ETSI TS 123 228 V19.6.0 (2026-03)

Digital cellular telecommunications system (Phase 2+) (GSM); Universal Mobile Telecommunications System (UMTS); LTE; 5G; IP Multimedia Subsystem (IMS); Stage 2 (3GPP TS 23.228 version 19.6.0 Release 19)

RTS/TSGS-0223228vj60

General Information

• Status: Not Published
• Technical Committee: 3GPP SA 2 - Architecture
• Current Stage: 8 - Draft receipt by ETSI Secretariat
• Completion Date: 13-Mar-2026

Standards Content (Sample)

TECHNICAL SPECIFICATION
Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS);
LTE;
5G;
IP Multimedia Subsystem (IMS);
Stage 2
(3GPP TS 23.228 version 19.6.0 Release 19)

3GPP TS 23.228 version 19.6.0 Release 19 1 ETSI TS 123 228 V19.6.0 (2026-03)

Reference
RTS/TSGS-0223228vj60
Keywords
5G,GSM,LTE,UMTS
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ETSI
3GPP TS 23.228 version 19.6.0 Release 19 2 ETSI TS 123 228 V19.6.0 (2026-03)
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Legal Notice
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ETSI
3GPP TS 23.228 version 19.6.0 Release 19 3 ETSI TS 123 228 V19.6.0 (2026-03)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 20
1 Scope . 21
2 References . 21
3 Definitions, symbols and abbreviations . 25
3.1 Definitions . 25
3.2 Symbols . 28
3.3 Abbreviations . 29
4 IP multimedia subsystem concepts . 30
4.0 General . 30
4.1 Relationship to CS domain and the IP-Connectivity Access Network . 32
4.2 IMS services concepts . 32
4.2.1 Home-network based services . 32
4.2.1.1 Support of CAMEL or IN . 32
4.2.1.2 Support of OSA . 32
4.2.1.3 Dynamic services interactions handling . 32
4.2.1.3.1 Service information exchanged between Application Servers . 32
4.2.1.3.2 Handling by the Application Server . 32
4.2.1.3.3 Deletion of services interaction information . 33
4.2.2 Support of numbers in non-international format in the IMS . 33
4.2.3 Support of roaming users . 34
4.2.4 IP multimedia Subsystem Service Control Interface (ISC) . 35
4.2.4a HSS to service platform Interface . 38
4.2.4b S-CSCF Service Control Model . 39
4.2.4c I-CSCF to AS reference point (Ma) . 40
4.2.5 The QoS requirements for an IM CN subsystem session. 41
4.2.6 QoS Requirements for IM CN subsystem signalling . 42
4.2.7 Support of SIP forking . 43
4.2.7.1 SIP Forking . 43
4.2.7.2 Forking within and outside the IM CN Subsystem . 43
4.2.7.3 Support for forked requests . 44
4.3 Naming and addressing concepts . 44
4.3.1 Address management . 44
4.3.2 Void . 44
4.3.3 Identification of users . 44
4.3.3.0 General . 44
4.3.3.1 Private User Identities . 44
4.3.3.2 Public User Identities . 45
4.3.3.2a Globally Routable User Agent URI (GRUU) . 46
4.3.3.2a.1 Architecture Requirements . 46
4.3.3.2b Wildcarded Public User Identity . 47
4.3.3.3 Routing of SIP signalling within the IP multimedia subsystem . 47
4.3.3.3a Handling of dialled number formats . 48
4.3.3.3b Termination of session with the TEL URI format Public User Identity . 48
4.3.3.4 Relationship of Private and Public User Identities . 48
4.3.3.5 Relationship of Public User Identities, GRUUs and UEs . 49
4.3.4 Identification of network nodes . 50
4.3.5 E.164 address to SIP URI resolution in an IM CN subsystem . 50
4.3.5.1 ENUM/DNS translation mechanism . 50
4.3.5.2 Handling of Tel URIs . 50
4.3.5.3 Handling of SIP URIs representing a telephone number . 51
4.3.6 Public Service Identities . 51
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4.4 Signalling concepts. 51
4.5 Mobility related concepts . 52
4.6 Roles of Session Control Functions . 53
4.6.0 General . 53
4.6.1 Proxy-CSCF . 53
4.6.2 Interrogating-CSCF . 54
4.6.2.0 General . 54
4.6.2.1 Void. 55
4.6.3 Serving-CSCF . 55
4.6.4 Breakout Gateway Control Function . 57
4.6.5 Void . 58
4.7 Multimedia Resource Function . 58
4.7a Media Resource Broker . 59
4.8 Security Concepts . 59
4.9 Charging Concepts . 59
4.10 IMS group management concepts . 59
4.10.0 General . 59
4.10.1 IMS group administration . 59
4.10.2 Group identifiers . 60
4.11 Relationship to 3GPP Generic User Profile (GUP) . 60
4.12 Network Address Translation traversal in access network . 60
4.13 Identification of IMS communication Services . 60
4.13.1 General . 60
4.13.2 Identification of IMS communication Services . 60
4.13.3 Identification of IMS applications . 62
4.14 Border Control concepts . 63
4.15 IMS in transit network scenarios . 64
4.15.1 General concepts . 64
4.15.2 IMS transit network configurations . 64
4.15.3 Providing IMS application services in transit network scenarios . 65
4.15a Roaming Architecture for Voice over IMS with Local Breakout . 65
4.15b Roaming Architecture for Voice over IMS with home routed traffic . 66
4.16 Support of multimedia telephony . 66
4.16.1 Telephony Application Server . 66
4.16.2 Identification of multimedia telephony . 67
4.16.3 Session setup principles . 67
4.17 Support of short message service . 67
4.17.1 IP Short Message Gateway (IP-SM-GW) . 67
4.18 Support of Number portability . 67
4.18.1 Number portability . 67
4.19 Support of Preferred Circuit Carrier Access and Per Call Circuit Carrier Selection . 68
4.19.1 Preferred Circuit Carrier Access and Per Call Circuit Carrier Selection . 68
4.20 Support of IMS Service Centralization and Continuity . 68
4.21 Support of Overlap Signalling . 69
4.22 Support of Explicit Congestion Notification (ECN) . 69
4.22.1 General . 69
4.22.2 CS GERAN/UTRAN Interworking at MGCF/IM-MGW . 69
4.22.3 Interworking with non-ECN IP network and/or terminal at IBCF/TrGW . 70
4.22.4 Interworking with non-3GPP ECN IP terminal at IBCF/TrGW . 71
4.22.5 ECN support at IMS-ALG/IMS-AGW . 71
4.22.6 ECN support at MRFC/MRFP . 71
4.22.7 CS GERAN/UTRAN Interworking at the MSC Server enhanced for ICS/MSC Server enhanced for
SRVCC with SIP/CS-MGW . 72
4.23 Support of Load Balancing . 72
4.23.1 General . 72
4.23.2 Registration-based load balancing of S-CSCFs . 72
4.23.3 Registration independent load balancing of Transit Functions . 72
4.24 Support of Restoration Procedures . 72
4.25 Support of Overload Control . 73
4.25.1 General . 73
4.25.2 Next-hop monitoring of overload . 73
4.25.3 Filter based Overload Control . 73
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4.26 Support for Business Trunking . 74
5 IP multimedia subsystem procedures . 74
5.0 General . 74
5.0a Session-unrelated procedures . 74
5.1 CSCF related procedures . 74
5.1.0 Establishing IP-Connectivity Access Network bearer for IM CN Subsystem Related Signalling . 74
5.1.1 Procedures related to Proxy-CSCF discovery . 75
5.1.1.0 General . 75
5.1.1.1 DHCP/DNS procedure for P-CSCF discovery . 75
5.1.1.2 Void. 76
5.1.2 Procedures related to Serving-CSCF assignment . 76
5.1.2.1 Assigning a Serving-CSCF for a user . 76
5.1.2.2 Cancelling the Serving-CSCF assignment . 77
5.1.2.3 Void. 77
5.1.3 Procedures related to Interrogating-CSCF . 77
5.1.4 Procedures related to Proxy-CSCF . 77
5.1.5 Subscription Updating Procedures . 77
5.1.5.0 General . 77
5.1.5.1 Subscription updating information flow . 77
5.2 Application level registration procedures . 78
5.2.0 General . 78
5.2.1 Requirements considered for registration . 78
5.2.1a Implicit Registration . 79
5.2.1a.0 General . 79
5.2.1a.1 Implicit Registration for UE without ISIM or IMC . 81
5.2.2 Registration flows . 81
5.2.2.1 Requirements to consider for registration . 81
5.2.2.2 Assumptions . 81
5.2.2.3 Registration information flow – User not registered . 81
5.2.2.4 Re-Registration information flow – User currently registered . 83
5.2.2.5 Stored information. . 85
5.3 Application level de-registration procedures . 86
5.3.1 Mobile initiated de-registration. 86
5.3.2 Network initiated de-registration . 87
5.3.2.0 General . 87
5.3.2.1 Network Initiated Application (SIP) De-registration, Registration Timeout . 88
5.3.2.2 Network Initiated Application (SIP) De-registration, Administrative. 88
5.3.2.2.0 General . 88
5.3.2.2.1 Network Initiated De-registration by HSS, administrative . 89
5.3.2.2.2 Network Initiated De-registration by Service Platform . 90
5.4 Procedures for IP multi-media sessions . 91
5.4.0 General . 91
5.4.1 Bearer interworking concepts . 91
5.4.2 Interworking with Internet . 91
5.4.2a IP version interworking . 92
5.4.3 Interworking with PSTN . 92
5.4.4 Requirements for IP multi-media session control . 93
5.4.5 Session Path Information . 94
5.4.5.1 Session Path Information during Registration and Session Initiation . 94
5.4.5.2 P-CSCF in the Session Path . 94
5.4.5.3 S-CSCF in the Session Path . 94
5.4.6 End-user preferences and terminal capabilities . 94
5.4.6.0 General . 94
5.4.6.1 Objectives. 94
5.4.6.2 End-user expectations . 95
5.4.6.3 Mechanism for bearer establishment . 95
5.4.6.4 Session progress indication to the originating UE . 97
5.4.7 Interaction between QoS and session signalling . 97
5.4.7.0 General . 97
5.4.7.1 Authorize QoS Resources . 98
5.4.7.1a Resource Reservation with Policy and Charging Control . 98
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5.4.7.2 Enabling of Media Flows . 99
5.4.7.3 Disabling of Media Flows . 99
5.4.7.4 Revoke Authorization for IP-Connectivity Access Network and IP Resources . 99
5.4.7.5 Indication of IP-Connectivity Access Network bearer release . 99
5.4.7.6 Authorization of IP-Connectivity Access Network bearer modification. 99
5.4.7.7 Indication of IP-Connectivity Access Network bearer modification . 100
5.4.7.8 Sharing of Resources for Network Detected Concurrent Sessions . 100
5.4.7.8.1 Network Detected Concurrent Sessions . 100
5.4.7.8.2 Initiating Resource Sharing for Network Detected Concurrent Sessions . 100
5.4.7.8.3 Void . 101
5.4.7.9 Priority sharing for concurrent sessions . 101
5.4.8 QoS-Assured Preconditions . 101
5.4.9 Event and information distribution . 102
5.4.9.0 General . 102
5.4.9.1 Subscription to event notifications . 103
5.4.10 Void . 105
5.4.11 Signalling Transport Interworking . 105
5.4.12 Configuration and Routing principles for Public Service Identities . 105
5.4.12.0 General . 105
5.4.12.1 PSIs on the originating side . 105
5.4.12.2 PSIs on the terminating side . 105
5.4.12.3 Subdomain based PSIs . 106
5.4.12.4 PSI configuration in the HSS . 106
5.4.12.5 Requests originated by the AS hosting the PSI . 106
5.4.13 Transcoding concepts . 107
5.4a Overview of session flow procedures . 107
5.4a.1 End-to-End session flow procedures . 107
5.4a.2 Transit network session flow procedures . 110
5.5 Serving-CSCF/MGCF to serving-CSCF/MGCF procedures . 112
5.5.0 General . 112
5.5.1 (S-S#1) Different network operators performing origination and termination . 112
5.5.2 (S-S#2) Single network operator performing origination and termination . 114
5.5.3 (S-S#3) Session origination with PSTN termination in the same network as the S-CSCF. . 117
5.5.4 (S-S#4) Session origination with PSTN termination in a different network from the S-CSCF. . 119
5.6 Origination procedures . 121
5.6.0 General . 121
5.6.1 (MO#1) Mobile origination, roaming . 121
5.6.2 (MO#2) Mobile origination, home . 124
5.6.3 (PSTN-O) PSTN origination. 126
5.6.4 (NI-O) Non-IMS Origination procedure from an external SIP client . 127
5.6.5 Application Server Origination Procedure . 129
5.6.5.1 (AS-O) Origination at Application Server . 129
5.6.5.2 Void. 131
5.6.5.3 S-CSCF selection by I-CSCF for AS Originating call procedures . 131
5.7 Termination procedures . 133
5.7.0 General . 133
5.7.1 (MT#1) Mobile termination, roaming . 133
5.7.2 (MT#2) Mobile termination, home . 136
5.7.2a (MT#3) Mobile termination, CS Domain roaming . 138
5.7.3 (PSTN-T) PSTN termination . 138
5.7.4 (NI-T) Non-IMS Termination to an external SIP client . 140
5.7.5 (AS-T#1) PSI based Application Server termination – direct . 142
5.7.6 (AS-T#2) PSI based Application Server termination – indirect. 142
5.7.7 (AS-T#3) PSI based Application Server termination – DNS routing . 143
5.7.8 (AST#4) Termination at Application Server based on service logic . 144
5.7a Procedures for the establishment of sessions without preconditions . 145
5.7a.1 General . 145
5.7a.2 Procedures for the establishment of sessions without preconditions - no resource reservation
required before session becomes active . 147
5.7a.3 Void . 149
5.8 Procedures related to routing information interrogation. 149
5.8.0 General . 149
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5.8.1 User identity to HSS resolution . 149
5.8.2 SLF on register . 150
5.8.3 SLF on UE invite . 151
5.8.4 SLF on AS access to HSS . 152
5.9 Routing of mid-session signalling . 152
5.10 Session release procedures . 153
5.10.0 General . 153
5.10.1 Terminal initiated session release . 153
5.10.2 PSTN initiated session release . 155
5.10.3 Network initiated session release .
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