Description
The InterWorking Function (IWF) is a conceptual and functional entity within 3GPP architectures whose primary role is to ensure interoperability between disparate network systems. It is not a single, monolithic node but a logical function that can be implemented in various physical network elements depending on the specific interworking scenario. The IWF performs necessary adaptations, including protocol conversion, message mapping, signal translation, and media transcoding, to bridge technological gaps. It essentially hides the differences between the interconnected networks, allowing them to exchange information and deliver services as if they were part of a homogeneous system.
Architecturally, an IWF sits at the boundary between two network domains. For example, in early circuit-switched core networks, an IWF was used to interwork between GSM's MAP signaling and ISDN's DSS1 signaling. In the context of IP Multimedia Subsystem (IMS), an IWF can facilitate interworking between SIP-based IMS networks and legacy circuit-switched telephony networks, handling the conversion between SIP messages and ISUP/BICC signaling protocols. Another key instance is the Interworking IMS (IW IMS) function defined for service continuity between IMS and non-IMS domains. The IWF contains the necessary application logic and state machines to interpret incoming messages from one domain, map parameters and information elements to their equivalents in the target domain, and generate the corresponding outgoing messages.
From an implementation perspective, the IWF function may be integrated into existing nodes like a Media Gateway (MGW) for media plane interworking or a Media Gateway Control Function (MGCF) for signaling interworking in IMS. It manages critical tasks such as address translation, codec negotiation and adaptation, bearer establishment coordination, and supplementary service mapping. Its operation is defined in numerous 3GPP specifications covering areas like core network signaling, IMS, emergency services, and messaging. The IWF is a fundamental enabler for service continuity, roaming, and the phased migration from legacy networks to new 3GPP systems, ensuring that subscribers can communicate and access services across technological generations.
Purpose & Motivation
The IWF exists to solve the fundamental problem of heterogeneity in telecommunications. Networks evolve through generations (2G, 3G, 4G, 5G) and coexist with other fixed and wireless technologies. Without interworking functions, these networks would be isolated islands, unable to communicate or share services. The IWF was motivated by the need for backward compatibility during network transitions and for enabling seamless service delivery across multi-vendor, multi-technology environments.
Historically, one of the first major needs for an IWF arose with the introduction of GSM, which required interconnection with the existing global PSTN/ISDN fixed networks. This required translating between GSM-specific MAP signaling and the ISUP signaling used in fixed networks. As networks evolved to packet-switched cores and IMS, new interworking challenges emerged, such as connecting SIP-based VoIP services to the legacy circuit-switched voice network. The IWF addresses the limitations of previous siloed approaches by providing a standardized point of adaptation, defined in 3GPP specs, which allows for predictable and interoperable connectivity.
Furthermore, the IWF concept enables convergence and service innovation. It allows new network capabilities (like rich communication services in IMS) to be made available to users on legacy networks, and vice-versa. It is crucial for emergency calls, lawful interception, and roaming scenarios where a user on one network type must access services in another. By abstracting the complexity of protocol differences, the IWF reduces integration costs for operators and ensures a consistent user experience, which was a key commercial and technical driver for its pervasive use across 3GPP standards.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (85 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the InterWorking Function (IWF) saw enhancements for group call handling during imminent peril situations and fixes for Short Data Service (SDS). It also introduced new key management procedures for inter-system interconnection (InterSD) and defined the mapping of extended AccessRestrictionData within the IWF. These updates improved interworking reliability and security between 3GPP systems and other networks like WLANs.
- Interconnection, Interworking media and signaling TS 33.180CR0045
- Interworking SeGy clarification TS 33.180CR0048
- IWF actions on Imminent peril group calls TS 23.283CR0008
- IWF SDS fix TS 23.283CR0017
- IWF actions on Imminent peril group calls TS 23.783CR0008
- IWF SDS fix TS 23.783CR0017
+ 3 more changes
In Release 16, the IWF (InterWorking Function) introduced new capabilities for managing group communications with Land Mobile Radio (LMR) systems, including support for preconfigured groups, temporary group calls, and preconfigured broadcast group calls. Enhancements also provided more detailed location information within call setup and floor control messages, such as adding talker location to group call requests. Furthermore, specific procedures were defined for user regrouping and for handling implicit floor requests in group call and imminent peril scenarios.
- Analogue FM interworking TS 23.283CR0030
- Functional Alias management for interworking between MC service system and LMR system TS 23.283CR0035
- IWF preconfigured groups TS 23.283CR0038
- IWF add user to temporary pre-configured group regroup TS 23.283CR0039
- IWF temporary group calls TS 23.283CR0041
- IWF user regroup with pre-configured group TS 23.283CR0042
+ 37 more changes
In Release 17, the IWF (InterWorking Function) saw specific enhancements for interworking with GSM-R and Land Mobile Radio (LMR) systems. New capabilities were introduced, including first-to-answer functionality for GSM-R interworking and enhancements for interworking MCPTT group calls and MCData SDS with GSM-R SMS. Furthermore, the release added features for functional alias restoration and defined procedures for private call and floor control interworking between MC service systems and LMR systems.
- Functional alias for private call interworking between an MC service system and an LMR system TS 23.283CR0036
- Functional alias for floor control interworking between MC service system and LMR system TS 23.283CR0037
- IWF functional alias restoration TS 23.283CR0046
- Add first-to-answer for interworking with GSM-R TS 23.283CR0048
- Add enhancements for interworking of MCData SDS with GSM-R SMS TS 23.283CR0050
- Add enhancements for interworking of MCPTT group calls with GSM-R TS 23.283CR0049
+ 9 more changes
In Release 18, the InterWorking Function (IWF) saw new standardization for location-based services through the introduction of LMR-3GPP Location Interworking. This enhancement specifically defines interworking procedures between 3GPP systems and other networks, such as WLANs, for location management. It builds upon the existing framework for I-WLAN and system area interworking to enable public land mobile location services.
- LMR-3GPP Location Interworking TS 23.283CR0066
In Release 19, the IWF function was enhanced with new capabilities for interworking between different service domains, specifically introducing support for IMS Data Channel (DC) interworking with MTSI UEs and detailed procedures for ad hoc group call interworking. The release also included updates and corrections to these DC interworking procedures via the DC Application Server and introduced formal documentation for interworking with systems like MCX-LMR and GSM-R. These refinements focused on the signaling procedures and the participating role of the IWF in managing these interworking scenarios.
- Support of IMS data channel interworking between DCMTSI UE and MTSI UE TS 23.228CR1418
- KI#3: DC interworking with MTSI UE TS 23.228CR1479
- Interworking ad hoc group call TS 23.283CR0076
- Interworking of private call with GSM-R TS 23.283CR0077
- Interworking ad hoc group call based on criteria TS 23.283CR0078
- Introduce annex about MCX-LMR interworking TS 23.283CR0080
+ 8 more changes
In Release 20, the IWF (InterWorking Function) introduced new interworking support for ad hoc group emergency alerts, specifically for scenarios initiated by an MCPTT user or by Land Mobile Radio (LMR) systems. This enhancement defined the necessary information flows and procedures to enable these critical alerting mechanisms across the interworking architecture. The updates focused on the functional coordination between the IWF and the involved systems to manage the air interface user rate and related communication paths for these emergency services.
Explore further
Broader topics and technologies where IWF plays a role.
Defining Specifications
3GPP specifications that define or reference IWF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TR 22.945 v1300 | Fax Services Guidance for GSM/UMTS | Rel-4 |
| TS 23.054 v1300 | Shared Interworking Function (SIWF) Stage 2 | Rel-4 |
| TS 23.146 vj00 | 3G Facsimile Group 3 Technical Realization | Rel-19 |
| TS 23.228 vj50 | IMS Stage-2 Service Description | Rel-19 |
| TS 23.231 vj00 | SIP-I based CS core network stage 2 | Rel-19 |
| TS 23.283 vk00 | Mission Critical Communication Interworking | Rel-20 |
| TS 23.782 vf00 | Interworking between LTE MC and non-LTE MC systems | Rel-15 |
| TR 23.783 vi00 | Technical Report on Mission Critical Services over 5GS | Rel-18 |
| TR 23.910 v1400 | UMTS Circuit Switched Bearer Services Overview | Rel-5 |
| TR 23.976 vj00 | Push Service Requirements Analysis | Rel-19 |
| TS 24.229 vj50 | IMS call control protocol based on SIP and SDP | Rel-19 |
| TS 24.523 vj00 | NGCN-NGN Interconnection Scenarios | Rel-19 |
| TS 24.525 vj00 | Business Trunking Architecture & Requirements | Rel-19 |
| TS 24.883 vg00 | MCPTT Interworking with LMR Systems | Rel-16 |
| TS 25.424 vj00 | UTRAN Iur Interface Data Transport & Signalling | Rel-19 |
| TR 26.930 vj00 | WebRTC Enhancements for Immersive RTC over 5G | Rel-19 |
| TS 28.702 vj00 | Core Network NRM IRP Information Service | Rel-19 |
| TS 29.305 vj00 | Interworking Functions for EPS-Legacy Systems | Rel-19 |
| TS 29.368 vj00 | Tsp Reference Point Stage 3 Specification | Rel-19 |
| TS 29.379 vj00 | MCPTT call control interworking with LMR systems | Rel-19 |
| TS 29.421 v810 | IMS Interworking with External IP Networks | Rel-8 |
| TS 29.805 v800 | IWF for MAP-Diameter Interworking | Rel-8 |
| TS 32.632 vb00 | Core Network Resources IRP: Network Resource Model | Rel-11 |
| TS 32.732 vb00 | IMS Network Resource Model IRP: Information Service | Rel-11 |
| TS 33.180 vk00 | Security of Mission Critical (MC) Service | Rel-20 |