Signal Transfer Point (STP)

SS7 is a critical component of modern telecommunications systems. SS7 is a communications protocol that provides signaling and control for various network services and capabilities. While the Internet, wireless data, and related technology have captured the attention of millions, many forget or don’t realize the importance of SS7. Not every call in every network is dependent on SS7 but big portion of them are (interworking). Likewise, every mobile phone user is dependent on SS7 to allow inter-network roaming. SS7 is also the “glue” that sticks together circuit switched (traditional) networks with Internet protocol based networks.
SS7 Technology

SS7 signaling is a form of packet switching. Unlike circuit switching, which utilizes dedicated data “pipes” for transmission of information, packet switching dynamically assigns “routes” based on availability and “least cost” algorithms. Another example of packet switching is TCP/IP, the protocol used for routing messages over the Internet. Unlike the Internet, which utilizes a vast public “web” of interconnecting facilities and routing equipment, SS7 networks are private and logically self-contained. The private nature of SS7 networks is critical for security and reliability.
SS7 involves two different types of signaling: connection oriented signaling and connectionless oriented signaling. Connection oriented signaling refers to the establishment of switch-to-switch facilities call inter-office trunks. These trunks carry voice communications. The ISDN User Part (ISUP) part of the SS7 protocol is utilized to establish trunks between switches. In contrast, the Transaction Capability Application Part (TCAP) is utilized for connectionless signaling which typically entails switch-to-database or database-to-database communications. An example of connectionless signaling is HLR to VLR TCAP signalling.


SS7 is comprised of a series of interconnected network elements such as switches, databases, and routing nodes. Each of these elements is interconnected with links, each of which has a specific purpose. The routing nodes are the heart of the SS7 network and are called Signal Transfer Points (STP). STPs are connected to Service Switching Points (SSP) that are switches equipped with SS7 control logic. SSP switches are connected to the STPs via Access links (A links). STPs also connect to databases called Service Control Points (SCP) via A links.

Typical SS7 Architecture?


OpenSTP is a Release14 effort to create a flexible low cost STP and Signaling Gateway. Most modern network cores have already made their transition to IP networking, therefore our current feature set revolves around M3UA SIGTRAN. M2UA support has already been placed in our roadmap, and will be available in the next major release. SCTP is a mandatory component for reliable and fast IP networking.

We accomplish full redundancy by deploying logical SGN nodes on multiple physical nodes. The underlying platform consists of multiple Signaling (SGN) and Filtering (FGN) nodes. Most of the actual decision making and routing of traffic is done by Filtering nodes, keeping Signaling nodes offloaded as much as possible to accommodate for maximum throughput.

Our platform’s routing engine is very flexible, giving our users the possibility to base their routing rules on any of the following layers and their supported attributes: M3UA, SCCP, GSM MAP and SMS TPDU.

For more detailed description of the underlying platform, please check our openSMSF product description.