SS7 and SIGTRAN are protocols that were designed to carry over the signaling information of voice calls. For a call to be made, signaling information has to be carried over the network, along with the audio, to provide directions for the call to reach its destination, whether it is a carrier or a receiving endpoint. So, the SS7 and SIGTRAN protocols were created to allow this information to be carried over without competing for space or available voice channels. In other words, an “additional path” was created to carry over the call signaling information.
The SS7 signaling system
When you use the SS7 protocol, instead of having all the digital information related to the call competing for space with the information required for making connections, there is a specific channel for sending the signaling information, which is common in several calls.
The Common Channel Signaling System or Signaling System Number 7 (SS7) is a set of protocols designed to enable telephony networks to exchange information, so that calls can be made between telephones and be charged correctly in a transparent manner. In other words, SS7 is directly responsible for interconnecting telephones and telephone exchanges with each other.
We can say that SS7 is a packet-switched network, such as those used by the X.25 packet-switched network, for example. The X.25 network uses ISDN as the transmission medium for long-distance networks. It is integrated into the telecommunications network, adding new features and services to it, such as increased bandwidth for call signaling, feasibility for 0800 calls, ISDN connectivity, advanced services for smart networks and many others.
Because SS7 is basically a network that is integrated into the existing telecommunications network, some began to wonder if it would be possible to use an Internet Protocol (IP) network instead, replacing the traditional 64-kbit signaling links. As a result of this, SIGTRAN was created.
SIGTRAN for signaling over IP
SIGTRAN (Signaling Transport Working Group) is a group that pertains to the IETF (Internet Engineering Task Force) and is dedicated to studying signaling information transport over IP networks. It has developed a family of protocols that provide a more reliable service for SS7, being actually an extension of the SS7 family of protocols.
SIGTRAN handles the same applications and call management functions performed by the SS7. However, SIGTRAN uses the IP (Internet Protocol) and SCTP (Stream Control Transmission Protocol) transport, instead of TCP or UDP, mainly due to security issues.
SCTP is responsible for carrying the PSTN signaling information over the IP and for the reliable transfer of messages between its users.
The way SIGTRAN works is that it essentially refers to an entity called media gateway.
The media gateway works as an interface between different media formats across public and data networks, and it also connects the public analog network to a network of data packets.
Learn more about media gateways in this other Khomp Blog post.