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3. SS7 (Signaling System #7)SS7 was designed to replace Signaling System #5 (SS5) and Signaling System #6 (SS6) and R2 defined by the ITU. It is the intelligence in the advanced intelligent network (AIN) that the telephone companies have been deploying. SS7 is commonly used name in North America and often referred to Common Channel Signaling System 7 (CCSS7), CCITT7, or CCIS7 in Europe and Asia. The SS7 protocols have been developed by AT&T since 1975 and defined as standard by ITU during 1981 in ITU's Q.7XX-series recommendations.
The SS7 architecture consists of three components with interconnection links between them as shown in Figure 3-1. Three components are Signal Switching Point (SSP), Signal Transfer Point (STP) and Signal Control Point (SCP). SSPs are local exchange switches that originate, terminate, or tandem calls. STPs are routers/or gateways that route each incoming message to an outgoing signaling link based on routing information contained in the SS7 message. There are three levels of STPs ??National, International and gateway STP. SCPs are computers used as a front end interfaces to applications such as databases. The hardware and software functions of the SS7 protocol are divided into functional abstractions called "levels". These levels may map to the Open Systems Interconnect (OSI) 7-layer model defined by the International Standards Organization (ISO) as shown in Figure 3-2.
The Message Transfer Part (MTP) occupies the lower layers. MTP is divided into three levels. The lowest level, MTP Level 1, is equivalent to the OSI Physical Layer. MTP Level 1 defines the physical, electrical, and functional characteristics of the digital signaling link. MTP Level 2 is concerned with the transmission of data from one node to the next, not to its final destination in the network. Level 2 implements flow control, message sequence validation, and error checking. MTP Level 3 provides message routing between signaling points in the SS7 network. MTP Level 3 handles message discrimination, message distribution, message routing, traffic management, route management, and link management. Signaling Connection Control Part (SCCP) offers enhancements to MTP level 3 to provide connectionless and connection-oriented network services, as well as to address translation capabilities. The ISDN User Part (ISUP) supports basic telephone call connect/disconnect between end offices. ISUP is used for both ISDN and non-ISDN calls. ISUP also links the cellular and PCS network to the PSTN. Broadband ISDN User Part (BISUP) defines broadband ISDN networks. B-ISDN user part messages are carried on the ATM signaling link by means of S-AAL service data units. Telephone User Part (TUP) is an analog protocol that carries the telephone user messages on the signaling data link by means of signal units. It has been replaced by ISUP, but is still used in some parts of the world. Transaction Capabilities Applications Part (TCAP) supports the exchange of non-circuit related information between applications across the SS7 network using the SCCP connectionless service. Queries and responses sent between SSPs and SCPs are carried in TCAP messages. SCP also uses TCAP to access and interface a database application. In mobile networks (IS-41 and GSM), TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming are carried by TCAP. The ITU definition of SS7 allows for national variants such as the American National Standards Institute (ANSI) and the European Telecommunications Standards Institute (ETSI) standard used in Europe. Table 3-1 lists SS7 standards in ITU, ETSI and ANSI. SS7 Related Standards ITU Recommendations: General Q.700 : Introduction to CCITT Signaling System No. 7. This Recommendation provides a general description of functions and capabilities of the Message Transfer Part (MTP), Signaling Connection Control Part (SCCP), Telephone User Part, ISDN User Part (ISUP), and Transaction Capabilities (TC). MTP (Message Transfer Part) Q.701 : Functional description of the message transfer part (MTP) of Signaling System No. 7. The MTP provides the functions that enable User Part significant information passed to the MTP to be transferred across the SS7 network to the required destination. The MTP of SS7 is specified in Recommendations Q.701 to Q.709. Q.704 : Signaling network functions and messages. This Recommendation describes the functions and procedures for and relating to the transfer of messages between the signaling points. Q.2210 : Message transfer part level 3 functions and messages using the services of ITU Recommendation Q.2140. This Recommendation specifies MTP Part level 3 functions and messages that are suitable for the control of signaling links that provide the services specified in Q.2140 (B-ISDN ATM adaptation layer - Service specific coordination function for signaling at the network node interface (SSCF at NNI)) TUP (Telephone User Part) Q.721 : Functional description of the Signaling System No. 7 Telephone User Part (TUP). The TUP specified in these specifications defines the necessary telephone signaling functions for use of SS7 for international telephone call control signaling. Recommendations Q.721 to Q.725 specify TUP. ISUP (ISDN User Part) Q.761 : Signaling System No. 7 - ISDN User Part functional description. This Recommendation specifies the signaling functions required to support basic bearer services and supplementary services for voice and non-voice applications in an ISDN. Recommendations Q.761 to Q.764 specify ISDN User Part signaling. Q.730 : ISDN user part supplementary services. Q.767 : Application of the ISDN User Part of CCITT signaling system No. 7 for international ISDN interconnections. This Recommendation specifies international ISUP. Q.2761: Functional description of the B-ISDN user part (B-ISUP) of signaling system No. 7. Recommendations Q.2762, Q.2763, Q.2764, and Q.2765 specify B-ISUP. SCCP (Signal Connection Control Part) Q.711 : Functional description of the signaling connection control part. The SCCP provides, above the MTP network or networks, connectionless, connection-oriented, routing and management services. Recommendations Q.711 to Q.714 and Q.716 specify SSCP. TCAP Q.771 : Functional description of transaction capabilities. This Recommendation specifies a set of communication capabilities that provide an interface between applications and a network layer service. Recommendations Q.771 to Q.775 specify TCAP. BICC (Bearer Independent Call Control) Q.1901: Bearer Independent Call Control protocol. This Recommendation describes the adaptation of the narrow-band ISDN User Part (ISUP) for the support of narrow-band ISDN services independent of the bearer technology and signaling message transport technology used. Recommendations Q.1902.1 to Q.1902.6, 1912.1 to 1912.5, 1922.2 to 1922.3, Q.1930, Q.1950, Q.1970, Q.1980.1 and Q.1990 specify BICC related protocols interworking with other protocols such as H.323 and ISP. OMAP (Operations, Maintenance and Administration Part) Q.750 : Overview of Signaling System No. 7 management. This series of Recommendations on the Operations, Maintenance and Administration Part (OMAP) define the functions, procedures and entities for managing the SS7 network. Recommendations Q.750, Q.752, Q.754 and Q.756 specify OMAP. Table 3-1: SS7 Standards
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