SS7 signaling system 7

SS7, also known as Signaling System 7, is a set of telephony signaling protocols used for the setup, control, and tear-down of telephone calls in Public Switched Telephone Networks (PSTNs) worldwide. It is a common protocol suite used in traditional circuit-switched networks, allowing for the exchange of signaling messages between different network elements.

SS7 was developed in the 1970s by the International Telecommunication Union (ITU) and became the standard for signaling in digital telecommunication networks. It provides a reliable and efficient means of managing call setup and routing, as well as offering various supplementary services such as call forwarding, caller ID, and call waiting.

The primary purpose of SS7 is to carry signaling information separate from the actual voice or data traffic. Signaling messages are exchanged between signaling points in the network, known as Signaling Transfer Points (STPs). These messages contain information about call setup, routing, billing, and other network-related functions.

SS7 operates on a separate network known as the Common Channel Signaling (CCS) network, which is physically distinct from the voice or data network. The CCS network uses out-of-band signaling, meaning that the signaling messages are transmitted on separate channels dedicated solely to signaling, ensuring the reliability and efficiency of the signaling process.

The SS7 protocol stack is divided into three levels:

1. Message Transfer Part (MTP): This is the lowest level of the SS7 protocol stack and provides the foundation for message transfer between signaling points. MTP consists of three sublayers: MTP Level 1 handles the physical transmission of signaling messages, MTP Level 2 manages error detection and correction, and MTP Level 3 is responsible for message routing.
2. Signaling Connection Control Part (SCCP): SCCP is a layer above MTP and provides additional services for routing and addressing within the SS7 network. It introduces Global Title Translation (GTT), which allows for the translation of phone numbers into network addresses and vice versa.
3. Telephone User Part (TUP): TUP is the highest level of the SS7 protocol stack and is responsible for call setup, teardown, and call control functions. It enables features such as call routing, call diversion, and call forwarding.

The SS7 protocol suite supports various types of signaling messages, including:

• ISUP (ISDN User Part): Used for call setup, teardown, and control in the Public Switched Telephone Network (PSTN).
• SCCP (Signaling Connection Control Part): Used for network addressing, routing, and management of signaling messages.
• TCAP (Transaction Capabilities Application Part): Enables the exchange of non-circuit-related information between network nodes, allowing for the implementation of supplementary services.
• MAP (Mobile Application Part): Used in mobile networks for signaling between different network elements, such as Home Location Registers (HLRs) and Mobile Switching Centers (MSCs).
• INAP (Intelligent Network Application Part): Facilitates the implementation of intelligent network services, such as prepaid calling cards and toll-free numbers.

SS7 has played a crucial role in the development and operation of telecommunication networks for several decades. However, with the advent of modern IP-based networks, SS7 has faced security vulnerabilities and has been subject to exploitation by malicious actors. This has led to efforts to improve the security of SS7 networks and the development of alternative signaling protocols, such as Diameter, in newer network technologies like 4G and 5G.