The Beginning of the PSTN

The first voice transmission, sent by Alexander Graham Bell, was accomplished in 1876 through what is called a ring-down circuit. A ring-down circuit means that there was no dialing of numbers, Instead, a physical wire connected two devices. Basically, one person picked up the phone and another person was on the other end (no ringing was involved).

Over time, this simple design evolved from a one-way voice transmission, by which only one user could speak, to a bi-directional voice transmission, whereby both users could speak. Moving the voices across the wire required a carbon microphone, a battery, an electromagnet, and an iron diaphragm.

It also required a physical cable between each location that the user wanted to call. The concept of dialing a number to reach a destination, however, did not exist at this time.

To further illustrate the beginnings of the PSTN, see the basic four-telephone network shown in Figure 1-1. As you can see, a physical cable exists between each location.

Figure 1-1. Basic Four-Phone Network

Location A Location B

Figure 1-1. Basic Four-Phone Network

Location D

Place a physical cable between every household requiring access to a telephone, however, and you'll see that such a setup is neither cost-effective nor feasible (see Figure 1-2). To determine how many lines you need to your house, think about everyone you call as a value of N and use the following equation: N x (N-1)/2. As such, if you want to call 10 people, you need 45 pairs of lines running into your house.

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