 other hand, takes ad- vantage of the best of both and yields a network of least cost to the user. 6.6 SHARED RESOURCES Communication networks grew, primarily, from a need to make the resources of a computer-based system available to a geographically dispersed community of users. The earliest networks were simple cir- cuit-switched channels interconnecting the computer to the users on a point-to-point basis. As private networks grew, it became more evident that sporadic use of dedicated circuits often could not be economically justified. The user was being allocated costs based on time and distance. Circuit idle time was often high. A better solution was needed-one which would allow users to share the cost of ex- pensive communication facilities. By the mid-1960's, technology had progressed to the point where the first networks based on the con- cepts of shared use of the communication facility among users be- came practical (see Fig. 6-10). The shared use of communication facilities by an entire commu- nity of users led naturally to the consideration of providing these facilities as a public rather than a private resource. From this arose the development of the so-called public data networks (PDN) (see Fig. 6-11). Public data networks allow users to share expensive com- NETWORK STRUCTURES 97 User Z User A Shared resources User A User Z Fig. 6-10. Shared community resources. munication facilities. The user, called a subscriber in PDN termi- nology, has dedicated access only to the first node of the network, called a data switching exchange (DSE). The rest of the facilities are shared among all subscribers on an as-required demand basis. Public data networks are spreading rapidly around the globe. Most countries now offer this type of service. In the United States the two major public networks are operated by GTE Telenet and TYMNET. Other specialized public networks are in operation and more are on the horizon. Shared trunk Access line S: Subscriber DSE: Data switching exchange Fig. 6-11. Public data network. 98 T