2.4.1. Telegraph

Telegraph services use both wire line and wireless media for transmissions. Soon after the introduction of the telegraph in 1844, telegraph wires spanned the country. Telegraph companies maintained a system of wires and offices located in numerous cities. A message sent by telegraph was called a telegram. Telegrams were printed on paper and delivered to the receiving party by the telegraph company. With the invention of the radio in the early 1900s, telegraph signals could also be sent by radio waves. Wireless telegraphy made it practical for oceangoing ships as well as aircraft to stay in constant contact with land-based stations.

2.4. Some Telecommunication Systems, Operations and Applications

Individual people, businesses, and governments use many different types of telecommunications systems. Some systems, such as the telephone system, use a network of cables, wires, and switching stations for point-to-point communication. Other systems, such as radio and television, broadcast radio signals over the air that can be received by anyone who has a device to receive them. Some systems make use of several types of media to complete a transmission. For example, a telephone call may travel by means of copper wire, fiber-optic cable, and radio waves as the call is sent from sender to receiver. All telecommunications systems are constantly evolving as telecommunications technology improves. Many recent improvements, for example, offer high-speed broadband connections that are needed to send multimedia information over the Internet.

2.3. Telecommuting

Telecommuting, e-commuting, e-work, telework, working from home (WFH), or working at home (WAH) is a work arrangement in which employees enjoy flexibility in working location and hours. In other words, the daily commute to a central place of work is replaced by telecommunication links. Many work from home, while others, occasionally also referred to as nomad workers or web commuters utilize mobile telecommunications technology to work from coffee shops or myriad other locations. Telework is a broader term, referring to substituting telecommunications for any form of work-related travel, thereby eliminating the distance restrictions of telecommuting. All telecommuters are teleworkers but not all teleworkers are telecommuters. A frequently repeated motto is that "work is something you do, not something you travel to”. A successful telecommuting program requires a management style which is based on results and not on close scrutiny of individual employees. This is referred to as management by objectives as opposed to management by observation. The terms telecommuting and telework were coined by Jack Nilles in 1973.

All of the electronic links among the people in a modern office can be extended beyond the building walls to workers at home or in satellite offices. This capability has led to a sharp increase in telecommuting. In 1991 an estimated 5.5 million U.S. workers worked at least part of the time outside the main office, a 38 percent increase over 1990. Managers and professional employees were the major participants in this trend. Early reports of increased productivity among people who no longer spent hours traveling from home to office indicated that further increases in telecommuting were likely.

iv. Satellites

Communications Satellite is any earth-orbiting spacecraft that provides communication over long distances by reflecting or relaying radio-frequency signals. Communication satellites are a kind of Line of Sight communication. Communications satellites provide a means of transmitting telecommunications all over the globe, without the need for a network of wires and cables. The communication is carried through uplinks and downlinks and downlinks. Uplinks and downlinks are also called earth stations because they are located on the earth. They orbit Earth at a speed that enables them to stay above the same place on Earth at all times. This type of orbit is called geostationary or geosynchronous orbit because the satellite’s orbital speed operates in synchronicity with Earth’s rotation. The satellites receive transmissions from Earth and transmit them back to numerous Earth station receivers scattered within the receiving coverage area of the satellite. This relay function makes it possible for satellites to operate as “bent pipes”—that is, wireless transfer stations for point-to-point and point-to-multipoint transmissions. Communications satellites are used by telephone and television companies to transmit signals across great distances. Ship, airplane, and land navigators also receive signals from satellites to determine geographic positions.
Hundreds of active communications satellites are now in orbit. They receive signals from one ground station, amplify them, and then retransmit them at a different frequency to another station. Satellites use ranges of different frequencies, measured in hertz (Hz) or cycles per second, for receiving and transmitting signals. Many satellites use a band of frequencies of about 6 billion hertz, or 6 gigahertz (GHz) for upward, or uplink, transmission and 4 GHZ for downward, or downlink, transmission. Another band at 14 GHZ (uplink) and 11 or 12 GHZ (downlink) is also much in use, mostly with fixed (non mobile) ground stations. A band at about 1.5 GHZ (for both uplink and downlink) is used with small, mobile ground stations (ships, land vehicles, and aircraft). Solar energy cells mounted on large panels attached to the satellite provide power for reception and transmission.

A technique called frequency reuse allows satellites to communicate with a number of ground stations using the same frequency by transmitting in narrow beams pointed toward each of the stations. Beam widths can be adjusted to cover areas as large as the entire United States or as small as a state like Maryland. Two stations far enough apart can receive different messages transmitted on the same frequency. Satellite antennas have been designed to transmit several beams in different directions, using the same reflector.

A method for interconnecting many ground stations spread over great distances was demonstrated in 1993 with the launch of NASA's ACTS (Advanced Communications Technology Satellite). The satellite uses what is known as the hopping spot beam technique to combine the advantages of frequency reuse, spot beams, and TDMA. By concentrating the energy of the satellite's transmitted signal, ACTS can use ground stations that have smaller antennas and reduced power requirements. The concept of multiple spot beam communications was successfully demonstrated in 1991 with the launch of Italsat, developed by the Italian Research Council. With six spot beams operating at 30 GHZ (uplink) and 20 GHZ (downlink), the satellite interconnects TDMA transmissions between ground stations in all the major economic centers of Italy. It does this by demodulating uplink signals, routing them between up- and downlink beams, and combining and remodulating them for downlink transmission. Laser beams can also be used to transmit signals between a satellite and the earth, but the rate of transmission is limited because of absorption and scattering by the atmosphere. Lasers operating in the blue-green wavelength, which penetrates water, have been used for communication between satellites and submarines.

The latest development in satellites is the use of networks of small satellites in low earth orbit (2,000 km (1,200 mi) or less) to provide global telephone communication. The Iridium system uses 66 satellites in low earth orbit, while other groups have or are developing similar systems. Special telephones that communicate with these satellites allow users to access the regular telephone network and place calls from anywhere on the globe. Anticipated customers of these systems include international business travelers and people living or working in remote areas.

iii. Infrared

Infrared offers a great unbound photonic solution. Like fiber optic cabling, infrared communication uses light, and not being bound by the limitations of electricity. The wavelengths of infrared radiation are shorter than those of radio waves and longer than those of light waves. They range between approximately 10-6 and 10-3 (about 0.0004 and 0.04 in). Infrared radiation may be detected as heat, and instruments such as bolometer are used to detect it.

ii. Microwave

Microwaves are high-frequency radio waves lying roughly between very-high-frequency (infrared) waves and conventional radio waves. Microwaves thus range in length from about 1 mm to 30 cm (about 0.04 to 12 in). They are generated in special electron tubes, such as the klystron and the magnetron, with built-in resonators to control the frequency or by special oscillators or solid-state devices. Microwaves have many applications: in radio and television, radar, meteorology, satellite communications, distance measuring, and research into the properties of matter. Microwave Communications, in short, is a system to use of a high-frequency electromagnetic wave to transmit information. Microwave transmission can be described as microwave radio or microwave telegraphy. It involves receiving and resending microwave signals between relay stations. They are line of sight transmission.

i. Radio wave

Radio waves can operate on single or multiple frequency bands. In this case, the signals are carried aver carrier waves which have frequencies in the range of radio frequency spectrum. There are three main types of Radio Frequency (RF), namely, ground wave, ionospheric and line of sight. Wireless telecommunications use radio waves, sent through space from one antenna to another, as the medium for communication. Radio waves are used for receiving AM and FM radio and for receiving television. Cordless telephones and wireless radio telephone services, such as cellular radio telephones and pagers, also use radio waves. Telephone companies use microwaves to send signals over long distances. Microwaves use higher frequencies than the radio waves used for AM, FM, or cellular telephone transmissions and they can transmit larger amounts of data more efficiently. Microwaves have characteristics similar to those of visible light waves and transmit pencil-thin beams that can be received using dish-shaped antennas. Such narrow beams can be focused to a particular destination and provide reliable transmissions over short distances on Earth. Even higher and narrower beams provide the high-capacity links to and from satellites. The high frequencies easily penetrate the ionosphere (a layer of Earth’s atmosphere that blocks low-frequency waves) and provide a high-quality signal.