Local Loops Trunks and Interswitch Communication

The telephone infrastructure starts with a simple pair of copper wires running to your home. This physical cabling is known as a local loop . The local loop physically connects your home telephone to the central office switch (also known as a Class 5 switch or end office switch ). The communication path between the central office switch and your home is known as the phone line, and it normally runs over the local loop.

The communication path between several central office switches is known as a trunk . Just as it is not cost-effective to place a physical wire between your house and every other house you want to call, it is also not cost-effective to place a physical wire between every central office switch. You can see in Figure 1-7 that a meshed telephone network is not as scalable as one with a hierarchy of switches.

Figure 1-7. Meshed Network Versus Hierarchical Network

Figure 1-7. Meshed Network Versus Hierarchical Network

Switches are currently deployed in hierarchies. End office switches (or central office switches) interconnect through trunks to tandem switches (also referred to as Class 4 switches). Higher-layer tandem switches connect local tandem switches. Figure 1-8 shows a typical model of switching hierarchy.

Figure 1-8. Circuit-Switching Hierarchy

Figure 1-8. Circuit-Switching Hierarchy

Central office switches often directly connect to each other. Where the direct connections occur between central office switches depends to a great extent on call patterns. If enough traffic occurs between two central office switches, a dedicated circuit is placed between the two switches to offload those calls from the local tandem switches. Some portions of the PSTN use as many as five levels of switching hierarchy.

Now that you know how and why the PSTN is broken into a hierarchy of switches, you need to understand how they are physically connected, and how the network communicates.

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