COMMUNICATION TECHNOLOGIES

Archive for July, 2009

The most common data communication equipment is the data communications modem. Alternate names include data-sets, data phones, or simply modems. The word modem is a contraction derived from the words modulators and demodulators. The only transmissions facilities available at the time were analog voice-band telephone circuits. Telephone circuits were designed for transporting analog voice signals within a bandwidth of approximately 300 Hz to 3000 Hz. In addition, telephone circuits often included amplifiers and other analog devices that could not propagate digital signals.

Data communications modems designed to operate over for the limited bandwidth of the public telephone network are called voice-band modems. Digital signals cannot be transported directly over analog transmission media, the primary purpose of a data communications modem is to interface computers, computers networks, and over other digital terminal equipment to analog communications facilities. Modems are also used when computers are too far apart to be directly interconnected using standard computer cables.

In the transmitter section of a modem digital signals are encoded onto an analog carrier. The digital signals modulate the carrier, producing digital modulated analog signals that are capable of being transported through the analog communication media. The most common modems available are ones intended to be used to interface DTE through a serial interface standard voice-band telephone lines and provide reliable data transmissions rates from 36 bps to 56 bps. These types’ modems sometimes called telephone loop modems or POT modems, as they are connected to the telephone company through the same local loops that are used for voice telephone circuits.

Modem is in essence a transparent repeater that converts electrical signals received in digital from to electrical signals in analog from and vice versa. A modem is transparent, as it does not interpret or change the information contained in the data. It is a repeater, as it not destination for data it simply repeats or retransmits data. A modem is physically located between digital terminal equipment and the analog communication channel. Modems work in pairs with to be manufactured by the came company, however they must use compatible modulation schemes, data encoding formats and transmissions rates.

Most of the satellites mentioned thus far are called orbital satellites, which are Nonsynchronous. Nonsynchronous satellites rotate around earth in elliptical or circular pattern. In a circular orbit, the speed or rotation is constant; however, in elliptical orbits the speed depends on the height the satellite is above earth. The speed of the satellite is greater when it is close to earth than it is farther away.

If the satellite is orbiting in the same direction as earth’s rotation and at an angular velocity greater than that of earth the orbit is called a prograde or posigrade orbit. If the satellite is orbiting in the opposite direction as earth’s rotation or in the same direction with an angular velocity less than that of earth the orbit is called a retrograde orbit.

Satellite Elevation Categories:

Satellites are generally classified as having either low earth, medium earth orbit or geosynchronous earth orbit. Most satellites operate in the 1.0 GHz to 2.5 GHz frequency range. Motorola’s satellite based mobile-telephone system, iridium, is a Leo system utilizing a 66-satellite constellation orbiting approximately 480 miles above earth’s surface. The main advantage of LEO satellites is that the path loss between earth stations and space vehicles is much lower than for satellites revolving in medium-or high-altitude orbits. Less path loss equates to lower transmit powers, small antennas, and less weight.

Satellite Orbital Patterns:

Although there is an infinite number of an orbital path only three are useful for communication satellites. A satellite can follow as it rotates around earth: inclined, equatorial, or polar. All satellites rotate around earth in an orbit that forms a plane that passes through the center of gravity of earth called the geocenter. Inclined orbits are virtually all orbits except those that travel directly above the equator or directly over the north and south poles. The angle of inclination is the angle between the earth’s equatorial plane and the orbital plane of a satellite measured counter clock wise at the point in the orbit where it crosses the equatorial plane traveling from south to north. To provide coverage to regions of high latitudes, inclined orbits are generally elliptical.

Details:

Mobile satellite systems provide the vehicle for a new generation of wireless telephone services called personal communications satellite systems. Universal wireless telephone coverage is a developing MSS Service that promises to deliver mobile subscribers both traditional and enhanced telephone features while wide-area global coverage. MSS satellite are in essence, radio repeaters in the sky, and their usefulness for mobile communications depends on several factors, such as the space-vehicle altitude, orbital pattern, transmit power, receiver sensitivity, modulation technique, antenna radiation pattern, and number of satellites in its constellation.

Satellite communications systems have traditionally provided both narrowband and wideband voice, data, video, facsimile, and networking services using large and very expensive, high-powered earth station transmitters communicating via high-altitude, geosynchronous earth-orbit satellites. Personal communications satellite services, however, use low earth-orbit and medium earth earth-orbit satellites that communicate directly with small, Low-power mobile telephone units. The intension of PCSS mobile telephone is to provide the same features and services offered by traditional, terrestrial cellular telephone providers.

Advantage and Disadvantages:

The primary and probably most obvious advantage of PCSS mobile telephone is that it provides mobile telephone coverage and a host of other integrated services virtually anywhere in the world to a truly global customer base. PCSS can fill the vacancies between land-based cellular and PCS telephone systems and provide wide-area coverage on a regional or global basis. PCSS is ideally suited to fixed cellular telephone applications, as it can provide a full complement of telephone services to places where cables can never go because of economical, technical, or physical constraints. PCSS can also provide complementary and backup telephone services to large companies and organizations with multiple operations in diverse locations, such as retail, manufacturing, finance, transportation, government, military, and insurance.

Most of the disadvantages of PCSS are closely related to economics, with the primary disadvantage being the risk associated with the high costs of designing, building, and launching satellites. There is also a high cost for the terrestrial-based networking and interface infrastructure necessary to maintain, coordinate, and manage the network once it is in operation. In addition, the intricate low-power, dual-mode transceivers are more cumbersome and expensive than most mobile telephone units used with terrestrial cellular and PCS system.