Network Topology

The topology of a network describes the various network nodes and how they interconnect. Regulatory policies play a major role in exactly how voice network topologies are defined in each country, but general similarities exist. While topologies in competitive markets represent an interconnection of networks owned by different service providers, monopolistic markets are generally an interconnection of switches owned by the same operator.

Depending on geographical region, PSTN nodes are sometimes referred to by different names. The three node types we discuss in this chapter include:

• End Office (EO)— Also called a Local Exchange. The End Office provides network access for the subscriber. It is located at the bottom of the network hierarchy.

• Tandem— Connects EOs together, providing an aggregation point for traffic between them. In some cases, the Tandem node provides the EO access to the next hierarchical level of the network.

• Transit— Provides an interface to another hierarchical network level. Transit switches are generally used to aggregate traffic that is carried across long geographical distances.

There are two primary methods of connecting switching nodes. The first approach is a mesh topology, in which all nodes are interconnected. This approach does not scale well when you must connect a large number of nodes. You must connect each new node to every existing node. This approach does have its merits, however; it simplifies routing traffic between nodes and avoids bottlenecks by involving only those switches that are in direct communication with each other. The second approach is a hierarchical tree in which nodes are aggregated as the hierarchy traverses from the subscriber access points to the top of the tree. PSTN networks use a combination of these two methods, which are largely driven by cost and the traffic patterns between exchanges.

Figure 5-1 shows a generic PSTN hierarchy, in which End Offices are connected locally and through tandem switches. Transit switches provide further aggregation points for connecting multiple tandems between different networks. While actual network topologies vary, most follow some variation of this basic pattern.

Figure 5-1. Generic PSTN Hierarchies

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