VoIPFundamentals_PSTN - 78701686 CH01 Page 4 Wednesday,...

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Unformatted text preview: 78701686 CH01 Page 4 Wednesday, October 24, 2001 4:09 PM C H A P T E R 1 Overview of the PSTN and Comparisons to Voice over IP The Public Switched Telephone Network (PSTN) has been evolving ever since Alexander Graham Bell made the first voice transmission over wire in 1876. But, before explaining the present state of the PSTN and what’s in store for the future, it is important that you understand PSTN history and it’s basics. As such, this chapter discusses the beginnings of the PSTN and explains why the PSTN exists in its current state. This chapter also covers PSTN basics, components, and services to give you a good introduction to how the PSTN operates today. Finally, it discusses where the PSTN could be improved and ways in which it and other voice networks are evolving to the point at which they combine data, video, and voice. 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. 78701686 CH01 Page 5 Wednesday, October 24, 2001 4:09 PM 6 Chapter 1: Overview of the PSTN and Comparisons to Voice over IP Figure 1-1 Basic Four-Phone Network 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 × (N–1)/2. As such, if you want to call 10 people, you need 45 pairs of lines running into your house. Figure 1-2 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....
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This note was uploaded on 08/08/2011 for the course CS 310 taught by Professor Aartisingh during the Spring '11 term at National Institute of Technology, Calicut.

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VoIPFundamentals_PSTN - 78701686 CH01 Page 4 Wednesday,...

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