SIGTRAN (for Signaling Transport) is the standard telephony protocol used to transport Signaling System 7 (SS7) signals over the Internet. SS7 signals consist of special commands for handling a telephone call.
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Internet telephony uses the Internet Protocol's packet-switched connections to exchange voice, fax, and other forms of information that have traditionally been carried over the dedicated circuit-switched connections of the public switched telephone network (PSTN). Calls transmitted over the Internet travel as packets of data on shared lines, avoiding the tolls of PSTN.
A telephone company switch transmits SS7 signals to a signaling gateway. The gateway, in turn, converts the signals into SIGTRAN packets for transmission over IP to either the next signaling gateway or, if the packet destination is not another PSTN, to a softswitch.
The SIGTRAN protocol is actually made up of several components (this is what is sometimes referred to as a protocol stack): standard IP; a common signaling transport protocol (used to ensure that the data required for signaling is delivered properly), such as the Stream Control Transport Protocol (SCTP); and an adaptation protocol that supports "primitives" (a basic interface or segment of code that can be used to build more sophisticated program elements or interfaces) that are required by another protocol.