SMS Function

Description

The SMS Function (SMSF) is a 5G core network function introduced in 3GPP Release 15 as part of the Service-Based Architecture (SBA). It is defined in TS 23.501 (System architecture for the 5G System) and further specified in TS 23.540 (Technical realization of SMS in 5G). The SMSF is responsible for the delivery of Short Message Service (SMS) messages over the Non-Access Stratum (NAS) between the UE and the 5G core network. It acts as an intermediary between the Short Message Service Centre (SMSC) and the Access and Mobility Management Function (AMF), enabling SMS delivery without requiring the UE to support IMS or circuit-switched fallback.

Architecturally, the SMSF communicates with the AMF over the N20 interface (or service-based interface Nsmsf). The AMF is responsible for relaying SMS messages between the UE and the SMSF using NAS signalling. When a UE needs to send an SMS (mobile-originated), it encapsulates the SMS message in a NAS message and sends it to the AMF. The AMF forwards the message to the SMSF, which then delivers it to the SMSC using the appropriate interface (e.g., MAP or Diameter). For mobile-terminated SMS, the SMSC sends the message to the SMSF, which forwards it to the AMF, and the AMF delivers it to the UE via NAS signalling. The SMSF also interacts with the Unified Data Management (UDM) to retrieve subscriber data related to SMS services, such as whether the subscriber is authorized for SMS over NAS.

The SMSF supports both SMS over NAS and SMS over IP. For SMS over NAS, the UE must indicate its capability to support SMS over NAS during registration. The AMF selects an SMSF based on the subscriber's profile and network configuration. The SMSF then manages the SMS delivery, including store-and-forward functionality if the UE is temporarily unreachable. The SMSF can also generate delivery reports and handle error cases, such as when the UE is not reachable or when the message is too large.

The SMSF is a key enabler for SMS in 5G, particularly for devices that do not support IMS or for networks that do not deploy IMS. It ensures that SMS remains available as a basic service in 5G, which is critical for many use cases, including IoT devices, legacy applications, and two-factor authentication. The SMSF also supports roaming, allowing subscribers to send and receive SMS while visiting foreign 5G networks. The architecture ensures that SMS delivery is efficient and does not require the overhead of establishing an IMS session for each message.

Since its introduction in Rel-15, the SMSF has been maintained through Rel-20 with no changes to its fundamental definition. However, its specifications have been refined and extended to cover new deployment scenarios, such as SMS over NAS in non-standalone (NSA) and standalone (SA) 5G networks, and interworking with 4G networks. The SMSF is a critical component of the 5G core network, ensuring that SMS continues to be a reliable and ubiquitous service in the 5G era.

Purpose & Motivation

The SMSF was created to address the need for SMS support in 5G networks without relying on legacy circuit-switched infrastructure or IMS. In earlier generations, SMS delivery was tightly coupled with the circuit-switched domain (2G/3G) or required IMS deployment (LTE with SMS over IP). As 5G networks were designed as all-IP, service-based architectures, there was a need for a dedicated network function to handle SMS delivery natively over NAS. The SMSF fills this gap by providing a standardized, efficient mechanism for SMS transport in 5G.

The SMSF solves several problems. First, it enables SMS for UEs that do not support IMS, such as low-cost IoT devices or legacy devices. Second, it avoids the complexity and overhead of establishing IMS sessions for SMS delivery, which is particularly important for machine-type communications where efficiency is critical. Third, it provides a clear separation of concerns, allowing the AMF to focus on mobility management while the SMSF handles SMS-specific procedures. This modular design aligns with the 5G core network's service-based architecture.

Furthermore, the SMSF ensures that SMS remains a reliable service in 5G, supporting essential use cases such as two-factor authentication, service notifications, and person-to-person messaging. It also facilitates roaming by providing a standardized interface for SMS delivery across networks. The introduction of the SMSF in Rel-15 was a key milestone in the evolution of SMS, ensuring its continued relevance in the 5G era. Its maintenance through Rel-20 reflects its stable design and ongoing importance.