SMF — Session Management Function¶

Learning objectives¶

After reading this page, you will be able to:

• Explain in one sentence the role the SMF plays in the 5GC (PDU session management and UPF control).
• Diagram which interfaces (N4/N7/N10/N11-equivalent and others) the SMF terminates and which NFs it works with.
• Explain the positioning of the SMF as the "control side" in CUPS (the separation of the control-plane SMF and the forwarding-plane UPF).
• Explain how the SMF controls the UPF over N4(PFCP) and how it handles IP address assignment and QoS.
• Explain an overview of how the SMF behaves during the PDU Session Establishment procedure (details are in the PDU Session chapter).
• Explain the correspondence with the 4G (EPC) PGW-C/SGW-C (plus part of the MME).

Prerequisites¶

It will go more smoothly if you understand the following first:

• Basics of SBA (Service Based Architecture), NFs, and SBI → Curriculum
• The overall picture of PDU sessions (the procedure in which the SMF plays the lead role) → PDU Session Establishment
• The positioning of NAS (SM NAS also passes through here) → Protocol dictionary
• The role of the AMF (the party that relays SM NAS to the SMF) → AMF

What you will learn in this chapter¶

We first enter from Why (what goes wrong without the SMF), then organize What (the SMF's role, retained information, and APIs) with tables and Mermaid diagrams, and finally move to How (its appearance in PDU session establishment, comparison with EPC, and reference specs).

This page focuses on the SMF's role as an NF, its SBI services, and its architectural position. The procedural details of PDU Session Establishment (call flow, QoS Flow, the two-stage N4/N3 establishment, etc.) are consolidated in PDU Session Establishment, so we direct you there.

Why — why it is needed¶

Even after a UE completes Registration and Authentication, user data does not flow by that alone. To actually pass data, you need to set up, maintain, and eventually release a logical pipe (PDU session) that goes from the UE through the UPF (User Plane Function) to the DN (Data Network). A dedicated NF that single-handedly manages this session lifecycle is needed. That is the SMF.

Another key point is CUPS (Control and User Plane Separation). In 5G, the control (brain) that decides "how to set up the session and how to make it forward" and the forwarding (pipe) that actually passes packets were clearly separated. The SMF is the control side (brain), and the UPF is the forwarding side (pipe). Without the SMF, there would be no entity to tell the UPF "forward this packet this way," and the pipe would remain just an empty tube.

Analogy: The SMF is a factory's production line manager (line manager). The manager does not carry goods on the conveyor belt (UPF) themselves, but they decide "which line to bring up, at what speed and priority to run it, and where to ship it," and hand the field (UPF) an instruction sheet (the N4/PFCP rules). Carrying is done by the field, deciding by the manager — this division of command and execution is CUPS.

Overview¶

The SMF (Session Management Function) is the control-plane NF responsible for the PDU session lifecycle management. It receives session management NAS (SM NAS / 5GSM) from the UE via the AMF and terminates it, performing UPF selection, UPF control via N4(PFCP), IP address assignment to the UE, and application of QoS/charging policy (PCC: Policy and Charging Control) in cooperation with the PCF. It is the control side in CUPS, leaving forwarding to the UPF and dedicating itself to instructing it "forward this way."

Basic Concept — explanation for beginners¶

Think of the SMF as a travel arranger (coordinator). When you (the UE) apply saying "I want to go to my destination (DN)" (a PDU session request), the arranger (SMF) springs into action. First it confirms the contract details (subscriber data) with the HR window (UDM), and if needed queries the terms department (PCF) about the service quality rules (QoS/charging policy). On that basis it selects which means of transport (which UPF) to use, assigns your seat number (IP address), and hands the operating company (UPF) the operation instruction sheet (the N4/PFCP rules).

The point to grasp here is that the arranger (SMF) does not drive itself. What actually carries the packets is the UPF. The SMF only decides and instructs "who to carry, where, and at what quality" — a division of labor (CUPS).

The main things the SMF manages are these four:

• PDU session … the logical pipe that carries data. Manages the entire lifecycle of establishment, modification, and release.
• UPF control … selects which UPF to use and sets forwarding rules over N4(PFCP).
• IP address … manages the IP assigned to the UE (per PDU session).
• QoS/charging policy … applies the rules received from the PCF (PCC rules) to the UPF.

Network Function details¶

About the termination of SM NAS (5GSM): The UE's session management messages (PDU Session Establishment Request, etc.) are SM NAS (5GSM), nested within MM NAS (5GMM) and passing through N1. The AMF does not interpret the contents of this SM NAS; it performs SMF selection and relays it to the SMF. The entity that actually terminates and processes SM NAS is the SMF (basis: TS 24.501 §6.4 / TS 23.502 §4.3.2.2. Section numbers vary by version; to be confirmed).

Architecture¶

The SMF is positioned in the 5GC control plane as the command tower of PDU session management, receiving requests from the AMF, controlling the UPF, and cooperating with the UDM/PCF/CHF.

flowchart LR
  AMF["AMF"]
  SMF["SMF"]
  UPF["UPF (forwarding)"]
  UDM["UDM"]
  PCF["PCF"]
  CHF["CHF (charging)"]
  DN[(Data Network)]

  AMF -- "N11-equivalent (Namf/Nsmf)" --> SMF
  SMF -- "N10 (Nudm)" --> UDM
  SMF -- "N7 (Npcf)" --> PCF
  SMF -- "N40 (Nchf) *optional" -.-> CHF
  SMF == "N4 (PFCP)" ==> UPF
  UPF == "N6" ==> DN

How to read the diagram: The SMF receives SM NAS and session requests from the AMF (via N11-equivalent Namf/Nsmf). It retrieves subscription data from the UDM (N10) and QoS/charging policy from the PCF (N7), and cooperates with the CHF (N40) for charging as needed. On that basis it controls the UPF over N4(PFCP) (bold line), and the UPF forwards packets to the DN over N6. The key to CUPS is that the SMF dedicates itself to control, while the actual data flows on the UPF side (the bold-line section).

Interface¶

The main interfaces terminated by the SMF and their roles.

About the treatment of N11: The AMF⇔SMF path is shown as the reference point N11 in the 3GPP architecture diagrams, but in the SBA (servicification) it is exchanged in practice as the SBI services Namf/Nsmf. On this site we denote this as "N11-equivalent (Namf/Nsmf)." See the Interface dictionary for details.

You can also refer to the individual interface pages (N4 / N7 / N40). For terminology see the Interface dictionary, and for protocols such as PFCP see the Protocol dictionary.

Appearance in procedures¶

In PDU Session Establishment, the SMF becomes the primary agent of the procedure. Roughly, it behaves as follows:

1. Receives SM NAS and a session request from the AMF via Nsmf_PDUSession_CreateSMContext.
2. Retrieves session management subscription data (allowed DNN/S-NSSAI, default QoS, etc.) from the UDM (N10).
3. (Optionally/conditionally) retrieves the SM Policy (PCC rule / authorized QoS) from the PCF (N7).
4. Assigns the UE's IP address and performs UPF selection.
5. Sets forwarding rules (PDR/FAR/QER) on the UPF over N4(PFCP).
6. Returns PDU Session Establishment Accept to the UE via the AMF, prompting the gNB side to configure U-Plane resources.
7. Once the gNB-side N3 information comes back, reflects it to the UPF via N4 Modification and brings the user plane through.

The detailed call flow, message IEs, QoS Flow, and the two-stage N4/N3 establishment of these are consolidated in PDU Session Establishment. Please refer there for the procedural details.

Comparison with EPC¶

• In 4G: Session management (bearer control) was handled mainly by the PGW-C / SGW-C, and part of that control was also handled by the MME. Forwarding was handled by the PGW-U / SGW-U, and the separation of the two (CUPS) was introduced as a retrofitted option (the Sx interface).
• In 5G: The control functions for session management were consolidated into the SMF and clearly separated from the AMF for access/mobility management. Furthermore, forwarding was split off to the UPF, with the two connected by N4(PFCP), making CUPS the standard architecture from the start. This allows control and forwarding to scale independently.

Release differences¶

• Rel-15: Defines the SMF's basic functions (PDU session management, SM NAS termination, UPF selection and N4(PFCP) control, IP address assignment, PCF cooperation).
• Rel-16: Functional extensions (e.g., TSC (Time-Sensitive Communication), Ethernet PDU, introduction of I-SMF (Intermediate SMF), etc.). The specific section numbers and scope of incorporation are to be confirmed.
• Rel-17: Further extensions. Specific items and section numbers are to be confirmed.

3GPP Specification¶

Summary¶

• The SMF is the control-plane NF that manages the PDU session lifecycle (establishment, modification, release).
• It terminates SM NAS (5GSM), selects the UPF, and controls it over N4(PFCP) — the control side (brain) of CUPS. It leaves forwarding to the UPF (the pipe).
• The core of its role is UPF control, IP address assignment, and application of QoS/charging policy (PCC) (PCF cooperation).
• Its terminating interfaces are N4 (UPF), N7 (PCF), N10 (UDM), and N11-equivalent (AMF). Charging (N40/CHF) is optional/implementation-dependent.
• It corresponds to the 4G PGW-C/SGW-C (plus part of the MME), splitting forwarding off to the UPF and standardizing CUPS.
• The SMF plays the lead role in the PDU session establishment procedure. For the detailed procedure, see PDU Session Establishment.

Next Step¶

• PDU Session Establishment — the procedure in which the SMF plays the lead role (procedural details)
• AMF — the party that relays SM NAS to the SMF
• UDM — the party the SMF retrieves session management subscription data from
• UPF / PCF / CHF — the UPF is the forwarding entity controlled over N4, the PCF is for policy, the CHF is for charging
• NF dictionary / Interface dictionary / Protocol dictionary — for checking terminology