Telephony 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).
The first telephones were connected directly in pairs. Each user had a separate telephone wired to the locations he might wish to reach. This quickly became inconvenient and unmanageable when people wanted to communicate with more than a few people. The inventions 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 via one wire pair, the local loop. Nearby exchanges in other service areas were connected with trunk lines and long distance service could be established by relaying the calls through multiple exchanges.
Initially the switchboards were manually operated by an attendant, a switchboard operator. When a customer cranked a handle on the telephone, it turned on an indicator on the board in front of the operator who would 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 complete the station-to-station circuit. Trunk calls were made with the assistance of other operators at other exchangers in the network.
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 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).
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 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 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. The first implementation of this, ISDN, permitted all data transport from end-to-end speedily over telephone lines. This service was later made much less important due to the ability to provide digital services based on the IP protocol.
Since the advent of personal computer technology in the 1980s, computer telephony integration 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 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.
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.
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, 8 kHz standard is developed; Nyquist's theorem and the standard 3.5 kHz telephony bandwidth
- DS0 as the basic digital telephony bitstream standard
- non-linear quantization: A-law vs. ?-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.
Main article: Voice over IP
A commercial IP telephone, with keypad, control keys, and screen functions to perform configuration and user features.
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 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 compete 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.