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.
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