geographically distributed computers were established in the mid-1960s (Davies and Barber, 1973; Marrill and Roberts, 1966). The ARPANET, which was to become the largest operational network in the world and to remain so for many years, was started as a four-node system by the Advanced Research Projects Agency of the U.S. Department of Defense in 1969 (Heart, 1975; Heart et al., 1978). According to Pool (1993), its successor, the Internet, connected about 1.7 million host computers and between 5 million and 15 million users as of 1993, and the numbers have been doubling annually.
The establishment and proliferation of computer networks have been accompanied—indeed made possible—by an ever-increasing blurring of the distinction between computer and communication technologies. The Internet and the many smaller networks that connect to it depend on computing resources for all aspects of their operation and for the provision of the various services, such as electronic mail and bulletin boards, teleconferencing, information utilities, and the many others that they offer.
Today there are several types of networks: local networks, long-distance networks that use telephone lines, satellite networks that communicate by radio transmission, and network complexes that use a variety of means of transmission. Some networks are designed to connect only the terminals in a single building or office complex; at the other extreme are those that connect facilities in different countries and regions of the world. Networks have been established to serve the interests of government agencies, business corporations, educational institutions, and the general public.
Not only have networks been rapidly increasing in number and size; the bandwidth or "throughput" capacity of the individual links of which they are composed has been expanding greatly as well. Wide-area networks now typically operate at 1.5 megabits (million bits) per second, and many local-area networks have transmission rates of 10 megabits per second. Systems that use optical fibers as the transmission medium support rates of 100 megabits and, in a few cases, 1 gigabit (billion bit).
Network enhancements will come from the development of increasingly powerful computing devices, many of which are especially designed for network applications, as well as from improvements in the methods for transmitting information from point to point and from the development of new network configurations and new ways of linking networks together.
Probably the most significant predictable trend in networking is a continuing increase in network bandwidths at all levels of network operations.