Sec.1.2 Messages and Switching 9 unchanged.Such a geographically centralized but logically distributed structure would both allow for shared memory between the computers and for centralized repair.Why, then,are data networks with geographically distributed computational and data base facilities growing so quickly in importance?One major reason is the cost and delay of communication.With distributed computers,many computational tasks can be handled locally.Even for remote tasks.communication costs can often be reduced significantly by some local processing.Another reason is that organizations often acquire computers for local automation tasks,and only after this local automation takes place does the need for remote interactions arise.Finally,organizations often wish to have control of their own computer systems rather than be overly dependent on the pricing policies, software changes,and potential security violations of a computer utility shared with many organizations. Another advantage often claimed for a network with distributed computational facilities is increased reliability.For the centralized system in Fig.1.2 there is some truth to this claim,since the failure of a communication link could isolate a set of sites from all access to computation.For the geographically centralized but logically distributed network,especially if there are several disjoint paths between each pair of sites,the failure of a communication link is less critical and the question of reliability becomes more complex.If all the large computers and data bases in a network were centralized,the network could be destroyed by a catastrophe at the central site.Aside from this possibility,however,a central site can be more carefully protected and repairs can be made more quickly and easily than with distributed computational sites.Other than these effects,there appears to be no reason why geographically distributed computational facilities are inherently more or less reliable than geographically centralized (but logically distributed)facilities.At any rate,the main focus in what follows will be on networks as in Figs.1.3 and 1.4,where the communication subnet is properly viewed as the center of the entire network. 1.2 MESSAGES AND SWITCHING 1.2.1 Messages and Packets A message in a data network corresponds roughly to the everyday English usage of the word.For example,in an airline reservation system,we would regard a request for a reservation,including date,flight number,passenger names,and so on,as a message. In an electronic mail system,a message would be a single document from one user to another.If that same document is then forwarded to several other users,we would sometimes want to regard this forwarding as several new messages and sometimes as forwarding of the same message,depending on the context.In a file transfer system.a message would usually be regarded as a file.In an image transmission system (i.e.,pic- tures,figures,diagrams,etc.).we would regard a message as an image.In an application requiring interactive communication between two or more users,a message would be one unit of communication from one user to another.Thus,in an interactive transaction,Sec. 1.2 Messages and Switching 9 unchanged. Such a geographically centralized but logically distributed structure would both allow for shared memory between the computers and for centralized repair. Why, then, are data networks with geographically distributed computational and data base facilities growing so quickly in importance? One major reason is the cost and delay of communication. With distributed computers, many computational tasks can be handled locally. Even for remote tasks, communication costs can often be reduced significantly by some local processing. Another reason is that organizations often acquire computers for local automation tasks, and only after this local automation takes place does the need for remote interactions arise. Finally, organizations often wish to have control of their own computer systems rather than be overly dependent on the pricing policies, software changes, and potential security violations of a computer utility shared with many organizations. Another advantage often claimed for a network with distributed computational facilities is increased reliability. For the centralized system in Fig. 1.2 there is some truth to this claim, since the failure of a communication link could isolate a set of sites from all access to computation. For the geographically centralized but logically distributed network, especially if there are several disjoint paths between each pair of sites, the failure of a communication link is less critical and the question of reliability becomes more complex. If all the large computers and data bases in a network were centralized, the network could be destroyed by a catastrophe at the central site. Aside from this possibility, however, a central site can be more carefully protected and repairs can be made more quickly and easily than with distributed computational sites. Other than these effects, there appears to be no reason why geographically distributed computational facilities are inherently more or less reliable than geographically centralized (but logically distributed) facilities. At any rate, the main focus in what follows will be on networks as in Figs. 1.3 and lA, where the communication subnet is properly viewed as the center of the entire network. 1.2 MESSAGES AND SWITCHING 1.2.1 Messages and Packets A message in a data network corresponds roughly to the everyday English usage of the word. For example, in an airline reservation system, we would regard a request for a reservation, including date, flight number, passenger names, and so on, as a message. In an electronic mail system, a message would be a single document from one user to another. If that same document is then forwarded to several other users, we would sometimes want to regard this forwarding as several new messages and sometimes as forwarding of the same message, depending on the context. In a file transfer system, a message would usually be regarded as a file. In an image transmission system (i.e., pic￾tures, figures, diagrams, etc.), we would regard a message as an image. In an application requiring interactive communication between two or more users, a message would be one unit of communication from one user to another. Thus, in an interactive transaction