Voice over Internet Protocol (VoIP) is a protocol optimized for the transmission
of voice through the Internet or other packet switched networks. VoIP is often used abstractly
to refer to the actual transmission of voice (rather than the protocol implementing it).
VoIP is also known as IP Telephony, Internet telephony, Broadband telephony,
Broadband Phone and Voice over Broadband. "VoIP" is pronounced voyp.
Companies providing VoIP service are commonly referred to as providers, and protocols which are
used to carry voice signals over the IP network are commonly referred to as Voice over IP
or VoIP protocols. They may be viewed as commercial realizations of the experimental Network
Voice Protocol (1973) invented for the ARPANET providers. Some cost savings are due to utilizing a
single network to carry voice and data, especially where users have existing underutilized network
capacity that can carry VoIP at no additional cost. VoIP to VoIP phone calls are sometimes free,
while VoIP to public switched telephone networks, PSTN, may have a cost that is borne by the VoIP user.
Voice over IP protocols carry telephony signals as digital audio, typically reduced in data rate
using speech data compression techniques, encapsulated in a data packet stream over IP.
There are two types of PSTN to VoIP services: Direct Inward Dialing (DID) and access numbers.
DID will connect the caller directly to the VoIP user while access numbers require the caller
to input the extension number of the VoIP user.
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