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Chapter 1: Introduction To Telecommunication Systems
1-1. Introduction
Telecommunications has played a vital role in the advancement of engineering and science. In addition to its importance in public switched telephone network (PSTN), radio and television networks, the Internet, and ATM networks, telecommunications has become an important and integral part of modern society. For example, telecommunications is essential in worldwide airline reservation systems. It is also essential in commercial data processing industries, in remote accessing of data bases, in information and financial services, in inventory control systems, in automatic teller systems, and in automated order systems.
Since advancement in the theory and practice of telecommunications provide the means for attaining good quality of service in voice and data communications, sharing data bases and computer resources, etc., most engineers and scientists should have a good understanding of the fundamental concepts of telecommunications.
Historical Overview. The first significant work in telecommunications was F.B. Morse's code for telegraphy. Other significant works in the early stages of the development of telecommunications were due to A.G. Bell, G. Marconi and C.E. Shannon, among many others. In 1876, Bell invented a telephone system. In 1897, Marconi patented a wireless telegraph system. Teletypewriter service was initiated in 1931. Satellite communication was proposed by J. Pierce in 1953 and began service in 1962. During the decade of 1960, TV systems and cable TV services were offered commercially. Starting in 1969, the ARPA net created the world's first network for computer communications. The fundamental basis of the ARPA net is that messages are transmitted in a store-and-forward manner.
Store-and-forward packet switching was invented by P. Baran in 1961 in the United States. He proposed that if messages are divided into smaller units called packets for transmission, it would be more reliable, less susceptible to nuclear annihilation, and more economical than the PSTN. By the early 1980s, computer communication networks, such as the ARPA net, the Systems Network Architecture (SNA) of IBM, and the DEC net of Digital Equipment Corporation, etc. were interconnected to form the Internet.
In 1968, A.G. Fraser proposed the concepts of virtual circuits and fixed-size packets for Spider, the first asynchronous time-division multiplexing (ATDM) network. Fraser's work laid the foundation for asynchronous transfer mode (ATM) networking. By the late 1980s, CCITT agreed in principle that the broad-band integrated services digital network (B-ISDN) would be built using ATDM, and the international standard was named ATM (asynchronous transfer mode).
Unlike the PSTN and the Internet, ATM technology is still under development. ATM networks try to combine the ideas of the telephone network, such as connection-oriented service and end-to-end quality of service, and those of the Internet, such as the virtual circuits, fixed-smallsize packets and integrated services.
Definitions. In order to provide the service which permits people or machines to communicate at a distance, a telecommunication system must provide the means and facilities for connecting the user terminals or telephones at the beginning of the service and disconnecting them when the service is completed.
In telephone systems, each subscriber telephone is connected by a subscriber line to a central switching point, called a switching center, the main function of which is to provide immediate connection between pairs of subscribers. The term telephone call, or simply call, means the demand to set up a connection. A call can be regarded as a series of dialing attempts to the same number, where the last attempt is either abandoned or results in a successful connection. The subscriber line, also known as the subscriber loop, is the pair of wires connecting the subscriber to the local switching center. The subscriber loop provides a path for the two-way speech signals, ringing, switching and supervisory signals, and is permanently associated with a particular subscriber. The end office is the local central switching center typically serving only subscriber lines. To set up a connection, the end office must perform switching, signaling, and transmitting functions.
In automatic switching centers, the connections are performed by means of switching elements, generally referred to as switches. A telephone switch actually has two parts: the switching hardware carrying voice, and the switch controller handling call set-up and tear-down signals.
In studying telecommunication engineering, we need to define additional terms that are necessary to describe telecommunication systems:
Trunks. A trunk in telephone systems is a communication path that contains shared circuits that are used to interconnect central offices; in general, 2-wire lines are used for interlocal centers and 4-wire lines for intertoll centers.
Trunk Groups. A trunk group is a group of trunks between two points, both of which are switching centers and/or individual message distribution points, and which employ the same multiplex terminal equipment.
Junctors. A junctor is a connection between networks in the same central office. Its function is equivalent to an interoffice trunk.
Links. A link is a transmission path used to make a connection between successive stages of a switching network.
Network. A network is a set of communication links and switches which, when activated, is capable of supporting a multiplicity of distinct transmission paths for voice or data transmissions.
Central Offices. A central office is a central switching center that comprises a switching network and its control and support equipment.
Toll Points. A toll point is a class 4 (switching) office not offering operator assistance for incoming calls.
Toll Centers. A toll center is a class 4 office offering operator assistance for incoming calls.
Tandem Offices. A tandem office is a switching center between local offices, reducing the number of direct trunks required.
Blocking. Blocking is the state of a group of paths between two points in a network in which all the paths are occupied and hence no further interconnections are possible.
Blocking Network. A blocking network is a network that, under certain conditions, may be unable to set up a connection from one end of tie network to the other. In general, all networks used in communications are of the blocking type.
BORSCHT Circuit. A BORSCHT circuit is a line circuit in the office. It is used as a mnemonic for the functions that must be performed by the circuit: Battery, Overvoltage, Ringing, Supervision, Coding (in a digital office), Hybrid and Testing.
Modems. A modem (modulator/demodulator) is a device which modulates and demodulates signals transmitted over communication facilities. The modulator is for transmission and the demodulator is for reception. A modem is used to permit digital signals to be sent over analog lines.
Multiplexing. Multiplexing is a means for the division of a transmission facility into a number of channels either by splitting the frequency band of the channel into narrower bands, each of which is used as a distinct channel (FDM, frequency-division-multiplexing), or by allotting the entire channel to a number of users, one at a time on a timeshared basis (TDM, time-division-multiplexing).
Packets. A packet is a group of binary digits including data and control information arranged in a specified format which is transmitted as a basic unit.
Protocols. A protocol is a well-defined set of rules or conventions which governs the format and control of information that is transmitted through a network or that is stored in a data base. The information may be voice, text, data or image.
Telecommunications. The term telecommunications is a service that permits people and machines to communicate at a distance. It includes telephony, video-telephony, data transmission, teleconferencing, e-mail, etc.
Stations. A station in a telecommunication system is the device used as a means of communication by the user to the system and to other users. It includes telephone sets, terminals, printers, computers, or other types of data-communicating and data-handling devices...
