Voice over IPwikipedia
Voice over Internet Protocol (also voice over IP, VoIP or IP telephony) is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet.
VoIPvoice over IPvoice over Internet Protocolvoice-over-IPinternet telephonyvoiceIP telephonyVoice over IP (VoIP)VoIP telephonyvoice calls

Internet

internetonlinethe Internet
Voice over Internet Protocol (also voice over IP, VoIP or IP telephony) is a methodology and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet.
The Internet carries a vast range of information resources and services, such as the inter-linked hypertext documents and applications of the World Wide Web (WWW), electronic mail, telephony, and file sharing.

Telephony

telephonydigital telephonytelephone
The steps and principles involved in originating VoIP telephone calls are similar to traditional digital telephony and involve signaling, channel setup, digitization of the analog voice signals, and encoding.
In this context the technology is specifically referred to as Internet telephony, or voice over Internet Protocol (VoIP).

VoIP phone

IP PhoneIP phonesEthernet-based phone
In addition to VoIP phones, VoIP is also available on many personal computers and other Internet access devices. These protocols can be used by a VoIP phone, special-purpose software, a mobile application or integrated into a web page.
A VoIP phone or IP phone uses voice over IP technologies for placing and transmitting telephone calls over an IP network, such as the Internet, instead of the traditional public switched telephone network (PSTN).

Session Initiation Protocol

SIPsession initiation protocolSession Initiation Protocol (SIP)
These include RTCP Extended Report (RFC 3611), SIP RTCP Summary Reports, H.460.9 Annex B (for H.323), H.248.30 and MGCP extensions.
SIP is used for signaling and controlling multimedia communication sessions in applications of Internet telephony for voice and video calls, in private IP telephone systems, in instant messaging over Internet Protocol (IP) networks as well as mobile phone calling over LTE (VoLTE).

Web-based VoIP

web-based VoIPintegrated into a web page
These protocols can be used by a VoIP phone, special-purpose software, a mobile application or integrated into a web page.
Web-based VoIP is the integration of voice over IP technologies into the facilities and methodologies of the World-Wide Web.

Speech coding

speech codingspeech codecspeech encoding
Various codecs exist that optimize the media stream based on application requirements and network bandwidth; some implementations rely on narrowband and compressed speech, while others support high-fidelity stereo codecs.
The two most important applications of speech coding are mobile telephony and voice over IP.

Federated VoIP

federated VoIP
Third-generation providers, such as Google Talk, adopted the concept of federated VoIP—which is a departure from the architecture of the legacy networks.
Federated VoIP is a form of packetized voice telephony that uses voice over IP between autonomous domains in the public Internet without the deployment of central virtual exchange points or switching centers for traffic routing.

Media Gateway Control Protocol

MGCPmedia gateway control protocol
These include RTCP Extended Report (RFC 3611), SIP RTCP Summary Reports, H.460.9 Annex B (for H.323), H.248.30 and MGCP extensions.
The Media Gateway Control Protocol (MGCP) is a signaling and call control communications protocol used in voice over IP (VoIP) telecommunication systems.

Analog telephone adapter

analog telephone adapterATAanalog telephony adapter
The fax machine may be a standard device connected to an analog telephone adapter (ATA), or it may be a software application or dedicated network device operating via an Ethernet interface.
An analog telephone adapter (ATA) is a device for connecting traditional analog telephones, fax machines, and similar customer-premises devices to a digital telephone system or a voice over IP telephony network.

XMPP

XMPPJabberJabber/XMPP
Designed to be extensible, the protocol has been used also for publish-subscribe systems, signalling for VoIP, video, file transfer, gaming, the Internet of Things (IoT) applications such as the smart grid, and social networking services.

Unified communications

unified communicationsUnified Communicationunif
VoIP solutions aimed at businesses have evolved into unified communications services that treat all communications—phone calls, faxes, voice mail, e-mail, web conferences, and more—as discrete units that can all be delivered via any means and to any handset, including cellphones.
Unified communications (UC) is a business and marketing concept describing the integration of enterprise communication services such as instant messaging (chat), presence information, voice (including IP telephony), mobility features (including extension mobility and single number reach), audio, web & video conferencing, fixed-mobile convergence (FMC), desktop sharing, data sharing (including web connected electronic interactive whiteboards), call control and speech recognition with non-real-time communication services such as unified messaging (integrated voicemail, e-mail, SMS and fax).

Jingle (protocol)

JinglelibjingleGoogle Talk voice calls
Jingle is an extension to the Extensible Messaging and Presence Protocol (XMPP) which adds peer-to-peer (P2P) session control (signaling) for multimedia interactions such as in Voice over IP (VoIP) or videoconferencing communications.

G.729

Some popular codecs include μ-law and a-law versions of G.711, G.722, an open source voice codec known as iLBC, a codec that uses only 8 kbit/s each way called G.729, and many others.
Because of its low bandwidth requirements, G.729 is mostly used in voice over Internet Protocol (VoIP) applications when bandwidth must be conserved, such as for conference calls.

Quality of service

quality of serviceQoSquality-of-service
It is a best-effort network without fundamental Quality of Service (QoS) guarantees.
In particular, developers have introduced Voice over IP technology to allow computer networks to become as useful as telephone networks for audio conversations, as well as supporting new applications with even stricter network performance requirements.

H.323

RASH.323
These include RTCP Extended Report (RFC 3611), SIP RTCP Summary Reports, H.460.9 Annex B (for H.323), H.248.30 and MGCP extensions.
A call model, similar to the ISDN call model, eases the introduction of IP telephony into existing networks of ISDN-based PBX systems, including transitions to IP-based PBXs.

Internet access

broadband internetbroadbandbroadband Internet access
Mass-market VoIP services use existing broadband Internet access, by which subscribers place and receive telephone calls in much the same manner as they would via the public switched telephone network (PSTN).
VDSL is capable of supporting applications such as high-definition television, as well as telephone services (voice over IP) and general Internet access, over a single physical connection.

Real-time Transport Protocol

RTPreal-time protocolReal-Time Transport Protocol
RTP is one of the technical foundations of Voice over IP and in this context is often used in conjunction with a signaling protocol such as the Session Initiation Protocol (SIP) which establishes connections across the network.

Inter-Asterisk eXchange

IAX2IAX
It is used for transporting VoIP telephony sessions between servers and to terminal devices.

Skype protocol

Skype
The Skype network is not interoperable with most other Voice over IP (VoIP) networks without proper licensing from Skype.

Internet Low Bitrate Codec

iLBCGIPS codecInternet low Bitrate Codec
Some popular codecs include μ-law and a-law versions of G.711, G.722, an open source voice codec known as iLBC, a codec that uses only 8 kbit/s each way called G.729, and many others.
It is suitable for VoIP applications, streaming audio, archival and messaging.

Business telephone system

PBXprivate branch exchangekey telephone system
In 2008, 80% of all new Private branch exchange (PBX) lines installed internationally were VoIP.
The modern key system is usually fully digital, although analog variants persist and some systems implement VOIP services.

Session border controller

session border controllerSBCSession Border Controllers
Private session border controllers are often employed to enable VoIP calls to and from protected networks.
A session border controller (SBC) is a device regularly deployed in Voice over Internet Protocol (VoIP) networks to exert control over the signaling and usually also the media streams involved in setting up, conducting, and tearing down telephone calls or other interactive media communications.

Transmission Control Protocol

TCPTCP/IPACK
This signals a transport protocol like TCP to reduce its transmission rate to alleviate the congestion.
Therefore, it is not particularly suitable for real-time applications such as Voice over IP.

Caller ID

caller IDcaller line identificationCLI
Voice over IP protocols and equipment provide caller ID support that is compatible with the facility provided in the public switched telephone network (PSTN).
Caller ID (caller identification, CID), also called calling line identification (CLID), Calling Line Identification (CLI), calling number delivery (CND), calling number identification (CNID), calling line identification presentation (CLIP), or call display, is a telephone service, available in analog and digital telephone systems, including VoIP, that transmits a caller's telephone number to the called party's telephone equipment when the call is being set up.

Internet telephony service provider

VoIP servicesVoIP telephone companiesITSPs
In such cases, the Internet telephony service provider (ITSP) knows only that a particular user's equipment is active.
An Internet telephony service provider (ITSP) offers digital telecommunications services based on Voice over Internet Protocol (VoIP) that are provisioned via the Internet.