Description
Digital Subscriber Signalling System No. 1 (DSS1), standardized by ITU-T in the Q.930-Q.939 series and adopted by 3GPP, is a layer 3 signalling protocol for the ISDN D-channel. It operates between the Terminal Equipment (TE) or Terminal Adapter (TA) at the subscriber premises and the ISDN exchange (Local Exchange - LE) in the network. DSS1 is a peer-to-peer protocol that uses the reliable data link layer service provided by LAPD (Link Access Procedure on the D-channel, Q.921) on the User-Network Interface (UNI).
Architecturally, DSS1 is part of the control plane for circuit-switched services. Its primary function is call control, managing the setup, maintenance, and teardown of circuit-switched connections (Bearer Channels - B-channels). It uses a message-oriented structure with key messages like SETUP, CALL PROCEEDING, ALERTING, CONNECT, and DISCONNECT. Beyond basic call control, DSS1 supports a rich set of supplementary services through facility information elements and specific messages. These include Call Forwarding, Call Waiting, Calling Line Identification Presentation/Restriction (CLIP/CLIR), and User-to-User Signalling (UUS).
In 3GPP contexts, DSS1's role has been primarily in the interworking between mobile core networks (like the MSC) and fixed ISDN networks. While 3GPP's native mobile signalling is based on MAP (for core) and RRC/NAS (for access), DSS1 is referenced for scenarios where a GSM/UMTS MSC needs to interface with a PSTN/ISDN using ISDN User Part (ISUP) or directly emulate ISDN terminal behaviour. Its inclusion in 3GPP specs (e.g., on charging and interworking) ensures compatibility and smooth interoperation with the vast installed base of fixed ISDN infrastructure during the transition to all-IP networks.
Purpose & Motivation
DSS1 was created by the ITU to provide a standardized, feature-rich signalling system for ISDN, the digital successor to the analog PSTN. Its purpose was to enable not just basic voice calls, but a wide array of digital services (data, video, text) over the same subscriber line, with in-band signalling moved to a separate, dedicated D-channel for greater efficiency and flexibility.
Within 3GPP, DSS1 is referenced to address the problem of interworking between emerging digital cellular networks and the existing fixed digital network infrastructure (ISDN). Mobile networks needed to appear as, or connect to, ISDN subscribers and exchanges to provide seamless national and international roaming and call routing. Understanding and supporting DSS1 allowed 3GPP systems to offer compatible supplementary services (like caller ID) and to utilize standardized procedures for call establishment across network boundaries. It represents a crucial piece of legacy protocol support that ensured backward compatibility during the multi-decade evolution from circuit-switched to packet-switched all-IP cores (IMS).
Key Features
- Call control procedures for circuit-switched B-channel establishment and release
- Support for a comprehensive set of supplementary services via information elements
- Operates over the reliable LAPD data link layer on the ISDN D-channel
- Structured message set for state-driven call control (Q.931)
- Capability for User-to-User Signalling (UUS) for limited data transfer
- Standardized interface (UNI) between user equipment and the ISDN exchange
Evolution Across Releases
Referenced in 3GPP specifications primarily for interworking and charging purposes. Its inclusion acknowledged the need for GSM/UMTS networks to interface with and emulate behaviour of fixed ISDN networks. The initial focus was on ensuring MSC could understand and map DSS1/ISUP signalling for call routing and supplementary service support towards the PSTN/ISDN.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |
| TS 29.007 | 3GPP TS 29.007 |
| TS 29.078 | 3GPP TS 29.078 |
| TS 32.101 | 3GPP TR 32.101 |
| TS 32.102 | 3GPP TR 32.102 |