Softphone

A softphone is a software program for making telephone calls over the Internet using a general purpose computer, rather than using dedicated hardware. The softphone can also be installed on a piece of equipment such as a workstation, portable computer, tablet or even a cellphone and allows the user to place and receive calls without requiring an actual telephone set.^[1] 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 with which the user can interact. A softphone is usually used with a headset connected to the sound card of the PC, or with a USB phone.^[2]

Applications

See Comparison of VoIP software

Communication protocols

To communicate, both end-points must support the same Voice-over-IP protocol, and at least one common audio codec.

Many service providers use the Session Initiation Protocol (SIP) standardized by the Internet Engineering Task Force (IETF). Skype, a popular service, uses proprietary protocols, and Google Talk leverages the Extensible Messaging and Presence Protocol (XMPP).

Some softphones also support the Inter-Asterisk eXchange protocol (IAX), a protocol supported by the open-source software application Asterisk.

Features

A typical softphone has all standard telephony features (DND, Mute, DTMF, Flash, Hold, Transfer etc.) and often additional features typical for online messaging, such as user presence indication, video, wide-band audio. Softphones provide a variety of audio codecs, a typical minimum set is G.711 and G.729.

Requirements

To make voice calls via the Internet, a user typically requires the following:

A modern PC with a microphone and speaker, or with a headset, or with USBphone.
Reliable high-speed Internet connectivity like Digital subscriber line (DSL), or cable service.
Account with an Internet telephony service provider or IP PBX provider.
Mobile or landline phone.

References

Telecommunications

History	Beacon Broadcasting Communications satellite Computer network Drums Electrical telegraph Fax Heliographs Hydraulic telegraph Internet Mass media Mobile phone Optical telecommunication Optical telegraphy Photophone Prepaid mobile phone Radio Radiotelephone Satellite communications Smoke signals Telecommunications history Telegraphy Telephone The Telephone Cases Television Timeline of communication technology Undersea telegraph line Videoconferencing Videophone Videotelephony

Pioneers	Edwin Howard Armstrong John Logie Baird Alexander Graham Bell Tim Berners-Lee Jagadish Chandra Bose Vint Cerf Claude Chappe Lee de Forest Philo Farnsworth Reginald Fessenden Elisha Gray Innocenzo Manzetti Guglielmo Marconi Antonio Meucci Alexander Stepanovich Popov Johann Philipp Reis Nikola Tesla Camille Papin Tissot Alfred Vail Charles Wheatstone Vladimir K. Zworykin

Transmission media	Electromagnetic radiation Electrical cable Free-space optical communication Molecular communication Optical fiber

Network topology and switching	Links Nodes Terminal node Network switching (circuit packet) Telephone exchange

Multiplexing	Space-division Frequency-division Time-division Polarization-division Orbital angular-momentum Code-division

Networks	ARPANET BITNET Cellular network Computer Ethernet FidoNet Internet ISDN LAN Mobile NGN Public Switched Telephone Radio Telecommunications equipment Television Telex WAN Wireless World Wide Web

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