Protocols and the OSI Model

Introduction to Networks

In the 1960s and 1970s, before the PC was invented, a company would typically have only one central computer, a mainframe. Users connected to the mainframe through terminals on their desks. These terminals had no intelligence of their owntheir only function was to display a text-based user interface provided by the mainframe. Hence, they were usually called dumb terminals. The only network was the connection between the terminals and the mainframe.

In 1981, the IBM PC was releasedthis event changed the industry significantly. The PC had intelligence of its own, allowing users to do tasks on their desktops that previously required a mainframe. Networks were introduced to interconnect these distributed PCs.

The term network is used in many ways. For example, people network with one another, telephones are networked in the public telephone system, and data networks connect different computers. These uses of the term have a common thread: Networks provide the ability for people or devices to communicate with each other.

A data network is a network that allows computers to exchange data. The simplest data network is two PCs connected through a cable. However, most data networks connect many devices.

An internetwork is a collection of individual networks that are connected by networking devices and that function as a single large network. The public Internet is the most common exampleit is a single network that connects millions of computers. Internetworking refers to the industry and products that design, implement, and administer internetworks.

The first networks were LANs; they enabled multiple users in a relatively small geographical area to exchange files and messages and to access shared resources such as printers and disk storage. WANs were introduced to interconnect these LANs so that geographically dispersed users could also share information. The "LANs and WANs" section, later in this appendix, further describes these two types of networks.

Note

Appendix D, "Abbreviations," lists many of the acronyms that appear in this book.