The Signaling System 7 (SS7) network was designed as a management tool to automatically organize call routing by assessing at a fraction of the time it would take to achieve manually the most appropriate direction in which a call should be sent with the support of all telephony services including 800 numbers, call forwarding, caller ID and local number portability. The SS7 Protocol was adopted as a standard by ITU-T in 1981 in its ITU-T Q7XX-series.
SS7 provides an out-of-band signaling system that improves upon the in-band system used by SS5 and SS6. In addition SS7 provides improved security enhancements that negate the problems previously experienced with earlier systems. ITU standardized an international version of SS7 while in the United States ANSI controls the standard for SS7. Out-of-band signaling possesses advantages over traditional in-band signaling which relates to a call that begins and ends over the same path. Out-of-band is less restrictive in relation to the amount of data able to be sent at higher speeds. Sin addition signaling does not need to be sent at the beginning of a call but may occur at anytime. Also out-of-band signaling allows signaling to network elements with no direct trunk.
Thanks to SS7 network maintenance, messaging, interfacing and the provision of a universal telephony network structure are made available. Setting up a call, swapping user information, call direction, billing structures and the support of Intelligent Network (IN) services are fundamental to the functions expected of SS7. [N technology brought the concept of splitting the signaling planes to the environment with significant uses of IN technology directed to simple number translation services as well as complex CLASS and prepaid telephone calls. The SS7 network comprises link types together with three signaling echoes — Service Switching Point (SSPs), Signal Transfer Point (STPs) and Service Control Point with each code uniquely identified and links between nodes being full-duplex 56kbit/s and or 64kbit/s.
SS7 separates the signaling planes from the voice circuits provide the Voice Over Internet Protocol (VoIP) and Public Switched Telephone Network (PSTN) network connection while also playing a role in the cellular networks such as GSM and UMTS for circuit switched (voice) and Packet Switched (data) applications. By identifying end-to-end addresses SS7 is able to control all the routing judgments necessary for a seamless communications. Additionally SS7 also supports all the telephony services including 800 numbers, call forwarding, caller ID and local number portability. The SS7 Protocol consists of four Message Transfer Parts (MTP) categorized as MTP 1, MTP2 and MTP3 with 4 comprising several different user parts such as TUP, ISUP, TCAP with INAP, MAP and SCCP. MTP controls the transfer protocols associated with the network interface, information transfer, message handling and the routing to higher levels. MTP3 handles the management services and end-to-end contact and routing connection less messages whilst ISUP completes the circuit based protocol to connect, maintain and disconnect the call. MAP is performed by TCAP which resolves database queries and handles advanced network functionalities or links to Intelligent Networks.