Signaling used in Public Switched Telephone Networks (PSTN) calls relate to the telephony signals which circulate the globe irrespective of international boundaries. In order that telephone services providers are able to communicate at an international level it has been necessary for their engineers to interact with those in other countries. Appreciating what an engineer operating in a foreign country is talking about may not be immediately obvious and because advances in technology were looming closer on the horizon in 1975 AT&T began to define what have developed into standard protocols that could be recognized at an international level. Even so the references adopted are not always immediately recognizable to anyone who operates outside the engineering environment for example in the United States SS7 is referred to as CCS7 and in the UK C7 (CCITT number 7).

Giving a better understanding of systems operating in foreign countries and the specific identities of respective signaling equipment between foreign telephone engineers was seen as a necessary stage in order to create a more efficient and thereby consumer friendly global telephony environment. In 1981 ITU adopted the SS7 Protocols as a standard in its ITU-T Q7XX-series recommendations. SS7 utilizes’ an out-of-band signaling system that is an improvement over the in-band system used by SS5 and SS6 while at the same time enhances security levels. What this means in practice is that when a telephone call is set up between subscribers, it is highly probable that several telephone exchanges will be drawn in, even across international boundaries. In order that a call is correctly set up, the switches signal call related information such as calling or called party number sent to the next switch in the network will use ISUP messages.

ISUP or ISDN is a part of the complex signaling system SS7 uses to set up calls through the PSTN and is also able to provide information relating to timeslots. ISUP will ensure that should no outbound CIC be available a blocking message be routed back to the previous switch in order that a new direction be attempted. Also the circuit based protocol to establish, maintain and end connection for calls is resolved by 1SUP. In North America ITU-T set the criteria to be followed by ISUP while in Europe that task was carried out by ETSI. Both ISUP specifications provide the blueprints for national ISUP variants.

Today the SS7 network, which separates the signaling planes from the voice circuits, provides the link between VOIP traffic and the PSTN network and also plays a role in cellular networks such as GSM and UMTS for circuit switched (voice) and Packet switched (data) applications. The SS7 network manages calls by identifying end-to-end addressing and from there controls all the routing judgments necessary for a seamless operation. In addition SS7 supports all the telephony services such as 800 numbers, call forwarding, caller ID and local number portability. The SS7 Protocol consists of four Message transfer divisions with ISUP handling the circuit based protocol in order to establish, maintain and end connection for calls.