How telecommunications work

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Half a century later, it was superseded by the electric telegraph.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.

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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.

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.

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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.

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

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.

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.

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. .