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Digital telephony is the use of digital electronics in the provision of digital telephone services and systems. Since the 1960s a digital core network has almost entirely replaced the old analog system, and much of the access network 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 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.

See also

• Telephone
• Telephony
• Multiplexing

Further reading

• John Bellamy. Digital Telephony (3rd Edition, 2000).
• Andrew Wheen. DOT-DASH TO DOT.COM: How Modern Telecommunications Evolved from the Telegraph to the Internet (Springer, 2011)

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