SIGNALLING SYSTEM NO. 7 (SS7) is a set of telephony signaling
protocols developed in 1975, which is used to set up and tear down
most of the world's public switched telephone network (PSTN) telephone
calls. It also performs number translation, local number portability ,
Short Message Service
In North America it is often referred to as CCSS7, abbreviated for
Common Channel Signalling System 7. In the
The only international SS7 protocol is defined by ITU-T 's Q.700-series recommendations in 1988. Of the many national variants of the SS7 protocols, most are based on variants of the international protocol as standardized by ANSI and ETSI . National variants with striking characteristics are the Chinese and Japanese (TTC ) national variants.
Internet Engineering Task Force
* 1 History
* 2 Functionality
* 2.1 Signaling modes
* 3 Physical network * 4 SS7 protocol suite * 5 Protocol security vulnerabilities * 6 See also * 7 References * 8 Further reading
SS5 and earlier systems used in-band signaling , in which the call-setup information was sent by playing special multi-frequency tones into the telephone lines, known as bearer channels. As the bearer channel was directly accessible by users, it was exploited with devices such as the blue box , which played the tones required for call control and routing. As a remedy, SS6 and SS7 implemented out-of-band signaling, carried in a separate signaling channel, :141 thus keeping the speech path separate. SS6 and SS7 are referred to as common-channel signaling (CCS) protocols, or Common Channel Interoffice Signalling (CCIS) systems.
Since 1975, CCS protocols have been developed by major telephone
companies and the International
Internet Engineering Task Force
Signaling in telephony is the exchange of control information associated with the setup and release of a telephone call on a telecommunications circuit. :318 Examples of control information are the digits dialed by the caller and the caller's billing number.
When signaling is performed on the same circuit as the conversation of the call, it is termed channel-associated signaling (CAS). This is the case for earlier analogue trunks, multi-frequency (MF) and R2 digital trunks, and DSS1/DASS PBX trunks.
In contrast, SS7 uses common channel signaling , in which the path and facility used by the signaling is separate and distinct from the telecommunications channels that carry the telephone conversation. With CCS, it becomes possible to exchange signaling without first seizing a voice channel, leading to significant savings and performance increases in both signaling and channel usage.
Because of the mechanisms used by signaling methods prior to SS7 (battery reversal, multi-frequency digit outpulsing , A- and B-bit signaling ), these older methods could not communicate much signaling information. Usually only the dialed digits were signaled, and merely during call setup. For charged calls, dialed digits and charge number digits were outpulsed. SS7, being a high-speed and high-performance packet-based communications protocol, can communicate significant amounts of information when setting up a call, during the call, and at the end of the call. This permits rich call-related services to be developed. Some of the first such services were call management related, call forwarding (busy and no answer) , voice mail , call waiting , conference calling , calling name and number display , call screening , malicious caller identification , busy callback . :Introduction xx
The earliest deployed upper layer protocols in the SS7 suite were dedicated to the setup, maintenance, and release of telephone calls. The Telephone User Part (TUP) was adopted in Europe and the Integrated Services Digital Network (ISDN) User Part (ISUP ) adapted for public switched telephone network (PSTN) calls was adopted in North America. ISUP was later used in Europe when the European networks upgraded to the ISDN. As of 2015 North America has not accomplished full upgrade to the ISDN, and the predominant telephone service is still the older Plain Old Telephone Service . Due to its richness and the need for an out-of-band channel for its operation, SS7 is mostly used for signaling between telephone switches and not for signaling between local exchanges and customer-premises equipment .
Because SS7 signaling does not require seizure of a channel for a conversation prior to the exchange of control information, non-facility associated signalling (NFAS) became possible. NFAS is signaling that is not directly associated with the path that a conversation will traverse and may concern other information located at a centralized database such as service subscription, feature activation, and service logic. This makes possible a set of network-based services that do not rely upon the call being routed to a particular subscription switch at which service logic would be executed, but permits service logic to be distributed throughout the telephone network and executed more expediently at originating switches far in advance of call routing. It also permits the subscriber increased mobility due to the decoupling of service logic from the subscription switch. Another ISUP characteristic SS7 with NFAS enables is the exchange of signaling information during the middle of a call. :318
SS7 also enables Non-Call-Associated Signaling, which is signaling not directly related to establishing a telephone call. :319 This includes the exchange of registration information used between a mobile telephone and a home location register database, which tracks the location of the mobile. Other examples include Intelligent Network and local number portability databases. :433
Apart from signaling with these various degrees of association with call set-up and the facilities used to carry calls, SS7 is designed to operate in two modes: associated mode and quasi-associated mode.
When operating in the associated mode, SS7 signaling progresses from switch to switch through the Public Switched Telephone Network following the same path as the associated facilities that carry the telephone call. This mode is more economical for small networks. The associated mode of signaling is not the predominant choice of modes in North America.
When operating in the quasi-associated mode, SS7 signaling progresses from the originating switch to the terminating switch, following a path through a separate SS7 signaling network composed of signal transfer points . This mode is more economical for large networks with lightly loaded signaling links. The quasi-associated mode of signaling is the predominant choice of modes in North America.
SS7 separates signalling from the voice circuits. An SS7 network must be made up of SS7-capable equipment from end to end in order to provide its full functionality. The network can be made up of several link types (A, B, C, D, E, and F) and three signaling nodes - Service Switching Points (SSPs), Signal Transfer Points (STPs), and Service Control Points (SCPs). Each node is identified on the network by a number, a signalling point code. Extended services are provided by a database interface at the SCP level using the SS7 network.
The links between nodes are full-duplex 56, 64, 1,536, or 1,984 kbit/s graded communications channels. In Europe they are usually one (64 kbit/s) or all (1,984 kbit/s) timeslots (DS0s ) within an E1 facility; in North America one (56 or 64 kbit/s) or all (1,536 kbit/s) timeslots (DS0As or DS0s) within a T1 facility. One or more signaling links can be connected to the same two endpoints that together form a signaling link set. Signaling links are added to link sets to increase the signaling capacity of the link set.
In Europe, SS7 links normally are directly connected between switching exchanges using F-links. This direct connection is called associated signaling. In North America, SS7 links are normally indirectly connected between switching exchanges using an intervening network of STPs. This indirect connection is called quasi-associated signaling, which reduces the number of SS7 links necessary to interconnect all switching exchanges and SCPs in an SS7 signaling network.
SS7 links at higher signaling capacity (1.536 and 1.984 Mbit/s, simply referred to as the 1.5 Mbit/s and 2.0 Mbit/s rates) are called high speed links (HSL) in contrast to the low speed (56 and 64 kbit/s) links. High speed links are specified in ITU-T Recommendation Q.703 for the 1.5 Mbit/s and 2.0 Mbit/s rates, and ANSI Standard T1.111.3 for the 1.536 Mbit/s rate. There are differences between the specifications for the 1.5 Mbit/s rate. High speed links utilize the entire bandwidth of a T1 (1.536 Mbit/s) or E1 (1.984 Mbit/s) transmission facility for the transport of SS7 signaling messages.
SS7 PROTOCOL SUITE
SS7 protocol suite SS7 protocols by OSI layer
NETWORK MTP Level 3 + SCCP
DATA LINK MTP Level 2
PHYSICAL MTP Level 1
* v * t * e
The SS7 protocol stack may be partially mapped to the
The Message Transfer Part (MTP) covers a portion of the functions of the OSI network layer including: network interface, information transfer, message handling and routing to the higher levels. Signalling Connection Control Part (SCCP) is at functional Level 4. Together with MTP Level 3 it is called the Network Service Part (NSP). SCCP completes the functions of the OSI network layer: end-to-end addressing and routing, connectionless messages (UDTs), and management services for users of the Network Service Part (NSP). Telephone User Part (TUP) is a link-by-link signaling system used to connect calls. ISUP is the key user part, providing a circuit-based protocol to establish, maintain, and end the connections for calls. Transaction Capabilities Application Part (TCAP) is used to create database queries and invoke advanced network functionality, or links to Intelligent Network Application Part (INAP) for intelligent networks, or Mobile Application Part (MAP) for mobile services.
PROTOCOL SECURITY VULNERABILITIES
Several SS7 vulnerabilities that allow cell phone users to be secretly tracked were publicized in 2008. In 2014, the media reported a protocol vulnerability of SS7 by which both government agencies and non-state actors can track the movements of cell phone users from virtually anywhere in the world with a success rate of approximately 70%. In addition, eavesdropping is possible by using the protocol to forward calls and also facilitate decryption by requesting that each caller’s carrier release a temporary encryption key to unlock the communication after it has been recorded. Karsten Nohl (de) created a tool (SnoopSnitch) which can warn when certain SS7 attacks occur against a phone and detect IMSI-catchers .
In February 2016, 30% of the network to the largest mobile operator
In April 2016, US congressman Ted Lieu called for an oversight committee investigation, saying:
The applications for this vulnerability are seemingly limitless, from criminals monitoring individual targets to foreign entities conducting economic espionage on American companies to nation states monitoring US government officials. ... The vulnerability has serious ramifications not only for individual privacy , but also for American innovation, competitiveness and national security . Many innovations in digital security – such as multi-factor authentication using text messages – may be rendered useless.
In May 2017, O2 Telefonica, a German mobile service provider, confirmed that cybercriminals had exploited SS7 vulnerabilities to bypass two-factor authentication (2FA) to do unauthorized withdrawals from users' bank accounts. The criminals first infected the account holders' computers in an attempt to steal their bank account credentials and phone numbers. Then the attackers purchased access to a fake telecom provider and set up redirects for the victims' phone numbers to handsets controlled by them. Finally, the attackers logged into victims' online bank accounts and requested for the money on the accounts to be withdrawn to accounts owned by the criminals. 2FA confirmation codes were routed to phone numbers controlled by the attackers and the criminals transferred the money out.
* ^ A B C
ITU-T Recommendation Q.700
* ^ A B C Ronayne, John P (1986). The Digital Network Introduction
to Digital Communications Switching (1 ed.). Indianapolis: Howard W.
Sams & Co., Inc. ISBN 0-672-22498-4 .
* ^ RFC 2719 - Framework Architecture for Signaling Transport
* ^ A B C D E F Russell, Travis (2002). Signaling System #7 (4
ed.). New York: McGraw-Hill. ISBN 978-0-07-138772-9 .
ITU-T Recommendation Q.700,03/93, Section 3.2.1, p. 7.
ITU-T Recommendation Q.700, p. 4.
* ^ (Dryburgh 2004 , pp. 22–23).
* ^ (Dryburgh 2004 , p. 23).
ITU-T Recommendation Q.700, Section 2.2.3, "signalling modes",
* ^ A B "
ITU-T Recommendation Q.703, Annex A, Additions for a
national option for high speed signalling links". International
* Dryburgh, Lee; Hewitt, Jeff (2004). Signalling System No. 7 (SS7/C7): Protocol, Architecture, and Services. Indianapolis: Cisco Press. ISBN 1-58705-040-4 . * Ronayne, John P. (1986). "The Digital Network". Introduction to Digital Communications Switching (1st ed.). Indianapolis: Howard W. Sams & Co., Inc. ISBN 0-672-22498-4 . * Russell, Travis (2002). Signaling System #7 (4th ed.). New York: McGraw-Hill. ISBN 978-0-07-138772-9 .
* v * t * e
Cable protection system
Edwin Howard Armstrong
* Terminal node
* Network switching (circuit
* packet )
* Space-division * Frequency-division * Time-division * Polarization-division * Orbital angular-momentum * Code-division