NG Setup Procedure — What the gNB and AMF Exchange When Establishing N2¶

Learning objectives¶

After reading this chapter, you will be able to:

• Explain that NG Setup is a procedure between the gNB and the AMF (non-UE-associated), not a per-UE procedure
• Explain the bring-up of N2 in layers: IP → SCTP → NG Setup → UE-associated NGAP
• Explain the meaning of the main IEs of NG Setup Request / Response / Failure (Global RAN Node ID, Supported TA List, Served GUAMI List, Relative AMF Capacity, Cause, Time to Wait)
• Isolate, layer by layer, the case "SCTP is ESTABLISHED but Registration does not reach the AMF"

Prerequisites¶

• GUAMI / AMF Region / Set / Pointer — the contents of the Served GUAMI List
• N2 — basics of NGAP over SCTP
• Supplementary: NGAP Dictionary / Network Slicing (S-NSSAI) / Registration (UE signaling that flows after NG Setup)

Why — why is a "handshake" needed before any UE?¶

Just connecting a cable and IP between the gNB and the AMF does not tell either of them anything about the other.

• Unless the AMF knows "which PLMN and which Tracking Areas this gNB serves," it cannot decide where to send Paging.
• Unless the gNB knows "which GUAMIs this AMF serves and how much capacity it has," it cannot decide which AMF to send a UE to (previous chapter).

So, before any UE signaling flows, the gNB and the AMF need a procedure to introduce themselves and exchange capabilities. That is NG Setup (TS 38.413 Clause 8.7.1).

Analogy: NG Setup is "the activation procedure between a new branch (gNB) and the head office (AMF)." The branch says, "I am branch XX, and I cover this area and these services," and the head office replies, "I am counter YY at head office; these are my numbers, and this is roughly my capacity." Until activation is complete, customers' (UEs') paperwork cannot be processed.

What — where NG Setup fits¶

A non-UE-associated, Class 1 procedure¶

NGAP procedures are divided into UE-associated procedures, tied to a specific UE, and non-UE-associated procedures, which concern the nodes as a whole. NG Setup is the latter. It is also a Class 1 procedure, in which a request gets a response (success/failure) (TS 38.413 Clause 8.1).

N2 comes up in four layers¶

flowchart TD
    L1["① IP reachability
(routing / VRF / FW)"] --> L2["② SCTP Association
INIT → INIT ACK → COOKIE ECHO → COOKIE ACK"]
    L2 --> L3["③ NG Setup
NGAP non-UE-associated"]
    L3 --> L4["④ UE-associated NGAP
Initial UE Message, etc."]
    L4 --> L5["⑤ NAS procedures
Registration / Authentication / Security"]

SCTP provisions for N2 (TS 38.412 Clause 7)¶

How — the information exchanged¶

Call Flow¶

sequenceDiagram
    participant gNB
    participant AMF
    gNB->>AMF: SCTP INIT
    AMF-->>gNB: SCTP INIT ACK
    gNB->>AMF: SCTP COOKIE ECHO
    AMF-->>gNB: SCTP COOKIE ACK
    Note over gNB,AMF: SCTP Association ESTABLISHED (N2 not yet established)
    gNB->>AMF: NG Setup Request
(Global RAN Node ID, RAN Node Name, Supported TA List, Default Paging DRX)
    AMF->>AMF: checks PLMN etc. against its own configuration
    alt success
        AMF-->>gNB: NG Setup Response
(AMF Name, Served GUAMI List, Relative AMF Capacity, PLMN Support List)
        Note over gNB,AMF: N2 Ready → UE-associated NGAP can flow
    else failure
        AMF-->>gNB: NG Setup Failure
(Cause, Time to Wait)
        Note over gNB: if Time to Wait is present, wait at least that long before retrying
    end

NG Setup Request — gNB → AMF (TS 38.413 Clause 9.2.6.1)¶

The gNB tells the AMF "who I am and what I serve."

This is an excerpt of the main IEs and not exhaustive (optional IEs are added depending on the Release).

Global RAN Node ID¶

Global RAN Node ID (CHOICE)
├─ Global gNB ID
│   ├─ PLMN Identity
│   └─ gNB ID        ← 22–32 bits (length chosen by the operator)
├─ Global ng-eNB ID
└─ Global N3IWF ID   (TNGF / TWIF / W-AGF etc. added depending on the Release)

The operator chooses the gNB ID length in the range 22–32 bits, and it forms the upper part of the NR Cell Identity (36 bits) (TS 38.413 Clause 9.3.1.6, TS 38.300 Clause 8.2; both clause numbers to be confirmed). It is the most important IE for identifying "which gNB this NG Setup came from" in a trace.

Supported TA List — the most important IE in this chapter¶

Supported TA List
└─ Supported TA Item (per TA)
    ├─ TAC
    └─ Broadcast PLMN List
        └─ Broadcast PLMN Item (per PLMN)
            ├─ PLMN Identity
            └─ TAI Slice Support List
                ├─ S-NSSAI #1  (SST, SD)
                └─ S-NSSAI #2

The gNB declares to the AMF: "In this TA, I can provide these S-NSSAIs of this PLMN."

Why is it a "List"? A Tracking Area is a unit that groups multiple cells and is the building block of the Registration Area (TAI List) (TS 23.501 Clause 5.3.2.3). The TAC is broadcast per cell (strictly, per PLMN-IdentityInfo within SIB1) in SIB1 (TS 38.331 Clause 6.2.2 / Clause 6.3.2, to be confirmed). Because the cells of a single gNB may belong to different TAs, the gNB enumerates and declares each TA its cells belong to.

gNB-1
├─ Cell A ─┐
├─ Cell B ─┴─ TAC 0x000001 ─┐
└─ Cell C ─── TAC 0x000002 ─┴─► Supported TA List
                                 ├─ Supported TA Item (TAC 0x000001)
                                 └─ Supported TA Item (TAC 0x000002)

If this declaration and the TAC broadcast in each cell's SIB1 disagree, the TAs known to the AMF and the TA the UE sees in its serving cell diverge (for the impact on Paging and the Registration Area, see Tracking Area / TAI / Registration Area). The relationship between TAs and cells is covered in detail there as well.

A Tracking Area is the unit that makes up a UE's Registration Area (TAI List). From this list the AMF learns "which gNB serves which TA," and sends Paging for a UE in CM-IDLE to the gNBs serving the TAs included in that UE's Registration Area (TS 38.413 Clause 8.5 / TS 23.502 Clause 4.2.3.3, to be confirmed).

What does the AMF check?¶

The rejection condition that is explicitly specified is when the AMF cannot identify any of the PLMNs (including SNPNs from Rel-16 onward) indicated by the gNB (TS 38.413 Clause 8.7.1.4, to be confirmed). Anything else falls under the generic rule "if the AMF cannot accept the setup, it responds with NG Setup Failure" (Clause 8.7.1.3) and depends on the AMF implementation.

flowchart TD
    R["NG Setup Request received"] --> P{"Can the AMF handle any of
the indicated PLMNs?"}
    P -- "No" --> F["NG Setup Failure
(Cause: Unknown PLMN /
Unknown PLMN or SNPN)"]
    P -- "Yes" --> O{"Are TAC / S-NSSAI etc.
consistent with the AMF configuration?"}
    O -- "decision depends on AMF implementation/configuration" --> S["NG Setup Response
or Failure"]

NG Setup Response — AMF → gNB (TS 38.413 Clause 9.2.6.2)¶

This is an excerpt of the main IEs and not exhaustive (optional IEs are added depending on the Release).

Relative AMF Capacity — not CPU utilization¶

Relative AMF Capacity is the weight (weight factor) NG-RAN uses when selecting an AMF (TS 23.501 Clause 5.19.3; clause number to be confirmed). New UEs are distributed to the AMFs with probabilities proportional to this value.

AMF-1: Relative AMF Capacity = 255
AMF-2: Relative AMF Capacity = 128
→ new UEs are distributed roughly 2:1 (details of the distribution are gNB implementation dependent)

• It is not a real-time value of CPU utilization or load. It is a relative value set by the operator according to the AMFs' capacity. It can be changed later with AMF Configuration Update (TS 38.413 Clause 8.7.3).
• It does not affect existing UEs, because UEs with a 5G-GUTI / 5G-S-TMSI are returned to their previous AMF by the GUAMI.

NG Setup Failure — AMF → gNB (TS 38.413 Clause 9.2.6.3)¶

If Time to Wait is included, the gNB must not resend NG Setup to the same AMF until that time has elapsed (TS 38.413 Clause 8.7.1.3). This prevents many gNBs from retrying at once and overwhelming the AMF, for example right after an AMF restart.

Configuration changes after NG Setup¶

The information exchanged in NG Setup can be updated without re-establishing N2.

From NG Setup to UE Registration¶

sequenceDiagram
    participant UE
    participant gNB
    participant AMF
    Note over gNB,AMF: Step 1: SCTP Association
    Note over gNB,AMF: Step 2: NG Setup Request / Response
    UE->>gNB: RRC connection + Registration Request (NAS)
    gNB->>AMF: Initial UE Message (NAS-PDU)
    Note over AMF: Step 3: per-UE procedures begin
(Authentication / Security / Registration Accept)

Before the Initial UE Message, which is the first thing to appear in Registration, SCTP → NG Setup is always already done. The Served GUAMI List and Relative AMF Capacity that the gNB uses to select the AMF for a UE are also information received in this NG Setup.

Packet Analysis (Wireshark)¶

Display filters¶

The numeric NGAP procedureCode values may be displayed differently depending on the dissector and version, so this page does not assert them. Checking message names (NGSetupRequest, etc.) in the GUI's protocol hierarchy is the reliable approach.

Decode example (conceptual)¶

The following is a conceptual representation of the display structure; the values are a fictitious example with the test PLMN (001-01) (not an actual capture).

NGAP NGSetupRequest
 ├─ GlobalRANNodeID
 │   └─ globalGNB-ID
 │       ├─ pLMNIdentity: 001-01
 │       └─ gNB-ID: 0x000001 (22 bits)
 ├─ RANNodeName: gnb-lab-01
 ├─ SupportedTAList
 │   └─ SupportedTAItem
 │       ├─ tAC: 0x000001
 │       └─ broadcastPLMNList
 │           └─ BroadcastPLMNItem
 │               ├─ pLMNIdentity: 001-01
 │               └─ tAISliceSupportList
 │                   └─ s-NSSAI: sST=1, sD=0x010203
 └─ DefaultPagingDRX: v128

Order of checks¶

flowchart LR
    A["① IP
ping / routes"] --> B["② SCTP
4-way handshake
any ABORT/SHUTDOWN?"]
    B --> C["③ NG Setup
Request → Response / Failure"]
    C --> D["④ UE-associated NGAP
Initial UE Message"]

When "SCTP is ESTABLISHED but no Registration Request reaches the AMF," check ③ whether NG Setup succeeded before suspecting the UE.

Build a comparison table¶

When analyzing an NG Setup Failure, it is efficient to put the values sent by the gNB side by side with the AMF configuration.

Trouble Shooting¶

Narrow down from the Cause category¶

When you see an NG Setup Failure, first look at the Cause category to separate "a configuration mismatch / the AMF's state / a problem with the message itself." The list of Cause values is in TS 38.413 Clause 9.3.1.2 (see also the Cause Dictionary).

Do not confuse the SCTP association with the NG connection¶

             IP-A ────────── IP-C
            /                    \
gNB ===== SCTP Association ====== AMF      ← one association can have multiple IP paths (multihoming)
            \                    /
             IP-B ────────── IP-D

• SCTP multihoming lets one association have multiple IP paths; if one path goes down, the association is maintained (RFC 9260).
• Multiple TNL associations means having multiple SCTP associations themselves between the gNB and the AMF (TS 38.412 Clause 7, TS 38.413 Clause 8.7.3).
• Both are a different layer from the NGAP NG connection (the relationship established by NG Setup). In troubleshooting, separate the layers: "IP → SCTP → NGAP (NG Setup)."

Boundary between 3GPP specifications and vendor implementations¶

When NG Setup fails, separate the question into a 3GPP signaling problem / a transport or SCTP problem / a vendor implementation problem.

Comparison with EPC (4G)¶

The skeleton is almost the same as S1 Setup; the big differences in 5G are the addition of slice (S-NSSAI) information and non-3GPP access nodes (N3IWF, etc.).

3GPP Specification¶

Summary¶

• NG Setup is a non-UE-associated procedure between the gNB and the AMF, and the first NGAP procedure performed after SCTP is established
• N2 comes up as IP → SCTP → NG Setup → UE-associated NGAP. "SCTP ESTABLISHED," "NG Setup succeeded" and "Registration succeeded" are separate stages
• The gNB introduces itself with the Global RAN Node ID / Supported TA List (TAC, PLMN, S-NSSAI)
• The AMF returns the Served GUAMI List / Relative AMF Capacity / PLMN Support List, which the gNB uses for AMF selection
• The rejection condition explicitly specified is a PLMN mismatch. Handling of TAC and slice mismatches is implementation dependent
• On failure, look at the Cause (use its category to separate "configuration mismatch / AMF state / message encoding") and Time to Wait. Redoing NG Setup erases UE contexts unless UE Retention has been agreed

Comprehension check¶

Next Step¶

• The contents of the GUAMI received in the NG Setup Response → GUAMI / AMF Region / Set / Pointer
• UE signaling that flows after NG Setup → Registration / SUPI / SUCI / 5G-GUTI
• Mechanisms where the TAC in the Supported TA List matters → Tracking Area / TAI / Registration Area / Paging / Mobility Registration Update
• S-NSSAI and slice selection → Network Slicing
• The whole picture of the N2 interface → N2 / NGAP Dictionary