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5G NR Roaming Registration Call Flow

call-flow 5G NR | Roaming | NAS | Registration | Authentication

5G NR Roaming Registration explains how a UE becomes registered while attached to a visited network.

It brings together visited PLMN access, home-side subscriber validation, and roaming-aware NAS security into one end-to-end call flow.

Introduction

This page is the umbrella roaming registration story for 5G. It is broader than a specific home-routed or local-breakout service model.

The practical engineering pattern is to start with the visited PLMN and UE identity, then follow the validation path toward Registration Accept and the service limits that come with it.

What Is NR Roaming Registration in Simple Terms?

What starts the procedure: A UE in a visited PLMN needs fresh 5GS registration.
What the UE and network want to achieve: Complete roaming registration and reach allowed service state.
What success looks like: The UE registers cleanly and can move on to permitted sessions and services.
What failure means: Roaming policy, identity, or authentication blocks the registration path.

Why this procedure matters

Roaming issues often look like generic registration failures until you include the home-side validation path. This page keeps the full cross-network procedure in view.

Quick Fact Sheet

Procedure name	5G NR Roaming Registration
Domain	5G roaming registration across visited and home network functions
Main trigger	A UE enters or powers on in a visited PLMN and needs 5GS registration
Start state	UE has visited-network radio coverage but no valid roaming registration context
End state	The UE becomes registered in roaming and can continue to allowed services
Main nodes	UE, visited gNB, visited AMF, home UDM/AUSF, roaming policy functions
Main protocols	RRC, NAS, NGAP, roaming subscriber and authentication signaling
Main success outcome	The UE completes registration in the visited network under the permitted roaming model
Main failure outcome	Registration is blocked by roaming policy, authentication failure, or identity mismatch
Most important messages	Registration Request, Authentication Request/Response, Security Mode, Registration Accept, Registration Complete
Main specs	TS 23.501, TS 23.502, TS 24.501, TS 29.510 family, TS 38.300

Preconditions

The UE has visited-network coverage and can reach the access layer.
A valid roaming relationship exists between home and visited networks.
The UE can provide usable identity context for roaming registration.
Home-side subscriber and authentication systems are reachable.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Selects the visited PLMN, starts registration, and completes authentication and security in the roaming context.
Visited gNB	Provides NR access and relays the early registration path toward the visited core.
Visited AMF	Anchors the roaming registration attempt and coordinates with home-side functions.
Home UDM / AUSF	Provide subscriber validation and authentication support from the home network.
Roaming policy functions	Determine what services, slices, and session behavior are allowed in roaming.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> visited gNB	Carries radio access for the roaming registration attempt.
N1	UE <-> visited AMF	Carries NAS registration, authentication, and security signaling.
N2	visited gNB <-> visited AMF	Relays the access-side UE context for registration.
Roaming control interfaces	visited core <-> home core	Carry subscriber, policy, and authentication coordination.

End-to-End Call Flow

UE             visited gNB         visited AMF        home functions
|                  |                  |                    |
|-- Registration Request -----------> |                    |
|                  |----------------->|-- validation ------>|
|                  |                  |<-- allow / vectors --|
|<-- Authentication / Security ------|                    |
|<-- Registration Accept ------------|                    |
|-- Registration Complete ---------->|                    |
|==== roaming registration active =======================>|

Major Phases

Phase	What happens
1. Visited PLMN access	The UE selects and camps on the visited network.
2. Roaming registration start	The UE sends Registration Request through the visited access.
3. Subscriber and roaming validation	Visited and home functions decide whether roaming registration may continue.
4. Security and registration accept	The network authenticates the UE, activates security, and returns Registration Accept.
5. Registration completion	The UE returns Registration Complete and can proceed to allowed services.

Step-by-Step Breakdown

The UE selects the visited network

Sender -> receiver: UE -> visited gNB

Message(s): Cell selection and access establishment

Purpose: Prepare the roaming registration path through the visited PLMN.

State or context change: The UE is ready to begin NAS registration in a foreign network.

Note: Correct visited PLMN selection is the first checkpoint in every roaming trace.

The roaming registration attempt begins

Sender -> receiver: UE -> visited gNB -> visited AMF

Message(s): Registration Request

Purpose: Provide identity and context so the visited core can start roaming validation.

State or context change: The visited AMF creates the initial roaming registration context.

Note: Identity form, registration type, and requested NSSAI strongly influence what happens next.

Visited and home networks validate the UE

Sender -> receiver: Visited AMF <-> home UDM / AUSF

Message(s): Subscriber data and authentication coordination

Purpose: Determine whether the subscriber is allowed to register while roaming.

State or context change: The registration either progresses toward security or stalls on policy and authorization.

Note: Many apparent RAN or NAS issues are really home-side entitlement problems here.

Security is activated and registration is accepted

Sender -> receiver: Visited AMF -> UE

Message(s): Authentication Request, security procedure, Registration Accept

Purpose: Protect the UE and deliver the accepted roaming registration context.

State or context change: The UE becomes registered under the roaming model allowed by the networks.

Note: Registration success still needs to be compared with allowed slice and service restrictions.

The UE finalizes registration

Sender -> receiver: UE -> visited AMF

Message(s): Registration Complete

Purpose: Close the registration transaction cleanly before session setup starts.

State or context change: The UE reaches stable roaming registration state.

Note: Without Registration Complete, later service troubleshooting becomes much harder to read correctly.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Registration Request	NAS	UE -> visited AMF	Starts the roaming registration path.	Inspect identity, roaming context, and requested capabilities.
Authentication Request	NAS	Network -> UE	Shows that subscriber validation is progressing toward secure registration.	Use it to separate authorization failure from later issues.
Registration Accept	NAS	visited AMF -> UE	Confirms successful roaming registration.	Inspect allowed NSSAI and timers relevant to the roaming state.
Registration Complete	NAS	UE -> visited AMF	Closes the transaction and confirms UE acceptance.	Verify it reaches the network cleanly.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
SUPI / SUCI / GUTI context	Identity used for the roaming attempt.	Registration Request	Crucial for home-side subscriber correlation.	Wrong identity handling causes confusing reject behavior.
Visited PLMN	Foreign network where the UE is trying to register.	Access selection and NAS context	Determines which roaming agreement and policy should apply.	Wrong PLMN choice invalidates later debugging.
Allowed NSSAI	Slices available to the roaming UE after registration.	Registration Accept	Explains what sessions can succeed later.	Registration may succeed while later sessions fail due to slice limits.
Authentication outcome	Whether home-side validation and AKA completed.	Authentication phase	Separates entitlement issues from service issues.	Do not treat every failure here as radio trouble.
Roaming model restrictions	Service limits applied in the visited network.	Policy and acceptance context	Important for realistic post-registration expectations.	Ignored restrictions create misleading follow-up troubleshooting.

Success Criteria

The visited network receives a valid roaming Registration Request.
Home-side subscriber and authentication checks succeed.
The UE completes security and accepts the roaming registration context.
Later service procedures follow the expected roaming restrictions and entitlements.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Registration is rejected before security completes	Roaming is not allowed or the identity could not be validated.	Registration Request, identity, and home-side policy context.	Registration Request / Reject	N1, roaming interfaces	Start with authorization, not radio blame.
Authentication stalls in roaming	Cross-network subscriber or security coordination is incomplete.	Authentication timing and home/visited coordination.	Authentication Request / Response	N1, roaming interfaces	This is often a cross-domain integration issue.
Registration succeeds but later service is limited	The roaming model accepted registration but not all slices or services.	Allowed NSSAI and roaming restrictions.	Registration Accept	N1, policy	Very common in real roaming deployments.
UE never sends Registration Complete	The UE did not converge on the accepted context or the return was lost.	Accept-to-Complete phase and UE state.	Registration Complete	N1, NR-Uu	A half-finished registration can look deceptively healthy.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is 5G NR Roaming Registration?

It is the registration procedure used when a UE registers to 5GS while attached to a visited network.

How is it different from local registration?

Visited access is used, but home-side validation and roaming policy strongly influence the outcome.

What proves success?

Registration Accept, Registration Complete, and later allowed service behavior under the roaming profile.

What should I inspect first?

Start with the visited PLMN, UE identity, and home-side subscriber validation path.

Why can registration succeed but service still fail later?

Because roaming restrictions may allow registration but limit slices, sessions, or certain services.