SMSF — Short Message Service Function¶

Difficulty: Intermediate ／ Estimated study time: 12 min ／ Related NFs: AMF (NAS relay), UDM (SMSF registration) ／ Related interfaces: SBI (Nsmsf), N1 (NAS)

Learning objectives¶

Understand the role of the SMSF, namely that it is the processing entity for SMS carried over NAS (N1) (SMS over NAS).
Understand the flow in which the SMSF coordinates with the AMF and relays the UE's NAS messages as SMS.
Understand the mechanism by which, when the UE supports SMS, the AMF selects an SMSF and performs SMSF registration with the UDM.
Understand the role by which the SMSF relays with the SMSC (SMS Center)/IP-SM-GW and completes the actual SMS delivery.

Prerequisites¶

Basics of the AMF and NAS signaling: AMF ／ N1
The UDM responsible for subscriber data management: UDM
The concept of SBA/SBI: Curriculum

What you will learn in this chapter¶

Why: Because SMS is important as a basic service that delivers authentication codes and various notifications even in situations where voice/data are unavailable, and a 5G entity is needed to process SMS carried over NAS.
What: Learn that the SMSF takes charge of SMS via NAS, retains SMS context and SMS subscription information, and provides the Nsmsf API.
How: Learn the sequence of SMSF selection and UDM registration at registration, NAS relay from the AMF when sending/receiving SMS, and handoff to the SMSC/IP-SM-GW.

Why — Why it is needed¶

Even in situations where voice calls or data communication are unavailable, SMS is a basic and important service that delivers authentication codes (one-time passwords) and various system notifications. In 5G, a scheme to carry this SMS over NAS (N1) signaling (SMS over NAS) is provided, and an entity dedicated to processing it is needed. The SMSF fulfills this role and bridges between the UE and the SMSC/IP-SM-GW.

Analogy: The SMSF is the "mail-sorting counter" for SMS. It receives SMS that arrives through the passage called NAS and sorts it to the correct destination (the SMSC). Without this counter, SMS via NAS could not be delivered.

Overview¶

At registration, the UE indicates that it supports SMS (SMS over NAS).
Receiving this, the AMF selects an SMSF and performs SMSF registration with the UDM.
When sending/receiving SMS, the AMF relays the UE's NAS messages to the SMSF.
The SMSF hands off the received SMS to the SMSC/IP-SM-GW, completing the actual delivery.

Basic Concept — Explanation for beginners¶

The SMSF is the "counter" that sorts SMS arriving through the passage called NAS and hands it to the SMSC (the SMS collection/delivery office). From the UE's viewpoint, SMS is carried on the NAS exchange with the AMF. The AMF passes that SMS portion to the SMSF, and the SMSF bridges it to the SMSC. The reverse direction (SMSC→UE) is similar: the SMSF places it on NAS via the AMF and delivers it to the UE.

Network Function details¶

Item	Content
Role	Processes SMS carried over NAS (N1) (SMS over NAS) and relays between the AMF and the SMSC/IP-SM-GW.
Retained information	SMS context, the UE's SMS subscription information, SMSC connection information.
Main APIs used	Retrieving SMS subscription data from the UDM and SMSF registration with the UDM (Nudm); NAS relay from the AMF.
Provided APIs	Nsmsf_SMService (Activate / Deactivate / UplinkSMS, etc. individual operations to be confirmed) — TS 29.540.
Related interfaces	SBI (Nsmsf); N1 (NAS) via the AMF.
Impact on failure	SMS via NAS becomes unavailable. Voice/data communication is unaffected, and the impact is limited to SMS.

Architecture¶

flowchart LR
    UE["UE"] -->|"NAS / N1"| AMF["AMF (NAS relay)"]
    AMF -->|"Nsmsf"| SMSF["SMSF (SMS processing)"]
    SMSF --> SMSC["SMSC / IP-SM-GW"]
    AMF -->|"SMSF registration"| UDM["UDM (subscriber data)"]
    SMSF -->|"retrieve SMS subscription data"| UDM

How to read the diagram: The UE's SMS arrives at the AMF on NAS (N1), and the AMF relays it to the SMSF via Nsmsf. The SMSF hands it off to the SMSC/IP-SM-GW to complete delivery; at registration the AMF performs SMSF registration with the UDM, and the SMSF retrieves SMS subscription data from the UDM.

Interface¶

Interface	Connection	Main role
SBI (Nsmsf)	AMF ⇔ SMSF	SMS relay (SMService)
N1 (NAS)	UE ⇔ AMF	SMS transport

For details, see the Interface dictionary.

Appearance in procedures¶

At Registration: When the UE indicates SMS (SMS over NAS) support, the AMF selects an SMSF and performs SMSF registration with the UDM.
When sending/receiving SMS: The AMF relays the UE's NAS message (the SMS portion) to the SMSF, and the SMSF hands it off to the SMSC/IP-SM-GW.

For the specific flow, see Registration and AMF. The detailed procedure description is to be confirmed.

Comparison with EPC¶

In 4G/EPC, SMS was provided by schemes such as SMS over SGs (MME ⇔ MSC) and SMS in MME (via SGd / IP-SM-GW). In 5G, the SMSF takes charge of SMS via NAS (SMS over NAS) (the precise correspondence is to be confirmed).

Generation	SMS provisioning scheme
4G/EPC	SMS over SGs (MME⇔MSC) / SMS in MME (via SGd)
5G (SMSF)	SMS over NAS (the SMSF processes SMS via NAS)

Release differences¶

The SMSF was defined as a basic function in Rel-15. Extensions in subsequent releases are to be confirmed.

3GPP Specification¶

TS 23.501 §6.2.13 defines the SMSF.
TS 23.502: Specifies SMS-related procedures (registration, sending/receiving) (to be confirmed).
TS 29.540: Specifies Nsmsf (SMService) (individual section numbers to be confirmed).

Summary¶

The SMSF is the entity that processes SMS carried over NAS (N1) (SMS over NAS).
In sending/receiving SMS, the AMF relays the UE's NAS messages to the SMSF.
When the UE supports SMS, the AMF selects an SMSF and performs SMSF registration with the UDM.
The SMSF relays with the SMSC/IP-SM-GW and completes the actual SMS delivery.
The impact of a failure is limited to SMS and does not extend to voice/data communication.