Electro-Optical Transmitters

The efficiency of an electro-optical transmitter is determined by many factors, but the most important are the following: spectral line width, which is the width of the carrier spectrum and is zero for an ideal monochromatic light source; insertion loss, which is the amount of transmitted energy that does not couple into the fibre; transmitter lifetime; and maximum operating bit rate.

Two kinds of electro-optical transmitters are commonly used in optical fibre links—the light-emitting diode and the semiconductor laser. The light-emitting diode (LED) is a broad-line width light source that is used for medium-speed, short-span links in which dispersion of the light beam over distance is not a major problem. The LED is lower in cost and has a longer lifetime than the semiconductor laser. However, the semiconductor laser couples its light output to the optical fibre much more efficiently than the LED, making it more suitable for longer spans, and it also has a faster “rise” time, allowing higher data transmission rates. Laser diodes are available that operate at wavelengths in the proximity of 0.85, 1.3, and 1.5 micrometre and have spectral line widths of less than 0.003 micrometre. They are capable of transmitting at over 10 gigabits per second. LEDs capable of operating over a broader range of carrier wavelengths exist, but they generally have higher insertion losses and linewidths exceeding 0.035 micrometre.

 

Optoelectronic Receivers

The two most common kinds of optoelectronic receivers for optical links are the positive-intrinsic-negative (PIN) photodiode and the avalanche photodiode (APD). These optical receivers extract the baseband signal from a modulated optical carrier signal by converting incident optical power into electric current. The PIN photodiode has low gain but very fast response; the APD has high gain but slower response.

UNIT 19

Optical Fibres

 

An optical fibre consists of a transparent core sheathed by a transparent cladding and by an opaque plastic protective coating. The core and the cladding are dielectrics with different indexes of refraction, the cladding having a lower index than the core. According to a standard adopted by the International Telegraph and Telephone Consultative Committee (CCITT), the outer diameter of a high-performance clad fibre is approximately 125 micrometres, while the core diameter typically ranges from 8 to 50 micrometres. The abrupt change in refractive index between the core and the cladding makes the inside of the core-to-cladding interface highly reflective to light rays that graze the interface. The fibre therefore acts like a tubular mirror, confining most of the propagating rays of light to the interior of the core.

The bandwidth of an optical fibre is limited by a phenomenon known as multimode dispersion, which is described as follows. Different reflection angles within the fibre core create different propagation paths for the light rays. Rays that travel nearest to the axis of the core propagate by what is called the zeroth order mode; other light rays propagate by higher-order modes. It is the simultaneous presence of many modes of propagation within a single fibre that creates multimode dispersion. Multimode dispersion causes a signal of uniform transmitted intensity to arrive at the far end of the fibre in a complicated spatial “interference pattern,” and this pattern in turn can translate into pulse “spreading” or “smearing” and intersymbol interference at the optoelectronic receiver output. Pulse spreading worsens in longer fibres.

When the index of refraction is constant within the core, the fibre is called a stepped-index (SI) fibre. Graded-index (GI) fibre reduces multimode dispersion by grading the refractive index of the core so that it smoothly tapers between the core centre and the cladding. Another type of fibre, known as single-mode (SM) fibre, eliminates multimode dispersion by reducing the diameter of the core to a point at which it passes only light rays of the zeroth order mode. Typical SM core diameters are 10 micrometres or less, while standard SI core diameters are in the range of 50 micrometres. Single-mode fibres have become the dominant medium in long-distance optical fibre links.

Other important causes of signal distortion in optical fibres are material dispersion and waveguide dispersion. Material dispersion is a phenomenon in which different optical wavelengths propagate at different velocities, depending on the refractive index of the material used in the fibre core. Waveguide dispersion depends not on the material of the fibre core but on its diameter; it too causes different wavelengths to propagate at different velocities. As is the case in multimode dispersion, described above, material and waveguide dispersion cause spreading of the received light pulses and can lead to intersymbol interference.

Since a transmitted signal always contains components at different wavelengths, material dispersion and waveguide dispersion are problems that affect not only SI and GI fibres but also SM fibres. For SM fibres, however, there exists a transmission wavelength at which the material dispersion exactly cancels the waveguide dispersion. This “zero dispersion” wavelength can be adjusted by modifying the material composition (and hence the refractive index) as well as the diameter of the fibre core. In this way SM fibres are designed to exhibit their zero dispersion wavelength near the intended optical carrier wavelength. For a CCITT standard SM fibre with an 8-micrometre core, the zero dispersion wavelength occurs near the 1.3-micrometre wavelength of certain laser diodes. Other SM fibres have been developed with a zero dispersion wavelength of 1.55 micrometres.

Noise in an optical fibre link is introduced by the photoelectric conversion process at the receiver. Losses in signal power are primarily caused by radiation of light energy to the cladding as well as absorption of light energy by silica and impurities in the fibre core.

The production process for manufacturing optical fibre is extremely demanding, requiring very close tolerances on core and cladding thickness. Although the manufacture of low-grade fibre from transparent polymer materials is not uncommon, most high-performance optical fibres are made of fused silica glass. The refractive index of either the core or the cladding is modified during the manufacturing process by diluting pure silica glass with fluorine or germanium in a process known as doping. Several fibres can be bundled into a common sheath around a central strengthening member to form a fibre-optic cable. For fibre cables that must operate in adverse environments — for instance, undersea cables — other layers of strengthening and protecting materials may be added. These layers may include single-fibre buffer tubes, textile binder tape, moisture barrier sheathing, corrugated steel tape, and impact-resistant plastic jackets.

 

UNIT 20

Telecommunications Industry

Until the 1980s the world telecommunications system had a relatively simple administrative structure. In the United States telephone service was supplied by a regulated monopoly, American Telephone and Telegraph (AT&T). Telegraph service was provided mainly by the Western Union Corporation. In almost all other countries both services were the monopolies of government agencies known as PTTs (for Post, Telephone, and Telegraph).

In the United States, however, beginning in 1983, the situation became far more complex. As a result of an antitrust suit launched by the federal government, AT&T agreed in a court settlement to divest itself of the local operating companies that provided basic telephone service. They remained regulated local monopolies, grouped together into eight regional companies. AT&T now offers long-distance service in competition with a half dozen major and many minor competitors while retaining ownership of a subsidiary that produces telephone equipment, computers, and other electronic devices.

During the same period Great Britain's national telephone company was sold to private investors as was Japan's NTT telephone monopoly. For telegraphy and data transmission, Western Union was joined by many other major companies, while many leading multinational firms formed their own telecommunications services that link offices scattered throughout the world.

New technology also brought continuing changes in the providers of telecommunications. Private companies such as Comsat in the United States were organized to provide satellite communications links within the country. An international organization called Intelsat, which is jointly owned by the various PTTs and private communications companies, furnished the global links in the satellite telecommunications networks.

The introduction of more widespread competition into the highly integrated telecommunications networks proved a controversial move. Supporters praised it as a way of liberating the field from monopolistic practices that retarded technology and kept rates uneconomically high. But critics pointed out, at least in the early years of deregulation, that rates for the vast majority of users rose sharply and that in some respects technical progress became far more difficult. When the world's largest US telecommunications network was under the control of a single regulated corporation, system-wide changes and long-term planning were possible.

In February 1996, the U.S. passed a law approving landmark telecommunications reform. The measure scrapped many rules limiting competition in telephone service, television, and radio markets; relaxed restrictions on media ownership; mandated that television manufacturers equip new sets with the so-called “v-chip”, which would allow parents to block violent television programming; and set criminal penalties for the distribution of “indecent” material over the Internet computer network.

According to industry experts, American consumers stood to benefit the most from provisions in the legislation increasing telephone competition. The bill allowed national long-distance telephone companies to compete in local phone markets currently controlled by the so-called baby Bells, the companies created in the 1984 court-ordered breakup of AT&T's telephone monopoly. In return, the Bells would be able to offer national long-distance service to their customers.

The legislation's provisions setting criminal penalties for distributing pornography over the Internet were extremely controversial. Supporters of the bill said that children had unfettered access to explicit material on the Internet and some measure of control was needed. Opponents of the antipornography measure criticized it as a blow to free speech and said that because the Internet was a global computer network, the United States regulations on it would be impossible to enforce.

The United States spearheaded a 1997 agreement among 67 nations of the World Trade Organization (WTO) that would open telecommunication markets in those countries to free competition. Designed to end state monopolies over national telecommunications, the agreement was applauded by officials from the nations involved as a measure that would drastically cut the costs of global telecommunications services. It was believed that the agreement would save the countries involved more than $1 trillion by the year 2010. The agreement was hailed by observers as the most significant legislation negotiated by the two-year-old World Trade Organization. According to the agreement, the WTO would become the arbitrator in any disputes between member nations and international corporations, in order to ensure that the elimination of market barriers was conducted uniformly. Critics, however, questioned whether the WTO, which relied primarily on the goodwill of the member nations and the threat of economic penalties against non-cooperating members, held enough authority or power to mediate effectively in the event of serious international disputes.

CONTENTS

Unit 1. Telecommunication…………………………………………………………	..5
Unit 2. Analog-to-Digital Conversion ……………………………………………...	..6
Unit 3. Source Encoding…………………………………………………................	..7
Unit 4. Channel Encoding……………………………………..……..………….….	..9
Unit 5. Convolutional Encoding…………………………………………………….	11
Unit 6. Modulation………………………………………………………………….	11
Unit 7. Multiplexing………………………………………………………...............	13
Unit 8. Multiple Access……………………………………………………………..	15
Unit 9. Telecommunications Network……………………………………………...	17
Unit 10. Network Access…………………………………………………………….	18
Unit 11. Open Systems Interconnection……………………………………………..	19
Unit 12. Telecommunications Media………………………………………………..	20
Unit 13. Wire Transmission…………………………………………………………	21
Unit 14. Radio Transmission………………………………………………………...	23
Unit 15. The Radio-Frequency Spectrum……………………………………………	25
Unit 16. Line-of-sight Microwave Links……………………………………………	28
Unit 17. Satellite Links………………………………………………………………	29
Unit 18. Optical Transmission……………………………………………………….	30
Unit 19. Optical Fibres………………………………………………………………	32
Unit 20. Telecommunications Industry……………………………………………...	34

 

 

Учебное издание

Свиридова Людмила Аниновна

Сорокина Марина Михайловна

ТЕЛЕКОММУНИКАЦИИ

Учебно-методическое пособие

по профессионально-ориентированному чтению и обработке информации

 

Редактор Л.И. Сергейчик

Корректор Л.И. Сергейчик

 

Подписано к печати 10.11.2008. Формат 60х84/16.

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