e, messages can easily be broken into component parts and addressed properly to lower nodes. Going in the opposite direction, information gathered at lower nodes can be aggregated, analyzed, and transmitted to higher nodes. The tree structure is a fundamental aspect of the structure of information. All outlines of books, articles, or even plays have a hierarchical structure of sections, subsections, and other subdivisions. The tree or outline concept permeates com- puters through the use of directories and subdirectories that are used to organize databases or to arrange information in computer memory. The directory structure of databases and computers mirrors the structure of traditional information-filing systems. The overwhelming advantage of a tree structure is its ease of use for infor- mation storage and retrieval, because users can quickly select the appropriate label of the tree, scan the root directory, move to the appropriate subdirectory, and find the folder or file that contains the desired information. Internet search engines depend on the use of the tree structure in information storage. A ring network connects each node to a circuit. This can be represented by a circuit graph; see Figure 1.12. In such a network, the number of connections corresponds to the number of links, usually reducing the length of each of the transmission links. More switching is needed, however, because the shortest trans- mission or transportation between nodes 1 and 3 necessarily must pass through node 2. More generally, Peter Huber defined the geodesic network structure as being one in which all nodes are connected along a geodesic - a path of minimum length. Because the nodes in a data network are computerized switches, the cost of switching is considerably lower than the cost of human or mechanical switching in a traditional telephone network. As Huber (1987, pp. 1.3, 1.14) observes, "When The Structure and Functions of Networks Figure 1.12. A circuit graph representing a ring network. switching is cheap and transmission