the modem dial- up connection
Dial-up Internet has been around since the 1980s via public providers such as
NSFNET-linked universities and was first offered commercially in July 1992 by
Sprint.
[1]
the modem
Modems are generally classified by the amount of data they can send in a given
unit of time, usually expressed in
bits per second (symbol
bit/s, sometimes abbreviated "bps"), or
bytes per second (symbol
B/s). Modems can also be classified by their
symbol rate, measured in
baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used
audio frequency shift keying with two possible frequencies, corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols (two bits per symbol), transmitted 1,200 bits by sending 600 symbols per second (600 baud) using
phase shift keying.
the demodulator
A radio frequency modulator (or RF modulator) takes a base band input signal and then outputs a radio frequency modulated signal. This is often a preliminary step in signal transmission, either by antenna or to another device such as a television.
A demodulator is a circuit that is used in amplitude modulation and frequency modulation receivers in order to separate the information that was modulated onto the carrier from the carrier itself. A demodulator is the analog part of the modulator. A modulator puts the information onto a carrier wave at the transmitter end and then a demodulator pulls it so it can be processed and used on the receiver end.
broadband
In the context of
Internet access broadband is used much more loosely to mean any high-speed Internet access that is always on and faster than traditional
dial-up access.
cable
An electrical
cable is made of two or more
wires running side by side and bonded, twisted, or braided together to form a single assembly, the ends of which can be connected to two devices, enabling the transfer of electrical signals from one device to the other. Cables are used for a wide range of purposes, and each must be tailored for that purpose. Cables are used extensively in electronic devices for power and signal circuits. Long-distance communication takes place over
undersea cables.
Power cables are used for bulk transmission of alternating and direct current power, especially using
high-voltage cable. Electrical cables are extensively used in
building wiring for lighting, power and control circuits permanently installed in buildings. Since all the circuit conductors required can be installed in a cable at one time, installation labor is saved compared to certain other wiring methods.
The term originally referred to a nautical line of specific length where multiple
ropes, each laid clockwise, are then laid together anti-clockwise and shackled to produce a strong thick line, resistant to water absorption, that was used to anchor large ships. In
mechanics, cables, otherwise known as
wire ropes, are used for lifting, hauling, and towing or conveying force through tension. In
electrical engineering cables are used to carry
electric currents. An optical cable contains one or more
optical fibers in a protective jacket that supports the fibers.
DSL
Digital subscriber line (
DSL; originally
digital subscriber loop) is a family of technologies that are used to transmit
digital data over
telephone lines. In telecommunications marketing, the term DSL is widely understood to mean
asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for
Internet access. DSL service can be delivered simultaneously with
wired telephone service on the same telephone line. This is possible because DSL uses higher
frequency bands for data. On the customer premises, a
DSL filter on each non-DSL outlet blocks any high-frequency interference to enable simultaneous use of the voice and DSL services.
The
bit rate of consumer DSL services typically ranges from 256 kbit/s to over 100 Mbit/s in the direction to the customer (
downstream), depending on DSL technology, line conditions, and service-level implementation. Bit rates of 1 Gbit/s have been reached in trials,
[1] but most homes are likely to be limited to 500-800 Mbit/s. In ADSL, the data throughput in the
upstream direction (the direction to the service provider) is lower, hence the designation of
asymmetric service. In
symmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal. Researchers at
Bell Labs have reached speeds of 10
Gbit/s, while delivering 1 Gbit/s symmetrical broadband access services using traditional copper
telephone lines. These higher speeds are lab results, however.
[2][3][4] A 2012 survey found that "DSL continues to be the dominant technology for broadband access" with 364.1 million subscribers worldwide.
[5]
satellite
In the context of
spaceflight, a
satellite is an artificial object which has been intentionally placed into
orbit. Such objects are sometimes called
artificial satellites to distinguish them from
natural satellites such as Earth's
Moon.
The world's first artificial satellite, the
Sputnik 1, was launched by the
Soviet Union in 1957. Since then, thousands of satellites have been launched into orbit around the
Earth. Some satellites, notably
space stations, have been launched in parts and assembled in orbit. Artificial satellites originate from more than 40 countries and have used the satellite launching capabilities of ten nations. A few hundred satellites are currently operational, whereas thousands of unused satellites and satellite fragments orbit the Earth as
space debris. A few
space probes have been placed into orbit around other bodies and become artificial satellites to the Moon,
Mercury,
Venus,
Mars,
Jupiter,
Saturn,
Vesta,
Eros,
Ceres,
[1] and the
Sun.
About 6,600 satellites have been launched. The latest estimates are that 3,600 remain in orbit.
[2] Of those, about 1,000 are operational;
[3][4] the rest have lived out their useful lives and are part of the
space debris. Approximately 500 operational satellites are in
low-Earth orbit, 50 are in
medium-Earth orbit (at 20,000 km), the rest are in
geostationary orbit (at 36,000 km).
[5]
Satellites are usually semi-independent computer-controlled systems. Satellite subsystems attend many tasks, such as power generation,
thermal control, telemetry,
attitude control and orbit control.
wireless
Wireless communication is the
transfer of information between two or more points that are not connected by an electrical conductor.
The most common wireless technologies use
radio. With radio waves distances can be short, such as a few meters for
television or as far as thousands or even millions of kilometers for deep-space radio communications. It encompasses various types of fixed, mobile, and portable applications, including
two-way radios,
cellular telephones,
personal digital assistants (PDAs), and
wireless networking. Other examples of applications of radio
wireless technology include
GPS units,
garage door openers, wireless
computer mice,
keyboards and
headsets,
headphones,
radio receivers,
satellite television,
broadcast television and
cordless telephones.
Somewhat less common methods of achieving wireless communications include the use of other
electromagnetic wireless technologies, such as light, magnetic, or electric fields or the use of sound.
It should be noted that the term
wireless has been used twice in communications history, with slightly different meaning. It was initially used from about 1890 for the first primitive radio transmitting and receiving technology, as in
wireless telegraphy, until the new word
radio replaced it around 1920. The term was revived in the 1980s and 1990s mainly to distinguish digital devices that communicate without wires, such as the examples listed in the previous paragraph, from those that require wires. This is its main usage today.
the modulator
modulator (or radio frequency modulator) is an electronic device whose input is a baseband signal which is used to modulate a radio frequency source.
