Integrated Services Digital Network

Description

Integrated Services Digital Network (ISDN) is a circuit-switched telephone network system that provides digital transmission of voice and data over ordinary copper telephone lines, offering better quality and higher data rates than the traditional analog system. Within the 3GPP architecture, ISDN is not a 3GPP-defined technology but is a crucial legacy system that early GSM and UMTS networks were designed to interconnect with. 3GPP specifications extensively reference ISDN protocols, numbering, and service concepts to ensure seamless interworking between mobile networks and the fixed ISDN infrastructure. This interworking is primarily handled in the core network, specifically through the circuit-switched domain, where mobile switching centers (MSCs) interface with ISDN exchanges using signaling protocols like ISDN User Part (ISUP).

Architecturally, ISDN defines two main interface types: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides two 64 kbps bearer (B) channels for voice or data and one 16 kbps delta (D) channel for signaling, suitable for small offices or residential use. PRI, typically used for larger connections, offers 23 B channels (30 in Europe) and one D channel at 64 kbps, aggregating to 1.544 Mbps (T1) or 2.048 Mbps (E1). In the context of 3GPP, the mobile core network emulates these interfaces. For example, the MSC acts as an ISDN exchange for mobile subscribers, providing ISDN-like services such as circuit-switched voice calls, facsimile, and supplementary services like call forwarding and caller ID. The signaling between the MSC and the external ISDN network uses ISUP, which is part of the Signaling System No. 7 (SS7) suite, to set up, manage, and tear down calls.

How it works in interoperation: When a mobile subscriber makes a call to a fixed ISDN phone, the MSC receives the call request via the mobile air interface (e.g., through BSSAP signaling from the base station). The MSC then translates this request into an ISUP Initial Address Message (IAM) containing the called party number and other parameters, which it sends over the SS7 network to the ISDN exchange. The ISDN exchange routes the call to the destination. Similarly, for incoming calls from ISDN to a mobile subscriber, the ISDN exchange sends an ISUP message to the MSC, which pages the mobile station and establishes a radio channel. Throughout the call, voice is carried over 64 kbps circuits (PCM encoded) between the MSC and the ISDN network. This circuit-switched paradigm is mirrored in the mobile network's circuit-switched core.

Key components in the 3GPP-ISDN interworking include the MSC, the Gateway MSC (GMSC) for interfacing with external networks, the Home Location Register (HLR) for subscriber data, and the SS7 signaling infrastructure with ISUP. ISDN numbering, defined by ITU-T E.164, is also used as the basis for mobile subscriber numbers (MSISDN). Additionally, ISDN supplementary services, standardized in Q.931 (layer 3) and related protocols, are supported in 3GPP through Customized Applications for Mobile networks Enhanced Logic (CAMEL) and other mechanisms to provide features like call waiting, hold, and barring. As 3GPP evolved, the importance of direct ISDN interworking diminished with the rise of all-IP networks (IMS and VoLTE), but it remains critical for backward compatibility, especially in regions where ISDN is still in use, and for ensuring service continuity during the transition to packet-switched networks.

Purpose & Motivation

ISDN was developed to address the limitations of the analog telephone network, which was inefficient for data transmission and limited to voice services. Its purpose was to create a fully digital network that could integrate voice, data, text, and video over the same lines, offering higher quality, faster setup times, and greater bandwidth. For the telecommunications industry, ISDN represented the first step towards digital convergence. In the context of 3GPP and mobile networks, the purpose of referencing ISDN was to ensure that new digital mobile systems (starting with GSM) could seamlessly interconnect with the existing fixed-line digital infrastructure. This was essential for providing ubiquitous national and international calling, as the fixed network was predominantly ISDN in many developed countries during the 1990s and early 2000s.

The problems ISDN solved included the poor quality and low speed of analog modems for data calls, the inability to use the same line for multiple simultaneous services, and the slow call setup times. By providing end-to-end digital connectivity, ISDN enabled reliable 64 kbps data channels, which were revolutionary for dial-up internet, video conferencing, and fax. For 3GPP, the challenge was to design a mobile system that could offer equivalent services and quality. By adopting ISDN principles—such as 64 kbps bearer channels, E.164 numbering, and ISUP signaling—GSM could present itself as just another access network to the ISDN core, simplifying interworking and allowing mobile users to access the same supplementary services as fixed ISDN subscribers.

Historically, ISDN served as the blueprint for the circuit-switched core in 2G and 3G networks. However, its limitations, such as the inflexibility of circuit switching for bursty data traffic and the complexity of managing separate networks for voice and data, motivated the move towards packet-switched all-IP architectures in 3GPP Release 4 and beyond with the IP Multimedia Subsystem (IMS). Despite this evolution, ISDN interworking remains specified to support legacy connectivity, especially for international roaming and in areas where ISDN has not been fully phased out. The extensive list of 3GPP specs referencing ISDN, spanning from Release 99 to Release 19, underscores its enduring role in ensuring backward compatibility and a smooth migration path from circuit-switched to packet-switched services in mobile telecommunications.