Theme 1 Telephones and Mobile Phones
Unit 5
Unit 5 Telephone Systems (Rev 1.0)
Telephone Systems
5.1 Introduction
Although most people take it for granted, a telephone system is extremely complicated in reality.
It is a voice communication system that has been developed using analog transmission. Today,
however, most parts of a telephone system use digital communications.
A telephone system can be divided into four main parts:
• instruments like handsets or fax machines;
• local loop;
• switching facilities; and
• trunk circuits.
5.2 The Local Loop
The local loop is the pair of wires that go from a telephone office to each home. Figure 5.2.1
shows how these wires go from a telephone office (called a central office) to an area that the
office is serving. Figure 5.2.2 shows a telephone box which connects to one end of a local loop.
A cable from a telephone instrument can be plugged into the socket on the box for connection to
the telephone system.
Figure 5.2.1 The local loop.
1
Theme 1 Telephones and Mobile Phones
Unit 5 Telephone Systems (Rev 1.0)
Figure 5.2.2 A telephone box.
5.3 Telephone Switching
To see why telephone switching is needed, see Figure 5.3.1. If there are only two or three
telephones, calls can be made between any two of them if they have direct wires connecting them.
In this case, no switching is required. However, if the number of telephones increases, the
number of wires connecting the telephones will also need to increase at a very fast pace. Since it
is not possible to install so many wires in practice, switching is required.
Figure 5.3.1 Switching is required if there are many telephones to be connected together.
The concept on switching is simple – setting up and releasing connections between two
telephones when needed. However, there are over 600 million telephones in the world. Switching
for so many telephones is not an easy task. In a telephone system, switchi ng is carried out by
telephone offices. The telephone offices are connected together by trunk circuits in a hierarchical
structure as shown in Figure 5.3.2. In the figure, there are two levels of telephone offices. The
squares are the higher level ones called toll offices. Each of these telephone offices serves a large
geographical area. All the lower level telephone offices in an area, called local central offices (or
2
Theme 1 Telephones and Mobile Phones
Unit 5 Telephone Systems (Rev 1.0)
simply central offices), will be connected to the toll office of the area via the trunk circuits. With
the help of the local loops, telephones are in turn connected to the nearest local central offices.
Figure 5.3.2 Hierarchical switching used in telephone systems.
5.4 Advanced Telephone Services
Today, telephone systems provide many services other than voice service. These services include
Interactive Voice Response (IVR), computer telephony, and others.
An IVR system is a system that allows users to input data through the telephone touch-tone
keypads. When a user dials an IVR system, he/she actually converses with a computer. By using
the keypad on a telephone, the user can instruct the computer to perform operations as desired.
Many IVR services are in use today. Examples include banking services, information inquiry and
voice mailing services.
Figure 5.4.1 An IP phone.
3
Theme 1 Telephones and Mobile Phones
Unit 5 Telephone Systems (Rev 1.0)
Computer telephony is the new trend in providing voice telephone service. In computer telephony,
computers are used for voice communication instead of telephones. The advantage of using
computers to replace telephones is that in addition to voice conversations, there are many other
types of communications provided, such as video. Since the Internet is widely used today,
Internet enabled computers can also be used to provide this kind of service. This is called the
Internet Phone service. The telephones that connect to the Internet and provide voice service are
often called the IP Phones (see Figure 5.4.1).
5.5 Summary
In summary, a telephone system is a system that connects telephones together. Its main function
is to provide voice conversation service to the users. To properly perform this function, switching
is required. Nowadays telephone systems use a hierarchical switching structure. Although
telephone systems were developed using analog transmission, they are almost fully digitized
today. Due to the digitization of telephone systems and the advances in computer and
communication technologies, telephone systems can now provide many advanced services beside
the basic voice communication service.
Key Terms
Local Loop: The link that connects a home telephone to a telephone office.
Telephone Switch: A device that connects multiple telephone lines together and provides
communication paths for the lines.
Trunk Circuits: The communication circuits between two switching offices.
Switching: The setting up and releasing of connections between two telephones when needed.
Hierarchical Switching: The switching approach that organizes the switches in a hierarchical
(tree-like) structure.
Toll Office: A higher level telephone office that usually provides long-distance connections.
Central Office: A lower level telephone office that connects to user telephones.
IVR: An IVR system is a system that allows users to input data through the telephone touch-tone
keypads.
IP: Internetworking Protocol, the computer protocol that is widely used in the Internet.
Additional Resources
Web of fixed network operators
OFTA
Telephone Country code
PPS
4