Sound is nothing more than the banging of air molecules together at a rapid rate. As we generate sound with our vocal chords, we are banging these air waves together to produce intelligible information that can be used and understood by others. This is in the form of air pressure changes. Hold your hand in front of your mouth as you speak. Feel the air pressure hitting your hand? This is the effect of sound as we generate the changes in air pressure. Our vocal chords, therefore, are moving back and forth and banging the molecules of air together. There is an old question, If a tree falls in the forest, and no one is there to hear it, does it create sound? The answer must be an obvious yes. Although no one is there to hear the information, the air pressure changes from the tree falling to the ground—regardless of where it falls—must still be producing sound. If we put a tape recorder in the forest and the tree falls, it is most likely that the sound will be captured on the recorder. Even though no one is there, the recorder still captures the noises created from the air changes. The telephone converts these sound waves to their analog equivalent in the form of electrical pulses, which then are carried across the telephone network. We can, therefore, assume that in order to communicate across these wires, we must have some form of sound. The sound will be converted into electricity. The electricity is then sent across the telephone wires. Sound is the banging together of air molecules at a rapid pace. This is called compression and rarefaction. Regardless, the human voice produces sound at a constantly changing set of frequencies (speed) and amplitudes Cloudness). The human voice changes these variables of frequency and amplitude in cycles per second. The vocal chords compress the air molecules at a rate of between 100 and 5000 times per second. To recreate the sound in good faith, the sound waves (or air pressure changes) are converted from sound into electricity.
This is what the telephone set is doing for us. As the electrical equivalent of sound is created, compare the sound wave to that of an electrical wave. Electricity is typically generated in an analog form by rotating the electromagnetic energy around a center point. As the energy is on the rise, it increases the amplitude to a peak level in decibels, then begins to fall. Because the wave is concurrent, it will have both a positive and a negative side of the electrical field. Therefore, as the signal decreases from the peak of the positive energy, it moves back toward the zero line. As with a magnet there are two poles, the positive and negative sides.
Therefore, as the wave gets to the zero line, it will continue to fall to the negative side of the voltage line until it hits some peak at the bottom side of the energy field. From there, it will in turn rise back up to the zero line (value) again. This, in effect, constitutes a 360° cycle around the base line, or one complete rotation. This rotation is called a sinusoidal wave. This sinusoidal wave is the analogous recreation of the human sound wave in its electrical form. This form of one wave is called one hertz, named after the gentleman who discovered this concept. Hertz is normally abbreviated as Hz. The human sound wave will have a constantly changing variable energy in both signal strength (the amplitude) and the number of rotations around the baseline over a period of time (the frequency). Voice creates these The human ear is responsive to variations of frequency and amplitude at rates from 25 to 22,000 hertz. This means that the ear can receive and discern all of the information contained in the human voice. This is important in telephony. Differences exist in human responsiveness to sound. For example, older humans can discern sounds ranging up to 7 to 8 kHz, as a result of abuses and deterioration of the eardrums.
A person who has fired a weapon (rifle or handgun) without proper protection of their ears will have damaged the upper and lower frequency responses. Whereas, a younger person who has not had the chance to damage the ears, will be able to receive and discern sound waves in the 16- to 18-kHz range. Although when we see these youngsters carrying those boomboxes on their shoulder, with the box blaring away and sitting directly next to the youngster’s ear, we can only imagine how the ear will respond in a matter of time. They are inadvertently destroying the frequency responses that the ear will be able to selectively respond to. The telephone company realized that the majority of usable information in a human conversation will fit in a 3-kHz range; therefore, it provides a pair of telephone wires, typically made of copper, that will carry all of the usable information on it. The telephone networks were built to carry speech, and the most commonly carried signal on the network is in the electrical equivalent of speech (voice). The telephone transmitter converts the acoustic signal (sound wave) that is generated in the human speaker’s larynx into electrical waves. Actually, the analog waves can be represented in frequency and analog changes over a broad spectrum (or band) from approximately 30 Hz to about 10 kHz. However, most of the usable and understandable energy falls in the spectrum of 200 to 3500 Hz. If we subtract the differences between the high end and the low end, the spectrum is 3300 Hz wide, or 3.3 kHz. It is not necessary to recreate all of the speech waveforms precisely to get an acceptable transmission of human speech across the telephone network. This is because the ear is not really that sensitive to very fine distinctions in the frequency changes, and the human brain can make up for any variations in the speech form by interpretation. Of course, if something does not come across the wire clearly enough, the human brain will intervene and cause the mouth to say What? This in turn will cause the transmitting end to regenerate the signal over again by repeating themselves. Because the cost of transmitting the signal across a telephone network is directly proportional to the amount of energy that must be carried, the telephone company uses a bandpass filter on the circuit. Commercially acceptable and usable information is transmitted in what is called a band-limited channel. This means that all of the usable information is allowed to pass onto the circuit, but the extraneous information that does not add significantly to the conversation is filtered off. If the extra energy is put onto the wire and carried from end to end, the costs will go up at an equal rate; this is wasteful. The human sound wave produces both frequency and amplitude changes.
This introduction is meant to teach you about the functions and technology of a Central Office.
Analog to Digital Bandwidth
The Telephone Network
A Topology of Connection
Network Hierarchy (pre 1984)
Network Hierarchy (post 1984)
North American Numbering Plan
The Subscriber Extension
Local Access and Transport Areas
Wiring Connections: Hooking Things Up
Types of Communication
Lines Vs. Trunks
Foreign Exchange Signal