 of a group of inter- connected nodes or switching centers. There are a number of different ways we can interconnect switches in a telecommunication network. If every switch in a network is connected to all other switches (or nodes) in the network, we call this "pattern" a full-mesh network. Such a network is shown in Figure 1.9a. The figure has eight nodes. 8 In the 1970s, Madrid (Spain) had 82 switching centers connected in a full-mesh network! A full-mesh network is very survivable because of a plethora of possible alter- native routes. Figure 1.9b shows a star network. It is probably the least survivable. However, it is one of the most economic nodal patterns both to install and to administer. Figure 1.9c shows a multiple star network. Of course we are free to modify such networks by adding direct routes. Usually we can apply the 20-erlang rule in such situations. If a certain traffic relation has 20 erlangs or more of BH traffic, a direct route is usually justified. The term traffic relation simply means the traffic intensity (usually the BH traffic intensity) we can expect between two known points. For instance, between Albany, NY,9 and New York Figure 1.9a A full-mesh network connecting eight nodes. The reader is challenged to redraw the figure, adding just one node for a total of nine nodes. Then add a tenth, and SO on. The increasing complexity becomes very obvious. 9 Albany is the capital of the state of New York. 1.3 INTRODUCTORY TOPICS IN TELECOMMUNICATIONS Figure 1.9b A star network. Figure 1.9c A higher-order or multiple-star network. Note the direct route between 2B1 and 2B , and note another direct route between 3A5 and 3A6. City there is a traffic relation. On that relation we'd probably expect thousands of erlangs during the busy hour. Figure 1.9d shows a hierarchical network. It is a natural outgrowth of the multiple star network shown in Figure 1.9c. The PSTNs of the world universally used a hierarchical network; CCITT recommended such a network for international application. Today there is a 