SS-7 Signaling System Number 7

SS7 (Signaling System Number 7) is a set of protocols used in telecommunication networks to control the setup, routing, and tear-down of voice and data calls. It is a signaling network used by traditional public switched telephone networks (PSTN), as well as mobile networks, to facilitate the exchange of signaling messages between various network elements.

Overview of SS7:

SS7 is a hierarchical and packet-based signaling protocol that operates on a separate network dedicated to signaling. It runs on a packet-switched network using digital signaling links, such as T1 or E1 lines. The protocol stack of SS7 consists of various levels, each responsible for a specific function in the signaling process.

Functionality:

SS7 provides a range of services essential for telecommunication networks, including call setup, routing, and teardown. It enables the exchange of information about the availability and status of network elements, such as switches, mobile devices, and service platforms. SS7 supports features like call forwarding, call waiting, short messaging service (SMS), and caller ID.

Components of SS7:

The SS7 network consists of multiple components that work together to facilitate signaling between network elements. Some key components include:

a. Service Switching Point (SSP): An SSP is a network element responsible for originating or terminating calls. It generates and sends signaling messages to other components in the SS7 network for call setup and teardown.

b. Service Control Point (SCP): SCP acts as a centralized database that stores and provides service-related information. It is responsible for call routing decisions based on the information received from the SSP.

c. Signal Transfer Point (STP): STPs act as the backbone of the SS7 network, routing signaling messages between different SSPs and SCPs. They ensure efficient and reliable delivery of signaling messages across the network.

d. Home Location Register (HLR) and Visitor Location Register (VLR): HLR and VLR are components specific to mobile networks. HLR stores subscriber-related information, such as location and services subscribed, while VLR keeps track of the current location of mobile subscribers.

Signaling Messages:

SS7 signaling messages carry information necessary for call setup, routing, and teardown. Some commonly used signaling messages include:

a. ISUP (ISDN User Part): ISUP messages are used for call setup and teardown in circuit-switched networks. They carry information about the called and calling party numbers, call status, and call control commands.

b. SCCP (Signaling Connection Control Part): SCCP provides additional services on top of the basic message routing provided by MTP (Message Transfer Part). It includes functions like global title translation, connectionless and connection-oriented signaling, and routing to specific subsystems.

c. TCAP (Transaction Capabilities Application Part): TCAP enables the exchange of non-circuit-related information between network elements. It allows for the implementation of advanced services such as intelligent network (IN) features and service-related queries.

Security and Concerns:

SS7 has faced security concerns in recent years due to vulnerabilities that can be exploited for unauthorized interception, tracking, and fraud. Some of these vulnerabilities include the ability to redirect calls, intercept SMS messages, and track the location of mobile devices. Efforts are underway to address these security issues through enhancements and the adoption of more secure signaling protocols like Diameter and SIGTRAN.

Evolution:

As telecommunication networks evolve, SS7 is gradually being replaced by newer signaling protocols. IP-based networks, such as those used in Voice over IP (VoIP) and 4G/5G mobile networks, use protocols like SIP (Session Initiation Protocol) and Diameter for signaling purposes. However, SS7 remains an integral part of legacy networks and continues to play a crucial role in interconnecting different telecommunication networks.

In conclusion, SS7 is a signaling protocol used in telecommunication networks to facilitate call setup, routing, and teardown. It is a hierarchical and packet-based protocol that operates on a separate network dedicated to signaling. Despite the security concerns, SS7 remains an essential component of traditional telecommunication networks, while newer signaling protocols are adopted in IP-based networks.