Signalling System 7
Encyclopedia
Signalling System No. 7 (SS7) is a set of telephony
signaling protocols which are used to set up most of the world's public switched telephone network
telephone calls. The main purpose is to set up and tear down
telephone call
s. Other uses include number translation, local number portability
, prepaid billing mechanisms, short message service
(SMS), and a variety of other mass market services.
It is usually referenced as Signalling System No. 7 or Signalling System #7, or simply abbreviated to SS7. In North America it is often referred to as CCSS7, an abbreviation for Common Channel Signalling System 7. In some European countries, specifically the United Kingdom
, it is sometimes called C7 (CCITT number 7) and is also known as number 7 and CCIS7 (Common Channel Interoffice Signaling 7).
There is only one international SS7 protocol defined by ITU-T
in its Q.700-series recommendations. There are however, many national variants of the SS7 protocols. Most national variants are based on two widely deployed national variants as standardized by ANSI
and ETSI
, which are in turn based on the international protocol defined by ITU-T
. Each national variant has its own unique characteristics. Some national variants with rather striking characteristics are the China (PRC
) and Japan (TTC
) national variants.
The Internet Engineering Task Force
(IETF) has also defined level 2, 3, and 4 protocols that are compatible with SS7:
but use a Stream Control Transmission Protocol
(SCTP) transport mechanism. This suite of protocols is called SIGTRAN
.
protocols have been developed by major telephone companies and the ITU-T
since 1975 and the first international Common Channel Signaling protocol was defined by the ITU-T
as Signalling System No. 6
(SS6) in 1977. Signalling System No. 7 was defined as an international standard by ITU-T
in its 1980 (Yellow Book) Q.7XX-series recommendations. SS7 was designed to replace SS6, which had a restricted 28-bit signal unit that was both limited in function and not amenable to digital systems. SS7 has substantially replaced SS6
, Signalling System No. 5 (SS5), R1
and R2
, with the exception that R1
and R2
variants are still used in numerous nations.
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 in the parlance of the telecom industry. This led to security problems with blue boxes. SS6 and SS7 implement out-of-band signaling protocols, carried in a separate signaling channel, explicitly keep the end-user's audio path—the so-called speech path—separate from the signaling phase to eliminate the possibility that end users may introduce tones that would be mistaken for those used for signaling. See falsing
. SS6 and SS7 are referred to as so-called Common Channel Interoffice Signalling Systems (CCIS) or Common Channel Signaling
(CCS) due to their hard separation of signaling and bearer channels. This required a separate channel dedicated solely to signaling, but the greater speed of signaling decreased the holding time of the bearer channels, and the number of available channels was rapidly increasing anyway at the time SS7 was implemented.
The common channel signaling paradigm was translated to IP via the SIGTRAN
protocols as defined by the IETF. While running on a transport based upon IP, the SIGTRAN
protocols are not an SS7 variant, but simply transport existing national and international variants of SS7.
, when used in telephony, refers to the exchange of control information associated with the establishment of a telephone call on a telecommunications circuit. An example of this control information is the digits dialed by the caller, the caller's billing number, and other call-related information.
When the signaling is performed on the same circuit that will ultimately carry the conversation of the call, it is termed channel associated signaling (CAS). This is the case for earlier analogue trunks, MF
and R2
digital trunks, and DSS1/DASS
PBX trunks.
In contrast, SS7 signaling is termed Common Channel Signaling (CCS) in that the path and facility used by the signaling is separate and distinct from the telecommunications channels that will ultimately carry the telephone conversation. With CCS, it becomes possible to exchange signaling without first seizing a facility, leading to significant savings and performance increases in both signaling and facility 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 only 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 services that many take for granted today: 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 signaling 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
. (North America never accomplished full upgrade to the ISDN and the predominant telephone service is still the older POTS
.) Due to its richness and the need for an out-of-band channel for its operation, SS7 signaling is mostly used for signaling between telephone switches
and not for signaling between local exchanges
and customer-premises equipment
(CPE).
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 characteristic of ISUP
made possible by SS7 with NFAS is the exchange of signaling information during the middle of a call.
Also possible with SS7 is Non-Call-Associated Signaling, which is signaling that is not directly related to the establishment of a telephone call. An example of this is the exchange of the registration information used between a mobile telephone and a home location register (HLR) database: a database that tracks the location of the mobile. Other examples include Intelligent Network
and local number portability
databases.
When operating in the associated mode, SS7 signaling progresses from switch to switch
through the PSTN
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 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.
s that carry the voice or data. This separation extends onto the physical network by having circuits that are solely dedicated to carrying the SS7 links. Clearly splitting the signaling plane and voice circuits. An SS7 network has to be made up of SS7-capable equipment from end to end in order to provide its full functionality. The network is made up of several link types (A, B, C, D, E, and F) and three signaling nodes - Service switching point
(SSPs), signal transfer point
(STPs), and service control point
(SCPs). Each node is identified on the network by a number, a 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 (DS0A
s 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. Quasi-associated signaling 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.
SIGTRAN
provides signaling using SCTP associations over the Internet Protocol
. The protocols for SIGTRAN
are M2PA, M2UA, M3UA
and SUA.
borrows partially from the OSI Model
of a packetized digital protocol stack. OSI layers 1 to 3 are provided by the Message Transfer Part
(MTP) and the Signalling Connection Control Part (SCCP) of the SS7 protocol (together referred to as the Network Service Part (NSP)); for circuit related signaling, such as the Telephone User Part (TUP)
or the ISDN User Part
(ISUP), the User Part provides layer 7. Currently there are no protocol components that provide OSI layers 4 through 6. The Transaction Capabilities Application Part
(TCAP) is the primary SCCP User in the Core Network, using SCCP in connectionless mode. SCCP in connection oriented mode provides the transport layer for air interface protocols such as BSSAP and RANAP. TCAP provides transaction capabilities to its Users (TC-Users), such as the Mobile Application Part
, the Intelligent Network Application Part
and the CAMEL Application Part
.
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. ISDN User Part
(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.
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
signaling protocols which are used to set up most of the world's public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
telephone calls. The main purpose is to set up and tear down
Clearing (telecommunications)
Clearing, in telecommunications means:* A sequence of events used to disconnect a call and return to the ready state. It is sometimes, particularly in the context of common channel signaling, called teardown....
telephone call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
s. Other uses include number translation, local number portability
Local number portability
Local number portability for fixed lines, and full mobile number portability for mobile phone lines, refers to the ability to transfer either an existing fixed-line or mobile telephone number assigned by a local exchange carrier and reassign it to another carrier...
, prepaid billing mechanisms, short message service
Short message service
Short Message Service is a text messaging service component of phone, web, or mobile communication systems, using standardized communications protocols that allow the exchange of short text messages between fixed line or mobile phone devices...
(SMS), and a variety of other mass market services.
It is usually referenced as Signalling System No. 7 or Signalling System #7, or simply abbreviated to SS7. In North America it is often referred to as CCSS7, an abbreviation for Common Channel Signalling System 7. In some European countries, specifically the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, it is sometimes called C7 (CCITT number 7) and is also known as number 7 and CCIS7 (Common Channel Interoffice Signaling 7).
There is only one international SS7 protocol defined by ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
in its Q.700-series recommendations. There are however, many national variants of the SS7 protocols. Most national variants are based on two widely deployed national variants as standardized by ANSI
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
and ETSI
European Telecommunications Standards Institute
The European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...
, which are in turn based on the international protocol defined by ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
. Each national variant has its own unique characteristics. Some national variants with rather striking characteristics are the China (PRC
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...
) and Japan (TTC
Telecommunication Technology Committee
The Telecommunication Technology Committee was established in 1985. This standardization organization is authorized by Japan's Ministry of Internal Affairs and Communications to conduct research and to develop and promote standards for telecommunications in Japan.The TTC is a founding partner...
) national variants.
The Internet Engineering Task Force
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
(IETF) has also defined level 2, 3, and 4 protocols that are compatible with SS7:
- MTP2 (M2UA and M2PA)
- MTP3 (M3UA)
- Signalling Connection Control Part (SCCP) (SUA)
but use a Stream Control Transmission Protocol
Stream Control Transmission Protocol
In computer networking, the Stream Control Transmission Protocol is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol and User Datagram Protocol...
(SCTP) transport mechanism. This suite of protocols is called SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
.
History
Common Channel SignalingCommon Channel Signaling
In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...
protocols have been developed by major telephone companies and the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
since 1975 and the first international Common Channel Signaling protocol was defined by the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
as Signalling System No. 6
SS6
SS6 or CCITT No 6 Signalling System was introduced in the 1970s as an early common channel signalling method for telephone trunks between International Switching Centres...
(SS6) in 1977. Signalling System No. 7 was defined as an international standard by ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
in its 1980 (Yellow Book) Q.7XX-series recommendations. SS7 was designed to replace SS6, which had a restricted 28-bit signal unit that was both limited in function and not amenable to digital systems. SS7 has substantially replaced SS6
SS6
SS6 or CCITT No 6 Signalling System was introduced in the 1970s as an early common channel signalling method for telephone trunks between International Switching Centres...
, Signalling System No. 5 (SS5), R1
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
and R2
R2 signalling
R2 is a 1950s- and 1970s-era channel-associated-signalling signalling protocol used outside of the former Bell System to convey information along a telephone trunk between two telephone switches in order to establish a single telephone call along that trunk....
, with the exception that R1
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
and R2
R2 signalling
R2 is a 1950s- and 1970s-era channel-associated-signalling signalling protocol used outside of the former Bell System to convey information along a telephone trunk between two telephone switches in order to establish a single telephone call along that trunk....
variants are still used in numerous nations.
SS5 and earlier systems used in-band signaling
In-band signaling
In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...
, in which the call-setup information was sent by playing special multi-frequency
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
tones into the telephone lines, known as bearer channels in the parlance of the telecom industry. This led to security problems with blue boxes. SS6 and SS7 implement out-of-band signaling protocols, carried in a separate signaling channel, explicitly keep the end-user's audio path—the so-called speech path—separate from the signaling phase to eliminate the possibility that end users may introduce tones that would be mistaken for those used for signaling. See falsing
Falsing
In telecommunications, falsing describes a decoder assuming that it is detecting a valid input when one is not present. This is also known as a false decode...
. SS6 and SS7 are referred to as so-called Common Channel Interoffice Signalling Systems (CCIS) or Common Channel Signaling
Common Channel Signaling
In telephony, Common Channel Signaling , in the US also Common Channel Interoffice Signaling , is the transmission of signaling information on a separate channel from the data, and, more specifically, where that signaling channel controls multiple data channels.For example, in the public switched...
(CCS) due to their hard separation of signaling and bearer channels. This required a separate channel dedicated solely to signaling, but the greater speed of signaling decreased the holding time of the bearer channels, and the number of available channels was rapidly increasing anyway at the time SS7 was implemented.
The common channel signaling paradigm was translated to IP via the SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
protocols as defined by the IETF. While running on a transport based upon IP, the SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
protocols are not an SS7 variant, but simply transport existing national and international variants of SS7.
Functionality
The term signalingSignalling (telecommunications)
In telecommunication, signaling has the following meanings:*the use of signals for controlling communications...
, when used in telephony, refers to the exchange of control information associated with the establishment of a telephone call on a telecommunications circuit. An example of this control information is the digits dialed by the caller, the caller's billing number, and other call-related information.
When the signaling is performed on the same circuit that will ultimately carry the conversation of the call, it is termed channel associated signaling (CAS). This is the case for earlier analogue trunks, MF
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
and R2
R2 signalling
R2 is a 1950s- and 1970s-era channel-associated-signalling signalling protocol used outside of the former Bell System to convey information along a telephone trunk between two telephone switches in order to establish a single telephone call along that trunk....
digital trunks, and DSS1/DASS
Digital Access Signalling System
The Digital Access Signalling System is the United Kingdom proprietary standard created by British Telecom to provide ISDN services in the United Kingdom. The first incarnation DASS1 is now obsolete and has been replaced by DASS II. This itself will become obsolete over the coming years as Q.931,...
PBX trunks.
In contrast, SS7 signaling is termed Common Channel Signaling (CCS) in that the path and facility used by the signaling is separate and distinct from the telecommunications channels that will ultimately carry the telephone conversation. With CCS, it becomes possible to exchange signaling without first seizing a facility, leading to significant savings and performance increases in both signaling and facility usage.
Because of the mechanisms used by signaling methods prior to SS7 (battery reversal, multi-frequency digit outpulsing
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...
, A- and B-bit signaling), these older methods could not communicate much signaling information. Usually only the dialed digits were signaled, and only 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 services that many take for granted today: call forwarding (busy and no answer)
Call forwarding
Call forwarding , in telephony, is a feature on some telephone networks allowing an incoming call to a called party to be redirected to a third party. For example, the third party may be a mobile telephone, voicemail box or other telephone number where the desired called party is situated. It was...
, voice mail, call waiting
Call waiting
Call waiting , in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new...
, conference calling, calling name and number display
Caller ID
Caller ID , also called calling line identification or calling number identification or Calling Line Identification Presentation , is a telephone service, available in analog and digital phone systems and most Voice over Internet Protocol applications, that transmits a caller's number to...
, call screening
Call screening
Call screening is the process of evaluating the characteristics of a telephone call before deciding how or whether to answer it.Some methods may include:* listening to the message being recorded on an answering machine or voice mail...
, malicious caller identification
Malicious Caller Identification
Malicious Caller Identification , also called Malicious Call Trace or Caller Activated Malicious Call Trace is a facility which enables a phone call recipient to mark a phone call as malicious The phone system will then automatically trace the call...
, busy callback
Automatic callback
In telecommunication, an automatic callback is a computer telephony calling feature that permits a user, when encountering a busy condition or other condition where the called individual is unavailable, to instruct the system to retain the called number and to establish the call when there is an...
.
The earliest deployed upper layer protocols in the SS7 signaling 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
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
(ISDN) User Part (ISUP
ISDN User Part
The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
) adapted for public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
(PSTN) calls was adopted in North America. ISUP
ISDN User Part
The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
was later used in Europe when the European networks upgraded to the ISDN
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
. (North America never accomplished full upgrade to the ISDN and the predominant telephone service is still the older POTS
Plain old telephone service
Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....
.) Due to its richness and the need for an out-of-band channel for its operation, SS7 signaling is mostly used for signaling between telephone switches
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
and not for signaling between local exchanges
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
and customer-premises equipment
Customer-premises equipment
Customer-premises equipment or customer-provided equipment is any terminal and associated equipment located at a subscriber's premises and connected with a carrier's telecommunication channel at the demarcation point...
(CPE).
Because SS7 signaling does not require seizure of a channel for a conversation prior to the exchange of control information, non-facility associated signalling
Non-Facility Associated Signalling
Non-Facility Associated Signalling or NFAS is a Primary Rate Interface configuration whereby multiple T1 carriers share a signaling channel ....
(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 characteristic of ISUP
ISDN User Part
The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
made possible by SS7 with NFAS is the exchange of signaling information during the middle of a call.
Also possible with SS7 is Non-Call-Associated Signaling, which is signaling that is not directly related to the establishment of a telephone call. An example of this is the exchange of the registration information used between a mobile telephone and a home location register (HLR) database: a database that tracks the location of the mobile. Other examples include Intelligent Network
Intelligent network
The Intelligent Network , is the standard network architecture specified in the . It is intended for fixed as well as mobile telecom networks...
and local number portability
Local number portability
Local number portability for fixed lines, and full mobile number portability for mobile phone lines, refers to the ability to transfer either an existing fixed-line or mobile telephone number assigned by a local exchange carrier and reassign it to another carrier...
databases.
Signaling modes
As well as providing for 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
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
through the PSTN
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
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
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
to the terminating switch, following a path through a separate SS7 signaling network composed of signal transfer point
Signal Transfer Point
A Signal Transfer Point is a router that relays SS7 messages between signaling end-points and other signaling transfer points . Typical SEPs include service switching points and service control points . The STP is connected to adjacent SEPs and STPs via signaling links...
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.
Physical network
SS7 is an out-of-band signaling protocol, i.e. separate from the bearer channelBearer channel
A bearer channel is a DS-0 that carries call content i.e. one that does not carry signaling.In the Common Channel Signaling scheme for telecommunications, signaling is sent out-of-band, while all other traffic rides bearer channels...
s that carry the voice or data. This separation extends onto the physical network by having circuits that are solely dedicated to carrying the SS7 links. Clearly splitting the signaling plane and voice circuits. An SS7 network has to be made up of SS7-capable equipment from end to end in order to provide its full functionality. The network is made up of several link types (A, B, C, D, E, and F) and three signaling nodes - Service switching point
Service switching point
In telephony, a service switching point is the telephone exchange that initially responds, when a telephone caller dials a number, by sending a query to a central database called a service control point so that the call can be handled...
(SSPs), signal transfer point
Signal Transfer Point
A Signal Transfer Point is a router that relays SS7 messages between signaling end-points and other signaling transfer points . Typical SEPs include service switching points and service control points . The STP is connected to adjacent SEPs and STPs via signaling links...
(STPs), and service control point
Service Control Point
A service control point is a standard component of the Intelligent Network telephone system which is used to control the service. Standard SCPs in the telecom industry today are deployed using SS7, Sigtran or SIP technologies. The SCP queries the service data point which holds the actual...
(SCPs). Each node is identified on the network by a number, a 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
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
facility; in North America one (56 or 64 kbit/s) or all (1,536 kbit/s) timeslots (DS0A
DS0A
DS0A is the interface most commonly used for SS7 networks in the US. It is a 56/64kbps channel typically located in a DS1 or larger facility. The DS0A electrical interface usually only exists inside a central office environment, and only exists for the sole purpose of connecting into a channel bank...
s or DS0s) within a T1
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
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. Quasi-associated signaling 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.
SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
provides signaling using SCTP associations over the Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
. The protocols for SIGTRAN
SIGTRAN
SIGTRAN is the name, derived from signaling transport, of the former Internet Engineering Task Force working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for Signaling System 7 and ISDN communications protocols....
are M2PA, M2UA, M3UA
M3UA
M3UA stands for MTP Level 3 User Adaptation Layer as defined by the IETF SIGTRAN working group in RFC 4666 . M3UA enables the SS7 protocol's User Parts to run over IP instead of telephony equipment like ISDN and PSTN...
and SUA.
SS7 protocol suite
The SS7 protocol stackProtocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
borrows partially from the OSI Model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
of a packetized digital protocol stack. OSI layers 1 to 3 are provided by the Message Transfer Part
Message Transfer Part
The Message Transfer Part is part of the Signaling System 7 used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners....
(MTP) and the Signalling Connection Control Part (SCCP) of the SS7 protocol (together referred to as the Network Service Part (NSP)); for circuit related signaling, such as the Telephone User Part (TUP)
Telephone User Part (TUP)
Telephone User Part provides conventional PSTN telephony services across the SS7network. TUP was the first layer 4 protocol defined by the standards bodies and as such did notprovision for ISDN services. It has now largely been replaced by ISUP...
or the ISDN User Part
ISDN User Part
The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
(ISUP), the User Part provides layer 7. Currently there are no protocol components that provide OSI layers 4 through 6. The Transaction Capabilities Application Part
Transaction Capabilities Application Part
Transaction Capabilities Application Part, from ITU-T recommendations Q.771-Q.775 or ANSI T1.114 is a protocol for Signalling System 7 networks. Its primary purpose is to facilitate multiple concurrent dialogs between the same sub-systems on the same machines, using Transaction IDs to differentiate...
(TCAP) is the primary SCCP User in the Core Network, using SCCP in connectionless mode. SCCP in connection oriented mode provides the transport layer for air interface protocols such as BSSAP and RANAP. TCAP provides transaction capabilities to its Users (TC-Users), such as the Mobile Application Part
Mobile Application Part
The Mobile Application Part is an SS7 protocol which provides an application layer for the various nodes in GSM and UMTS mobile core networks and GPRS core networks to communicate with each other in order to provide services to mobile phone users...
, the Intelligent Network Application Part
INAP
The Intelligent Network Application Part is a signalling protocol used in the intelligent network architecture. It is part of the SS7 protocol suite, typically layered on top of TCAP.It can also be termed as logic for controlling telecommunication services migrated from traditional switching...
and the CAMEL Application Part
Camel Application Part
The CAMEL Application Part is a signalling protocol used in the Intelligent Network architecture. CAP is a Remote Operations Service Element user protocol, and as such is layered on top of the Transaction Capabilities Application Part of the SS#7 protocol suite...
.
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. ISDN User Part
ISDN User Part
The ISDN User Part or ISUP is part of the Signaling System #7 which is used to set up telephone calls in Public Switched Telephone Networks...
(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.
External links
- Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services. Full HTML version of the 2004 edition of the Dryburgh/Hewitt book (above).
- SS7 Protocol layer architecture overview, tutorials and discussion forum
- SS7 - A Brief Comparison with TCP/IP
- www.protocols.com: practical overview
- SS7 open source project
- Mobicents SS7 open source project
- Introduction to signaling. A tutorial about signaling.
- Excellent SS7 Tutorial
- SeveNTest online SS7 decoder