Middle tier Technical Specification

Description

The 'middle tier' Technical Specifications (TSs) are a defined group of 3GPP charging specifications within the 32.250 to 32.279 numbering range. They provide detailed, service-specific and domain-specific charging requirements that enable operators to monetize different network services and subsystems. Examples include TS 32.250 for the Circuit Switched (CS) domain, TS 32.251 for the Packet Switched (PS) domain, TS 32.270 for the Multimedia Messaging Service (MMS), and TS 32.272 for the IP Multimedia Subsystem (IMS). These specifications define how charging data is generated, collected, and processed for each particular service or network domain, covering both online (prepaid, real-time) and offline (postpaid, batch) charging mechanisms.

Architecturally, the middle tier TSs operate under the umbrella of the 'tier 1' charging specification, TS 32.240, which establishes the overall charging architecture, principles, and logical charging functions. The middle tier documents then specialize these principles for a given domain or service. For instance, TS 32.251 details the charging architecture for the PS domain, specifying the Charging Trigger Function (CTF), Charging Data Function (CDF), and Charging Gateway Function (CGF) interactions, as well as the structure of Charging Data Records (CDRs) for packet data sessions. Similarly, TS 32.272 defines the charging framework for IMS, including the use of the IMS Charging Identifier (ICID) and the mapping of SIP signalling to charging events.

These specifications also rely on a set of 'top tier' TSs in the 32.29x range, which define common charging aspects applicable across all middle tier specifications. These include TS 32.295 for the charging data record (CDR) parameter definitions, TS 32.297 for the common billing domain interface, and TS 32.299 for the Diameter charging applications. The middle tier TSs reference these common specifications to ensure consistency and interoperability across different services and domains.

The middle tier TSs are essential for enabling flexible and accurate billing in multi-service networks. They allow operators to define charging models that reflect the specific characteristics of each service, such as volume-based charging for data, time-based charging for voice, or event-based charging for messaging. By separating domain-specific charging logic from the common architecture and enablers, 3GPP ensures that new services can be integrated into the charging system without redefining the entire framework. This modular approach has been maintained and evolved across multiple releases, from Rel-11 through Rel-19, adapting to new technologies like 5G and network slicing.

Purpose & Motivation

The middle tier TSs exist to provide a structured, modular approach to charging in 3GPP networks. Before their introduction, charging specifications were often fragmented and service-specific, leading to inconsistencies and integration challenges. The tiered architecture, with TS 32.240 as the overarching architecture and the 32.29x series for common enablers, allows for a clear separation of concerns: the middle tier focuses on the unique charging aspects of each domain or service, while leveraging common mechanisms for data formats, interfaces, and protocols.

This structure solves the problem of scalability and extensibility. As new services are introduced (e.g., MMS, IMS, 5G slice-based services), only a new middle tier specification is needed, reusing the common charging functions and interfaces. This reduces development effort and ensures interoperability between different vendors' charging implementations. It also enables operators to deploy a unified charging system that can handle multiple services with consistent procedures for online and offline charging.

Historically, the middle tier concept was formalized in Rel-8, but the term 'Middle tier TS' as a distinct entry was introduced in Rel-11. The need for real-time charging and flexible billing models drove the development of these specifications. Over successive releases, the middle tier TSs have been updated to incorporate new charging requirements, such as those for 5G, network slicing, and edge computing, demonstrating the adaptability of the tiered approach.