Sec.1.1 Historical Overview 7 1012 bits/sec.Optical fiber is becoming widespread in use and is expected to be the dominant mode of transmission in the future.One consequence of this is that network costs are not expected to be dominated by transmission costs in the future.Another consequence is that network link capacities will increase dramatically;as discussed later, this will change the nature of network applications. Third,for local area networks,the cost of a network has never been dominated by transmission costs.Coaxial cable and even a twisted pair of wires can achieve relatively high-speed communication at modest cost in a small geographic area.The use of such me- dia and the desire to avoid relatively expensive switching have led to a local area network technology in which many nodes share a common high-speed communication medium on a shared multiaccess basis.This type of network structure is discussed in Chapter 4. 1.1.3 Applications of Data Networks With the proliferation of computers referred to above,it is not difficult to imagine a growing need for data communication.A brief description of several applications requir- ing communication will help in understanding the basic problems that arise with data networks. First,there are many applications centered on remote accessing of central storage facilities and of data bases.One common example is that of a local area network in which a number of workstations without disk storage use one or more common file servers to access files.Other examples are the information services and financial services available to personal computer users.More sophisticated examples,requiring many interactions between the remote site and the data base and its associated programs, include remote computerized medical diagnoses and remote computer-aided education. In some of these examples,there is a cost trade-off between maintaining the data base wherever it might be required and the communication cost of remotely accessing it as required.In other examples,in which the data base is rapidly changing,there is no alternative to communication between the remote sites and the central data base. Next,there are many applications involving the remote updating of data bases, perhaps in addition to accessing the data.Airline reservation systems,automatic teller machines,inventory control systems,automated order entry systems,and word pro- cessing with a set of geographically distributed authors provide a number of examples. Weather tracking systems and military early warning systems are larger-scale examples. In general,for applications of this type,there are many geographically separated points at which data enter the system and often many geographically separated points at which outputs are required.Whether the inputs are processed and stored at one point (as in Figs.1.1 and 1.2)or processed and stored at many points (as in Fig.1.3),there is a need for a network to collect the inputs and disseminate the outputs.In any data base with multiple users there is a problem maintaining consistency (e.g.,two users of an airline reservation system might sell the same seat on some flight).In geographically distributed systems these problems are particularly acute because of the networking delays. The communication requirements for accessing files and data bases have been increasing rapidly in recent years.Part of the reason for this is just the natural growthSec. 1.1 Historical Overview 7 1012 bits/sec. Optical fiber is becoming widespread in use and is expected to be the dominant mode of transmission in the future. One consequence of this is that network costs are not expected to be dominated by transmission costs in the future. Another consequence is that network link capacities will increase dramatically; as discussed later, this will change the nature of network applications. Third, for local area networks, the cost of a network has never been dominated by transmission costs. Coaxial cable and even a twisted pair of wires can achieve relatively high-speed communication at modest cost in a small geographic area. The use of such me￾dia and the desire to avoid relatively expensive switching have led to a local area network technology in which many nodes share a common high-speed communication medium on a shared multiaccess basis. This type of network structure is discussed in Chapter 4. 1.1.3 Applications of Data Networks With the proliferation of computers referred to above, it is not difficult to imagine a growing need for data communication. A brief description of several applications requir￾ing communication will help in understanding the basic problems that arise with data networks. First, there are many applications centered on remote accessing of central storage facilities and of data bases. One common example is that of a local area network in which a number of workstations without disk storage use one or more common file servers to access files. Other examples are the information services and financial services available to personal computer users. More sophisticated examples, requiring many interactions between the remote site and the data base and its associated programs, include remote computerized medical diagnoses and remote computer-aided education. In some of these examples, there is a cost trade-off between maintaining the data base wherever it might be required and the communication cost of remotely accessing it as required. In other examples, in which the data base is rapidly changing, there is no alternative to communication between the remote sites and the central data base. Next, there are many applications involving the remote updating of data bases, perhaps in addition to accessing the data. Airline reservation systems, automatic teller machines, inventory control systems, automated order entry systems, and word pro￾cessing with a set of geographically distributed authors provide a number of examples. Weather tracking systems and military early warning systems are larger-scale examples. In general, for applications of this type, there are many geographically separated points at which data enter the system and often many geographically separated points at which outputs are required. Whether the inputs are processed and stored at one point (as in Figs. 1.1 and 1.2) or processed and stored at many points (as in Fig. 1.3), there is a need for a network to collect the inputs and disseminate the outputs. In any data base with multiple users there is a problem maintaining consistency (e.g., two users of an airline reservation system might sell the same seat on some flight). In geographically distributed systems these problems are particularly acute because of the networking delays. The communication requirements for accessing files and data bases have been increasing rapidly in recent years. Part of the reason for this is just the natural growth