Figure 12 Physical Cable Between All Telephone Users

Due to the cost concerns and the impossibility of running a physical cable between everyone on Earth who wanted access to a telephone, another mechanism was developed that could map any phone to another phone. With this device, called a switch , the telephone users needed only one cable to the centralized switch office, instead of seven.

At first, a telephone operator acted as the switch. This operator asked callers where they wanted to dial and then manually connected the two voice paths. Figure 1-3 shows how the four-phone network example would look today with a centralized operator to switch the calls.

Figure 1-3. Centralized Operator: The Human Switch

Location A Location B

Figure 1-3. Centralized Operator: The Human Switch

Location D

Now, skip ahead 100 years or so—the human switch is replaced by electronic switches. At this point, you can learn how the modern PSTN network is built.

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