Description
The Service Centralization and Continuity Application Server (SCC AS) is a critical functional entity within the IP Multimedia Subsystem (IMS) architecture, defined by 3GPP to manage service continuity and centralization for multimedia sessions. It operates as an application server that anchors IMS sessions, particularly those involving voice or video communications, to facilitate seamless handovers between different access technologies, such as transitioning a voice call from LTE (packet-switched) to GSM or UMTS (circuit-switched). The SCC AS achieves this by implementing the IMS Service Continuity procedures, which involve session renegotiation and media path updates to maintain active sessions without interruption. Architecturally, it interfaces with other IMS nodes like the Serving-Call Session Control Function (S-CSCF) for session control and the Home Subscriber Server (HSS) for subscriber data, ensuring it can apply appropriate service logic based on user profiles and network conditions.
In operation, the SCC AS employs mechanisms like Single Radio Voice Call Continuity (SRVCC) and enhanced SRVCC (eSRVCC) to handle mobility events. When a UE moves from a VoLTE-capable LTE coverage area to a legacy 2G/3G network, the SCC AS coordinates with the Mobility Management Entity (MME) and the MSC Server to transfer the session anchor point and update the media path, minimizing service disruption. It manages the Access Transfer Control Function (ATCF) and Access Transfer Gateway (ATGW) in eSRVCC scenarios, which localize the media anchor to reduce handover latency. The SCC AS also supports mid-call features, such as adding or removing media components during a session, by leveraging IMS session control protocols like SIP.
Key components within the SCC AS include logic for session anchoring, continuity management, and interworking with circuit-switched networks via the IMS Centralized Services (ICS) framework. Its role extends beyond handovers to include service centralization, where it acts as a centralized point for applying service logic, ensuring consistent user experiences across multiple devices and access types. This is particularly important for enabling features like call forwarding, simultaneous ringing, and multimedia telephony. By decoupling service execution from access technology, the SCC AS simplifies network evolution and supports the convergence of fixed and mobile services.
Purpose & Motivation
The SCC AS was introduced in 3GPP Release 8 to address the challenge of maintaining service continuity for IMS-based multimedia services as networks evolved towards all-IP architectures like LTE. Prior to its development, voice services were predominantly circuit-switched, and handovers between packet-switched and circuit-switched domains were not standardized, leading to dropped calls during mobility events. The SCC AS solves this by providing a standardized mechanism to anchor IMS sessions, enabling seamless transitions and ensuring that users do not experience interruptions when moving between LTE and legacy networks.
Its creation was motivated by the industry's shift to VoLTE and the need to support rich communication services while leveraging existing 2G/3G infrastructure for coverage. By centralizing service logic, the SCC AS also reduces complexity in the network, allowing operators to deploy new features uniformly across different access technologies. This addresses limitations of earlier approaches, where service continuity was handled in a fragmented manner, often requiring proprietary solutions that hindered interoperability and scalability.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (28 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the SCC AS function was enhanced to support SRVCC for IMS emergency sessions initiated during the early dialogue, alerting, or pre-alerting phases. Furthermore, clarifications were made regarding the usage of the pre-alerting SRVCC feature tag, and the MSC server's role was refined for ICS, including its use of the P-Called-Party-ID for the P-Asserted-Identity.
- PS to CS SRVCC for IMS emergency session in early dialogue phase TS 23.237CR0504
- PS to CS SRVCC for emergency session in alerting or pre-alerting phase TS 24.237CR1271
- PANI addition by MSC server enhanced for ICS TS 24.237CR1278
- MSC server using P-Called-Party-ID for PAI TS 24.292CR0352
- Clalrification on usage of pre Alerting SRVCC feature tag TS 24.237CR1280
In Release 16, the SCC AS function was enhanced to support 5G SRVCC procedures, enabling service continuity for IMS sessions from 5GS to EPS or to GERAN/UTRAN. This release also introduced support for SRVCC in deployments without IMS-level roaming interfaces and for emergency calls transferred to EPS. Furthermore, it specified that the SCC AS can retrieve SRVCC subscription data for a UE from the HSS using Service-Based Architecture (SBA).
- Introduction of support of multiple EATF instances in Emergency SRVCC procedures TS 23.237CR0509
- SRVCC in deployments without IMS-level roaming interfaces TS 24.237CR1285
- Introduction of 5G SRVCC TS 24.237CR1292
- Further introduce support for 5G-SRVCC TS 24.237CR1296
- Completing introducing of 5G SRVCC TS 23.237CR0510
- Corrections on ATCF TS 23.237CR0514
+ 4 more changes
In Release 17, the SCC AS function was updated to explicitly include applicability for 5G-SRVCC procedures, building upon the existing SRVCC architecture. The enhancements also addressed specific signaling scenarios, such as the handling of an "IMS session re-establishment required indicator" for SRVCC handover cancelled via NG-RAN. Furthermore, the release introduced updates to the applicability of test procedures for FR2 radio frequency phase continuity.
- SRVCC handover cancelled, IMS session re-establishment required indicator via NG-RAN TS 24.237CR1303
- g.3gpp.srvcc-alerting media feature tag missing in flows TS 24.237CR1304
- Applicability for 5G-SRVCC TS 38.522CR0085
- Addition of applicability for FR2 RF phase continuity test TS 38.522CR0265
- Addition of applicability for FR2 RF phase continuity test TS 38.522CR0297
- Update to applicability for FR2 RF phase continuity test TS 38.522CR0334
In Release 18, the SCC AS function was enhanced to support SRVCC for the IMS data channel, extending service continuity mechanisms to data sessions. Furthermore, charging descriptions were updated for the SCC AS application, and the specifications for phase continuity tests saw applicability updates to refine testing procedures. The release also introduced mechanisms for charging session continuity identification specifically during a V-SMF change.
- SRVCC for IMS data channel TS 24.237CR1305
- Rel-18 CR TS 32.255 Enhance Application Charging description TS 32.255CR0469
- Applicability updates for Phase continuity tests TS 38.522CR0362
- Applicability updates for Phase continuity tests TS 38.522CR0404
- Applicability updates for Phase continuity tests TS 38.522CR0441
- Rel-18 CR 32.255 QBC Charging Session Continuity Identification at V-SMF Change TS 32.255CR0466
In Release 19, the SCC AS function was enhanced to support volume-based charging for specific application scenarios. This introduces a new charging principle for Data Channel (DC) application download and usage, expanding the SCC AS's charging capabilities beyond traditional session-based models. The update integrates this charging method into the existing SCC AS architecture, which already handles procedures like PS-CS Access Transfer and collaborative session control.
- Rel-19 CR 32.255 Add volume based charging principle for DC application download and usage TS 32.255CR0575
Explore further
Broader topics and technologies where SCC plays a role.
Defining Specifications
3GPP specifications that define or reference SCC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.237 vj00 | IMS Service Continuity (ISC) Stage 2 | Rel-19 |
| TS 23.292 vj00 | IMS Centralized Services (ICS) Architecture | Rel-19 |
| TS 24.237 vj00 | IMS Service Continuity Protocol Details | Rel-19 |
| TS 24.292 vj00 | IMS Centralized Services (ICS) Protocol | Rel-19 |
| TS 24.294 vj00 | IMS Centralized Services (ICS) I1 Interface Protocol | Rel-19 |
| TS 24.337 vj00 | IMS Inter-UE Transfer Protocol Specification | Rel-19 |
| TS 24.802 vc10 | IMS II-NNI Traversal Scenario Determination Study | Rel-12 |
| TS 26.237 vj00 | IMS for PSS and MBMS Control | Rel-19 |
| TR 26.955 vj00 | Video Codec Analysis for 5G Services | Rel-19 |
| TR 29.949 vj00 | VoLTE IMS Roaming Architecture & Procedures | Rel-19 |
| TS 32.250 vj00 | Circuit Switched Offline Charging | Rel-19 |
| TS 32.255 vk10 | Telecom Management; Charging for 5G Data Connectivity | Rel-20 |
| TS 36.101 vj30 | LTE UE Radio Transmission & Reception Requirements | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.306 vj00 | E-UTRA UE Radio Access Capability Parameters | Rel-19 |
| TS 36.714 | 3GPP TR 36.714 | Rel-8 |
| TS 36.715 | 3GPP TR 36.715 | Rel-8 |
| TS 36.716 | 3GPP TR 36.716 | Rel-8 |
| TS 36.833 | 3GPP TR 36.833 | Rel-8 |
| TS 37.571 vj00 | UE Conformance for Positioning | Rel-19 |
| TR 37.901 vf10 | UE Application Layer Data Throughput Performance | Rel-15 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.522 vj11 | UE Conformance Test Applicability Statement | Rel-19 |
| TS 38.716 | 3GPP TR 38.716 | Rel-8 |
| TS 38.717 | 3GPP TR 38.717 | Rel-8 |