IP Phone

IP Phone

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A VoIP phone uses voice over IP
Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...

 (VoIP) technologies allowing telephone calls to be made over an IP network such as the Internet instead of the ordinary PSTN
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...

 system. Calls can traverse the Internet, or a private IP network such as that of a company. The phones use control protocols such as Session Initiation Protocol
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

 (SIP), Skinny Client Control Protocol
Skinny Client Control Protocol
The Skinny Call Control Protocol is a proprietary network terminal control protocol originally developed by Selsius Systems....

 (SCCP) or one of various proprietary protocol
Proprietary protocol
In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.-Enforcement:Proprietors may enforce restrictions through patents and by keeping the protocol specification a trade secret...

s such as that used by Skype
Skype
Skype is a software application that allows users to make voice and video calls and chat over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system...

.

softphones, smartphones, USB hard phones, PoE hard phones


VoIP phones can be simple software-based softphone
Softphone
A softphone is a software program for making telephone calls over the Internet using a general purpose computer, rather than using dedicated hardware. Often a softphone is designed to behave like a traditional telephone, sometimes appearing as an image of a phone, with a display panel and buttons...

s or purpose-built hardware devices that appear much like an ordinary telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 or a cordless phone. Ordinary PSTN phones are used as VoIP phones with analog telephone adapters (ATA).

Two combined signal-and-power wired cable interfaces are in common use to communicate between computer networks (or computers) and physically separate VoIP phones: USB and Power over Ethernet
Power over Ethernet
Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels...

. The latter is preferred in industry such as large scale call centers or PBX
PBX
PBX may refer to:*Pioneer Natural Resources' stock ticker symbol*Polymer-bonded explosive*Pre-B-cell leukemia homeobox*Private branch exchange, a telephone exchange that serves a particular business or office*PBX, a rewrite of the Project Builder IDE...

 replacement applications, because PoE has the following advantages over USB:
  • Much greater and more flexible power allocation: PoE switches typically support 4, 8, 16, 24, 48 or more ports each, with each device drawing up to 30W. By contrast, a USB 3.0 only supports up to 4.5W devices, insufficient for cordless, conference, videoconferencing and other common-use devices.
  • Much greater cord range. While DC power diminishes rapidly over long cables, and is inefficient to supply further than 3M for high-draw devices, the low power draw of modern indoor radios permits full use of cat5
    Cat5
    Cat5 may refer to:*Category 5 hurricane, used in the Saffir–Simpson Hurricane Scale*Category 5 cable, unshielded twisted pair type cable*Port McNeill Airport, the ICAO airport code...

     Ethernet cables of up to 100m length. By contrast USB supports under 3m and with extensions and boosters perhaps 10m maximum.
  • Single connector: PoE relies on the single uniform cat5/6 connector, a close relative of the proven cat3
    Cat3
    Cat3 may refer to:*Category 3 cable, an unshielded twisted pair cable*Nanaimo/Long Lake Water Aerodrome, the ICAO airport code *Cognitive Abilities Test 3, a test of reasoning abilities undertaken by students in the UK and Canada...

     connector used for analog phones. By contrast there are six USB connectors and each change to the USB standard so far has involved at least one new type of connector.
  • Ethernet protocol: Scalable from its original 1 megabit to modern 100 gigabit applications, copper Ethernet has no peer for scalability. It makes effective use of many different cable types (especially cat3
    Cat3
    Cat3 may refer to:*Category 3 cable, an unshielded twisted pair cable*Nanaimo/Long Lake Water Aerodrome, the ICAO airport code *Cognitive Abilities Test 3, a test of reasoning abilities undertaken by students in the UK and Canada...

     and coax) and has robust wireless equivalents (802.11/WiFi
    WIFI
    WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...

    ) when power and wired security is not required.


For these reasons, USB and softphone PC applications are considered transitional by many industrial users and makers of larger telephone switches, used only to build markets for VoIP that will eventually shift over to the more robust PoE technology shipped by Cisco
Cisco
Cisco may refer to:Companies:*Cisco Systems, a computer networking company* Certis CISCO, corporatised entity of the former Commercial and Industrial Security Corporation in Singapore...

, Siemens
Siemens
Siemens may refer toSiemens, a German family name carried by generations of telecommunications industrialists, including:* Werner von Siemens , inventor, founder of Siemens AG...

, Alcatel
Alcatel
Alcatel Mobile Phones is a brand of mobile handsets. It was established in 2004 as a joint venture between Alcatel-Lucent of France and TCL Communication of China....

 and other large switch makers.

A VoIP phone or application may have many features an analog phone doesn't support, such as e-mail-like IDs for contacts that may be easier to remember than names or phone numbers, or easy sharing of contact lists among multiple accounts. Generally the features of VoIP phones follow those of Skype
Skype
Skype is a software application that allows users to make voice and video calls and chat over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system...

, Google Voice
Google voice
Search by voice is a branded name for a technology to "search by voice on your [digital device]", such as a mobile phone or PC, i.e. have the device search for data upon entering information on what to search into the device by speaking....

 and other PC-based phone services, which have richer feature sets but (because they rely on mainstream operating systems' IP support) latency-related audio problems.

A competing view is that as mainstream operating systems become better at voice applications with appropriate Quality of Service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...

 (QoS) guarantees and 5G
5G
5G is a name used in some research papers and projects to denote the next major phase of mobile telecommunications standards beyond the 4G/IMT-Advanced standards effective since 2011...

 handoff (IEEE 802.21
IEEE 802.21
802.21 is an IEEE standard published in 2008. The standard supports algorithms enabling seamless handover between networks of the same type as well as handover between different network types also called Media independent handover or vertical handover...

 etc.) becomes available from outdoor wireless carriers, netbook
Netbook
Netbooks are a category of small, lightweight, legacy-free, and inexpensive laptop computers.At their inception in late 2007 as smaller notebooks optimized for low weight and low cost — netbooks omitted certain features , featured smaller screens and keyboards, and offered reduced computing...

s and smartphone
Smartphone
A smartphone is a high-end mobile phone built on a mobile computing platform, with more advanced computing ability and connectivity than a contemporary feature phone. The first smartphones were devices that mainly combined the functions of a personal digital assistant and a mobile phone or camera...

s will simply become the dominant interfaces. iPhone
IPhone
The iPhone is a line of Internet and multimedia-enabled smartphones marketed by Apple Inc. The first iPhone was unveiled by Steve Jobs, then CEO of Apple, on January 9, 2007, and released on June 29, 2007...

, Android and the QNX
QNX
QNX is a commercial Unix-like real-time operating system, aimed primarily at the embedded systems market. The product was originally developed by Canadian company, QNX Software Systems, which was later acquired by Canadian BlackBerry-producer Research In Motion.-Description:As a microkernel-based...

 OS used in 2012-and-later BlackBerry
BlackBerry
BlackBerry is a line of mobile email and smartphone devices developed and designed by Canadian company Research In Motion since 1999.BlackBerry devices are smartphones, designed to function as personal digital assistants, portable media players, internet browsers, gaming devices, and much more...

 phones are generally capable of VoIP performance even on small battery-charged devices. They also typically support the USB but not Ethernet or Power over Ethernet interfaces, at least as of late 2011.
According to this view, the smartphone becomes the dominant VoIP phone because it works both indoors and outdoors and shifts base stations/protocols easily to trade off access costs and call clarity and other factors personal to the user, and the PoE/USB VoIP phone is thus the transitional device.

Elements of a VoIP phone

  • Physical Hardware
  • DNS
    Domain name system
    The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...

     client
  • STUN client
  • DHCP
    Dynamic Host Configuration Protocol
    The Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...

     client (not commonly used)
  • Signalling stack (SIP
    Session Initiation Protocol
    The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

    , H.323
    H.323
    H.323 is a recommendation from the ITU Telecommunication Standardization Sector that defines the protocols to provide audio-visual communication sessions on any packet network...

    , Skinny, Skype
    Skype
    Skype is a software application that allows users to make voice and video calls and chat over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system...

    , or others)
  • RTP
    Real-time Transport Protocol
    The Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...

     stack
  • Codecs
    • Audio
      • G.711
        G.711
        G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...

      • GSM
      • iLBC
        ILBC
        Internet Low Bitrate Codec is an open source royalty-free narrowband speech codec, developed by Global IP Solutions formerly Global IP Sound . It was formerly licensed as a freeware with limited commercial use, but since 2011 it is available under an open source license as a part of the open...

      • Speex
        Speex
        Speex is a patent-free audio compression format designed for speech and also a free software speech codec that may be used on VoIP applications and podcasts. It is based on the CELP speech coding algorithm. Speex claims to be free of any patent restrictions and is licensed under the revised BSD...

      • G.729
        G.729
        G.729 is an audio data compression algorithm for voice that compresses digital voice in packets of 10 milliseconds duration. It is officially described as Coding of speech at 8 kbit/s using conjugate-structure algebraic code-excited linear prediction .Because of its low bandwidth requirements,...

      • G.722
        G.722
        G.722 is a ITU-T standard 7 kHz wideband speech codec operating at 48, 56 and 64 kbit/s. It was approved by ITU-T in November 1988. Technology of the codec is based on sub-band ADPCM ....

      • G.722.2 (AMR-WB)
      • other audio codecs
    • Video
      • H.263
        H.263
        H.263 is a video compression standard originally designed as a low-bitrate compressed format for videoconferencing. It was developed by the ITU-T Video Coding Experts Group in a project ending in 1995/1996 as one member of the H.26x family of video coding standards in the domain of the ITU-T.H.263...

      • H.263+
      • H.264
  • User interface

Hardware of a standalone VoIP phone



The overall hardware may look like a telephone or mobile phone. A VoIP phone has the following hardware components.
  • Speaker/ear phone and microphone
    Microphone
    A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

  • Key pad/touch pad to enter phone number and text (not used for ATAs).
  • Display
    Display device
    A display device is an output device for presentation of information in visual or tactile form...

     hardware to feedback user input and show caller-id/messages (not used for ATAs).
  • General purpose processor (GPP) to process application messages.
  • A voice engine
    Voice engine
    A voice engine is a software subsystem for bidirectional audio communication, typically used as part of a telecommunications system to simulate a telephone. It functions like a data pump for audio data, specifically voice data...

     or a digital signal processor
    Digital signal processor
    A digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...

     (DSP) to process RTP
    Real-time Transport Protocol
    The Real-time Transport Protocol defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and...

     messages. Some IC
    Integrated circuit
    An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

     manufacturers provides GPP and DSP in single chip
    Integrated circuit
    An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

    .
  • ADC
    Analog-to-digital converter
    An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...

     and DAC
    Digital-to-analog converter
    In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...

     converters: To convert voice to digital data and vice versa.
  • Ethernet
    Ethernet
    Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

     or wireless network
    Wireless network
    Wireless network refers to any type of computer network that is not connected by cables of any kind. It is a method by which homes, telecommunications networks and enterprise installations avoid the costly process of introducing cables into a building, or as a connection between various equipment...

     hardware to send and receive messages on data network.
  • Most IP phones have a PC data jack on the phone. This allows your computer and phone to share one data connection.
  • Power source might be a battery or DC source. Some VoIP phones receive electricity from Power over Ethernet
    Power over Ethernet
    Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels...

    .


For wireless VoIP phones
  • battery
    Battery (electricity)
    An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

  • Wireless network interface controller

Other devices


There are several Wi-Fi
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...

 enabled mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

s and PDA
Personal digital assistant
A personal digital assistant , also known as a palmtop computer, or personal data assistant, is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet...

s that come pre-loaded with SIP clients, or are capable of running IP telephony clients. Some VoIP phones also support PSTN phone lines directly.

Gateway devices


Analog telephony adapter
Analog telephony adapter
An analog telephony adapter, or analog telephone adapter, is a device used to connect one or more standard analog telephones to a digital telephone system or a non-standard telephone system....

s are connected to the internet or Local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

 using an Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 port and have sockets to connect one or more PSTN phones. Such devices are sent out to customers who sign up with various commercial VoIP providers allowing them to continue using their existing PSTN based telephones.

Another type of gateway device acts as a simple GSM base station and regular mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

s can connect to this and make VoIP calls. While a license is required to run one of these in most countries these can be useful on ship
Ship
Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s or remote areas where a low-powered gateway transmitting on unused frequencies is likely to go unnoticed.

Stun client


A STUN (Session Traversal Utilities for NAT
Network address translation
In computer networking, network address translation is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device....

) client is used on some SIP
Session Initiation Protocol
The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

-based VoIP phones as firewalls on network interface sometimes block SIP/RTP packets. Some special mechanism is required in this case to enable routing of SIP packets from one network to other. STUN is used in some of the sip phones to enable the SIP/RTP packets to cross boundaries of two different IP networks. A packet becomes unroutable between two sip elements if one of the networks uses private IP address range and other is in public IP address range. Stun is a mechanism to enable this border traversal. There are alternate mechanisms for traversal of NAT, STUN is just one of them. STUN or any other NAT traversal mechanism is not required when the two SIP phones connecting are routable from each other and no firewall exists in between.

DHCP client


A DHCP client may be used to configure the TCP/IP parameters and server details if a network segment uses dynamic IP address configuration. The DHCP client then provides central and automatic management of VoIP phones configuration.

Common features of VoIP phones

  • Caller ID
    Caller ID
    Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...

  • Dialing using name/ID (differs from speed dial
    Speed dial
    Speed dial is a function available on many telephone systems allowing the user to place a call by pressing a reduced number of keys. This function is particularly useful for phone users who dial certain numbers on a regular basis....

     in that no number is stored on the client)
  • Locally stored and network-based directories
  • Conference and multiparty call
  • Call park
  • Call transfer and call hold
  • Preserving user name/ number when choosing a different service provider (not widely supported).
  • Applications like weather report, Attendance in school and offices, Live news etc.

Disadvantages of VoIP phones

  • Requires Internet access to make calls outside the local area network
    Local area network
    A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

     (LAN) unless a compatible local PBX is available to handle calls to and from outside lines.
  • VoIP phones and the routers depend on mains electricity
    Mains electricity
    Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...

     for power, unlike PSTN phones, which are supplied with power from the telephone exchange
    Telephone exchange
    In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

    . However, this can be mitigated by installing a UPS
    Uninterruptible power supply
    An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails...

    . The Power over Ethernet
    Power over Ethernet
    Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels...

     interface simplifies this immensely since power can be "injected" at any connector (especially in passive mode where all devices are drawing the same voltage) or at the router. This is a major reason the dominant call center and PBX VoIP systems rely on PoE exclusively)
  • IP networks, particularly residential Internet connections are easily congested. This can cause poorer voice quality or the call to be dropped completely. As commercial grade routers begin to incorporate "managed" or "carrier
    Carrier Ethernet
    Carrier Ethernet is a marketing term for extensions to Ethernet to enable telecommunications network providers to provide Ethernet services to customers and to utilize Ethernet technology in their networks....

    " QoS features, this is less of an issue, but such features typically require expert configuration.
  • VoIP phones, like other network devices can be subjected to denial-of-service attack
    Denial-of-service attack
    A denial-of-service attack or distributed denial-of-service attack is an attempt to make a computer resource unavailable to its intended users...

    s as well as other attacks especially if the device is given a public IP address
    IP address
    An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

    ; This is especially significant as an issue with wireless devices using 802.11 protocols
  • Due to the latency induced by protocol overhead
    Protocol overhead
    Protocol overhead refers to metadata and network routing information sent by an application, which uses a portion of the available bandwidth of a communications protocol...

     and other factors they do not work as well on satellite Internet, analog cell ("edge" networks) and other high-latency Internet connections. Extremely latency sensitive applications (music, remote device control) as of 2012 simply cannot exploit VoIP protocols.
  • Proprietary vendors such as Skype
    Skype
    Skype is a software application that allows users to make voice and video calls and chat over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system...

     and Google Voice
    Google voice
    Search by voice is a branded name for a technology to "search by voice on your [digital device]", such as a mobile phone or PC, i.e. have the device search for data upon entering information on what to search into the device by speaking....

     focus on improving call quality between their own users to grow their user base, which to some degree competes and conflicts with the goal of better connections from Skype to Google Voice, or from either to the existing PSTN and cellular networks. The best codec
    Codec
    A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...

    s tend to be proprietary and not licensed to competitors, retarding the growth of the industry and causing incompatibility.

See also



  • List of SIP software
  • IP Multimedia Subsystem
    IP Multimedia Subsystem
    The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem is an architectural framework for delivering Internet Protocol multimedia services. It was originally designed by the wireless standards body 3rd Generation Partnership Project , as a part of the vision for evolving mobile...

     (IMS)
  • Voice over IP
    Voice over IP
    Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...

  • Media phone
    Media phone
    The media phone represents a new category of broadband multimedia device that has the potential to become the 4th screen in the home, complementing the PC, TV, and mobile phone handset.- Features :...

  • Mobile VoIP
    Mobile VoIP
    Mobile VoIP or simply mVoIP is an extension of mobility to a Voice over IP network. Two types of communication are generally supported: cordless/DECT/PCS protocols for short range or campus communications where all base stations are linked into the same LAN, and wider area communications using...

  • Private branch exchange (PBX)
  • ZRTP
    ZRTP
    ZRTP is a cryptographic key-agreement protocol to negotiate the keys for encryption between two end points in a Voice over Internet Protocol phone telephony call based on the Real-time Transport Protocol. It uses Diffie-Hellman key exchange and the Secure Real-time Transport Protocol for...

  • Session Initiation Protocol
    Session Initiation Protocol
    The Session Initiation Protocol is an IETF-defined signaling protocol widely used for controlling communication sessions such as voice and video calls over Internet Protocol . The protocol can be used for creating, modifying and terminating two-party or multiparty sessions...

     (SIP)
  • Softphone
    Softphone
    A softphone is a software program for making telephone calls over the Internet using a general purpose computer, rather than using dedicated hardware. Often a softphone is designed to behave like a traditional telephone, sometimes appearing as an image of a phone, with a display panel and buttons...

  • STUN