Service Capability Exposure Function Packet Data Network Connection

Description

The SCEF PDN Connection is a specialised Packet Data Network (PDN) connection in 3GPP networks that connects a User Equipment (UE) to the Service Capability Exposure Function (SCEF) for the purpose of transmitting non-IP data associated with a specific application. This concept was introduced in Release 13 as part of the Cellular Internet of Things (CIoT) enhancements to support Machine-Type Communication (MTC) devices that do not use traditional IP-based data transport. The connection is defined in 3GPP TS 23.007 (Restoration procedures), TS 24.301 (Non-Access-Stratum protocols for EPS), and TS 29.274 (Evolved GPRS Tunnelling Protocol Control plane).

Architecturally, the SCEF PDN Connection is established between the UE and the SCEF, traversing the Evolved Packet Core (EPC). The SCEF is a network entity that provides a means to securely expose the services and capabilities provided by 3GPP network interfaces to external third-party service providers. In the context of CIoT, the SCEF acts as an anchor point for non-IP data delivery (NIDD). When a UE needs to send non-IP data—such as small sensor readings, commands, or other application-specific payloads—it establishes a PDN connection to the SCEF rather than to a conventional Packet Data Network Gateway (P-GW) for IP connectivity. This PDN connection is dedicated to a specific application, identified by an Application Port ID or similar parameter.

The establishment of an SCEF PDN Connection involves several steps. The UE initiates a PDN Connectivity Request with a specific Access Point Name (APN) that resolves to the SCEF. The Mobility Management Entity (MME) processes this request and selects the SCEF as the anchor. The SCEF PDN Connection is then created, and the UE can send and receive non-IP data over it. The data is transported via the control plane using the EPS Mobility Management (EMM) and EPS Session Management (ESM) protocols, or via the user plane using GTP-U tunnels. The Non-IP Data Delivery (NIDD) procedure allows the UE to send data to the SCEF, which then forwards it to the appropriate Application Server (AS) via the SCEF's northbound APIs (e.g., T8 interface).

Key to the SCEF PDN Connection is the concept of non-IP data. Traditional PDN connections carry IP packets, requiring UEs to have an IP stack, which can be resource-intensive for constrained IoT devices. Non-IP data allows the UE to send raw application payloads without IP encapsulation, reducing overhead and complexity. The SCEF PDN Connection manages the mapping between the UE's non-IP data and the appropriate application server, ensuring that data is routed correctly based on the application identifier. This is particularly important for massive IoT deployments where millions of devices may need to communicate small amounts of data efficiently.

From a mobility and session management perspective, the SCEF PDN Connection is maintained across tracking area updates and handovers within the EPC. The MME and SCEF coordinate to ensure session continuity. The SCEF PDN Connection also supports features like small data transmission, power saving mode (PSM), and extended discontinuous reception (eDRX), which are critical for battery-constrained IoT devices. The connection can be released and re-established as needed, and restoration procedures (defined in TS 23.007) ensure that the connection state is recovered after network element failures.

The SCEF PDN Connection is a foundational element of 3GPP's CIoT architecture. It enables operators to offer value-added IoT services by exposing network capabilities to third-party application providers through the SCEF. It also supports the interworking of non-IP data with external networks and ensures that IoT devices can communicate reliably and securely. As IoT continues to grow, the SCEF PDN Connection remains a key mechanism for non-IP data delivery in LTE and is being adapted for 5G networks through the Network Exposure Function (NEF) and related 5G core procedures.

Purpose & Motivation

The SCEF PDN Connection was created to address the specific needs of Cellular IoT (CIoT) devices, which often have limited processing power, memory, and battery life. Traditional IP-based PDN connections require a full IP stack, which adds protocol overhead and consumes resources that constrained devices cannot afford. Additionally, many IoT applications generate small, infrequent data payloads that do not fit well with IP-based transport. The SCEF PDN Connection solves this by enabling non-IP data delivery (NIDD), allowing devices to send application-specific payloads without IP encapsulation. This reduces overhead, simplifies device implementation, and improves efficiency for massive IoT deployments.

The problem it addresses is twofold: technical and commercial. Technically, non-IP data transport eliminates the need for IP address management, header compression, and other IP-related overheads, which is crucial for devices that transmit only a few bytes at a time. Commercially, the SCEF PDN Connection enables operators to expose network capabilities to third-party application providers via the SCEF, creating new revenue streams. By anchoring non-IP data at the SCEF, operators can apply policies, charging, and security controls, and provide value-added services such as device triggering, group messaging, and data buffering. The SCEF acts as a secure gateway, ensuring that external applications can interact with devices without compromising network integrity.

The specifications TS 23.007, 24.301, and 29.274 were developed in Release 13 to standardise the SCEF PDN Connection and its associated procedures. This was part of a broader effort in 3GPP to support the diverse requirements of IoT, including low-power wide-area (LPWA) use cases. Previous approaches, such as using regular IP PDN connections, were inefficient for these scenarios. The SCEF PDN Connection filled this gap by providing a dedicated, optimised path for non-IP data. It also laid the groundwork for 5G's Network Exposure Function (NEF) and non-IP data delivery mechanisms, ensuring continuity as networks evolve.