388 Part 2 ½ Regions of Computer Space Section 5 ½ Networks
not only the communication data rate but also the allowable transmission delay.
Whether the interconnected components are identical (forming a homogeneous network, shown in Fig. 1a) or different (forming a heterogeneous network, Fig. 1b)-and for whatever reason, whether dictated by the application or by economics-can have a major impact on design decisions in other dimensions (e.g., host access and access connection)
The individual functions of a network (e.g., routing and switching) may be either centralized or distributed. Networks of the earliest form had all logical and physical functions centralized. These networks were called star networks (Fig. 2a), since everything was connected to a centralized node, usually the host computer. Centralized networks are conceptually easier to design but are limited by the capacity and reliability of the central resource. Alternatively, a network function can be distributed over several nodes in the network (as in the packet routing of the ARPANET, Fig. 2b).
Hierarchical networks are interconnections of several networks and may be carried out to any number of levels. Figure 2c illustrates the simple case of a hierarchical star network, while Fig. 2d depicts local distributed networks interconnected via a backbone network.
The physical proximity of the interconnected components determines the feasibility of several other parameters, including interconnection capacities and routing strategies. A geographically distributed network where nodes are more than a kilometer apart will be significantly different from a local network whose nodes are separated by a few meters.
These topology subdimensions are an attempt to abstract properties that might be shared by several networks. But the flexibility and evolution of networks, particularly of those where geographical distribution impacts the topology through local, autonomous decisions (i.e., those in which physical distance is greater than 1 kin), means that it is very unlikely that two networks will be exactly the same. Thus the definitive method of
describing the topology is to give the complete physical structure including node types, link types, and geometry.
Following the specification of the components to be interconnected and their topology, the next most important dimension is the means of interconnection. Historically, the majority of networks have had point-to-point links. Communications across the links
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