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Digital telephony is a technology used in the provision of digital telephone services and systems. Since the 1960s it has almost entirely replaced the old telephone system that used analog telephony. It was introduced to improve voice services, but was then found to be of great value to new network services that needed to speedily transfer data over telephone lines.

This is a highly compressed list of milestones in digital telephony, in roughly chronological order. This should be expanded into a fully developed article

• 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 E1-based system developed to carry digital telephony
• introduction of space-time switching in fully digital telephone exchanges
• 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 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 ite 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

• Telephone
• Telephony
• Multiplexing

• John Bellamy. Digital Telephony (3rd Edition, 2000).

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