Digital telephony is the use of digital electronics in the provision of digital telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 services and systems. Since the 1960s a digital core network

Core network

A core network, or network core, is the central part of a telecommunication network that provides various services to customers who are connected by the access network. One of the main functions is to route telephone calls across the PSTN....

 has almost entirely replaced the old analog

Analog signal

An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 system, and much of the access network

Access network

An access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...

 has also been digitized. Digital telephony was introduced to provide voice services at lower cost, but was then found to be of great value to new network services such as ISDN that could use digital facilities to transfer data speedily over telephone lines.

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
  Bitstream
  A bitstream or bit stream is a time series of bits.A bytestream is a series of bytes, typically of 8 bits each, and can be regarded as a special case of a 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
  T-carrier
  In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
  
   system and the European E-carrier
  E-carrier
  In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
  
   system developed to carry digital telephony
• introduction of space-time switching in fully digital electronic switching system
  Electronic switching system
  In telecommunications, an electronic switching system is:* A telephone exchange based on the principles of time-division multiplexing of digitized analog signals. An electronic switching system digitizes analog signals from subscriber loops, and interconnects them by assigning the digitized...
  
  s
• replacement of tone signaling with digital signaling for trunks
• in-band signaling
  In-band signaling
  In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...
  
   vs. out-of-band signaling
• the problem of bit-robbing
• development of SS7
  SS7
  SS-7 can stand for:* Signaling System #7, a set of telephone signaling protocols.* The R-16 missile, with NATO reporting name SS-7 Saddler.* China Railways SS7, an electric locomotive model in China.* Super Socket 7, a chip socket introduced by AMD...
  
  
• emergence of fiber optic network
  Optical fiber
  An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
  
  ing allows greater reliability and call capacity
• transition from plesiochronous
  Plesiochronous
  The term Plesiochronous is derived from the Greek plesio, meaning near, and chronos, time, and refers to the fact that plesiochronous systems run in a state where different parts of the system are almost, but not quite perfectly, synchronised....
  
   transmission to synchronous systems like SONET
  Sonet
  Sonet may refer to:* Sonet Records, European record label* Synchronous optical networking * Saab Sonett...
  
  /SDH
  SDH
  SDH may refer to:* Synchronous Digital Hierarchy, in telecommunications* Secure Digital Host Controller, in computing* Saradhna, a railway station in India* The Shubnikov-De Haas effect, also see Fermi surface...
  
  
• optical self-healing ring
  Self-healing ring
  A self-healing ring, or SHR, is a telecommunications term for loop network topology, a common configuration in telecommunications transmission systems. Like roadway and water distribution systems, a loop or ring is used to provide redundancy...
  
   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
  Intelligent network
  The Intelligent Network , is the standard network architecture specified in the . It is intended for fixed as well as mobile telecom networks...
  
   services
• digital speech coding
  Speech coding
  Speech coding is the application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting...
  
   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
  Primary rate interface
  The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....
  
   catches on, BRI
  Basic rate interface
  Basic Rate Interface is an Integrated Services Digital Network configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service...
  
   mostly does not, except in Germany
• the effects of digital telephony, and digital termination at the ISP, on modem performance
• voice over IP
  Voice over IP
  Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
  
   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 tariff
  Telecommunications tariffs
  A telecommunications tariff is an open contract between a telecommunications service provider and the public, filed with a regulating body such as a Public Utilities Commission...
  
  s, increasing moves towards flat rate
  Flat rate
  A flat fee, also referred to as a flat rate or a linear rate, refers to a pricing structure that charges a single fixed fee for a service, regardless of usage. Rarely, it may refer to a rate that does not vary with usage or time of use...
  
   pricing as the marginal cost
  Marginal cost
  In economics and finance, marginal cost is the change in total cost that arises when the quantity produced changes by one unit. That is, it is the cost of producing one more unit of a good...
  
  of telephony drops further and further.