Description
The IWK-SCEF (Interworking - Service Capability Exposure Function) is a functional entity introduced in 3GPP Release 13 as part of the Cellular Internet of Things (CIoT) enhancements. Its primary role is to facilitate the interconnection between the Service Capability Exposure Function (SCEF) and the Core Network mobility management nodes—specifically the Mobility Management Entity (MME) and the Serving GPRS Support Node (SGSN)—for certain CIoT services. The SCEF itself is the key node for securely exposing network services and capabilities to third-party Application Servers (AS), but direct interfaces between the SCEF and the MME/SGSN were not originally defined for all procedures.
Architecturally, the IWK-SCEF sits logically between the SCEF and the MME/SGSN. It acts as an intermediary or a proxy for the T6a/T6b interface, which is used for Non-IP Data Delivery (NIDD). NIDD is a CIoT optimization that allows small, infrequent data packets from IoT devices to be transmitted efficiently without the overhead of a full IP stack or PDP context. When an Application Server wants to send or receive NIDD data to/from an IoT device, it communicates with the SCEF over the API-based T8 interface. For the actual data transfer and session management with the device, the SCEF needs to interact with the MME (for LTE) or SGSN (for 2G/3G). The IWK-SCEF provides this interconnection point, relaying NIDD-related messages between the SCEF and the MME/SGSN.
How it works involves specific signaling procedures. For example, for mobile-originated NIDD, the device sends data to the MME. The MME forwards this data to the IWK-SCEF over the T6a interface. The IWK-SCEF then routes it to the appropriate SCEF, which delivers it to the destined Application Server. The reverse path is followed for mobile-terminated NIDD. The IWK-SCEF handles the mapping of identifiers, manages NIDD session contexts, and ensures the secure and reliable transfer of the small data packets. It is a critical component in the CIoT EPS architecture for enabling efficient, network-exposed IoT communication, particularly for devices using the Control Plane CIoT EPS optimization where data is carried over NAS signaling.
Purpose & Motivation
IWK-SCEF was created to resolve a specific architectural gap in the initial CIoT and SCEF framework defined in earlier releases. The SCEF was designed to expose network capabilities, but its direct interconnection with the MME and SGSN for the vital Non-IP Data Delivery (NIDD) service was not fully specified, creating an implementation hurdle for efficient IoT data transport.
The problem it addresses is enabling the SCEF-based exposure model for the new, optimized CIoT architectures. IoT devices often use power-efficient, latency-tolerant communication methods like NIDD over the control plane. To make these device services available to external applications via the SCEF's T8 API, a standardized and secure path from the SCEF to the MME/SGSN was necessary. Without the IWK-SCEF, operators would need proprietary integrations or alternative, less optimal data paths for CIoT devices using NIDD, undermining the efficiency gains of the CIoT optimizations.
Its introduction in Release 13 was motivated by the need to complete the end-to-end architecture for CIoT service exposure. It ensured that the benefits of the SCEF—such as secure API-based access, network capability abstraction, and simplified application development—could be fully realized for the massive IoT market segment. By defining the IWK-SCEF, 3GPP provided a clear, standardized interworking point that enabled scalable deployment of NIDD services, supporting the low-cost, long-battery-life use cases that are central to Cellular IoT.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (93 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-13, normative work from Rel-15.
In Release 15, the IWK-SCEF (InterWorking SCEF) was newly introduced as an optional function located in the VPLMN, as shown in architectural figures. Its introduction established new reference points for roaming, specifically the T6ai and T6bi interfaces to the serving MME and SGSN, and the T7 interface to the SCEF in the home network. This enabled the secure exposure of 3GPP network services and capabilities to external applications in roaming scenarios.
- Background Data Transfer Policy Activation via the SCEF TS 23.682CR0263
- Northbound APIs for SCEF - SCS/AS Interworking - Clause 1-3 enhancements TS 23.682CR0271
- Northbound APIs for SCEF - SCS/AS Interworking - Clause 4 enhancements TS 23.682CR0272
- Enabling the Routing of non-IP traffic between the UE and SCEF TS 23.682CR0277
- SCEF Behaviour in the NIDD Configuration and NIDD Authorisation Update Procedures TS 23.682CR0278
- SCEF Behaviour in the Mobile Terminated NIDD Procedure TS 23.682CR0279
+ 31 more changes
In Release 16, the IWK-SCEF saw enhancements including support for the RACS (Radio Capability Signaling) architecture for provisioning the UCMF, updates and corrections to Non-IP Data Delivery (NIDD) APIs and configuration procedures, and the addition of CHF (Charging Function) CDRs for the northbound exposure API. These changes also involved clarifications and corrections to the T8 interface APIs, such as capability changes based on API filters and handling of failure responses.
- EPS exposure architecture supporting RACS TS 23.682CR0447
- Update to NIDD APIs for RDS Dynamic Port Management TS 29.122CR0158
- RDS port mismatch in NIDD TS 29.122CR0205
- Support API capability change based on API filter TS 29.122CR0216
- Definition of charging parameter for interworking with EPC TS 32.298CR0722
- Addition of CHF CDR for exposure function northbound API TS 32.298CR0786
+ 11 more changes
In Release 17, the enhancements for the IWK-SCEF function specifically focused on refining the Non-IP Data Delivery (NIDD) API and improving the northbound interface towards the SCS/AS. Key updates included support for the PATCH method to update NIDD resources, improved error handling for the NIDD API, and support for redirection mechanisms for pure 4G SCEF northbound APIs. These changes provided more granular control and operational efficiency for the T8 interface between the SCEF and the SCS/AS.
- Support of Network Exposure to EAS via Local NEF TS 29.122CR0428
- Updates notification destination via PATCH operation in NIDD API TS 29.122CR0444
- Add the support for PATCH method for the update of a NIDD DL Data transfer resource TS 29.122CR0547
- Application Identifier for SCEF API ChargebleParty and AS Session with QoS TS 23.682CR0475
- Adding some missing description fields to data type definitions in OpenAPI specification files of the NIDD API TS 29.122CR0394
- Support redirection for pure 4G SCEF northbound APIs TS 29.122CR0406
+ 8 more changes
In Release 18, the IWK-SCEF function was enhanced with new exposure capabilities for application detection events and for location related to Ranging_SL. The release also introduced support for BAT window and BAT adaptation capability via the SCEF. These updates expanded the network services that could be securely exposed to external application servers over the T8 interface.
- Support of BAT window and capability for BAT adaptation TS 29.122CR0658
- Update application detection event exposure TS 29.122CR0723
- Location exposure for Ranging_SL TS 29.122CR0746
- Update Location exposure for Ranging_SL TS 29.122CR0762
- Update application detection event exposure TS 29.122CR0775
- Updates on application detection event exposure TS 29.122CR0812
+ 1 more changes
In Release 19, the IWK-SCEF function was enhanced with new capabilities for exposing network and UE information to external applications. Key additions included support for exposing energy consumption information with configurable thresholds and reporting periods, as well as the exposure of available data rate and RAT Type for a PDU Session. The release also introduced enhancements for QoS monitoring capabilities and provided clarifications on existing exposure procedures.
- QoS Monitoring enhancement on capability report TS 29.122CR0872
- QoS Monitoring capability report Editors note update TS 29.122CR0878
- MPS for Messaging Indication parameter provisioning via SCEF TS 29.122CR0880
- Support Energy related information exposure TS 29.122CR0898
- Support threshold and reporting time period for energy based exposure events TS 29.122CR0917
- Support S-NSSAI level exposure related to EnergySys feature TS 29.122CR0920
+ 12 more changes
Explore further
Broader topics and technologies where IWK-SCEF plays a role.
Defining Specifications
3GPP specifications that define or reference IWK-SCEF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.682 vj30 | 3GPP TS 23682: MTC Architecture Enhancements | Rel-19 |
| TS 29.122 vj40 | T8 Reference Point for Northbound APIs | Rel-19 |
| TS 29.128 vj10 | MME/SGSN-SCEF Diameter Interfaces for PDN Interworking | Rel-19 |
| TS 32.253 vj00 | Charging for Control Plane Data Transfer | Rel-19 |
| TS 32.278 vj00 | Monitoring Events Offline Charging Specification | Rel-19 |
| TS 32.298 vj30 | Charging Data Record (CDR) Parameter Specification | Rel-19 |
| TS 32.299 vj00 | Diameter Charging Applications for 3GPP | Rel-19 |