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2 Published in 2012 by Britannica Educational Publishing (a trademark of Encyclopædia Britannica, Inc.) in association with Rosen Educational Services, LLC 29 East 21st Street, New York, NY Copyright 2012 Encyclopædia Britannica, Inc. Britannica, Encyclopædia Britannica, and the Thistle logo are registered trademarks of Encyclopædia Britannica, Inc. All rights reserved. Rosen Educational Services materials copyright 2012 Rosen Educational Services, LLC. All rights reserved. Distributed exclusively by Rosen Educational Services. For a listing of additional Britannica Educational Publishing titles, call toll free (800) First Edition Britannica Educational Publishing Michael I. Levy: Executive Editor J.E. Luebering: Senior Manager Adam Augustyn, Assistant Manager, Encyclopædia Britannica Marilyn L. Barton: Senior Coordinator, Production Control Steven Bosco: Director, Editorial Technologies Lisa S. Braucher: Senior Producer and Data Editor Yvette Charboneau: Senior Copy Editor Kathy Nakamura: Manager, Media Acquisition Robert Curley: Manager, Science and Technology Rosen Educational Services Jeanne Nagle: Senior Editor Nelson Sá: Art Director Cindy Reiman: Photography Manager Brian Garvey: Designer, Cover Design Introduction by Laura Loria Library of Congress Cataloging-in-Publication Data Breakthroughs in telephone technology: from Bell to smartphones/edited by Robert Curley. 1st ed. p. cm. (Computing and connecting in the 21st century) "In association with Britannica Educational Publishing, Rosen Educational Services." Includes bibliographical references and index. ISBN (ebook) 1. Telephone History. 2. Telecommunication History. 3. Electronics History. I. Curley, Robert, 1955 TK6015.B dc On the cover: An antique rotary telephone and a contemporary smart phone. Shutterstock.com (left), Bloomberg via Getty Images Pages iii, 10, 14, 15, 16, 18, 37, 50, 79, 102, 106, Volodin; pp. v, vi, vii (background graphic), ix pp. viii-ix Tooga/The Image Bank/Getty Images; pp. 1, 21, 45, 71, 93, 118, 120, 122 Shutterstock.com; pp. 9, 72, 98 Dolenc; remaining interior background image Ramberg.

3 CONTENTS introduction viii 9 chapter 1: the development of the basic Landline telephone 1 Working Components 2 Power Source 2 Switch Hook 3 Dialer 4 Ringer 4 Transmitter 5 Receiver 7 Anti-Sidetone Circuit 7 Early Sound Transmitters 8 Johann Reis s Philosophical Toys 10 Gray and Bell s Race for a Patent 11 The Search for a Successful Transmitter 13 Alexander Graham Bell 14 Thomas Augustus Watson 16 From Hand-Cranked to Cordless: 100 Years of AT&T Telephones 17 Western Electric 18 chapter 2: the development of the public switched telephone network 21 Switching 22 Manual Switching 23 Electromechanical Switching 24 Electronic Switching 25 Digital Switching 26 The Switching Network 27 Signaling 29 Rotary Dialing 30 Push-Button Dialing 31 Interoffice Signaling 32 Transmission

4 From Single-Wire to Two-Wire Circuits 34 Problems of Interference and Attenuation 35 Analog and Digital Transmission 36 DSL 37 Coaxial Cable 38 Microwave Link 38 Optical-Fibre Cable 40 Terrestrial Radio 41 Undersea Cable 42 Satellite 43 chapter 3: the principles of telecommunications systems 45 Analog-to-Digital Conversion 46 Sampling 48 Quantization 49 Harry Nyquist 50 Bit Mapping 51 Source Encoding 52 Huffman Codes 53 The Lempel-Ziv Algorithm 54 Run-Length Codes 55 Channel Encoding 56 Repetition Codes 57 The Hamming Code 57 Convolutional Encoding 58 Modulation 59 Amplitude Modulation and Frequency Modulation 61 Digital Modulation 61 Multiplexing 64 Frequency-Division Multiplexing 64 Time-Division Multiplexing 66 Multiple Access 68

5 Frequency-Division Multiple Access 68 Time-Division Multiple Access 69 Code-Division Multiple Access 69 chapter 4: Faxes and modems 71 The Fax 71 Standard Fax Transmission 71 History of Fax Technology 74 Bits and Bytes 79 The Modem 80 Development of Voiceband Modems 81 Voiceband Modem Operating Standards 83 Cable Modems 88 DSL Modems 90 chapter 5: mobile phones, Videophones, and internet phones 93 The Cell Phone 94 Cellular Communication 95 Development of Cellular Systems 97 CUL8R: Texting 102 The Smartphone 103 The Satellite Phone 105 The ipho ne 106 The Videophone 109 Early Videophones 110 Analog Videophones 111 Digital Videophone Systems 112 Videoconferencing 113 VoIP Conclusion 117 Glossary 118 Bibliography 120 Index 122

6 Chapter Image still TK Chapter 1 THE DEVELOPMENT OF THE BASIC LANDLINE TELEPHONE T he word telephone, from the Greek roots tēle, far, and phonē, sound, was applied as early as the late 17th century to the string telephone familiar to children sound waves traveling over string pulled taut and capped on either end by tin cans or paper cups. The term was later used to refer to the megaphone and the speaking tube. In modern usage, however, it refers solely to electrical devices derived from the inventions of Alexander Graham Bell and others in the late 19th century. Business telephone, c Corbis 1

7 7 Breakthroughs in Telephone Technology: From Bell to Smartphones 7 Within 20 years of the 1876 Bell patent, the telephone instrument, as modified by Thomas Watson, Emil Berliner, Thomas Edison, and others, acquired a functional design that has not changed fundamentally in more than a century. Metal wiring and other heavy hardware have been replaced by lightweight and compact microcircuitry, and digital signaling has allowed the introduction of a number of smart features such as automatic redialing, call-number identification, wireless transmission, and visual display. Such advances supplement the basic telephone design, but they do not replace it. working components As it has since its early years, the telephone instrument is made up of numerous functional components. These include a power source, a switch hook, a dialer, a ringer, a transmitter, a receiver, and an anti-sidetone circuit. Power Source In the first experimental telephones, the electric current that powered the telephone circuit was generated at the transmitter by means of an electromagnet activated by the speaker s voice. Such a system could not generate enough voltage to produce audible speech in distant receivers, so every transmitter since Bell s patented design has operated on a direct current supplied by an independent power source. The first sources were batteries located in the telephone instruments themselves, but since the 1890s current has been generated at the local switching office. The current is supplied through a two-wire circuit called the local loop. The standard voltage is 48 volts. Cordless telephones represent a return to individual power sources in that their low-wattage radio transmitters 2

8 7 The Development of the Basic Landline Telephone 7 A woman uses a cordless telephone. The two metal strips visible at the bottom of the handset connect with contact strips in the phone s base to recharge the unit s battery. Comstock Images/Thinkstock are powered by a small (e.g., 3.6-volt) battery located in the portable handset. When the telephone is not in use, the battery is recharged through contacts with the base unit. The base unit is powered by a transformer connection to a standard electric outlet. Switch Hook The switch hook connects the telephone instrument to the direct current supplied through the local loop. In early telephones the receiver was hung on a hook that operated the switch by opening and closing a metal contact. This system is still common, though the hook has been replaced by a cradle to hold the combined handset, enclosing both receiver and transmitter. In some modern electronic 3

9 7 Breakthroughs in Telephone Technology: From Bell to Smartphones 7 instruments, the mechanical operation of metal contacts has been replaced by a system of transistor relays. When the telephone is on hook, contact with the local loop is broken. When it is off hook (i.e., when the handset is lifted from the cradle), contact is restored and current flows through the loop. The switching office signals restoration of contact by transmitting a low-frequency dial tone actually two simultaneous tones of 350 and 440 hertz. Dialer The dialer is used to enter the number of the party that the user wishes to call. Signals generated by the dialer activate switches in the local office, which establish a transmission path to the called party. The traditional rotary dialer, invented in the 1890s, has been replaced by push-button dialing, introduced in the 1960s. In push-button dialing, the pressing of each button on a keypad generates a dualtone signal that is specific to the number being entered. The tones can travel through the telephone system so that push-button telephones can be used to activate automated functions at the other end of the line. In both rotary and push-button systems, a capacitor and resistor prevent dialing signals from passing into the ringer circuit. Ringer The ringer alerts the user to an incoming call by emitting an audible tone or ring. Ringers are of two types, the traditional mechanical or the modern electronic. Both types are activated by a 20-hertz, 75-volt alternating current generated by the switching office. In North America the ringer is activated in two-second pulses, each pulse separated by a pause of four seconds. In Europe it is common 4

10 7 The Development of the Basic Landline Telephone 7 to hear two short, half-second pulses, each pair of pulses followed by a pause of two seconds. The traditional mechanical ringer was introduced with the early Bell telephones. It consisted of two closely spaced bells, a metal clapper, and a magnet. Passage of alternating current through a coil of wire produced alternations in the magnetic attraction exerted on the clapper, so it vibrated rapidly and loudly against the bells. Volume could be muted by a switch that placed a mechanical damper against the bells. In modern electronic ringers, introduced in the 1980s, the ringer current is passed through an oscillator, which adjusts the current to the precise frequency required to activate a piezoelectric transducer a device made of a crystalline material that vibrates in response to an electric current. The transducer may be coupled to a small loudspeaker, which can be adjusted for volume. The ringer circuit remains connected to the local loop even when the telephone is on hook. A larger voltage is necessary to activate the ringer because the ringer circuit is made with a high electrical impedance in order to avoid draining power from the transmitter-receiver circuit when the telephone is in use. A capacitor prevents direct current from passing through the ringer once the handset has been lifted off the switch hook. Transmitter The transmitter is essentially a tiny microphone located in the mouthpiece of the telephone s handset. It converts the vibrations of the speaker s voice into variations in the direct current flowing through the set from the power source. In traditional carbon transmitters, developed in the 1880s, a thin layer of carbon granules separated a fixed 5

11 7 Breakthroughs in Telephone Technology: From Bell to Smartphones 7 Voice vibrations travel through the small mouthpiece opening at the bottom of telephone handsets (as seen on these Japanese mobile units) to the transmitter within. Toru Yamanaka/AFP/Getty Images electrode from a diaphragm-activated electrode. Electric current flowed through the carbon against a certain resistance. The diaphragm, vibrating in response to the speaker s voice, forced the movable electrode to exert a fluctuating pressure on the carbon layer. Fluctuations in the carbon layer created fluctuations in its electrical resistance, which in turn produced fluctuations in the electric current. In modern electret transmitters, developed in the 1970s, the carbon layer is replaced by a thin plastic sheet that has been given a conductive metallic coating on one side. The plastic separates that coating from another metal electrode and maintains an electric field between them. Vibrations caused by speech produce fluctuations in the electric field, which in turn produce small variations in voltage. The voltages are amplified for transmission over the telephone line. 6