Perspectives on the PSTN

The Public Switched Telephone Network (PSTN) was built to support traffic between telephones—in other words, voice traffic. Three of the four access technologies covered in this chapter happen to use the PSTN, so a basic understanding of the PSTN can help you appreciate how modems, ISDN, and DSL work. If you already know a fair amount about the PSTN, feel free to jump ahead to the section titled "Analog Modems."

Sound waves travel through the air by vibrating the air. The human ear hears the sound because the ear vibrates as a result of the air inside the ear moving, which, in turn, causes the brain to process the sounds that were heard by the ear.

The PSTN, however, cannot forward sound waves. Instead, a telephone includes a microphone, which simply converts the sound waves into an analog electrical signal. The PSTN can send the electrical signal between one phone and another. On the receiving side, the phone converts the electrical signal back to sound waves using a speaker that is inside the part of the phone that you put next to your ear.

The analog electrical signals used to represent sound can be shown on a graph, as in Figure 15-1.

Figure 15-1 Analog Electrical Signal: Frequency, Amplitude, and Phase Voltage

Figure 15-1 Analog Electrical Signal: Frequency, Amplitude, and Phase Voltage

3 Wavelengths in Second = 3 Hz Frequency

The graph represents the three main components of the signal:

■ Frequency—Frequency is defined as how many times the signal would repeat itself, from peak to peak, in 1 second (assuming that the sound didn't change for a whole second.) The figure shows a frequency of 3 Hertz (Hz). The greater the frequency of the electrical signal is, the higher the pitch is of the sound being represented.

■ Amplitude—The amplitude represents how strong the signal is; a higher amplitude peak represents a louder sound.

■ Phase—Phase refers to where the signal is at a point in time—at the top, going down, at the bottom, going up, and so on.

The goal of the original PSTN was to create a circuit between any two phones. Each circuit consisted of an electrical path between two phones, which, in turn, supported the sending of an analog electrical signal in each direction, allowing the people on the circuit to have a conversation. Remember, the original PSTN, built by Alexander Graham Bell's new company, predated the first vacuum tube computers, so the concept of support data communication between computers wasn't a consideration for the original PSTN. It just wanted to get these analog electrical signals, which represented sounds, from one place to the other.

To set up a circuit, when the PSTN first got started, you picked up your phone. A flashing light at a switchboard at the local phone company office told the operator to pick up the phone, and then you told the operator who you wanted to talk to. If it was a local call, the operator completed the circuit literally by patching the cable at the end of the phone line connected to your house to the end of the phone line connected to the house of the person you were calling. Figure 15-2 depicts the basic concept.

Figure 15-2 Human Operator Setting Up a Circuit at a Switchboard


Figure 15-2 Human Operator Setting Up a Circuit at a Switchboard


In the figure, Sarah, the operator, picks up the phone when she sees a light flashing telling her that someone at Andy's house has picked up the phone. Andy might say something like, "Sarah, I want to talk to Barney." Because Andy, Sarah, and Barney probably all knew each other, that was enough. In a larger town, Andy might simply say, "Please ring phone number 555-1212," and Sarah would connect the call. In fact, patching the call on the switchboard is where we got the old American saying "patch me through."

Over the years, the signaling to set up a circuit got more sophisticated. Phones evolved to have a rotary dial on them, so you could just pick up the phone and dial the number you wanted to call. Later, 12-digit keypads replaced the dial so that you could simply press the numbers. For those of you who do not remember phones with dials on them, it would have taken you 20 seconds to dial a number that had lots of 8s, 9s, and 0s in them, so a keypad was a big timesaver!

The PSTN also evolved to use digital signals instead of analog signals inside the core of the PSTN. By using digital signals instead of analog, the PSTN could send more voice calls over the same physical cables, which, in turn, allowed it to grow while reducing the per-call-minute cost.

So, what is a digital signal? Digital signals represent binary numbers. Electrically, digital signals use a defined set of both positive and negative voltages, which, in turn, represent either a binary 0 or a binary 1. Encoding schemes define the rules as to which electrical signals mean a binary 0 and which ones mean a binary 1. The simplest encoding scheme might be to represent a binary 1 with +5V and a binary 0 with —5V; much more sophisticated encoding schemes are used today. Figure 15-3 shows an example of a graph of a digital signal over time, using the basic encoding scheme that was just described.

Figure 15-3 Example of a Digital Signal with a Simple Encoding Scheme Voltage


The sender of the digital signal simply varies the signal based on the encoding scheme. The receiver interprets the incoming signal according to the same encoding scheme, re-creating the digits. In the figure, if the receiver examined the signal at each point with an asterisk, the binary code would be 100101011.

So, if a device wanted to somehow send a set of binary digits to another device and there was a digital circuit between the two, it could send the appropriate digital signals over the circuit. To achieve a particular bit rate, the sender would make sure that the voltage level was at the right level at regular intervals, and the receiver would sample the incoming signal at the same rate. For instance, to achieve 28 kbps, the sender would change (as necessary) the voltage level every 1/28,000th of a second. The receiver would sample the incoming digital signal every 1/28,000th of a second as well.

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