2 Introduction and Layered Network Architecture Chap.1 Printer Terminal Central processor RJE Figure 1.1 Network with one central processor and a separate communication link to each device. control the communication to and from all the peripherals.This led to the more com- plex structure shown in Fig.1.2.The communication is automated in such systems,in contrast to telegraphy,for example,but the control of the communication is centrally exercised at the computer.While it is perfectly appropriate and widely accepted to refer to such a system as a data network or computer communication network,it is simpler to view it as a computer with remote peripherals.Many of the interesting problems associated with data networks,such as the distributed control of the system,the relaying of messages over multiple communication links,and the sharing of communication links between many users and processes,do not arise in these centralized systems. The ARPANET and TYMNET,introduced around 1970,were the first large-scale, general-purpose data networks connecting geographically distributed computer systems, users,and peripherals.Figure 1.3 shows such networks.Inside the "subnet"are a set of nodes,various pairs of which are connected by communication links.Outside the subnet are the various computers,data bases,terminals,and so on,that are connected via the subnet.Messages originate at these external devices,pass into the subnet,pass from node to node on the communication links,and finally pass out to the external recipient. The nodes of the subnet,usually computers in their own right,serve primarily to route the messages through the subnet.These nodes are sometimes called IMPs (interface message processors)and sometimes called switches.In some networks (e.g.,DECNET), nodes in the subnet might be physically implemented within the external computers using the network.It is helpful,however,to view the subnet nodes as being logically distinct from the external computers. It is important to observe that in Figs.1.1 and 1.2 the computer system is the center of the network,whereas in Fig.1.3 the subnet (i.e.,the communication part of the network)is central.Keeping this picture of external devices around a communication2 Introduction and Layered Network Architecture Chap. 1 Figure 1.1 Network with one central processor and a separate communication link to each device. control the communication to and from all the peripherals. This led to the more com￾plex structure shown in Fig. 1.2. The communication is automated in such systems, in contrast to telegraphy, for example, but the control of the communication is centrally exercised at the computer. While it is perfectly appropriate and widely accepted to refer to such a system as a data network or computer communication network, it is simpler to view it as a computer with remote peripherals. Many of the interesting problems associated with data networks, such as the distributed control of the system, the relaying of messages over multiple communication links, and the sharing of communication links between many users and processes, do not arise in these centralized systems. The ARPANET and TYMNET, introduced around 1970, were the first large-scale, general-purpose data networks connecting geographically distributed computer systems, users, and peripherals. Figure 1.3 shows such networks. Inside the "subnet" are a set of nodes, various pairs of which are connected by communication links. Outside the subnet are the various computers, data bases, terminals, and so on, that are connected via the subnet. Messages originate at these external devices, pass into the subnet, pass from node to node on the communication links, and finally pass out to the external recipient. The nodes of the subnet, usually computers in their own right, serve primarily to route the messages through the subnet. These nodes are sometimes called IMPs (interface message processors) and sometimes called switches. In some networks (e.g., DECNET), nodes in the subnet might be physically implemented within the external computers using the network. It is helpful, however, to view the subnet nodes as being logically distinct from the external computers. It is important to observe that in Figs. 1.1 and 1.2 the computer system is the center of the network, whereas in Fig. 1.3 the subnet (i.e., the communication part of the network) is central. Keeping this picture of external devices around a communication