No. 7 signalling system (SS7)

No. 7 signalling system (SS7)
Name	No. 7 signalling system (SS7)
Caption	Public switched telephone network signalling architecture
Introduced	1975
Developer	International Telegraph and Telephone Consultative Committee
Type	Signalling system
Used with	Public switched telephone network

No. 7 signalling system (SS7) is a global suite of telephony signalling protocols that underpins call setup, routing, billing, and supplementary services across the public switched telephone network. Developed in the 1970s and standardized by international bodies, it connects exchanges, databases, and service platforms to enable voice, SMS, number translation, and roaming. SS7’s layered architecture and defined network elements have been widely implemented by national operators and equipment manufacturers, influencing interoperability among carriers, mobile operators, and value-added service providers.

History and development

SS7 originated from specifications produced by the International Telegraph and Telephone Consultative Committee and successor bodies during the 1970s to replace earlier in-band signalling used in exchanges such as those by Western Electric and ITT. Early deployments involved national administrations including British Telecom, AT&T, and France Télécom to interconnect Class 4 and Class 5 switches like the Ericsson AXE, Siemens EWSD, and Nortel DMS. The protocol suite evolved through contributions from standards organizations including the International Telecommunication Union, European Telecommunications Standards Institute, and Comité Consultatif International Téléphonique et Télégraphique, aligning with implementations by equipment makers such as Alcatel-Lucent, Huawei, and Cisco Systems. During the 1980s and 1990s, SS7 supported the expansion of mobile networks operated by Vodafone, Telefónica, NTT, and China Mobile, enabling services standardized by GSMA, IEEE, and 3GPP to interoperate with legacy PSTN infrastructures.

Architecture and protocols

SS7 defines network elements such as Service Switching Points, Service Control Points, Signal Transfer Points, and Service Management Points used within signaling networks deployed by operators like BT, Verizon, Deutsche Telekom, and Orange. The stack includes Message Transfer Part levels used for link management and routing, Transaction Capabilities Application Part for database queries, ISDN User Part for circuit control, and Mobile Application Part for roaming between networks operated by T-Mobile, Sprint, and KDDI. Physical and transport layers may use links over T-carrier and E-carrier systems supplied by Ericsson, Nokia, or via IP-based SIGTRAN adaptation using IETF RFCs implemented by Juniper Networks and Huawei. Routing, point codes, and global title translation interwork with numbering plans administered by the International Telecommunication Union and national regulators such as the Federal Communications Commission and Ofcom.

Network functions and services

SS7 enables call setup and teardown for carriers including AT&T, BT, and NTT, supports supplementary services used by subscribers of Orange, Vodafone, and Telefónica, and provides signaling for number portability administered in countries like the United States, United Kingdom, and Japan. Supervisory and control messages orchestrate features such as caller ID, call forwarding, prepaid charging systems used by MTN and Airtel, and SMS routing between mobile operators under GSMA roaming frameworks. Database services implemented on SCPs interact with Home Location Registers operated by mobile network operators including Verizon Wireless, China Telecom, and Rogers Communications to authenticate subscribers and manage roaming, interworking with billing systems from Oracle and Amdocs.

Security and vulnerabilities

Historically designed for closed networks operated by national incumbents, SS7 lacks built-in authentication and encryption, exposing networks operated by Deutsche Telekom, Telefónica, and Sprint to signaling manipulation. Researchers and security teams at academic institutions and vendors such as Symantec, Kaspersky Lab, and FireEye have demonstrated exploitation techniques including location tracking, call interception, SMS interception for two-factor authentication bypass affecting platforms like Google, Microsoft, and Facebook, and fraud schemes targeting banks and payment processors including Visa and MasterCard. Regulatory authorities including the European Commission and Federal Communications Commission have issued advisories; carriers such as Vodafone, Deutsche Telekom, and AT&T have implemented mitigations like filtering, monitoring platforms by NetScout and Subex, and SIGTRAN over IPsec tunnels. Despite mitigations, nation-state intelligence agencies and law enforcement organizations such as NSA and GCHQ historically leveraged SS7 for lawful intercept under legal frameworks exemplified by the Mutual Legal Assistance Treaties and national surveillance statutes.

Implementations and global deployment

SS7 has been deployed by national incumbents and mobile operators worldwide: historically by Bell System successors such as AT&T and Bell Canada, by European operators like France Télécom and Deutsche Telekom, and by Asian carriers including NTT, China Mobile, and SK Telecom. Equipment vendors including Ericsson, Nokia, Huawei, Alcatel-Lucent, and Cisco Systems supply switches, signaling transfer points, and softswitches used in SS7 cores; database vendors such as Oracle and Huawei provide SCP and IN platforms. Migration strategies toward IP Multimedia Subsystem architectures used by Vodafone, Orange, and AT&T employ SIGTRAN and Diameter interworking to preserve services during transitions influenced by 3GPP releases and ETSI recommendations. Regional initiatives by GSMA and ASEAN carriers handle interconnection and roaming agreements, while development banks and international lenders have funded modernization in emerging markets where carriers such as MTN Group, Bharti Airtel, and América Móvil operate.

Regulatory and standardization bodies

Standards and regulatory oversight involve international organizations such as the International Telecommunication Union, European Telecommunications Standards Institute, 3GPP, GSMA, and Internet Engineering Task Force which produce specifications and RFCs impacting SS7 evolution. National regulators including the Federal Communications Commission, Ofcom, Autorité de Régulation des Communications Électroniques, and Japan’s MIC set interconnection and security requirements for operators like AT&T, BT Group, Orange, and NTT. Industry consortia and testing bodies such as ETSI, ITU-T Study Groups, and regional forums coordinate interoperability events attended by vendors including Ericsson, Nokia, Huawei, Cisco Systems, and ZTE to ensure global compatibility and compliance with numbering plans and lawful intercept provisions overseen by courts and legislative frameworks in jurisdictions such as the United States, United Kingdom, and European Union.

Category:Telecommunications protocols