N1 — UE ⇔ AMF (NAS Reference Point)¶

Learning objectives¶

After reading this page, you will be able to:

• Explain that N1 is the logical interface connecting the UE and the AMF.
• Distinguish the protocol N1 carries (NAS) and its two sublayers (5GMM / 5GSM).
• Explain, using a diagram, the mechanism whereby "N1 has no physical link of its own and rides encapsulated inside N2 (NGAP)."
• Explain the role N1 plays in procedures such as Registration.
• Explain how it corresponds to 4G (EPC) NAS (UE⇔MME).

Prerequisites¶

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

• The positioning of NAS (Non-Access Stratum) → Protocol dictionary
• The basics of NGAP and SBA → Protocol dictionary / Interface dictionary
• The NF that terminates N1 → AMF
• The paired lower-layer IF → N2

Why — why it is needed¶

A smartphone (UE) wants to deliver its requests themselves — "I want to register with the network," "I want to set up a session" — directly to the person in charge in the core network (the AMF), without the intervening base station peeking at the contents. Separate from "radio concerns" such as radio-link quality and handover, there needs to be a path for a conversation understood only by the terminal and the core.

This "conversation path completed solely between the UE and the core (AMF)" is N1. The base station (gNB) sticks to its role as a radio relay and, in principle, passes the contents of the NAS carried over N1 (registration requests, authentication responses) straight through. Without N1, the UE could not talk directly to the core, and registration, authentication, and session establishment could not even begin.

Overview¶

N1 is the logical interface connecting UE ⇔ AMF, and the protocol flowing on top of it is NAS (Non-Access Stratum). NAS is divided into two sublayers by purpose.

An important characteristic is that N1 has no physical link of its own. N1's NAS messages are carried over radio (RRC) between UE⇔gNB, and packed as a NAS-PDU inside N2 (NGAP) between gNB⇔AMF.

Basic Concept — a beginner-friendly explanation¶

Let us liken N1 to an exchange of letters.

Suppose you (the UE) send an application form (a NAS message) to a government office (the AMF). You want the contents of the envelope (the application details) delivered to the office without being read by the post office (the gNB) along the way. The post office merely carries the envelope; it does not open the contents.

• The contents of the letter are N1 (NAS) = read only by the UE and the office.
• The postal network carrying the letter is N2 or the radio = a delivery path that has nothing to do with the contents.

In other words, N1 is not a "dedicated road" but "an envelope that hitches a ride on the post office's (N2's) truck." This point — "N1 rides on N2" — is the first hurdle in understanding 5GC.

Protocol / Transport¶

Protocol stack (NAS rides in a different container per segment)¶

The same NAS message is carried encapsulated in a different lower-layer protocol depending on the segment. UE⇔gNB is over radio (RRC); gNB⇔AMF is the NAS-PDU of N2 (NGAP). The gNB does not open the NAS contents; it merely re-encapsulates them (transparent relay).

flowchart LR
    subgraph UEside["UE"]
        direction TB
        U_NAS["NAS
(5GMM / 5GSM)"]
        U_RRC["RRC"]
        U_L2["PDCP/RLC/MAC/PHY
(radio)"]
        U_NAS --- U_RRC --- U_L2
    end
    subgraph gNBside["gNB (transparent relay: does not open NAS contents)"]
        direction TB
        G_TOP["(merely re-encapsulates NAS)"]
        G_RRC["RRC ⇔ NGAP
conversion"]
        G_TOP --- G_RRC
    end
    subgraph AMFside["AMF"]
        direction TB
        A_NAS["NAS
(5GMM / 5GSM)"]
        A_NGAP["NGAP (encapsulated as NAS-PDU)"]
        A_SCTP["SCTP / IP"]
        A_NAS --- A_NGAP --- A_SCTP
    end

    U_L2 == "Uu (radio)" === G_RRC
    G_RRC == "N2 (NGAP / SCTP:38412)" === A_SCTP
    U_NAS -. "N1: logically the same NAS gets through" .- A_NAS

    classDef nas fill:#eef,stroke:#66f,stroke-width:2px;
    class U_NAS,A_NAS nas;

Architecture¶

This shows who N1 (NAS) connects and which lower-layer IF it rides on.

flowchart LR
  UE(("UE"))
  gNB["gNB (base station)"]
  AMF["AMF"]

  UE == "N1 (NAS) = logical conversation" ==> AMF
  UE -- "carried by radio RRC" --> gNB
  gNB -- "carried by NAS-PDU of N2 (NGAP)" --> AMF

  classDef logical fill:#eef,stroke:#66f,stroke-width:2px;

How to read the diagram: the thick line (==>) is N1's logical conversation (meaningful between UE⇔AMF). The thin lines are the actual transport paths. The NAS contents are not opened at the gNB and are passed transparently to the AMF via radio and N2.

Procedures that use it¶

N1 (NAS) is used in almost every procedure where the UE and the core talk directly.

• Registration: carries Registration Request/Accept/Complete. The starting point for the UE registering with the network.
• Authentication: carries Authentication Request/Response and Security Mode Command/Complete.
• Service Request: the NAS request when an idle UE re-establishes connectivity.
• PDU Session Establishment (5GSM): carries the session establishment request (the AMF relays it to the SMF).

Its role in Registration is especially central: the UE's first NAS (Registration Request) is sent out as N1, and the gNB puts it in an N2 Initial UE Message and delivers it to the AMF. For the detailed flow, see Registration.

Main Messages¶

These are representative NAS messages that flow over N1.

For details of each message, see the Message dictionary.

Packet Analysis (Wireshark)¶

N1 (NAS) does not appear on its own in a capture; it is decoded as a NAS-PDU inside an NGAP (N2) packet.

Decode highlights:

• Wireshark automatically double-decodes the NGAP NAS-PDU field as nas-5gs. Understanding progresses when you view it with the structure "NAS nested inside NGAP" in mind.
• You can also confirm that NAS from Security Mode Command onward is ciphered so the contents become invisible (a MAC for integrity protection is still attached).
• Strictly, the Security Mode Command itself is only integrity protected and not ciphered (integrity protected but not ciphered). Ciphering becomes effective from the message immediately after Security Mode Command in the DL direction, and from Security Mode Complete onward in the UL direction (TS 33.501 §6 / TS 24.501 §4.4).

Comparison with EPC¶

• In 4G: NAS was terminated between UE ⇔ MME and was split into EMM (mobility management) and ESM (session management) (TS 24.301). Transport was the NAS-PDU of S1AP (S1-MME).
• In 5G: NAS is terminated between UE ⇔ AMF and was reorganized into 5GMM / 5GSM (TS 24.501). Transport is the NAS-PDU of NGAP (N2).

3GPP Specification¶

• 3GPP TS 24.501 — NAS protocol for the 5G system (5GMM / 5GSM). Individual clause numbers are to be confirmed
• 3GPP TS 23.501 §4 — System architecture and reference points (definition of N1)
• 3GPP TS 33.501 — NAS security (ciphering, integrity protection). Individual clause numbers are to be confirmed

Summary¶

• N1 is the logical interface connecting UE ⇔ AMF, and its protocol is NAS.
• NAS has two sublayers: 5GMM (mobility management) and 5GSM (session management).
• N1 has no link of its own; UE⇔gNB is over radio, and gNB⇔AMF rides on the NAS-PDU of N2 (NGAP).
• NAS security (ciphering, integrity protection) is terminated at UE⇔AMF.
• It corresponds to 4G NAS (UE⇔MME, TS 24.301), and in 5G it was reorganized into 5GMM/5GSM.

Next Step¶

• N2 — the lower-layer interface that transports N1 (NGAP/SCTP)
• AMF — the NF that terminates N1
• Registration — the procedure where N1 plays an active role
• Message dictionary / Interface dictionary — to check terminology