Service Specific Entities

Description

Service Specific Entities (SSEs) are a standardized architectural concept within the 3GPP IP Multimedia Subsystem (IMS) framework. They represent dedicated application servers or functional entities that host and execute the logic for specific, value-added multimedia services beyond basic SIP session control. An SSE is not a single network node but a logical role that can be implemented on various physical platforms, such as a dedicated Application Server (AS), a part of a Telephony Application Server (TAS), or a specialized server for a particular service enabler. The core function of an SSE is to provide the service-specific processing, data manipulation, and interaction required for advanced IMS-based services like Push-to-talk over Cellular (PoC), multimedia conferencing, instant messaging, presence, or group management.

SSEs work by interfacing with the core IMS infrastructure via standardized reference points, primarily the ISC (IMS Service Control) interface using the SIP protocol. When a SIP request for a service (e.g., a PoC session invitation) arrives at the S-CSCF, the S-CSCF uses initial Filter Criteria (iFC) in the user's profile to determine if the request should be forwarded to a specific SSE. The request is then routed to the appropriate SSE. The SSE executes its service logic, which may involve interacting with other SSEs, querying databases (like a Group List Management Server), modifying SIP messages, managing media resources, or interfacing with external networks. After processing, the SSE typically returns control to the S-CSCF for further routing. For example, a PoC SSE would handle floor control, talker identification, and media distribution to group members.

Architecturally, SSEs promote a modular and scalable service layer. Key components within an SSE include the service logic execution environment, interfaces to the IMS core (ISC), interfaces to other enablers (e.g., to a Presence Server via the Ut interface for configuration), and potentially interfaces to media resources (Mr, Mp). The 3GPP specifications, particularly in the TS 26 series for codecs and media handling and TS 24 series for communication procedures, define the precise behavior, protocols, and information flows for standardized SSEs. This ensures interoperability between equipment from different vendors and across operator networks. The role of an SSE is to decouple service innovation from core network evolution, allowing new services to be introduced by adding or updating SSEs without major modifications to the CSCFs or other core IMS elements.

Purpose & Motivation

The concept of SSEs was introduced to address the challenge of service creation and standardization in the emerging all-IP IMS architecture. Prior to IMS, advanced telephony services were often tightly coupled to the circuit-switched network or implemented as proprietary, siloed solutions, leading to interoperability issues and slow innovation. The IMS vision was to create a common, reusable IP-based core for delivering a wide array of multimedia services. However, a core network alone cannot provide end-user services; it needs a standardized way to plug in service logic.

SSEs were created to fulfill this need. They provide the 'service layer' atop the IMS core, solving the problem of how to define, standardize, and deploy interoperable multimedia applications. By specifying SSEs for key service enablers (like PoC or instant messaging), 3GPP ensured that basic building blocks for complex services were uniformly available. This allowed operators to offer consistent services and enabled device manufacturers to implement compliant clients. The motivation was to avoid fragmentation and accelerate the adoption of IMS-based services by providing clear blueprints for service implementation.

Furthermore, SSEs facilitate service integration and composition. Different SSEs can work together; for instance, a conferencing service (an SSE) might utilize a group management service (another SSE) and a presence service (a third SSE). This modular approach solves the problem of service redundancy and complexity. Instead of each service implementing its own group list management, a standardized Group Management SSE can be used by multiple applications. This promotes network efficiency, reduces development time for new services, and creates a more cohesive user experience across different IMS applications.

Key Features

• Hosts service-specific logic for IMS-based multimedia applications (e.g., PoC, conferencing)
• Interfaces with the IMS core via the standardized ISC reference point using SIP
• Enables modular and decoupled service architecture within IMS
• Promotes interoperability through 3GPP-standardized behavior and procedures
• Allows for service composition by interacting with other SSEs and enablers
• Facilitates rapid deployment of new value-added services without core network changes

Evolution Across Releases

Defining Specifications