June 3, 2015

How telecommunications work

How telecommunications work

How telecommunications work

Since ancient times, people have wondered how to communicate over long distances. The term "telecommunications" refers to the transmission and remote reception of signals carrying all types of information, from simple text to images and sound. Unlike messengers or the postal service, telecommunications signals carry messages that have no physical substance.

Today, cables and radio waves carry all manner of information around the planet via the many networks that are constantly expanding and combining in infinite ways. Before the advent of modern communications, however, our ancestors had to use their ingenuity to send messages.

The human voice, light signals and smoke signals were the forebears of modern telegraphs, telephones, radio, television, faxes and the Internet, which are the result of rapid advances made possible by the advent of electricity and, more recently, computers.

The visual telegraph, developed by Claude Chappe. Courtesy of Musée de la Poste - Paris

Using increasingly sophisticated technology, these tools have become a vital part of daily life and are the culmination of human endeavours stretching back over the centuries

Did you know?

The International Telecommunication Union (ITU)
Founded in 1865, the ITU, which has around 195 member states, promotes international cooperation in telecommunications. Its activities include providing technical assistance for developing countries, regulating the industry, implementing standards and allocating radio frequencies. In France, the Autorité de Régulation des Télécommunications (ART) manages French telecommunications legislation and competition.

History of telecommunications

Since time immemorial, people have needed to communicate over long distances.

Early attempts at long-distance communications include torches and light beacons in Ancient Greece, drums and bugles on battlefields, tom-toms in the savannah, smoke signals used by Native Americans, horns at fortified castles and yodelling in the Alps.

However, light and sound signals can only carry a few miles. The first real communications network appeared at the end of the French Revolution. The visual telegraph, developed by Claude Chappe, linked Paris and Lille by a series of towers along which coded messages were sent using semaphore.

Half a century later, it was superseded by the electric telegraph. This transmitted signals using metal wires, meaning it could also be used at night and in adverse atmospheric conditions. The system was later improved by the introduction of Morse code.
The electric telegraph also prompted early research into the transmission of still images. Line-by-line analysis and transmission of photographs was extended to moving pictures, culminating in television, which appeared around 1920. Underwater cables made it possible to progressively establish links between continents.
The telephone also appeared around this time. Based on the concept of electromagnetism, it converts the sound vibrations of the human voice into electrical signals using a magnetic field.

Did you know?

Keeping track...
In France, telegraphs and trains ran side by side, as the country's telegraph network was set up in partnership with the rail companies. The telegraph lines ran alongside the railway tracks, so they could be monitored at all times. Railway workers also used them to send messages between stations. 




12 July 1793

August 1794


24 May 1844



18 August 1858


June 1876






10 novembre 1935



10 juillet 1962


10 juillet 1981



First messages transmitted using optical and acoustic systems in Greece, Egypt, Rome, Gaul and China

French engineer Claude Chappe unveils the visual telegraph

First transmission between Belleville and Saint-Martin-du-Tertre (near Paris), a distance of 35 kilometres

Opening of the visual telegraph line between Paris and Lille

Advent of the electric telegraph

First transmission of an electric telegram using a printing telegraph and Morse code

Opening of France's first operational electric telegraph line, between Paris and Lille.

First underwater electric cable laid between England and France

First transatlantic telegraph message

Transmission des messages en clair grâce au télégraphe électrique, sans codage.

First telephone conversation

First urban telephone networks established in New York

Wireless link between Cornwall and Newfoundland, a distance of 3,400 kilometres

First transmission of a photograph, between Munich and Nuremberg

Radiotelephone link between Brittany and Paris

Advent of television, based on the same principles used today

Official launch of television broadcasting in France

First operational use of analogue cordless telephones, in the United States

Telex introduced in France

Launch of Telstar, the first telecommunications satellite, in the United States

First commercial fibre-optic telephone link, in the United States

Launch of Minitel, France's viewdata service

Official adoption of standard Internet protocols

Market launch of second-generation mobile telephones, which use the GSM digital network

The advent of wireless telegraphy

The electric telegraph and the telephone gave a major boost to the development of telecommunications.

However, the use of cables to connect transmitters and receivers soon began to show its limitations. By the late 19th century, research was being conducted into electromagnetic waves. The laws of electromagnetic wave propagation showed that information could be transmitted without cables.

The advent of wireless telegraphy and telephony offered enormous potential, despite the poor quality of the first transmissions. Its many applications include the arrival of radiotelephony and the first cordless telephones, as well as radio equipment for maritime safety and military communications.

The relief of the Earth's surface and the huge distances involved (see picture) still posed a problem, though. Due to the number of relay stations required, the system soon proved expensive. It also had limited application for intercontinental links.

Electromagnetic waves travel in straight lines. The Earth, however, is round. This means that signals transmitted over long distances cannot reach the receiver, as the curve of the Earth's surface and other obstacles get in the way. To overcome this problem, relay stations are used. Satellites make an ideal relay, and are also cheaper.

Following the launch of the first Sputnik satellite in 1957, the idea of using satellites orbiting the Earth as radio relays emerged as a natural solution to the problem. Thus began a revolution in telecommunications. The potential offered by satellites far exceeded anything offered by ground-based systems, and the last geographic limits were finally overcome.

Since then, numerous satellite-based telecommunications networks have been developed to meet growing demand in a wide range of specialist areas.

Today's technologies

Although telecommunications technologies have come a long way since the early years, messages are still carried in the same way, using cables and electromagnetic waves. Copper wire, fibre-optic cable and radio waves form complementary networks.

Satellites are also used to relay messages between thousands of ground stations worldwide.

How a telephone call gets from A to B

When you ring someone, an electrical signal travels from your phone to your local telephone exchange along two copper wires. The signal is then directed through other exchanges to the person you are calling. Depending on the type of call, the electric signal travels from one exchange to another via fibre-optic cable or radio relay. In the latter case, 30 percent of calls are transmitted via satellite.

See animation (Site www.howstuffworks.com)

Telecommunications have also benefited from the development of information technology, which makes it possible to store, exchange, process and retrieve all types of information automatically. Such change has been facilitated by the use of digital signals, which were previously analogue.

Word watch

Analogue or digital?
Until recently, all telephones worked by sending analogue signals, i.e. an electric current that varies in amplitude continuously and proportionally in response to your voice. Today, most telephone systems are digital. Your voice is sampled at a rate of 8,000 samples per second, then binary coded (i.e. converted into a series of 0s and 1s). The signal is neither proportional nor continuous. Analogue signals are, however, still used for certain applications. 

The first communications networks, which paved the way for the Internet, were developed in the 1970s for military purposes. Used by scientists and businesses, these networks expanded and merged over the period of 10 years to form a global network called the Interconnecting Network, better known as the Internet.

Based on a common communications protocol called Internet Protocol (IP), the Internet connects local networks across the globe. By early 2000, the number ran into hundreds of thousands. The Internet's architecture evolved to meet increasing demand and offer different services such as e-mail, e-commerce, discussion forums, file transfer and, of course, browsing web pages for information.
These technology networks are constantly evolving and merging to provide access to a wide range of services, from mobile telephony, multimedia applications and television to geographic positioning, data collection and rescue systems.

Information carriers

Type of carrierDescription  Typical use
Copper wireMetallic wireFixed telephones (emission and final distribution)
Fibre-optic cableFilament of dielectric material such as glass or silica, capable of transmitting lightTransmission of digital information across high-speed networks, final distribution of television services
Radio wavesElectromagnetic radiation with a lower frequency than optical waves (i.e. with wavelengths over 1 mm). Each signal is also characterized by a bandwidth (or frequency range): 
 L band : 1,5/1,6 GHzRadionavigation, satellite-based mobile services
S band : 1,8/2,5 GHzSatellite-based mobile services for aviation, terrestrial and maritime applications
 C band : 4/6 GHzFixed telephones and radio broadcasting
 X band : 7/ 8 GHzEncrypted government and military communications
 Ku band : 11/14 GHzHigh-data-rate communications such as television broadcasting and multimedia applications (videoconferencing, file transfer, etc.)
 Ka band : 20/30 GHzHigh-speed civil communications
 EHF band : 20/40 GHzMilitary communications
 V band : 60 GHzIntersatellite links

The beginnings of satellite telecommunications

The first telecommunications satellites were passive satellites, designed simply to reflect signals from one ground station to another. These were superseded by active satellites, which receive radio signals, amplify them by a factor of several billion and then re-transmit them to the ground.

The first satellite links between France and the United States were established via an active satellite in 1965.

Did you know?

A balloon in orbit
The first experimental satellite, the precursor of modern telecommunications satellites, was launched on 12 August 1960. The American Echo 1 satellite was a huge aluminium-coated balloon, 30 metres in diameter, which was inflated in orbit. It was a passive satellite, designed simply to reflect radio waves received from ground stations. The reflected signal was very weak. 

The first satellites were non-synchronous satellites in low-Earth orbit. They were easy to orbit and close enough to Earth to act as passive radio relays. However, because non-synchronous satellites move with respect to the Earth, they can only establish communications for a few minutes during each pass.

There are three ways to establish permanent links:

  • Elliptical orbits. These are highly elongated orbits in which the apogee (40,000 km) is directly above the ground station, thus providing a useful coverage period ;
  • Constellations, made up of a large number of non-synchronous satellites, which together provide permanent links and global coverage ;
  • Geostationary orbits. These are circular orbits in which the position of the satellite is fixed with respect to Earth. This is the cheapest and most effective solution. A satellite orbiting at an altitude of 35,786 kilometres above the equator has a period of revolution around the Earth the same as the Earth's period of rotation, so it’s ground track is a point on the equator. Three geostationary satellites, correctly positioned, can cover the entire surface of the globe between the latitudes of +80° and –80°.

The Clarke Belt, which contains all telecommunications satellites in geostationary orbit.
Since 1957, telecommunications satellites have been placed in geostationary orbit. Despite its circumference of 264,000 kilometres, the Clarke Belt must be regularly cleared of spent satellites to avoid congestion and interference.

Most telecommunications satellites currently in orbit are geostationary. Constellations of non-synchronous satellites are essential for specific applications such as the Global Positioning System (GPS), which requires coverage of all points on the surface of the globe by several satellites at the same time.

How telecommunications satellites work

Cutaway of a telecommunications satellite (see Stentor) showing the transponders and antennas. Crédits : CNES

Telecommunications satellites rely on the properties of propagating electromagnetic waves to convey information, which is transmitted using carrier waves in clearly defined frequency bands.

A satellite may be a passive element of this transmission process, acting simply as a relay that amplifies signals and transposes frequencies. It may also be a switching node or signal adapter, in which case it plays a more active role. This function is performed by a device on the satellite called a transponder.

Ground-station antennas are used to both transmit and receive signals. At an altitude of 36,000 km, a satellite is little more than a tiny speck in the sky. The antenna consists of a feed, pointed very precisely at the satellite, and a parabolic reflector. The satellite is also fitted with antennas.

A ground receiving antenna. © CNES

To limit signal loss, the energy received is focused into narrow beams using reflectors, which can be several metres across.

The slightest change in the line of sight of the ground or satellite antennas can result in deterioration or loss of the signal. To maintain maximum precision, satellites are fitted with stabilizing and pointing systems to counter external disturbances and stay almost permanently in contact with the ground. .

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