The SCCP (Signaling Connection Control Part) layer builds on the services of the MTP layer to provide SS7 applications with a higher level transport subsystem. SCCP adds the following services to those provided by the MTP layers:
The ability to address individual applications or databases, known as subsystems, at a signaling point through a SCCP-level address consisting of a point code and a subsystem number.
An OSI-like connectionless transport service.
An OSI-like connection-oriented transport service.
An address translation mechanism called global title translation that can translate a string of digits (such as a telephone number or mobile identification number) into a point code or subsystem number, isolating applications from changes in the physical SS7 network structure.
A subsystem management layer that tracks the status of targeted subsystems at particular signaling points, known as concerned signaling points, and can optionally associate a backup signaling point with a subsystem for high availability applications.
Applications access these services either directly from the SCCP layer or indirectly through the TCAP layer.
SCCP implements services through the configuration of general parameters and the following entities:
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Entity |
Description |
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Define the interface between the user applications and the SCCP layer. One user SAP is defined for each application using the SCCP layer services. A user SAP is associated with a single subsystem number and protocol variant (ANSI or ITU-T). The user SAP defines whether the application is replicated on another node for reliability purposes and lists any concerned point codes (nodes that must be notified of any change in the status of the application). | |
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Define the interface between the SCCP layer and the MTP layer 3. One network SAP is defined for each MTP 3 layer interface that the SCCP layer uses. Typically the SCCP layer has only a single network SAP. If the same system supports multiple protocol variants (ANSI and ITU-T), the SCCP layer has a separate network SAP for each switch type. | |
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Define one route for each destination signaling point that the SCCP layer may be used to access. The route defines the destination point code of that signaling point and each subsystem of interest at that signaling point, as well as any backup point codes that replicate those subsystems. If the SCCP default routing feature is employed, all routing is deferred to the MTP layers and no SCCP routes need to be defined. Refer to Using default routing. | |
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Define how the SCCP layer is to translate or route between global titles, point codes, and subsystem numbers. Refer to Configuring global title translations. |
The following illustration shows the concept of user SAPs. When the application interfaces with the TCAP layer, the TCAP SAPs map one-for-one with an SCCP user SAP.
The following illustration shows the relationship between the SCCP configurable entities:
