Description
Digital Subscriber Signalling System No. 2 (DSS2) is a network-layer signalling protocol defined by 3GPP for use within the IP Multimedia Subsystem (IMS) and Next Generation Network (NGN) architectures. It operates over the Nc interface, which is the reference point between Call Session Control Functions (CSCFs) within the IMS core. DSS2 is based on the ITU-T Q.2931 standard, originally developed for Broadband Integrated Services Digital Network (B-ISDN), and is adapted by 3GPP to provide call and bearer control for packet-switched multimedia sessions. The protocol utilizes a message-oriented structure to exchange information elements between network entities, facilitating the establishment, modification, and teardown of connections with defined Quality of Service (QoS) parameters.
Architecturally, DSS2 functions within the control plane of the core network, separate from the user plane that carries the actual media traffic. It is a key component in the Media Gateway Control Function (MGCF) and CSCF interactions, particularly for interworking with legacy circuit-switched networks or within certain IMS call control scenarios. The protocol employs a reliable transport mechanism, typically over IP using Stream Control Transmission Protocol (SCTP) or MTP3, to ensure the delivery of signalling messages. Its operation involves the exchange of specific messages like SETUP, CALL PROCEEDING, ALERTING, CONNECT, and RELEASE, each containing mandatory and optional information elements that describe the call characteristics, bearer capabilities, and addressing.
The role of DSS2 in the 3GPP network is primarily as a legacy interworking and internal control protocol within the IMS architecture. It enables the CSCF to control media gateways and establish bearer paths for multimedia sessions. While newer protocols like the Session Initiation Protocol (SIP) have become dominant for IMS call control, DSS2 remains specified for specific interfaces and scenarios, ensuring backward compatibility and support for standardized interworking functions. Its detailed procedures cover error handling, state management, and negotiation of connection parameters, making it a robust, if specialized, component of the 3GPP signalling suite.
Purpose & Motivation
DSS2 was introduced into 3GPP specifications to provide a standardized, robust signalling mechanism for call and bearer control within the evolving packet-switched core network, particularly as the IMS architecture was being defined. It addressed the need for a proven, connection-oriented signalling protocol that could manage the establishment of guaranteed QoS bearers for multimedia services, a requirement that early packet protocols lacked. The motivation stemmed from the industry's experience with B-ISDN and the desire to leverage existing, well-specified signalling technology (ITU-T Q.2931) for the new IP-based networks, ensuring reliability and feature parity with legacy systems.
The creation of DSS2 within 3GPP was driven by the necessity for interworking between the new IMS domain and existing circuit-switched networks, as well as for internal control within certain IMS nodes. Previous approaches in pure IP networks often relied on simpler, best-effort signalling or were not designed for complex bearer negotiation with specific traffic parameters. DSS2 provided a formalized method to request a connection with explicit bandwidth, delay, and jitter characteristics, which was essential for voice and video services requiring guaranteed performance. Its adoption offered a migration path and a stable control plane technology during the transition from circuit-switched to all-IP core networks.
Historically, DSS2 represents a bridge between the telecom world of B-ISDN and the new world of 3GPP packet cores. While its usage has been superseded in many areas by more native IP protocols like SIP, its specification ensured that early IMS deployments could incorporate rigorous connection control and interwork with equipment using international telecommunication standards. It solved the problem of how to apply traditional telecom-grade signalling reliability and statefulness to packet-based bearer establishment.
Key Features
- Based on ITU-T Q.2931 standard for B-ISDN signalling
- Provides call control and bearer control functions for packet connections
- Operates over the Nc reference point between CSCFs in IMS
- Uses message-oriented signalling with defined information elements
- Supports negotiation of QoS parameters for the bearer channel
- Includes procedures for connection establishment, modification, and release
Evolution Across Releases
Initially standardized in 3GPP TS 29.414 for the Nc interface. DSS2 was adopted as the signalling protocol for the Nc interface reference point, defining the procedures for Bearer Independent Call Control (BICC) in the context of the IMS architecture. It provided the initial framework for call control between CSCF entities using this legacy telephony signalling adaptation.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.414 | 3GPP TS 29.414 |