Definition Edit

A communications infrastructure includes, but is not limited to, the organizations, personnel, procedures, facilities and networks employed to transmit and receive information by electrical or electronic means.

Overview Edit

Perhaps the earliest communications infrastructure was the road network of Rome, which carried not only the legions needed to enforce the emperor's will but also messengers to direct forces far from the capital. Ancient societies also developed systems that obviated the need for physical delivery of information. These systems operated within line-of-sight distances (later extended by telescope): smoke signals, torch signaling, flashing mirrors, signal flares, and semaphore flags. Observation stations were established along hilltops or roads to relay messages across great distances.

The first comprehensive infrastructure for transmitting messages faster than the fastest form of transportation was the optical telegraph, developed in 1793. Napoleon considered this his secret weapon because it brought him news in Paris and allowed him to control his armies beyond the borders of France. The optical telegraph consisted of a set of articulated arms that encoded hundreds of symbols in defined positions. Under a military contract, the signaling stations were deployed on strategic hilltops throughout France, linking Paris to its frontiers. By the mid-1800s, 556 stations enabled transmissions across more than 5,000 kilometers (km).

The optical telegraph was superseded by the electrical telegraph in 1838, when Samuel Morse developed his dot-and-dash code. Now information could be transmitted beyond visible distances without significant delay. In an 1844 demonstration on a government-funded research testbed, Morse sent the message "What Hath God Wrought?" from Baltimore to the U.S. Capitol.

The rapid deployment of telegraphic lines around the world was driven by the need of nineteenth-century European powers to communicate with their colonial possessions. High-risk technology investments were required. After the use of rubber coating was demonstrated on cables deployed across the Rhine River, the first transatlantic cable was laid in 1858, but it failed within months. A new cable designed by Lord Kelvin was laid in 1866 and operated successfully on a continuous basis. The result was a rapidly expanding telegraphic network that reached every corner of the globe. By 1870, Great Britain communicated directly with North America, Europe, the Middle East, and India. Other nations scrambled to duplicate that system's global reach, for no nation could trust its critical command messages to the telegraphic lines of a foreign power.

Within a few decades of its widespread deployment, telegraphy began to lose customers to a new technologyradio. In 1895 Guglielmo Marconi demonstrated that electromagnetic radiation could be detected at a distance. Great Britain's Royal Navy was an early and enthusiastic customer of the company that Marconi created to develop radio communications.

In 1901 Marconi bridged the Atlantic Ocean by radio, and regular commercial service was initiated in 1907. The importance of this new technology became evident with the onset of World War I. Soon after hostilities began, the British cut Germany's overseas telegraphic cables and destroyed its radio stations. Then Germany cut Britain's overland cables to India and those crossing the Baltic to Russia. Britain enlisted Marconi to put together a string of radio stations quickly to reestablish communications with its overseas possessions.

The original Marconi radios were soon replaced by more advanced equipment that exploited the vacuum tube's capability to amplify signals and operate at higher frequencies than did older systems. In 1915 the first wireless voice transmission between New York and San Francisco signaled the beginning of the convergence of radio and telephony. The first commercial radio broadcast followed in 1920. The use of higher frequencies (called shortwaves) exploited the ionosphere as a reflector, greatly increasing the range of communications. By World War II, shortwave radio had developed to the point where small radio sets could be installed in trucks or jeeps or carried by a single soldier. The first portable two-way radio, the Handie-Talkie, appeared in 1940. Two-way mobile communications on a large scale revolutionized warfare, allowing for mobile operations coordinated over large areas.

The telephone was first demonstrated in 1876. A telephone network based on mechanical switches and copper wires then grew rapidly. The high cost of the cables limited the number of conversations possible at any one time; as demand increased, multiplexing techniques, such as time division and frequency division, were developed.

A mix of independent operators ran telephone services in the early days. Subscribers to different services could not call each other even when in the same town. In 1913 the U.S. government allowed American Telephone and Telegraph (AT&T) to assume control of the national telephone network in return for becoming a regulated monopoly delivering "universal" service. Yet it was not until the 1950s that unified network signaling was offered to subscribers, allowing them to make direct-dial long-distance telephone calls. Since then, the rapid extension of the long-distance telephone network has been made possible by advances in photonic communications and network control technologies.

The concept of using geosynchronous satellites for communications purposes was first suggested in 1945 by the science fiction writer Arthur C. Clarke, then employed at Britain's Royal Aircraft Establishment, part of the Ministry of Defence. Satellites of this type are positioned above the equator and move in synch with Earth's rotation. In 1954 J.R. Pierce at AT&T's Bell Telephone Laboratories developed the concept of orbital radio relays and identified the key design issues for satellites: passive versus active transmission, station keeping, attitude control, and remote vehicle control. Pierce advocated an approach of reaching geostationary orbit in successive stages of technology development, starting with nonsynchronous, low-orbit satellites. Hughes Aircraft Company advocated a geostationary concept based on the company's patented station-keeping techniques.

In 1957 the Soviet Union launched Sputnik, the first satellite to be placed in orbit. Amateur radio operators were able to pick up its low-power transmissions all over the world. In 1960 the National Aeronautics and Space Administration (NASA) and Bell Laboratories launched the first U.S. communications satellite, Echo-1, in a low Earth orbit. The first satellite-based voice message was sent by President Dwight Eisenhower using passive transmission techniques.

The next advance in satellite technology was the successful launch of the TELSTAR system by NASA and Bell Laboratories. Using active transmission technology TELSTAR delivered the first television transmission across the Atlantic in 1962. Because it was placed in an elliptical orbit that varied from low to medium altitudes, the satellite was visible contemporaneously to Earth stations on both sides of the Atlantic for only about 30 minutes at a time. Clearly geostationary orbits were desirable if satellites were to be used for continuous telephone and television communications across long distances.

In 1963 Hughes Aircraft and NASA achieved geosynchronous orbit (known as GEO today) with the successful launch of the SYNCOM satellite]]. The satellite was placed in an orbit of approximately 36,210 km, a distance that allowed it to remain stationary over a given point on Earth's surface. SYNCOM led the way for the next several decades of satellite systems by demonstrating that synchronous orbit was achievable, and that station keeping and attitude control were feasible. Today most satellites, both military and commercial, are of the GEO variety.

COMSAT was formed by an act of Congress in 1962 and represented U.S. commercial interests in satellite technology development at Intelsat, established in 1964 as an international, government-chartered organization to coordinate worldwide satellite communications issues.

INTELSAT-II (Early Bird) was launched into a geosynchronous orbit in 1965 and supported 240 telephone links or one television channel. Channel capacities are now measured in the tens of thousands of voice channels (the INTELSAT-VI, launched in 1987, supports 80,000 voice channels).

The first military satellites, the DSCS-I group, were launched by the U.S. Air Force in 1966. Three launches placed 26 lightweight (100-pound) satellites in near-geosynchronous orbit. These systems supported digital voice and data communications using spread-spectrum technology. The satellites were replaced in the 1970s by the DSCS-II group, which increased channel capacity by using spot-beam antennas with high gain to boost the received power. The first cross-linked military satellites, the LES 8/9, were launched in 1976. This demonstration fostered a vision of space-based architectures — without vulnerable ground relays — for communication, navigation, surveillance, and reconnaissance.

Satellites offer several advantages over land-based communications systems. Rapid, two-way communications can be established over wide areas with only a single relay in space, and global coverage with only a few relay hops. Earth stations can now be set up and moved quickly.

Furthermore, satellite systems are virtually immune to impairments such as [[multipath fading. But with the rapid deployment of undersea fiber-optic links, the use of satellite channels for telephony has been on the decline. The high capacity of fiber provides for competitive costs, which, combined with low latency, have attracted consumers. The future of the satellite industry depends on the emergence of applications other than fixed telephony channels. A new generation of satellite systems is being deployed to provide mobile telephone services.

Source Edit