Editing Signalling System No. 7 (section)

==Functionality==
[[Signaling (telecommunications)|Signaling]] in telephony is the exchange of [[Protocol-control information|control information]] associated with the setup and release of a telephone call on a telecommunications circuit.<ref name=Russell>{{cite book|last=Russell|first=Travis|date=2002|title= Signaling System #7|edition=4|location= New York|publisher= McGraw-Hill|isbn= 978-0-07-138772-9}}</ref>{{rp|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 analogue trunks, [[multi-frequency]] (MF) and R2 digital trunks, and [[DASS1|DSS1/DASS]] [[Business telephone system#Private branch exchange|PBX]] trunks.{{citation needed|date=December 2014}}

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.{{citation needed|date=December 2014}}

Because of the mechanisms in use by signaling methods prior to SS7 (battery reversal, [[Multi-frequency|multi-frequency digit outpulsing]], [[Robbed-bit signaling|A- and B-bit signaling]]), these earlier methods cannot communicate much signaling information. Usually only the dialed digits are signaled during call setup. For charged calls, dialed digits and charge number digits are 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|call forwarding (busy and no answer)]], [[voice mail]], [[call waiting]], [[conference call]]ing, [[Caller ID|calling name and number display]], [[call screening]], [[malicious caller identification]], [[Automatic callback|busy callback]].<ref name=Russell/>{{rp|Introduction xx}}<!--page xx is in the introduction-->

The earliest deployed upper-layer protocols in the SS7 suite were dedicated to the setup, maintenance, and release of telephone calls.<ref>{{cite web|url=https://www.itu.int/rec/T-REC-Q.700-199303-I/en/|title=ITU-T Recommendation Q.700, section 3.2.1|page=7|date=1993-03-01}}</ref> The [[Telephone User Part]] (TUP) was adopted in Europe and the [[Integrated Services Digital Network]] (ISDN) User Part ([[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| 2020}} North America has not accomplished full upgrade to the ISDN, and the predominant telephone service is still [[Plain old telephone service|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 exchange|telephone switches]] and not for signaling between local exchanges and [[customer-premises equipment]].{{citation needed|date=December 2014}}

Because SS7 signaling does not require seizure of a channel for a conversation prior to the exchange of control information, [[Non-Facility Associated Signaling|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. Another ISUP characteristic SS7 with NFAS enables is the exchange of signaling information during the middle of a call.<ref name=Russell/>{{rp|318}}

SS7 also enables Non-Call-Associated Signaling, which is signaling not directly related to establishing a telephone call.<ref name=Russell/>{{rp|319}} This includes the exchange of registration information used between a mobile telephone and a [[Network switching subsystem#Home location register .28HLR.29|home location register]] database, which tracks the location of the mobile. Other examples include [[Intelligent Network]] and [[local number portability]] databases.<ref name=Russell/>{{rp|433}}

===Signaling modes===
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''.<ref>{{cite web|url=https://www.itu.int/rec/T-REC-Q.700-199303-I/en/|title=ITU-T Recommendation Q.700|page=4|date=1993-03-01}}</ref>

When operating in the ''associated mode'', SS7 signaling progresses from [[Telephone exchange|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.<ref>{{harv|Dryburgh|Hewitt|2004|pp=22–23}}.</ref>

When operating in the ''quasi-associated mode'', SS7 signaling progresses from the originating [[Telephone exchange|switch]] to the terminating switch, following a path through a separate SS7 signaling network composed of [[Signal Transfer Point|signal transfer point]]s. 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.<ref>{{harv|Dryburgh|Hewitt|2004|p=23}}.</ref>