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The Network Voice Protocol (NVP) was a pioneering computer network protocol for transporting human speech over packetized communications networks. It was an early example of Voice over Internet Protocol technology.
NVP was first implemented in December 1973 by computer networking researcher Danny Cohen of the Information Sciences Institute (ISI), University of Southern California, with funding from ARPA's Network Secure Communications (NSC) program. The project's stated goals as stated in IETF RFC 741, published 1977, were "to develop and demonstrate the feasibility of secure, high-quality, low-bandwidth, real-time, full-duplex (two-way) digital voice communications over packet-switched computer communications networks.... [and to] supply digitized speech which can be secured by existing encryption devices. The major goal of this research is to demonstrate a digital high-quality, low-bandwidth, secure voice handling capability as part of the general military requirement for worldwide secure voice communication."
NVP was used to send speech between distributed sites on the ARPANET using several different voice-encoding techniques, including linear predictive coding (LPC) and continuously variable slope delta modulation (CVSD). Cooperating researchers included Steve Casner, Randy Cole, and Paul Raveling (ISI); Jim Forgie (Lincoln Laboratory); Mike McCammon (Culler-Harrison); John Markel (Speech Communications Research Laboratory); and John Makhoul (Bolt, Beranek and Newman).
The protocol consisted of two distinct parts: control protocols and a data transport protocol. Control protocols included relatively rudimentary telephony features such as indicating who wants to talk to whom; ring tones; negotiation of voice encoding; and call termination. Data messages contained encoded speech. For each encoding scheme (vocoder) a frame was defined as a packet containing the negotiated transmission interval of a number of digitized voice samples.
NVP was used by experimental Voice Funnel equipment (circa February 1981), based on BBN Butterfly computers, as part of ongoing ARPA research into packetized audio. ARPA staff and contractors used the Voice Funnel, and related video facilities, to do three-way and four-way video conferencing among a handful of East and West Coast sites.
NVP was transported over the Internet Stream Protocol (ST) and a later version called Stream Protocol, version 2 (ST-II), both connection-oriented versions of the Internet Protocol (IP) and which carried the IP protocol version 5. These protocols may be viewed as early experiments in quality of service and connection-oriented network protocols such as Asynchronous Transfer Mode (ATM).