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Telephony (pron.: // 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. 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).
Brief overview 
Telephones were originally connected directly together in pairs. Each user would have a separate telephone wired to various places he might wish to reach. This became inconvenient when people wanted to communicate with many other people using telephones, so the telephone exchange was invented. Each telephone could then be connected to other local ones, thus inventing the local loop and the telephone call. Soon, nearby exchanges were connected by trunk lines, and eventually distant ones were as well.
In modern times, most telephones are plugged into telephone jacks. The jacks are connected by inside wiring to a drop wire 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 the user of a telephone 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 connected to each other, forming the public switched telephone network (PSTN). By the end of the 20th century almost all were stored program control exchanges.
After the middle of the 20th century, fax and data became important secondary users 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 has been digitized and has merged into digital telephony, the use of digital technology in the provision of telephone services and systems. Telephone calls can be provided digitally, but are restricted to cases in which the last mile is digital, or where the conversion between digital and analog signals takes place inside the telephone. This advancement has reduced costs in communication- and improved the quality of voice services. This has been found useful for new network services (ISDN) that transfer data speedily over telephone lines.
Another important concept that has been merged into telephony is computer telephony integration, which enables computers to know about and control phone functions such as making and receiving voice, fax, and data calls with telephone directory services and caller identification. The integration of telephone software and computer systems is a major development in the evolution of the automated office. 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's also sometimes used to describe 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). CTI is not a new concept and has been used in the past in large telephone networks, but only dedicated call centers could justify the costs of the required equipment installation. Primary telephone service providers are offering information services such as automatic number identification, which is a telephone service architecture that separates CTI services from call switching and will make it easier to add new services. Dialed Number Identification Service (DNIS) on a scale is wide enough for its implementation to bring real value to business or residential telephone usage. A new generation of applications (middleware) is being developed as a result of standardization and availability of low cost computer telephony links.
Recent developments 
The term's scope has been broadened with the advent of the different new communication technologies. In its broadest sense, the terms encompasses phone communication, Internet calling, mobile communication, faxing, voicemail and video conferencing. Telephony's initial idea returns to POTS, (an acronym for "plain old telephone service") technically called the PSTN (public-switched telephone network).
This system is being fiercely challenged by and to a great extent yielding to Voice over IP (VoIP) technology, which is also commonly referred to as IP Telephony and Internet Telephony. IP telephony is a modern form of telephony which uses the TCP/IP protocol popularized by the Internet to transmit digitized voice data. Also, 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.Using the Internet, calls travel as packets of data on shared lines, avoiding the tolls of the PSTN. The challenge in IP telephony is to deliver the voice, fax, or video packets in a dependable flow to the user. Much of IP telephony focuses on that challenge.
Digital telephony 
Starting with the introduction of the transistor, invented in 1947 by Bell Laboratories, to amplification and switching circuits in the 1950s, and through development of computer-based electronic switching systems, the public switched telephone network (PSTN) has gradually evolved towards automation and digitization of signaling and audio transmissions.
Digital telephony is the use of digital electronics in the operation and provisioning of telephony systems and services. Since the 1960s a digital core network has replaced the traditional analog transmission and signaling systems, and much of the access network has also been digitized.
Digital telephony has dramatically improved the capacity, quality, and cost of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading transmission networks with Digital Signal 1 (DS1/T1) carrier systems, designed to support the basic 3 kHz voice channel by sampling the bandwidth-limited analog voice signal and encoding using PCM. While digitization allows wideband voice on the same channel, the improved quality of a wider analog voice channel did not find a large market in the PSTN.
Later 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 multiplexed on the transmission medium. Today the end instrument often remains analog but the analog signals are typically converted to digital signals at the serving area interface (SAI), central office (CO), or other aggregation point. Digital loop carriers (DLC) place the digital network ever closer to the customer premises, relegating the analog local loop to legacy status.
Milestones in digital telephony 
- early experiments with pulse code modulation in telephony
- the 8-bit, 8kHz standard is developed; Nyquist's theorem and the standard 3.5kHz telephony bandwidth
- DS0 as the basic digital telephony bitstream standard
- non-linear quantization: A-law vs. μ-law, and transcoding between the two
- bit error rate and intelligibility
- first practical digital telephone systems put into service
- the U.S. T-carrier system and the European E-carrier system developed to carry digital telephony
- introduction of space-time switching in fully digital electronic switching systems
- replacement of tone signaling with digital signaling for trunks
- in-band signaling vs. out-of-band signaling
- the problem of bit-robbing
- development of SS7
- emergence of fiber optic networking allows greater reliability and call capacity
- transition from plesiochronous transmission to synchronous systems like SONET/SDH
- optical self-healing ring networks further increase reliability
- digital/optical systems revolutionize international long-distance networks, particularly undersea cables
- digital telephone exchanges eliminate moving parts, make exchange equipment much smaller and more reliable
- separation of exchange and concentrator functions
- roll-out of digital systems throughout the PSTN
- provision of intelligent network services
- digital speech coding and compression
- speech compression on international digital trunks
- phone tapping in the digital environment
- introduction of digital mobile telephony, specialized compression algorithms for high bit error rates
- direct digital termination to customers via ISDN; PRI catches on, BRI mostly does not, except in Germany
- the effects of digital telephony, and digital termination at the ISP, on modem performance
- voice over IP as a carrier strategy
- emergence of ADSL leads to voice over IP becoming a consumer product, and the slow demise of dial-up Internet access
- expected convergence of VoIP, mobile telephony, etc.
- flattening of telephony tariffs, increasing moves towards flat rate pricing as the marginal cost of telephony drops further and further.
IP telephony 
A specialization of digital telephony, Internet Protocol (IP) telephony involves the application of digital networking technology that were the foundation to the Internet to create, transmit, and receive telecommunications sessions over computer networks. 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." As of 2006 many VoIP companies offer service to consumers and businesses.
IP telephony uses an Internet connection and hardware IP Phones, analog telephone adapters, or softphone computer applications to transmit conversations encoded as data packets. In addition to replacing plain old telephone service (POTS), IP telephony services are also competing with mobile phone services by offering free or lower cost connections via WiFi hotspots. VoIP is also used on private networks which may or may not have a connection to the global telephone network.
Social impact research 
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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. 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. 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.
See also 
|Look up telephony in Wiktionary, the free dictionary.|
- Dictionary.com Telephony Definition
- What is CTI? TechTarget
- About.com VOIP and Telephony
- Sheridan, Barrett. "Newsweek - National News, World News, Health, Technology, Entertainment and more... - Newsweek.com". MSNBC. Retrieved 2010-05-23.[dead link]