Signaling System No. The protocol also performs number translation, local number portability , prepaid billing, Short Message Service SMS , and other services. The SS7 protocol is defined for international use by the Q. 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,  : thus keeping the speech path separate.
Signaling in telephony is the exchange of control information associated with the setup and release of a telephone call on a telecommunications circuit. When signaling is performed on the same circuit as the conversation of the call, it is termed channel-associated signaling CAS. In contrast, SS7 uses common channel signaling , in which the path and facility used by the signaling is separate and distinct from the 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 during call setup.
Signalling System No. 7
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.
The earliest deployed upper layer protocols in the SS7 suite were dedicated to the setup, maintenance, and release of telephone calls. 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 signaling 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.
SS7 also enables Non-Call-Associated Signaling, which is signaling not directly related to establishing a telephone call. Other examples include Intelligent Network and local number portability databases. 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.
T 3 6 SS7 Protocol stack
The quasi-associated mode of signaling is the predominant choice of modes in North America. SS7 separates signaling 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.
Each node is identified on the network by a number, a signaling point code. One or more signaling links can be connected to the same two endpoints that together form a signaling link set.
Ss7 protocol suite pdf files
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. 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. High speed links utilize the entire bandwidth of a T1 1. Currently there are no protocol components that provide OSI layers 4 through 6.
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.
ISUP is the key user part, providing a circuit-based protocol to establish, maintain, and end the connections for calls.
For each active mobile equipment one signalling connection is used by BSSAP having at least one active transactions for the transfer of messages.
In , several SS7 vulnerabilities were published that permitted the tracking of cell phone users.
The security vulnerabilities of SS7 have been highlighted in U. The perpetrators installed malware on compromised computers, allowing them to collect online banking account credentials and telephone numbers. They set up redirects for the victims' telephone numbers to telephone lines controlled by them. Confirmation calls of two-factor authentication procedures were routed to telephone numbers controlled by the attackers.
This enabled them to log into victims' online bank accounts and effect money transfers. In March , a method was published for the detection of the vulnerabilities, through the use of open-source monitoring software such as Wireshark and Snort.
From Wikipedia, the free encyclopedia. ITU-T recommendation. Indianapolis: Howard W.
Signaling System 7 4 ed. New York: McGraw-Hill. International Telecommunication Union. Retrieved Retrieved 19 April The Washington Post. Retrieved 27 December Retrieved 19 December Chaos Communication Congress. Google Play. August 15, Telenor ASA.
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