3GPP Architecture for Machine Type Communication: NB-IoT and LTE-M Roaming Explained
3GPP Architecture for Machine Type Communication (NB-IoT and LTE-M Roaming)
The rapid expansion of the Internet of Things (IoT) has led to a significant need for a standardized, scalable, and secure method to connect billions of devices across various networks. To meet this demand, 3GPP has developed Machine Type Communication (MTC) specifications, concentrating on NB-IoT (Narrowband IoT) and LTE-M (LTE-Cat-M1).
A key feature of IoT connectivity is roaming, which allows IoT devices to seamlessly transition between their home network (HPLMN) and a visited network (VPLMN) without losing service. The image uploaded illustrates the 3GPP roaming architecture for MTC, highlighting how different network functions like SCEF, MTC-IWF, GGSN/PGW, and application servers interact.
In this blog post, we’ll break down the architecture step-by-step, covering topics like control plane vs user plane signaling, the indirect and direct connectivity models, and the roles played by various network elements.
Key Components in NB-IoT and LTE-M Roaming Architecture
The architecture consists of user equipment (UE), access network (RAN), core network elements, and service enablers.
- User Equipment (MTC UE)
This represents the IoT device or an MTC UE Application.
It communicates with the RAN through the Um/Uu/LTE-Uu interface.
The device might be battery-powered, low mobility, and optimized for low-power wide-area (LPWA) applications.
- Radio Access Network (RAN)
This provides the connection between the UE and the core network.
It supports NB-IoT (via LTE cell) and LTE-M.
It’s responsible for mobility management, signaling, and allocating radio resources.
- Core Network Functions
The following nodes are crucial for MTC roaming:
Function | Role in MTC Architecture
MME | Handles NAS signaling, mobility, attach/detach, and bearer management.
SGSN | Supports legacy GPRS UEs within EPS/UMTS networks.
S-GW & P-GW (GGSN) | Provide user plane connectivity, IP address allocation, and enforce QoS.
MSC | Manages CS fallback (CSFB) and SGs interface for SMS over SGs.
IP-SM-GW | Gateway for delivering IP-based SMS.
HSS / MTC AAA | Stores subscriber data and authenticates UEs.
Service Enablers: SCEF and MTC-IWF
A major advancement for IoT in 3GPP is the introduction of SCEF (Service Capability Exposure Function) and MTC-IWF (Machine-Type Communication Interworking Function).
SCEF:
Securely exposes network services to application servers through an API.
Provides data access security, charging, and authorization.
Connects with the core network (MME, HSS, PGW) via standardized interfaces like Tsp, S6t, and API.
MTC-IWF:
Serves as a bridge between SCEF and legacy network nodes (like SGSN/MSC).
Ensures compatibility with 2G/3G networks.
Supports the routing of signaling messages for machine-type communication.
Control Plane vs User Plane
In the diagram, control plane signaling is illustrated with red dashed lines, while user plane data flows are shown as solid black lines.
Control Plane:
Manages session setup, authentication, bearer creation, mobility updates, and API calls.
Examples include the SGs interface (MSC-MME) and S6t signaling (HSS-SCEF).
User Plane:
Carries actual user data (application payloads).
Examples are the Gi/SGi interface between PGW and Application Server.
This separation allows for efficient signaling optimization for a large number of IoT devices, many of which send small bursts of data infrequently.
Roaming Models in 3GPP MTC Architecture
The architecture accommodates several roaming models to satisfy operator and enterprise needs.
- Indirect Model (Model 1)
The application server (AS) connects with SCEF in HPLMN.
SCEF in the HPLMN then interacts with SCEF in the VPLMN over the T7 interface.
This model is ideal when the home operator desires full control over data and signaling.
- Direct Model (Model 2)
The application server connects directly to SCEF in the VPLMN.
This offers lower latency and may reduce reliance on the HPLMN.
Often used when application traffic is localized within the visited country.
- Hybrid Model
This blends the Indirect and Direct Models.
It allows flexible deployment, where some services are based in the HPLMN while others are concluded in the VPLMN.
Interfaces and Protocols
Key points defined by 3GPP enable interoperability:
Interface | Connects | Purpose
T7 | HPLMN SCEF ↔ VPLMN SCEF | Enables service exposure in indirect model.
S6t | SCEF ↔ HSS | Queries subscriber data for policy and authorization.
API | SCEF ↔ SCS/AS | Service capability exposure for external apps.
SGi / Gi | PGW ↔ Application Server | Transfers user plane data.
Tsp | MTC-IWF ↔ SCEF | Signaling for interworking with legacy networks.
Application Servers and SCS
Application servers (AS) and Service Capability Server (SCS) are essential for IoT service delivery:
SCS: Gathers data from various IoT devices and applies service logic before sending it to the AS.
AS: Gets device data through SCEF exposure and can return commands to UEs.
This structure enables operators to offer network APIs to third-party IoT service providers securely.
Benefits of 3GPP MTC Architecture
Secure Service Exposure: APIs through SCEF ensure controlled access to network data.
Interoperability: Functions across multiple PLMNs (both home and visited).
Scalability for Massive IoT: Tailored for reduced signaling and small data transmissions.
Flexibility: Accommodates various roaming models (direct, indirect, hybrid).
Backward Compatibility: Thanks to MTC-IWF, it supports legacy 2G/3G UEs.
Challenges in NB-IoT and LTE-M Roaming
Roaming Agreements: Necessitates alignment between HPLMN and VPLMN operators.
Latency Issues: The indirect model might introduce extra hops, impacting delay-sensitive applications.
Increased Complexity: Arises when multiple interfaces (T7, S6t, Tsp) are involved.
Security Management: Proper AAA and API access control is crucial to prevent misuse.
Use Cases
This architecture enables a variety of IoT applications:
Smart Metering: Utility devices relay periodic updates to central servers.
Asset Tracking: Roaming trackers report locations across borders.
Fleet Management: Vehicles send telematics data regardless of the visited network.
Remote Monitoring: Industrial IoT sensors can alert cloud applications.
Conclusion
The 3GPP architecture for machine type communication is vital for enabling NB-IoT and LTE-M roaming. By integrating functions like SCEF, MTC-IWF, PGW, and flexible interfaces (T7, API, S6t), operators can securely expose network capabilities to IoT service providers while maintaining data and policy control.
The use of indirect, direct, and hybrid models gives service providers the flexibility to find the right mix of control, latency, and costs. As IoT continues to grow, this architecture guarantees interoperability, scalability, and security across networks — forming the backbone of global IoT connectivity.