-and- spoke networks with regional flights connecting cities to a central hub. Passengers traveling from a city on the periphery to another city on the periphery must make connections by first traveling to the central hub. The hub-and-spoke system is a way to reduce the number of links while also keeping switching to a minimum. All switching by the network takes place at the central hub. Another advantage of hub-and-spoke systems is that they can be connected through trunk lines to form a larger network. The hub-and-spoke network performs both local and long-distance switching while collecting traffic for transport or transmission on the trunk line. A tree network begins from an original node, with a series of branches reaching additional nodes or terminal nodes. There are no cycles in a tree. For a graph of a tree network see Figure 1.11. Tree networks are perhaps one of the most important types of networks. A tree network with a regular number of branches at each node, other than the terminal nodes, represents a process of exponential growth in the number of nodes and the number of links. For example, a tree with ten branches from each node has over a million links after only six steps! More precisely, 101 + 102 + 103 + 104 + 105 + 106 = 1,111,110 links. It is easy to see how a telecommunications network based on a tree structure can quickly cover a large number of households. Most telecommunications networks contain some trees within their network structure. Cable television networks tra- ditionally have been designed as trees, with transmission originating at the root node. The tree network is a powerful communications mechanism that lends itself in a natural way to both dissemination and gathering of information. The tree structure helps in the dissemination of information because each node need only contact a small number of nodes to pass along a message. With only a few iterations, the same Networks in Telecommunications Figure 1.11. A tree network. message reaches a large number of lower nodes. Furthermor