Signalling System Number 7

Description

Signalling System Number 7 (SS7) is a suite of telephony signaling protocols developed by ITU-T, adopted and profiled by 3GPP for use in mobile core networks, particularly the Circuit-Switched (CS) domain. It operates on an out-of-band signaling network separate from the voice traffic bearer channels, using dedicated signaling links. The architecture is based on a network of Signaling Transfer Points (STPs), Service Control Points (SCPs), and Service Switching Points (SSPs), which are implemented in network elements like Mobile Switching Centers (MSCs) and Home Location Registers (HLRs). Key protocols within the SS7 stack include the Message Transfer Part (MTP) for reliable routing and delivery, the Signaling Connection Control Part (SCCP) for enhanced addressing, and the Transaction Capabilities Application Part (TCAP) for supporting database queries and advanced services.

In a 3GPP context, SS7 is crucial for the CS domain of 2G (GSM) and 3G (UMTS) networks. It facilitates core mobility management and call control procedures. For instance, when a mobile user initiates a call, the MSC uses SS7 signaling to communicate with the HLR to authenticate the subscriber and retrieve routing information. It also communicates with the Visitor Location Register (VLR) and other MSCs to establish the call path. The ISDN User Part (ISUP) protocol within SS7 is responsible for setting up, managing, and releasing voice circuits between switches.

The role of SS7 extends beyond basic call control to enabling a wide array of intelligent network (IN) services. These include custom calling features like call forwarding, call waiting, and caller ID, as well as premium services like toll-free numbers (e.g., 800 numbers) and prepaid billing. Security in the traditional SS7 network was based on the closed, trusted nature of operator networks, with physical security of signaling links being a primary control. However, as networks evolved and interconnected globally, this trust model revealed vulnerabilities, leading to the development of security gateways in later releases.

Purpose & Motivation

SS7 was created to replace earlier in-band signaling systems (like SS5), which used tones within the voice channel for control. This was inefficient and prone to fraud (e.g., 'phreaking'). The purpose of SS7 was to provide a robust, high-performance, out-of-band signaling network that could handle the increasing complexity and volume of telephone traffic, support new services, and enable the automation of call setup and teardown. Its separation of control and bearer planes allowed for faster call setup times, more efficient use of trunk lines, and the foundation for intelligent network services.

In the context of 3GPP, starting with Release 99, SS7 was adopted as the core signaling protocol for the circuit-switched domain of GSM and UMTS networks. It solved the problem of enabling seamless mobility and roaming across large, geographically dispersed networks by providing a standardized method for MSCs, HLRs, and VLRs to exchange subscriber and call routing information. It was the glue that held together the global mobile telephony system before the full transition to IP-based signaling. The limitations it addressed were the manual switchboards and limited feature sets of electromechanical and early electronic switching systems, enabling the automated, feature-rich mobile networks of the 2G and 3G eras.

Key Features

• Out-of-band signaling separating control and bearer paths
• High-reliability message transfer via the Message Transfer Part (MTP)
• Global addressing and routing for network elements via Signaling Point Codes
• Support for transaction-oriented services via the Transaction Capabilities Application Part (TCAP)
• Call control and circuit management via the ISDN User Part (ISUP)
• Foundation for Intelligent Network (IN) and custom calling services

Evolution Across Releases

Defining Specifications