A commercial IP telephone, with keypad, control keys, and screen functions to perform configuration and user features.
Fixed telephone lines per 100 inhabitants 1997–2007

Field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties.

- Telephony

407 related topics


Pulse-code modulation

Method used to digitally represent sampled analog signals.

coding tube
Sampling and quantization of a signal (red) for 4-bit LPCM

It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications.

Analog transmission

Transmission method of conveying information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that information.

Antenna used for transmission of radio signals

Telephony and voice communication was originally primarily analog in nature, as was most television and radio transmission.

Local loop

A commercial IP telephone, with keypad, control keys, and screen functions to perform configuration and user features.

In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the common carrier or telecommunications service provider's network.


Semiconductor device used to amplify or switch electrical signals and power.

Size comparison of BJT transistor packages, from left to right: SOT-23, TO-92, TO-126, TO-3
Metal-oxide-semiconductor field-effect transistor (MOSFET), showing gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
Julius Edgar Lilienfeld proposed the concept of a field-effect transistor in 1925.
John Bardeen, William Shockley and Walter Brattain at Bell Labs in 1948. Bardeen and Brattain invented the point-contact transistor in 1947 and Shockley the bipolar junction transistor in 1948.
A replica of the first working transistor, a point-contact transistor invented in 1947
Herbert Mataré in 1950. He independently invented a point-contact transistor in June 1948.
Philco surface-barrier transistor developed and produced in 1953
A Darlington transistor opened up so the actual transistor chip (the small square) can be seen inside. A Darlington transistor is effectively two transistors on the same chip. One transistor is much larger than the other, but both are large in comparison to transistors in large-scale integration because this particular example is intended for power applications.
A simple circuit diagram to show the labels of an n–p–n bipolar transistor
BJT used as an electronic switch, in grounded-emitter configuration
Amplifier circuit, common-emitter configuration with a voltage-divider bias circuit
Operation of a FET and its Id-Vg curve. At first, when no gate voltage is applied, there are no inversion electrons in the channel, so the device is turned off. As gate voltage increases, the inversion electron density in the channel increases, current increases, and thus the device turns on.
Transistor symbol created on Portuguese pavement in the University of Aveiro
Assorted discrete transistors
Soviet KT315b transistors

The thermionic triode, a vacuum tube invented in 1907, enabled amplified radio technology and long-distance telephony.

Public switched telephone network

The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication.

A simplified example of an early circuit-switched telephone network.

The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephony operators.

Mobile phone

Portable telephone that can make and receive calls over a radio frequency link while the user is moving within a telephone service area.

Two decades of evolution of mobile phones, from a 1992 Motorola 8900X-2 to the 2014 iPhone 6 Plus
Martin Cooper of Motorola, shown here in a 2007 reenactment, made the first publicized handheld mobile phone call on a prototype DynaTAC model on 3 April 1973.
The Motorola DynaTAC 8000X. In 1983, it became the first commercially available handheld cellular mobile phone.
Dupuis and Haug during a GSM meeting in Belgium, April 1992
Personal Handy-phone System mobiles and modems, 1997–2003
Active mobile broadband subscriptions per 100 inhabitants.
Cellular networks work by only reusing radio frequencies (in this example frequencies f1-f4) in non adjacent cells to avoid interference
Typical mobile phone mini-SIM card
Android smartphones
A text message (SMS)
Growth in mobile phone subscribers per country from 1980 to 2009
Mobile phone subscribers per 100 inhabitants. 2014 figure is estimated.
Mobile payment system
A driver using two handheld mobile phones at once
A sign in the U.S. restricting cell phone use to certain times of day (no cell phone use between 7:30am-9:00am and 2:00pm-4:15pm)
Scrapped mobile phones

In addition to telephony, digital mobile phones (2G) support a variety of other services, such as text messaging, MMS, email, Internet access, short-range wireless communications (infrared, Bluetooth), business applications, video games and digital photography.

Digital subscriber line

Family of technologies that are used to transmit digital data over telephone lines.

A DSL modem
Example of a DSLAM from 2006
DSL Connection schematic
DSL Modem schematic

It is useful when the customer does not need the traditional telephony voice service because voice service is received either on top of the DSL services (usually VoIP) or through another network (E.g., mobile telephony).


Device that stores electrical energy in an electric field.

Battery of four Leyden jars in Museum Boerhaave, Leiden, the Netherlands
Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric (orange) reduces the field and increases the capacitance.
A simple demonstration capacitor made of two parallel metal plates, using an air gap as the dielectric
In the hydraulic analogy, a capacitor is analogous to a rubber membrane sealed inside a pipe – this animation illustrates a membrane being repeatedly stretched and un-stretched by the flow of water, which is analogous to a capacitor being repeatedly charged and discharged by the flow of charge.
Parallel plate capacitor model consists of two conducting plates, each of area A, separated by a gap of thickness d containing a dielectric.
The interleaved capacitor can be seen as a combination of several parallel connected capacitors.
A simple resistor–capacitor circuit demonstrates charging of a capacitor.
Several capacitors in parallel
Illustration of the parallel connection of two capacitors
Two different circuit models of a real capacitor
Capacitor materials. From left: multilayer ceramic, ceramic disc, multilayer polyester film, tubular ceramic, polystyrene, metalized polyester film, aluminum electrolytic. Major scale divisions are in centimetres.
Three aluminum electrolytic capacitors of varying capacity
Capacitor packages: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. Major scale divisions are cm.
Several axial-lead electrolytic capacitors
This mylar-film, oil-filled capacitor has very low inductance and low resistance, to provide the high-power (70 megawatt) and high speed (1.2 microsecond) discharge needed to operate a dye laser.
A 10,000 microfarad capacitor in an amplifier power supply
A high-voltage capacitor bank used for power-factor correction on a power transmission system
Polyester film capacitors are frequently used as coupling capacitors.
Example of a simple oscillator incorporating a capacitor
Swollen electrolytic capacitors – the special design of the capacitor tops allows them to vent instead of bursting violently.
This high-energy capacitor from a defibrillator has a resistor connected between the terminals for safety, to dissipate stored energy.
Catastrophic failure of a capacitor has scattered fragments of paper and metallic foil

Paper capacitors, made by sandwiching a strip of impregnated paper between strips of metal and rolling the result into a cylinder, were commonly used in the late 19th century; their manufacture started in 1876, and they were used from the early 20th century as decoupling capacitors in telephony.

Voice over IP

Method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet.

Example of residential network including VoIP
Asterisk-based PBX for small business

The steps and principles involved in originating VoIP telephone calls are similar to traditional digital telephony and involve signaling, channel setup, digitization of the analog voice signals, and encoding.


Telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly.

An old rotary dial telephone
AT&T push button telephone made by Western Electric, model 2500 DMG black, 1980
Alexander Graham Bell's Telephone Patent Drawing
Replica of the telettrofono, invented by Antonio Meucci and credited by several sources as the first telephone.
Bell placing the first New York to Chicago telephone call in 1892
Reis's telephone
Bell's first telephone transmitter, ca. 1876, reenacted 50 years later
Acoustic telephone ad, The Consolidated Telephone Co., Jersey City, New Jersey, 1886
1896 telephone from Sweden
Wooden wall telephone with a hand-cranked magneto generator
An IP desktop telephone attached to a computer network, with touch-tone dialing
Fixed telephone lines per 100 inhabitants 1997–2007
Ericsson DBH 1001 (ca. 1931), the first combined telephone made with a Bakelite housing and handset.
Telephone used by American soldiers (WWII, Minalin, Pampanga, Philippines)
Modern sound-powered emergency telephone
One type of mobile phone, called a cell phone

With the development of stored program control and MOS integrated circuits for electronic switching systems, and new transmission technologies such as pulse-code modulation (PCM), telephony gradually evolved towards digital telephony, which improved the capacity, quality, and cost of the network.