Introduction and Layered Network Architecture Chap.1 subnet in mind will make it easier both to understand network layering later in this chapter and to understand the issues of distributed network control throughout the book. The subnet shown in Fig.1.3 contains a somewhat arbitrary placement of links between the subnet nodes.This arbitrary placement (or arbitrary topology as it is often called)is typical of wide area networks(i.e.,networks covering more than a metropolitan area).Local area networks (i.e.,networks covering on the order of a square kilometer or less)usually have a much more restricted topology,with the nodes typically distributed on a bus,a ring,or a star. Since 1970 there has been an explosive growth in the number of wide area and local area networks.Many examples of these networks are discussed later,including as wide area networks,the seminal ARPANET and TYMNET,and as local area networks, Ethernets and token rings.For the moment,however,Fig.1.3 provides a generic model for data networks. With the multiplicity of different data networks in existence in the 1980s,more and more networks have been connected via gateways and bridges so as to allow users of one network to send data to users of other networks (see Fig.1.4).At a fundamental level, one can regard such a network of networks as simply another network,as in Fig.1.3, with each gateway,bridge,and subnet node of each constituent network being a subnet node of the overall network.From a more practical viewpoint,a network of networks is much more complex than a single network.The problem is that each constituent subnet has its own conventions and control algorithms (i.e.,protocols)for handling data,and the gateways and bridges must deal with this inhomogeneity.We discuss this problem later after developing some understanding of the functioning of individual subnets. Personal computer Wide-area network Subnet Local area network 型电 Terminal H坐 Local area network CPU CPU Figure 1.4 Network of interconnected networks.Individual wide area networks (WANs) and local networks (LANs)are connected via bridges and gateways.4 Introduction and Layered Network Architecture Chap. 1 subnet in mind will make it easier both to understand network layering later in this chapter and to understand the issues of distributed network control throughout the book. The subnet shown in Fig. 1.3 contains a somewhat arbitrary placement of links between the subnet nodes. This arbitrary placement (or arbitrary topology as it is often called) is typical of wide area networks (i.e., networks covering more than a metropolitan area). Local area networks (i.e., networks covering on the order of a square kilometer or less) usually have a much more restricted topology, with the nodes typically distributed on a bus, a ring, or a star. Since 1970 there has been an explosive growth in the number of wide area and local area networks. Many examples of these networks are discussed later, including as wide area networks, the seminal ARPANET and TYMNET, and as local area networks, Ethemets and token rings. For the moment, however, Fig. 1.3 provides a generic model for data networks. With the multiplicity of different data networks in existence in the 1980s, more and more networks have been connected via gateways and bridges so as to allow users of one network to send data to users of other networks (see Fig. 1.4). At a fundamental level, one can regard such a network of networks as simply another network, as in Fig. 1.3, with each gateway, bridge, and subnet node of each constituent network being a subnet node of the overall network. From a more practical viewpoint, a network of networks is much more complex than a single network. The problem is that each constituent subnet has its own conventions and control algorithms (i.e., protocols) for handling data, and the gateways and bridges must deal with this inhomogeneity. We discuss this problem later after developing some understanding of the functioning of individual subnets. Subnet o rr=-=n IL-JI CPf7 TI, Terminal Figure 1.4 Network of interconnected networks. Individual wide area networks (WANs) and local networks (LANs) are connected via bridges and gateways