Overview Of The Pstn And Comparisons To Voice Over Ip

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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 chapterdiscusses 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.

TheBeginning 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 designevolved 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 adestination, 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.

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Chapter 1: Overview of the PSTN and Comparisons to Voice over IP

Figure 1-1

Basic Four-Phone Network
Location A Location B

Location C Location D

Place a physicalcable 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. Atfirst, 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.

Understanding PSTN Basics

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Figure 1-3

Centralized Operator: The Human Switch
Location A Location B

Location C LocationD

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.

Understanding PSTN Basics
Although it is difficult to explain every component of the PSTN, this section explains the most important pieces that make the PSTN work. The following sections discuss how your voice is transmitted across adigital network, basic circuit-switching concepts, and why your phone number is 10 digits long.

Analog and Digital Signaling
Everything you hear, including human speech, is in analog form. Until several decades ago, the telephony network was based on an analog infrastructure as well. Although analog communication is ideal for human interaction, it is neither robust nor efficient at recovering...