Service Switching Function Management Entity

Description

The Service Switching Function Management Entity (SSME) is a core component within the 3GPP Customised Applications for Mobile network Enhanced Logic (CAMEL) architecture, defined as part of the Intelligent Network (IN) framework for mobile networks. It operates as the management and control interface for the Service Switching Function (SSF), which is typically embedded within the Mobile Switching Centre (MSC) or Gateway MSC. The SSME's primary function is to manage the detection points (DPs) and trigger mechanisms within the SSF that identify when a call or session requires CAMEL-based service logic. It acts as the intermediary, receiving and processing instructions from the Service Control Function (SCF), which hosts the actual service logic (e.g., for prepaid charging or call forwarding), and ensuring these instructions are correctly executed by the switching function.

Architecturally, the SSME is defined to handle the CAP (CAMEL Application Part) protocol interactions. When a call or session is established, the SSF, under the management of the SSME, monitors for specific triggering events defined in the subscriber's CAMEL Subscription Information (CSI). Upon detecting a trigger, the SSME formulates a CAP InitialDP message and sends it to the appropriate SCF. The SCF then returns CAP instructions (e.g., ApplyCharging, Connect, Continue) which the SSME interprets and enforces on the SSF to control call routing, charging, and other service features. This decouples service logic from basic call processing, allowing for flexible service deployment.

The SSME manages key resources and states associated with CAMEL dialogues. It is responsible for maintaining the state of the CAMEL relationship for each active call, handling timers for operations like charging control, and managing error conditions. Its role ensures that the network can provide advanced, operator-specific services consistently and reliably, even for subscribers roaming outside their home network, by standardizing the interaction between the switching and control layers. The specifications, particularly TS 23.078 and TS 29.078, detail its procedures and the CAP protocol it utilizes.

Purpose & Motivation

The SSME was introduced to provide a standardized, manageable interface for the Service Switching Function within the CAMEL framework. Prior to CAMEL, advanced telephony services were largely proprietary and tied to specific switch vendors, making it difficult for operators to deploy consistent services across their network, especially for roaming subscribers. The Intelligent Network concept aimed to separate service logic from switching hardware, and the SSME is the 3GPP realization of the SSF management needed to make this work in GSM and UMTS networks.

Its creation solved the problem of how to reliably trigger and execute external service logic during basic call control. By defining a dedicated management entity, 3GPP ensured that CAMEL services like prepaid, freephone numbers, and virtual private networks could be invoked in a standardized manner. The SSME abstracts the complexities of the underlying switch, presenting a uniform CAP interface to the SCF. This allows service logic developers and network operators to focus on creating services without worrying about the specifics of every MSC vendor's implementation, fostering innovation and interoperability in mobile service offerings.

Key Features

• Manages Detection Point (DP) triggering and event handling within the SSF
• Acts as the protocol endpoint for CAMEL Application Part (CAP) dialogue with the SCF
• Interprets and executes service control instructions (e.g., Connect, ApplyCharging) from the SCF
• Maintains state and timers for active CAMEL service sessions
• Handles error recovery and fallback procedures for CAMEL interactions
• Supports multiple CAMEL phases and service scenarios (e.g., MO, MT, SMS control)

Evolution Across Releases

Defining Specifications