson received the IEEE's Koji Kobayashi Computers and Communications Award "for development of the concept of the Aloha System, which led to modern local area networks."
Invention of Ethernet
Metcalfe realized that he could improve on the Aloha system of arbitrating access to a shared communications channel. He developed a new system that included a mechanism that detected when a collision occurred (collision detect). The system also included "listen before talk," in which stations listened for activity (carrier sense) before transmitting, and supported 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 more sophisticated backoff algorithm, which, in combination with the CSMA/CD protocol, allowed the Ethernet system to function at 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 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, Etbernet was born.
In 1976, Metcalfe drew the following diagram (Figure 1-1) "...to present Ethernet for the first time. It was used in his presentation 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. "
In July 1976, Bob Metcalfe and David Boggs published their landmark paper "Ethernet: Distributed Packet Switching for Local Computer Networks," in the Communications of the Association for Computing Machinery (CACM). In late 1977, Robert M. Metcalfe, David R. Boggs, Charles P. Thacker, and Butler W...