How Networks Work, Sixth Edition

Part 3: Local Area Networks (LANs)

Networks are for sharing. Sharing such things as word processing and spreadsheet files, printers, communication links to distant computers and networks, and electronic mail systems is the function of a network. Every sharing activity, from car pools to bank lines, has its own rules. In networking, we call these rules standards and protocols. Standards describe how things should be; typically they set a minimum performance level. Protocols are sets of rules and agreements; they describe how elements interact. The key to understanding networking is understanding the standards and protocols that make it possible to interoperate without losing or abusing the shared files and devices.

In this section, we'll talk about standards, protocols, and sharing. First, let's spend a little time learning how many of these standards and protocols came about; then we'll talk more about sharing. Although the standards and protocols for computer communications go back to the work of Morse and Bell at the beginning of the 1900s, the standards and protocols for computer interoperation did not emerge until the early 1980s. Three streams fed the computer networking flood: IBM, the U.S. Department of Defense (DoD), and the Xerox Corporation's Palo Alto Research Center. Later, other industry and professional organizations, particularly the Institute of Electrical and Electronic Engineers (the IEEE, pronounced i-triple-e) played an important part in developing standards, but the story starts with a computer system called SAGE.

The Semi-Automatic Ground Environment (SAGE) was developed by IBM for the DoD in the 1960s. SAGE, an air-defense system that operated until the mid-1980s, used vacuumtube computers with memory banks so large that two people could stand inside them. The computers were installed in pairs in blockhouse buildings, and the filaments from the tubes in a pair of SAGE computers supplied all the winter heat for large three-story buildings in places such as Great Falls, Montana, and Duluth, Minnesota. The SAGE program involved the efforts of all the best U.S. communications and computer scientists in the 1960s and resulted in a network of interoperating computers that stretched across the United States. The program was the equivalent of the Golden Spike that linked the railroads in 1869. SAGE proved the practicality of interoperating computer systems and stimulated development efforts, particularly by IBM and the federal government.

In the 1970s, the DoD-faced with an inventory of different computers that could not intemperate-pioneered the development of network software protocols that work on more than one make and model of computer. The major set of protocols established by the DoD is the Transmission Control Protocol/Internet Protocol (TCP/IP). As the name implies, these protocols are agreements on how transmission takes place across networks. Companies, particularly those that want the federal government's business, write software that conforms to those protocols.

At about the same time in the 1970s, IBM began making public the standards and protocols it used for its proprietary computer systems. The standards included detailed descriptions of cabling, and the protocols were designed to ensure accurate communications under heavy loads. This work led others to emulate IBM's techniques and raised the quality of network development in the entire industry. It also led to an uprising by other computer companies that objected to IBM's total control of the most widely used standards and protocols. Eventually, this uprising led to the flexibility and interoperability we enjoy today.

Computer interoperation involves moving a lot of data, but it's difficult to move a lot of anything, including data, over a long distance. So, computer interoperation usually begins with computers in the same office or the same building connected to a local network. The term local area network, or LAN (rhymes with pan), describes a group of computers typically connected by no more than 1,000 feet of cable, which interoperate and allow people to share resources.

In the 1970s, IBM and Digital Equipment Corporation developed ways for a few large computers to intemperate over local networks, but the most important work on LANs for a large number of computers was done at the Xerox Corporation's Palo Alto Research Center (PARC) in the late 1970s and early 1980s. At PARC, an important set of standards and protocols called Ethernet was conceived and developed to the point of becoming a commercial product. At about the same time, people working independently at Datapoint Corporation developed a standard called ARCnet, but Datapoint kept ARCnet as a proprietary set of specifications, so it didn't have the commercial success of Ethernet. Later, IBM developed the third major networking technology we use today, Token-Ring.

The early local area network architectures, such as Ethernet and ARCnet, combined inflexible hardware specifications with strict protocol descriptions. Specific types of copper cable, specific cable connectors, one physical configuration, and certain software functions were bundled together in each LAN definition...