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{{short description|Field of telecommunication services}}
'''Telephony''' ({{IPAc-en|t|ə|ˈ|l|ɛ|f|ə|n|i}} {{respell|tə|LEF|ə-nee}}) is the 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. The history of telephony is intimately linked to the invention and development of the [[telephone]].

Telephony is commonly referred to as the construction or operation of telephones and telephonic systems and as a system of telecommunications in which telephonic equipment is employed in the transmission of speech or other sound between points, with or without the use of wires.<ref>[http://dictionary.reference.com/browse/telephony Dictionary.com] Telephony Definition</ref> The term is also used frequently to refer to [[computer hardware]], [[software]], and [[computer network]] systems, that perform functions traditionally performed by telephone equipment. In this context the technology is specifically referred to as Internet telephony, or [[voice over Internet Protocol]] (VoIP).

==Overview==
The first telephones were connected directly in pairs. Each user had a separate telephone wired to each locations to be reached. This quickly became inconvenient and unmanageable when users wanted to communicate with more than a few people. The invention of the [[telephone exchange]] provided the solution for establishing telephone connections with any other telephone in service in the local area. Each telephone was connected to the exchange at first with one wire, later one wire pair, the [[local loop]]. Nearby exchanges in other service areas were connected with [[Trunking|trunk lines]], and long-distance service could be established by relaying the calls through multiple exchanges.

Initially, exchange [[Telephone switchboard|switchboards]] were manually operated by an attendant, commonly referred to as the "[[switchboard operator]]".  When a customer cranked a handle on the telephone, it activated an indicator on the board in front of the operator, who would in response plug the operator headset into that jack and offer service. The caller had to ask for the called party by name, later by number, and the operator connected one end of a circuit into the called party jack to alert them.  If the called station answered, the operator disconnected their headset and completed the station-to-station circuit. Trunk calls were made with the assistance of other operators at other exchangers in the network.

Until the 1970s, most telephones were permanently wired to the telephone line installed at customer premises. Later, conversion to installation of jacks that terminated the [[on-premises wiring|inside wiring]] permitted simple exchange of telephone sets with [[telephone plugs]] and allowed portability of the set to multiple locations in the premises where jacks were installed. The inside wiring to all jacks was connected in one place to the [[drop (telecommunication)|wire drop]] which connects the building to a cable. Cables usually bring a large number of drop wires from all over a district [[access network]] to one wire center or telephone exchange. When a telephone user wants to make a [[telephone call]], equipment at the exchange examines the dialed [[telephone number]] and connects that [[telephone line]] to another in the same wire center, or to a trunk to a distant exchange. Most of the exchanges in the world are interconnected through a system of larger switching systems, forming the [[public switched telephone network]] (PSTN).

In the second half of the 20th century, fax and data became important secondary applications of the network created to carry voices, and late in the century, parts of the network were upgraded with [[ISDN]] and [[DSL]] to improve handling of such traffic.

Today, telephony uses digital technology ([[digital telephony]]) in the provisioning of telephone services and systems. Telephone calls can be provided digitally, but may be restricted to cases in which the [[Last mile (telecommunications)|last mile]] is digital, or where the conversion between [[digital signal (signal processing)|digital]] and [[analog signal]]s takes place inside the telephone. This advancement has reduced costs in communication, and improved the quality of voice services. The first implementation of this, [[Integrated Services Digital Network|ISDN]], permitted all data transport from end-to-end speedily over telephone lines.<ref>{{Cite web |title=The Communications Museum Trust - eMuseum - History of Digital Switching -ISDN |url=https://www.communicationsmuseum.org.uk/emuseum/electronicswitching/digital/isdn/ |access-date=2022-08-25 |website=www.communicationsmuseum.org.uk}}</ref> This service was later made much less important due to the ability to provide digital services based on the [[Internet protocol suite]].<ref>{{Cite web |date=2022-03-07 |title=Why ISDN telephones are in decline - Sussex, Surrey, Brighton {{!}} Ingenio |url=https://ingeniotech.co.uk/isdn-telephones-decline/ |access-date=2022-08-25 |website=ingeniotech.co.uk |language=en-US}}</ref>

Since the advent of personal computer technology in the 1980s, [[computer telephony integration]] (CTI) has progressively provided more sophisticated telephony services, initiated and controlled by the computer, such as making and receiving voice, fax, and data calls with [[telephone directory]] services and [[caller identification]]. The integration of telephony software and computer systems is a major development in the evolution of office automation.  The term is used in describing the computerized services of call centers, such as those that direct your phone call to the right department at a business you're calling.  It is also sometimes used for the ability to use your personal computer to initiate and manage phone calls (in which case you can think of your computer as your personal call center).<ref>[http://searchnetworking.techtarget.com/definition/CTI What is CTI?] TechTarget</ref>

=={{anchor|Digital telephony}} Digital telephony==
'''Digital telephony''' is the use of [[digital electronics]] in the operation and provisioning of telephony systems and services. Since the late 20th century, a [[digital electronics|digital]] [[core network]] has replaced the traditional [[analog transmission]] and signaling systems, and much of the [[access network]] has also been digitized.

Starting with the development of [[transistor]] technology, originating from [[Bell Telephone Laboratories]] in 1947, to [[amplifier|amplification]] and [[switching circuit]]s in the 1950s, the [[public switched telephone network]] (PSTN) has gradually moved towards [[solid-state electronics]] and [[automation]]. Following the development of [[computer]]-based [[electronic switching system]]s incorporating [[metal–oxide–semiconductor]] (MOS) and [[pulse-code modulation]] (PCM) technologies, the PSTN gradually evolved towards the [[digital signal|digitization of signaling]] and [[digital audio|audio transmissions]]. Digital telephony has since dramatically improved the capacity, quality and cost of the network. Digitization allows [[wideband voice]] on the same channel, with improved quality of a wider analog voice channel.

===History===
The earliest end-to-end analog telephone networks to be modified and upgraded to transmission networks with [[Digital Signal 1]] (DS1/T1) carrier systems date back to the early 1960s. They were designed to support the basic 3&nbsp;kHz voice channel by sampling the bandwidth-limited analog voice signal and encoding using [[pulse-code modulation]] (PCM). Early PCM [[codec]]-[[Filter (signal processing)|filters]] were implemented as passive [[resistor]]{{ndash}}[[capacitor]]{{ndash}}[[inductor]] filter circuits, with [[analog-to-digital]] conversion (for digitizing voices) and [[digital-to-analog conversion]] (for reconstructing voices) handled by [[discrete device]]s. Early digital telephony was impractical due to the low performance and high costs of early PCM codec-filters.<ref name="Gibson26">{{cite book |last1=Floyd |first1=Michael D. |last2=Hillman |first2=Garth D. |chapter=Pulse-Code Modulation Codec-Filters |title=The Communications Handbook |edition=2nd |date=8 October 2018 |orig-year=1st pub. 2000 |pages=26-1, 26-2, 26-3 |publisher=[[CRC Press]] |isbn=9781420041163 |chapter-url=https://books.google.com/books?id=Tokk5bZxB0MC&pg=SA26-PA1}}</ref><ref name="Allstot">{{cite book |last1=Allstot |first1=David J. |chapter=Switched Capacitor Filters |editor-last1=Maloberti |editor-first1=Franco |editor-last2=Davies |editor-first2=Anthony C. |title=A Short History of Circuits and Systems: From Green, Mobile, Pervasive Networking to Big Data Computing |date=2016 |publisher=[[IEEE Circuits and Systems Society]] |isbn=9788793609860 |pages=105–110 |url=https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |access-date=2019-11-28 |archive-date=2021-09-30 |archive-url=https://web.archive.org/web/20210930151716/https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |url-status=dead }}</ref>

Practical [[digital telecommunication]] was enabled by the invention of the [[metal–oxide–semiconductor field-effect transistor]] (MOSFET),<ref name="Colinge2005">{{cite book |last1=Colinge |first1=Jean-Pierre |last2=Colinge |first2=C. A. |title=Physics of Semiconductor Devices |date=2005 |publisher=[[Springer Science & Business Media]] |isbn=9780387285238 |page=165 |url=https://books.google.com/books?id=-o5bAG5pR3AC&pg=PA165}}</ref> which led to the rapid development and wide adoption of PCM digital telephony.<ref name="Allstot"/> The MOSFET was invented by [[Mohamed M. Atalla]] and [[Dawon Kahng]] at [[Bell Telephone Laboratories]] in 1959, and the [[metal–oxide–semiconductor]] (MOS) [[integrated circuit]] (IC) chip was proposed soon after, but MOS technology was initially overlooked by Bell because they did not find it practical for analog telephone applications, before it was commercialized by [[Fairchild Semiconductor|Fairchild]] and [[RCA]] for [[digital electronics]] such as [[computers]].<ref>{{cite book |last1=Maloberti |first1=Franco |last2=Davies |first2=Anthony C. |chapter=History of Electronic Devices |title=A Short History of Circuits and Systems: From Green, Mobile, Pervasive Networking to Big Data Computing |date=2016 |publisher=[[IEEE Circuits and Systems Society]] |isbn=9788793609860 |pages=59-70 (65-7) |url=https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |access-date=2019-11-28 |archive-date=2021-09-30 |archive-url=https://web.archive.org/web/20210930151716/https://ieee-cas.org/sites/default/files/a_short_history_of_circuits_and_systems-_ebook-_web.pdf |url-status=dead }}</ref><ref name="Allstot"/> MOS technology eventually became practical for telephone applications with the MOS [[mixed-signal integrated circuit]], which combines analog and [[digital signal processing]] on a single chip, developed by former Bell engineer [[David A. Hodges]] with Paul R. Gray at [[UC Berkeley]] in the early 1970s.<ref name="Allstot"/> In 1974, Hodges and Gray worked with R.E. Suarez to develop MOS [[switched capacitor]] (SC) circuit technology, which they used to develop a [[digital-to-analog converter]] (DAC) chip, using [[MOS capacitor]]s and MOSFET switches for data conversion.<ref name="Allstot"/> MOS [[analog-to-digital converter]] (ADC) and DAC chips were commercialized by 1974.<ref name="US46">{{cite book |title=Electronic Components |date=1974 |publisher=[[U.S. Government Printing Office]] |page=46 |url=https://books.google.com/books?id=HikuAAAAMAAJ&pg=PA46}}</ref>

MOS SC circuits led to the development of PCM codec-filter chips in the late 1970s.<ref name="Allstot"/><ref name="Gibson26"/> The [[silicon-gate]] [[CMOS]] (complementary MOS) PCM codec-filter chip, developed by Hodges and W.C. Black in 1980,<ref name="Allstot"/> has since been the industry standard for digital telephony.<ref name="Allstot"/><ref name="Gibson26"/> By the 1990s, [[telecommunication network]]s such as the [[public switched telephone network]] (PSTN) had been largely digitized with [[very-large-scale integration]] (VLSI) CMOS PCM codec-filters, widely used in [[electronic switching system]]s for [[telephone exchanges]], [[private branch exchange]]s (PBX) and [[key telephone system]]s (KTS); user-end [[modems]]; [[data transmission]] applications such as [[digital loop carrier]]s, [[pair gain]] [[multiplexers]], telephone [[ADSL loop extender|loop extenders]], [[integrated services digital network]] (ISDN) terminals, digital [[cordless telephones]] and digital [[cell phones]]; and applications such as [[speech recognition]] equipment, voice [[data storage]], [[voice mail]] and digital tapeless [[answering machines]].<ref name="Gibson26"/> The bandwidth of digital telecommunication networks has been rapidly increasing at an exponential rate, as observed by [[Edholm's law]],<ref name="Cherry">{{cite journal |last1=Cherry |first1=Steven |title=Edholm's law of bandwidth |journal=IEEE Spectrum |date=2004 |volume=41 |issue=7 |pages=58–60 |doi=10.1109/MSPEC.2004.1309810|s2cid=27580722 }}</ref> largely driven by the [[MOSFET scaling|rapid scaling]] and [[miniaturization]] of MOS technology.<ref name="Jindal">{{cite book |last1=Jindal |first1=Renuka P. |title=2009 2nd International Workshop on Electron Devices and Semiconductor Technology |chapter=From millibits to terabits per second and beyond - over 60 years of innovation |date=2009 |pages=1–6 |doi=10.1109/EDST.2009.5166093 |isbn=978-1-4244-3831-0 |s2cid=25112828 |chapter-url=https://events.vtools.ieee.org/m/195547}}</ref><ref name="Allstot"/>

Uncompressed PCM [[digital audio]] with [[Audio bit depth|8-bit depth]] and 8{{nbsp}}[[kHz]] [[sample rate]] requires a [[bit rate]] of 64{{nbsp}}[[kbit/s]], which was impractical for early digital telecommunication networks with limited [[network bandwidth]]. A solution to this issue was [[linear predictive coding]] (LPC), a [[speech coding]] [[data compression]] algorithm that was first proposed by [[Fumitada Itakura]] of [[Nagoya University]] and Shuzo Saito of [[Nippon Telegraph and Telephone]] (NTT) in 1966. LPC was capable of [[audio data compression]] down to 2.4{{nbsp}}kbit/s, leading to the first successful real-time conversations over digital networks in the 1970s.<ref name="Gray">{{cite journal |last1=Gray |first1=Robert M. |title=A History of Realtime Digital Speech on Packet Networks: Part II of Linear Predictive Coding and the Internet Protocol |journal=Found. Trends Signal Process. |date=2010 |volume=3 |issue=4 |pages=203–303 |doi=10.1561/2000000036 |url=https://ee.stanford.edu/~gray/lpcip.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ee.stanford.edu/~gray/lpcip.pdf |archive-date=2022-10-09 |url-status=live |issn=1932-8346|doi-access=free }}</ref> LPC has since been the most widely used speech coding method.<ref>{{cite journal |last1=Gupta |first1=Shipra |title=Application of MFCC in Text Independent Speaker Recognition |journal=International Journal of Advanced Research in Computer Science and Software Engineering |date=May 2016 |volume=6 |issue=5 |pages=805–810 (806) |s2cid=212485331 |issn=2277-128X |url=https://pdfs.semanticscholar.org/2aa9/c2971342e8b0b1a0714938f39c406f258477.pdf |archive-url=https://web.archive.org/web/20191018231621/https://pdfs.semanticscholar.org/2aa9/c2971342e8b0b1a0714938f39c406f258477.pdf |url-status=dead |archive-date=2019-10-18 |access-date=18 October 2019}}</ref> Another [[audio data compression]] method, a [[discrete cosine transform]] (DCT) algorithm called the [[modified discrete cosine transform]] (MDCT), has been widely adopted for speech coding in [[voice-over-IP]] (VoIP) applications since the late 1990s.<ref name="Schnell">{{cite conference|last1=Schnell|first1=Markus|last2=Schmidt|first2=Markus|last3=Jander|first3=Manuel|last4=Albert|first4=Tobias|last5=Geiger|first5=Ralf|last6=Ruoppila|first6=Vesa|last7=Ekstrand|first7=Per|last8=Bernhard|first8=Grill|date=October 2008|title=MPEG-4 Enhanced Low Delay AAC - A New Standard for High Quality Communication|url=https://www.iis.fraunhofer.de/content/dam/iis/de/doc/ame/conference/AES-125-Convention_AAC-ELD-NewStandardForHighQualityCommunication_AES7503.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.iis.fraunhofer.de/content/dam/iis/de/doc/ame/conference/AES-125-Convention_AAC-ELD-NewStandardForHighQualityCommunication_AES7503.pdf |archive-date=2022-10-09 |url-status=live|conference=125th AES Convention|publisher=[[Audio Engineering Society]]|access-date=20 October 2019|website=[[Fraunhofer IIS]]}}</ref>

The development of transmission methods such as [[SONET]] and [[fiber optic]] transmission further advanced digital transmission. Although analog carrier systems existed that multiplexed multiple analog voice channels onto a single transmission medium, digital transmission allowed lower cost and more channels [[multiplexing|multiplexed]] on the transmission medium. Today the end instrument often remains analog but the analog signals are typically converted to [[Digital signal (signal processing)|digital signals]] at the [[serving area interface]] (SAI), [[Telephone exchange|central office]] (CO), or other aggregation point. [[Digital loop carrier]]s (DLC) and [[fiber to the x]] place the digital network ever closer to the customer premises, relegating the analog [[local loop]] to legacy status.

==IP telephony==
{{main|Voice over IP}}
[[File:Cisco7960G.jpg|thumb|A commercial IP telephone, with keypad, control keys, and screen functions to perform configuration and user features.]]
The field of technology available for telephony has broadened with the advent of new communication technologies. Telephony now includes the technologies of Internet services and mobile communication, including video conferencing.

The new technologies based on [[Internet Protocol]] (IP) concepts are often referred to separately as voice over IP (VoIP) telephony, also commonly referred to as IP telephony or Internet telephony. Unlike traditional phone service, IP telephony service is relatively unregulated by government. In the United States, the [[Federal Communications Commission]] (FCC) regulates phone-to-phone connections, but says they do not plan to regulate connections between a phone user and an IP telephony service provider.<ref>{{Cite web |title=Microsoft word - 37716 |url=https://docs.fcc.gov/public/attachments/FCC-04-97A1.pdf |website=docs.fcc.gov}}</ref>

A specialization of digital telephony, Internet Protocol (IP) telephony involves the application of digital networking technology that was the foundation to the [[Internet]] to create, transmit, and receive telecommunications sessions over [[computer network]]s. Internet telephony is commonly known as [[voice over Internet Protocol]] (VoIP), reflecting the principle, but it has been referred with many other terms. VoIP has proven to be a [[disruptive technology]] that is rapidly replacing traditional telephone infrastructure technologies. As of January 2005, up to 10% of telephone subscribers in [[Japan]] and [[South Korea]] have switched to this digital telephone service. A January 2005 ''[[Newsweek]]'' article suggested that Internet telephony may be "the next big thing".<ref>{{cite web|last=Sheridan |first=Barrett |url=http://msnbc.msn.com/id/6831938/site/newsweek/ |title=Newsweek - National News, World News, Health, Technology, Entertainment and more... - Newsweek.com |publisher=MSNBC |access-date=2010-05-23 |url-status=dead |archive-url=https://web.archive.org/web/20050118033848/http://msnbc.msn.com/id/6831938/site/newsweek/ |archive-date=January 18, 2005 }}</ref> As of 2006, many VoIP companies offer service to [[consumer]]s and [[business]]es.{{Update inline|date=September 2015}}

IP telephony uses an Internet connection and hardware [[IP phone]]s, analog telephone adapters, or [[softphone]] computer applications to transmit conversations encoded as [[data packet]]s. In addition to replacing plain old telephone service (POTS), IP telephony services compete with [[mobile phone]] services by offering free or lower cost connections via [[WiFi]] [[Hotspot (Wi-Fi)|hotspots]]. VoIP is also used on private networks which may or may not have a connection to the global telephone network.

[[File:Fixed telephone lines per 100 inhabitants 1997-2007 ITU.png|thumb|300px|Fixed telephone lines per 100 inhabitants 1997–2007]]

==Social impact research==
Direct person-to-person communication includes non-verbal cues expressed in facial and other bodily articulation, that cannot be transmitted in traditional voice telephony. [[Video telephony]] restores such interactions to varying degrees. Social Context Cues Theory is a model to measure the success of different types of communication in maintaining the non-verbal cues present in face-to-face interactions. The research examines many different cues, such as the physical context, different facial expressions, body movements, tone of voice, touch and smell.

Various communication cues are lost with the usage of the telephone. The communicating parties are not able to identify the body movements, and lack touch and smell. Although this diminished ability to identify social cues is well known, Wiesenfeld, Raghuram, and Garud point out that there is a value and efficiency to the type of communication for different tasks.<ref>{{cite web|title=Hosted PBX|url=http://www.hostedpbx.com.au|access-date=5 December 2017}}</ref> They examine work places in which different types of communication, such as the telephone, are more useful than face-to-face interaction.

The expansion of communication to mobile telephone service has created a different filter of the social cues than the [[land-line]] telephone. The use of instant messaging, such as ''texting'', on mobile telephones has created a sense of community.<ref>{{Cite journal|last1=Mesch|first1=Gustavo S.|last2=Talmud|first2=Ilan|last3=Quan-Haase|first3=Anabel|date=2012-09-01|title=Instant messaging social networks: Individual, relational, and cultural characteristics|journal=Journal of Social and Personal Relationships|language=en|volume=29|issue=6|pages=736–759|doi=10.1177/0265407512448263|s2cid=144874874 |issn=0265-4075}}</ref> In ''The Social Construction of Mobile Telephony'' it is suggested that each phone call and text message is more than an attempt to converse. Instead, it is a gesture which maintains the social network between family and friends. Although there is a loss of certain social cues through telephones, mobile phones bring new forms of expression of different cues that are understood by different audiences. New language additives attempt to compensate for the inherent lack of non-physical interaction.

Another social theory supported through telephony is the Media Dependency Theory. This theory concludes that people use media or a resource to attain certain goals. This theory states that there is a link between the media, audience, and the large social system.<ref>{{Cite web | url=http://communicationtheory.org/media-dependency-theory/ |title = Media Dependency Theory|date = 2012-02-12}}</ref> Telephones, depending on the person, help attain certain goals like accessing information, keeping in contact with others, sending quick communication, entertainment, etc.

==See also==
{{Wiktionary}}
{{portal|Telecommunications|Telephones}}
* [[List of telephony terminology]]
* [[History of the telephone]]
* [[Invention of the telephone]]
* [[Extended area service]]

==References==
{{Reflist}}

{{Telephony}}
{{Telecommunications}}

{{Authority control}}

[[Category:Telephony| ]]
[[Category:History of telecommunications]]
[[Category:Telecommunications]]