N11 — AMF ⇔ SMF (Nsmf_PDUSession / Namf_Communication reference point)¶

Learning objectives¶

After reading this page, you will be able to:

• Explain that N11 is an SBI (Service Based Interface), and that its substance is HTTP/2 + JSON over TLS.
• Explain that N11 consists of two services in two directions. That is, the AMF→SMF direction is Nsmf_PDUSession (provided by SMF), and the SMF→AMF direction is Namf_Communication_N1N2MessageTransfer (provided by AMF).
• Explain that the AMF does not interpret the contents of the SM NAS (Session Management NAS) but transparently relays it, and the master of SM (Session Management) is the SMF.
• Explain the positioning of N11 as the "carrier" of SM information toward the UE's N1 (NAS) and the gNB's N2 (NGAP).
• Explain the contrast between 4G (EPC), where SM/MM were unified within the MME, and 5G, where they were separated into MM=AMF and SM=SMF.

Prerequisites¶

Understanding the following first will make things smoother.

• The positioning of SBA / SBI and HTTP/2 · TLS → Protocol dictionary
• The NFs at both ends of N11 → AMF / SMF
• The concept of N1 (NAS) · N2 (NGAP) → Interface dictionary
• The assembly of a PDU session and how N11 is used → PDU Session

Why — why it is needed¶

5G separated AMF, which handles mobility management (MM), and SMF, which handles session management (SM), into distinct NFs. However, the NAS point of contact with the UE is consolidated into the single AMF (N1), and the NGAP point of contact with the gNB is also the AMF (N2). In other words, the SMF is not directly connected to the UE or the gNB.

Then, the NAS/NGAP information the SMF needs for session control (e.g., a PDU Session establishment request came from the UE, or conversely you want to return a PDU Session Accept, etc., to the UE) requires a path to carry it via the AMF. That is N11. Without N11, the separated MM and SM cannot cooperate, the UE's session request cannot be delivered to the SMF, and the SMF's responses cannot be returned to the UE/gNB.

An analogy: N11 is the internal line connecting the general reception of mobility management (AMF) and the session-specialist department (SMF). The point of contact interacting with customers (UE) and base stations (gNB) is consolidated at reception (AMF), and the session department's (SMF's) instructions reach the customer via reception. Reception merely forwards the envelope (SM NAS) without opening its contents; the department that creates and reads the contents is the session department.

Overview¶

N11 is the SBI reference point connecting AMF ⇔ SMF. Its greatest feature is that it is bidirectional, and a different service is used per direction.

• AMF → SMF direction: the AMF calls the service Nsmf_PDUSession (TS 29.502) provided by the SMF to request PDU-session operations. Representative operations are CreateSMContext (create) / UpdateSMContext (update) / ReleaseSMContext (release).
• SMF → AMF direction: the SMF calls the service Namf_Communication (TS 29.518) provided by the AMF, in particular N1N2MessageTransfer, to pass the N1 SM NAS it created (PDU Session Accept, etc.) and the N2 SM information (PDU session resource setup) to the AMF. The AMF delivers these to the UE (N1/NAS) / gNB (N2/NGAP).

Basic Concept — explanation for beginners¶

We liken N11 to the internal line connecting the general reception (AMF) and the session-specialist department (SMF).

When your phone tries to start data communication, the UE's request first arrives at the general reception, the AMF (NAS via N1). Since the AMF is in charge of mobility management, it leaves the session's contents to the specialist department, the SMF. So the AMF asks the SMF over the internal line (N11), "please create this session" (Nsmf_PDUSession_CreateSMContext).

The SMF assembles the session, prepares the envelope to return to the UE (N1 SM NAS = PDU Session Accept, etc.) and the setup to hand to the base station (N2 SM information), and returns them to the AMF over the internal line, saying "hand these to the customer and the base station" (Namf_Communication_N1N2MessageTransfer). The AMF forwards them as-is to the UE (N1) and to the gNB (N2) without opening the envelope's contents.

This internal-line exchange is not over a dedicated telephone line (a dedicated protocol like N2 or N4), but rather like throwing a request over an internal Web system (HTTP) and getting a reply. This is SBI (Service Based Interface) = the mechanism where NFs converse via Web APIs (HTTP/2), with both requests and responses exchanged in human-readable JSON. Giving each operation target (SM context, etc.) an "address (URI)" and throwing POST, etc., at it — this convention is called RESTful.

Protocol / Transport¶

Architecture¶

This shows the context in which N11 connects AMF and SMF, the AMF handles the point of contact with the UE/gNB (N1/N2), and the SMF controls the UPF over N4 and the PCF over N7.

flowchart LR
  UE["UE"]
  gNB["gNB"]
  AMF["AMF (MM · NAS/NGAP contact)"]
  SMF["SMF (SM master)"]
  UPF["UPF (forwarding)"]
  PCF["PCF"]

  UE -. "N1 (NAS)" .- AMF
  gNB -. "N2 (NGAP)" .- AMF
  AMF == "N11 (Nsmf_PDUSession / Namf_Communication)" ==> SMF
  SMF == "N4 (PFCP)" ==> UPF
  SMF -. "N7 (Npcf)" .-> PCF

  classDef ctrl fill:#efe,stroke:#3a3,stroke-width:2px;
  class AMF,SMF ctrl;

How to read the diagram: both the point of contact with the UE (N1/NAS) and the point of contact with the gNB (N2/NGAP) are consolidated into the AMF. The SMF is not directly connected to the UE/gNB; between it and the AMF is N11 (thick line). The AMF requests session operations of the SMF through N11 (Nsmf_PDUSession), and the SMF returns the N1 SM NAS / N2 SM information to the AMF over N11 to have it delivered (Namf_Communication). The SMF reflects the results it decides into the UPF over N4 (PFCP) (the thick line on the right), and queries the PCF for policy over N7 as needed.

Procedures used¶

N11 is used at the core of the PDU Session lifecycle. The most typical is PDU Session Establishment.

• PDU Session Establishment: when the UE's session request arrives at the AMF via N1, the AMF requests session creation from the SMF with Nsmf_PDUSession_CreateSMContext. The SMF configures the UPF over N4 (PFCP), prepares the N1 SM NAS to return to the UE (PDU Session Accept, etc.) and the N2 SM information toward the gNB (resource setup), and passes them to the AMF with Namf_Communication_N1N2MessageTransfer. The AMF delivers them to the UE (NAS/N1) and to the gNB (NGAP/N2).
• PDU Session Modification: for mid-session changes, Nsmf_PDUSession_UpdateSMContext is used, and the necessary updated N1/N2 information is likewise delivered via Namf_Communication.
• PDU Session Release: at session release, Nsmf_PDUSession_ReleaseSMContext is used.

The detailed flows of these procedures (message sequences, conditions, IEs, etc.) are consolidated in PDU Session and PDU Session Modification / Release. The procedure details (exact invocation order, method assignments) are to be confirmed; refer there.

Main Messages¶

Since N11 is an SBI, messages take the form of HTTP method + resource operation (RESTful operations, not a procedureCode). Representative service operations are as follows.

The HTTP-operation and direction mappings are a conceptual organization: the table above is a mapping for understanding; the exact method, resource URI, and invocation direction of each operation follow the OpenAPI definitions of TS 29.502 (Nsmf_PDUSession) / TS 29.518 (Namf_Communication). Strict method assignments are to be confirmed. For the details of each message, see the Message dictionary.

Packet Analysis (Wireshark)¶

Since N11 is an SBI, it is observed on the capture as HTTP/2 (no L4-dedicated protocols such as NGAP/SCTP appear).

Decode highlights:

• N11 is encrypted with TLS. To see the contents (HTTP/2 headers and JSON body) you need decryption keys (TLS session keys, etc.). Without keys, it appears only as an encrypted tls payload.
• Once decrypted, you can identify services and operations by the HTTP/2 headers (resource paths whose :path contains .../nsmf-pdusession/... or .../namf-comm/...).
• The body is JSON. What is important is that the N1 SM NAS and the N2 SM information are carried inside this JSON body (as binary elements). On N11 these are "cargo carried over HTTP/2," not N2 itself (NGAP/SCTP).
• N11 does not use SCTP. Since its observation method is fundamentally different from N2's NGAP (binary, over SCTP), be careful not to reuse SCTP-specific filters such as sctp.port.

Comparison with EPC¶

• In 4G: mobility management (MM) and session management (SM / bearer control) were handled together by the MME. The MME controlled the SGW over the S11 interface (GTP-C-based) and, via the SGW, set up the PGW's bearers — MM and SM were tightly coupled within the same node.
• In 5G: MM was separated into AMF and SM into SMF, and N11 (SBI) carries their cooperation. It took the form of two bidirectional services — AMF→SMF being Nsmf_PDUSession, and SMF→AMF being Namf_Communication.

The MM/SM separation is a fundamental 5G design: in 4G the MME held both MM and SM, but in 5G the functions were carved apart into MM=AMF and SM=SMF, and N11 (SBI) was newly created as their cooperation point. This separation is a fundamental 5G design philosophy supporting service-ization and scalability.

3GPP Specification¶

• 3GPP TS 29.502 — Nsmf_PDUSession service (CreateSMContext/UpdateSMContext/ReleaseSMContext, AMF→SMF). Individual clause numbers are to be confirmed
• 3GPP TS 29.518 — Namf_Communication service (N1N2MessageTransfer, etc., SMF→AMF). Individual clause numbers are to be confirmed
• 3GPP TS 29.500 / TS 29.501 — SBI framework (technical spec / design principles · OpenAPI). Individual clause numbers are to be confirmed
• 3GPP TS 23.501 §4 — system architecture and reference points (the definition of N11)
• 3GPP TS 23.502 §4.3 — use of N11 in PDU session procedures (procedural context). Individual clause numbers are to be confirmed
• 3GPP TS 33.501 — SBI security (TLS, etc.). Individual clause numbers are to be confirmed

Note (to be confirmed): that the substance of N11 is the two bidirectional services Nsmf_PDUSession (TS 29.502, AMF→SMF) and Namf_Communication (TS 29.518, SMF→AMF), and that the common SBI framework is the TS 29.500/29.501 family, are the general 3GPP-architecture organization. The detailed clause numbers, exact resource URIs, and method assignments of each service operation differ by Release, so each is individually to be confirmed.

Summary¶

• N11 is the SBI (HTTP/2 + JSON over TLS) reference point connecting AMF ⇔ SMF, and its core is that two services in two directions are involved.
• AMF → SMF direction = Nsmf_PDUSession (CreateSMContext/UpdateSMContext/ReleaseSMContext, provided by SMF, TS 29.502), SMF → AMF direction = Namf_Communication_N1N2MessageTransfer (provided by AMF, TS 29.518).
• The AMF does not interpret the contents of the SM NAS but transparently relays it, and the master of SM (Session Management) is the SMF.
• N11 functions as the "carrier" of SM information toward the UE's N1 (NAS) and the gNB's N2 (NGAP).
• In 4G, the MME handled MM + SM together (controlling the SGW over S11), but in 5G MM=AMF and SM=SMF were separated, and their cooperation became N11.

Next Step¶

• SMF — the SM master of N11. The NF that provides Nsmf_PDUSession and reflects into the UPF over N4
• AMF — the MM contact of N11. The NF that provides Namf_Communication and transparently relays the SM NAS
• PDU Session — the PDU Session establishment procedure where N11 is used at the core (details)
• PDU Session Modification / Release — the usage context of UpdateSMContext/ReleaseSMContext
• Interface dictionary — checking reference points including N1/N2/N4/N7