IMS-based PSTN/ISDN Emulation Sub-system

Description

The IMS-based PSTN/ISDN Emulation Sub-system (PES) is a comprehensive architectural framework defined by 3GPP to facilitate the migration of traditional telephony services to next-generation IP-based networks. It operates as a functional overlay within the IMS core, enabling the emulation of the full suite of Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN) services. This emulation is transparent to the end-user, who continues to use existing terminals (like traditional phones or ISDN terminals) and experiences the same service features, such as basic call handling, supplementary services (call waiting, call forwarding, etc.), and emergency calling. The PES architecture is built upon the IMS control plane, utilizing core IMS elements like the Call Session Control Function (CSCF) for session control and the Home Subscriber Server (HSS) for user data, but introduces specific PES-related functions to handle the unique signaling and media requirements of legacy services.

At the heart of PES are key functional entities defined in specifications like TS 23.517. The Access Gateway Control Function (AGCF) is a critical component that acts as a signaling interworking point. It terminates legacy access signaling protocols (like ISDN PRI or analog line signaling) from customer premises equipment or access networks, and translates them into IMS-compliant SIP signaling for communication with the IMS core. The AGCF also controls associated Media Gateways (MGWs) for bearer path interworking between Time-Division Multiplexing (TDM) circuits and IP-based Real-time Transport Protocol (RTP) streams. Another key entity is the PSTN/ISDN Emulation Edge (PEE), which handles interworking with external legacy PSTN/ISDN networks, ensuring seamless call routing between the emulated domain and the traditional network.

The PES subsystem works by establishing a clear separation between the access/legacy domain and the core IMS domain. When a legacy terminal initiates a call, its signaling is captured by the AGCF. The AGCF authenticates the user (often using data from the HSS), translates the legacy signaling into a SIP INVITE request, and forwards it into the IMS core. The IMS core then processes the session request according to standard IMS procedures, potentially involving Application Servers (AS) for service logic. For the media path, the AGCF instructs a Media Gateway to convert the user's TDM bearer into an IP/RTP stream. This architecture ensures that all service logic and session control resides in the IP/IMS domain, while the user's access remains unchanged. PES is a cornerstone of the "PSTN/ISDN migration" use case defined by 3GPP, providing a standardized, vendor-interoperable path for network transformation.

Purpose & Motivation

PES was created to address the fundamental challenge of migrating from aging, expensive-to-maintain circuit-switched telephony networks (PSTN/ISDN) to modern, efficient, and service-rich all-IP networks based on IMS. The traditional PSTN, while reliable, is based on dedicated TDM infrastructure that is costly to operate, scales poorly for data services, and limits the introduction of new multimedia features. The industry's move towards converged fixed and mobile networks demanded a unified IP-based control plane, which IMS provides. However, a "big bang" replacement of all customer premises equipment and access lines was economically and practically impossible for operators.

PES solves this by enabling a gradual, phased migration strategy. It allows network operators to introduce an IMS core and begin transitioning services without forcing an immediate upgrade of every end-user's terminal or access line. Customers can continue using their existing phones and services, while the operator benefits from consolidating network elements onto a single IP infrastructure, reducing operational costs, and freeing up valuable space and power previously consumed by legacy switches. PES provides a clear boundary where the legacy access network can be incrementally modernized (e.g., replaced by fiber or wireless access) while the service layer in IMS remains constant.

Furthermore, PES standardizes this migration path, preventing vendor lock-in and ensuring interoperability between different vendors' AGCFs, Media Gateways, and IMS cores. Before PES, operators faced proprietary or non-standard solutions for VoIP gateways, which complicated large-scale deployments and multi-vendor integration. By defining a complete subsystem within the 3GPP standards, PES gave operators a future-proof blueprint for network transformation, ensuring service continuity, regulatory compliance (especially for emergency services), and a clear evolution path towards full native IMS services (like VoLTE and VoNR) in the long term.

Key Features

• Emulation of complete PSTN/ISDN service feature set over IMS
• Interworking of legacy access signaling (e.g., ISDN, analog) to IMS/SIP
• Control of Media Gateways for TDM-to-IP bearer conversion
• Seamless interconnection with external legacy PSTN/ISDN networks
• Re-use of core IMS functions (CSCF, HSS) for control and subscriber data
• Support for regulatory services like emergency calling and lawful interception

Evolution Across Releases

Defining Specifications