Signalling System 7, often simply referred to as SS7 or even C7 can be used as a building block so that many new services can be introduced. In fact it is the signalling protocol of choice for VoIP (Voice over IP) and NGN (Next Generation Networks. The C7 network is structured around 3 key components or functions:
Terminating or originating points for C7 traffic.
Transfer points to enable routing of signalling messages
Control points that manage the traffic and various network databases.
C7 networks will incorporate all of these functions to ensure efficient transport and routing of signalling and data. Nodes in the network are interconnected via data links, and are usually deployed in pairs to provide a redundant structure to cope with link failures. The data links themselves are probably the most important aspect of a C7 network, as failure or loss of any of these links could result in call generation and clearance being compromised. Common nodes in the network are:
SSP – Service Switching Point
An SSP is, in effect the telephone exchange that responds when a telephone customer dials a number. The exchange will communicate with an SCP (Service Control Point) to determine how the call should be handled through the use of C7 protocols that initiate call setup, management and termination / clearance of calls between other SSPs.
SCP – Service Control Point
An SCP allow the use of services which form part of what has become known as an IN (Intelligent Network). These nodes are usually multi-protocol devices, running many different signalling systems to allow connectivity between VoIP networks, mobile networks and land-based networks. An example of an intelligent service is the use of Non-Geographic numbers such as 0800 numbers.
SDP – Service Data Point
Service Data Points, as used in mobile networks will hold information relating to subscribers, which can be accessed by other nodes.
STP – Signalling Transfer Point
An STP is the routing engine of the network that uses the addressing information in C7 messages to determine the route messages should take. They can be likened to routers in a standard data network. Instead of routing IP packets, they route Signalling System 7 messages between other nodes.
In a small network signalling can be performed directly between Exchanges or Service Switching Points, but in large telecommunications networks STPs are needed to ensure efficient routing. It also makes sense to build multiple routes between exchanges, and of course between operators in different countries.
There are many different variants of the C7 protocol which have evolved to accommodate associated signalling standards and variations of those standards. The majority have the same basic message format and incorporate what is known as the MTP (Message Transfer Part). In the upper layers of the protocol structure there are a number of user parts such as ISUP (ISDN User Part), TUP (Telephone User Part), MAP (Mobile Application Part and INAP (Intelligent Network Application Part) to name a few. The SCCP (Signal Connection Control Part) and TCAP (Transaction Application Part) operate together to enable setting up and management of connections between peer applications across the network. C7 is an extremely flexible protocol that was designed to accommodate new applications as and when they are developed.
This article on Signalling System 7 was written by David Christie, MD at NSTUK Ltd, Website http://www.nstuk.com .