Description
The Supplementary Service Application Part (SSAP) is a protocol defined within the 3GPP TS 29.013 specification, operating as a component of the Intelligent Network (IN) and CAMEL framework. It functions as an application-layer protocol that uses the Transaction Capabilities Application Part (TCAP) and the Signaling Connection Control Part (SCCP) for reliable message transfer over SS7 or IP-based signaling networks. SSAP defines the messages and procedures for the interaction between the gsmSCF (GSM Service Control Function) and the MSC (Mobile Switching Centre) or SGSN (Serving GPRS Support Node). This interaction allows the gsmSCF, which hosts the service logic for supplementary services, to control call and session handling in real-time. For instance, when a subscriber activates call forwarding, the MSC may invoke a CAMEL dialogue with the gsmSCF using SSAP messages to execute the specific forwarding logic, such as querying subscriber data or applying time-based rules.
Architecturally, SSAP is a service-specific adaptation layer that sits above TCAP. It defines a set of operations and associated parameters that model the control of supplementary services. These operations include requests from the MSC to the gsmSCF (e.g., to report an event like call setup) and instructions from the gsmSCF back to the MSC (e.g., to connect a call to a different number or play an announcement). The protocol ensures that the service logic in the gsmSCF can influence the call processing in the switch without requiring direct integration, enabling a clear separation between service control and switching functions. This separation is a cornerstone of the Intelligent Network concept, promoting flexibility and faster service deployment.
Key components of SSAP include its operation codes, error codes, and parameter structures defined in ASN.1. The protocol handles various supplementary service scenarios, including call diversion (CFU, CFB, CFNRy, CFNRc), call barring (BAOC, BOIC, BIC-Roam), and advice of charge. It works in conjunction with other CAMEL protocols like CAP (CAMEL Application Part) for basic call control and INAP for fixed-network interactions. While CAP is more broadly used for real-time service control like prepaid charging, SSAP is specifically tailored for the management of traditional telephony supplementary services within the GSM/UMTS circuit-switched domain. Its operation is tightly coupled with the subscriber's HLR profile, which indicates which supplementary services are active and whether CAMEL control is required.
Purpose & Motivation
SSAP was created to address the need for standardized, network-independent control of supplementary services within mobile networks, particularly as part of the CAMEL standard introduced in GSM Phase 2+. Prior to CAMEL and SSAP, supplementary services like call forwarding were implemented using proprietary, switch-vendor-specific logic within the MSC. This made it difficult for operators to introduce new services quickly or ensure consistent behavior across a multi-vendor network. The Intelligent Network (IN) concept, adopted by 3GPP as CAMEL, aimed to decouple service logic from switching hardware, allowing services to be developed and hosted on centralized platforms (the gsmSCF).
SSAP specifically solves the problem of how this centralized service logic can communicate with and control the MSC for supplementary service scenarios. It provides a standardized protocol so that an operator can deploy a gsmSCF from one vendor to control MSCs from another vendor, fostering interoperability and reducing vendor lock-in. This was especially important for enabling advanced, operator-specific services (like customized call forwarding rules based on time of day or caller identity) that go beyond the basic standardized supplementary services.
Historically, SSAP filled a niche within the broader CAMEL architecture. While CAP handled the fundamental call control for services like prepaid, SSAP addressed the more traditional telephony features. Its creation was motivated by the desire to extend the benefits of IN—flexibility, rapid service creation, and centralized control—to the entire suite of supplementary services, ensuring a cohesive framework for all value-added telephony features in 2G and 3G circuit-switched networks.
Key Features
- Standardized protocol for gsmSCF-to-MSC/SGSN communication for supplementary services
- Enables real-time control of call forwarding, barring, and other telephony features
- Uses TCAP and SCCP for reliable transaction-based signaling transport
- Defines specific operations and parameters in ASN.1 for precise service control
- Supports the CAMEL Intelligent Network architecture for service logic separation
- Facilitates multi-vendor interoperability for supplementary service deployment
Evolution Across Releases
SSAP was introduced in 3GPP Release 8 as part of the specification TS 29.013. Its initial architecture provided the protocol for controlling GSM supplementary services from a CAMEL Service Control Function (gsmSCF). It defined the basic operations for interaction between the gsmSCF and the MSC or SGSN, enabling centralized control of features like call diversion and call barring within the circuit-switched domain.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.013 | 3GPP TS 29.013 |