CIS 361 Data Communications and Networks

Chapter 5: Voice Communications



This chapter introduces concepts about voice communications. The objectives important to this chapter are:

  • explain the parts and functions of telephones
  • explain the difference between public and private switching equipment
  • describe analog signals
  • describe different telephone services and how to get them at discounts
The author suggests in this chapter that 80% of business telecommunications costs are from voice lines. As such, it makes sense to study these lines and equipment.

The first section of the chapter discusses telephone handsets. This is a generic term that the author uses to mean a telephone, not just the part you hold in your hand. The required functions for a handset are listed on page 148:

  • convert sound to electrical signals
  • convert electrical signals to sound
  • signal the phone company that you want to make a call or want to finish a call
  • tell the phone company what number you want to call
  • tell the user that a call is coming in
These basic needs are just a place to start. Many additional features are available.

The author discusses the function of the microphone in a telephone on page 149. His explanation of an analog signal is faulty, however. A signal can be digital or analog. An analog signal is one that varies from moment to moment as to its value, and can represent any value within a range of values. It cannot, however, jump from one value to another without passing through the values in between. A digital signal, on the other hand, can only be limited to representing certain, particular values. It is cleaner and more specific than an analog signal. Think of an analog signal as being like water coming out of a faucet. The user could make the water come out faster or slower, more or less, but cannot change it from full open to nothing without passing through every value in between. In a digital environment, the signal passing along the channel at any moment is not influenced by the one before or after it.

The illustration on page 151 is accurate. Sound is created by vibrations in air (or other medium). The waves of sound are typically bands of compression. These bands of compression compress the electric microphone in a telephone, and generate electrical waves which are transmitted across phone lines. The vocabulary word on page 151, sidetone, is notable. Sidetone is the portion of your signal that is passed to your own telephone speaker, allowing you to hear your own voice as the listener on the other end may hear it. This is to simulate the real world. When a human speaks, that human hears the sound as well as making it. If the phone did not work this way, people would feel unnatural.

The switchhook is the on/off switch for the phone. It is in the off-hook position when you signal the phone company that you want to make a call (or when you answer a call). Similarly, it is in the on-hook position when you hang up or end a call.

Two major systems exist for sending numbers to the phone company. Dial-pulsing is explained on page 153. It is used by systems that have rotary dials, and by those that have buttons that act like rotary dials. The number 1 on the dial is represented by a pulse for .05 seconds. The number 2 is represented by a .05 second pulse, a .05 second pause and a second .05 second pulse. The number 3 requires three pulses, separated by two pauses, and so on through 0 which needs ten pulses and nine pauses. (Footnote: you can actually simulate this by clicking the switchhook rapidly.)

Dual-Tone-Multifrequency dialing is usually called "touch tone". This system generates pairs of tones instead of pulses. A unique tone is assigned to each row and each column on a keypad, allowing us to get twelve unique button-tone combinations with only seven actual tones, as illustrated on page 155. Tone generated on this system need only last .05 seconds to be accepted by the phone company, so it is faster (unless you are dialing all 1's).

Signals pass from the user to a connection called the local loop to a phone company central office. The central office will have a mechanical or electronic central office switch. At this point, routing is first decided. Connections may have to be made to other central offices or through long distance offices.

The evolution of switching is discussed on page 157. Once, there was no switching, and phones had to be wired to each other. Switching allowed for operators to make connections with patch cords. Next came mechanical switching and electronic switching. Each evolution allowed more telephones to be serviced by one central office, using fewer people.

The idea of blocking is explained on page 159. This is when the central office cannot make your connection. Although the author says this is rare, it is very common in high use areas for cell phones.

On page 163, the author discusses the PSTN, Public Switched Telephone Network. This is the network available to all subscribers when they pick up a phone.

Analog signals are discussed starting on page 165. An analog signal has certain properties that characterize it. To understand this discussion, you must know some basic physics. Energy signals are often graphed as waves, sometimes called sine waves. The waves represent the pulsing nature of the energy. It is often graphed so that the pulsation shows as going from a positive value to zero, down to a negative value, back to zero, back to the positive value, and the pattern repeats. This is the cycle of the wave. A simple signal, like a musical note, will have a single frequency. Expressed as sound, this frequency is the pitch. Expressed as an electric signal, the frequency is how many cycles per second the signal generates. One cycle per second is called a Hertz, in honor of the German physicist Heinrich Hertz. Most signals generate many cycles per second, so we refer to kilohertz (thousands), megahertz (millions), gigahertz (billions) and so on.

A complex signal, like a voice signal, may contain may frequencies at the same time. It is possible for the human ear to hear sounds ranging from about 20 Hertz to about 20,000 Hertz. A telephone passes along signals that are within a certain band, in this case the band bordered by 300 Hertz and 3000 Hertz. This is adequate for most voice traffic, and give us a bandwidth of 2,700 Hertz.

Signals can also be measured by their signal strength. A way to think of this is "how loud is the signal?" All signals tend to fade over distance, electronic or sound signals. The fading process is called attenuation. If attenuation is too great, no usable signal is received. If the signal is too strong, however, crosstalk can occur. This is when the signal from one pair of wires interferes with signals on another pair of wires.

Another quality of a signal is the phase of it. I described a wave earlier as going from positive to zero, zero to negative, and so on. Each of these parts of the wave is logically followed by the next. However, we can vary a wave so that the parts occur out of sequence, out of phase.

So, we have determined that an phone conversation consists of a signal that has a bandwidth of about 2,700 Hertz. If we consider the wires we are using as having a potential bandwidth of 1 MHz, we are wasting a lot of capacity, unless we use multiplexing. This is the process of shifting a given set of signals to another band range, passing it along the wires and shifting it back down to its proper range at the other end. Theoretically, we could pass about 250 phone conversations across one pair of wires (page 172) but often only pass a dozen or two. This process is called Frequency Division Multiplexing (FDM) and is common on networks.

The act of changing a signal is called modulation (page 173). We can modulate the frequency, the amplitude and the phase of signals easily. Sometimes this is done in combinations. Multiplexing equipment is used to modulate signals so that more signals can be passed along a given channel.

The creation and assignment of phone numbers is discussed, beginning on page 176. It begins with nine geographic zones for the planet, then country codes, area codes or city codes. Next come exchange codes, the first three digits in a seven digit American phone number. Finally, the last four digits assign the actual account number.

The book next discusses dialing services. This topic includes local and long distance calls, calling through default service providers or designating a provider with the 10-10-XXX system. Other types of calls are described, such as 800 numbers (free to caller, billed to recipient), 900 numbers (charged at higher rates), and discounted calling plans.

The next major topic is Private Telephone Systems. The most common instance of this may be the PBX, or Private Branch Exchange, which serves to route calls and provide additional features to users within a business. PBX equipment is purchased by a business and serviced by entities other than a telephone company. The features commonly available to PBX customers are listed in the blue box on page 188. These features are explained through page 192.

It is not necessary to buy a PBX, however, since services of one may be leased through Centrex services, as described on page 192.

The remainder of the chapter discusses other telephone services: conferencing, wireless services, variations on cell phone systems, and phone systems for marine and aeronautical use.

Finally, the chapter ends with a brief mention of the use of voice lines for data transmission. Data is digital in nature, making voice lines a bad match. However, voice lines are all over, and they can and are used for this purpose.