What is the difference between Class 1 and Class 2 NGAP procedures?

Class 1 procedures have an initiating message and may have a successful or unsuccessful outcome message, while Class 2 procedures are one-way procedures without an explicit outcome message.

What are the main NGAP message roles?

The main NGAP message roles are initiating message, successful outcome, and unsuccessful outcome.

Why is the procedure model important when decoding NGAP?

The procedure model tells you what message should appear next, whether an outcome is expected at all, and which information elements matter at that stage.

What is the NGAP PDU structure used for?

The NGAP PDU structure tells you whether the received message is an initiating message, a successful outcome, or an unsuccessful outcome, and therefore how it fits into the procedure.

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NGAP Procedure Model and Message Structure

Q: What is an NGAP elementary procedure?

An NGAP elementary procedure is one defined signaling procedure made up of one or more NGAP messages with specific message roles and rules.

The NGAP procedure model and message structure define how NGAP elementary procedures are organized, how Class 1 and Class 2 procedures behave, how message roles are used, and how NGAP PDUs and information elements should be read during decode and troubleshooting.

Use it when you need more than a message name and want to understand where the message fits in the procedure flow, whether it starts a procedure, confirms success, reports failure, or belongs to a one-way signaling pattern.

Technology	5G
Area	NGAP procedure and message model
Scope	Elementary procedures, message roles, NGAP PDUs, and information-element structure
Main procedure split	Class 1 and Class 2 elementary procedures
Main message roles	Initiating message, successful outcome, unsuccessful outcome
Main use	Message decode, procedure pairing, failure reading, and message-to-procedure navigation
Related pages	NGAP Overview, NG Interface and Transport, Initial UE Message, Initial Context Setup, Paging, NG Reset, Error Indication

Overview
Elementary procedures
Class 1 and Class 2 procedures
Message roles
NGAP PDU structure
Information elements
Decode workflow
Procedure examples
Troubleshooting
References
FAQ

Overview

NGAP is easier to decode when you think in procedures first and messages second. A message name tells you what was sent, but the procedure model tells you why it was sent, what should appear next, and which fields matter at that stage.

This is especially important in NGAP because many procedures use closely related message names. A correct reading depends on knowing whether the message is an initiating step, a successful outcome, an unsuccessful outcome, or a one-way message with no explicit response.

Elementary procedures

NGAP is organized into elementary procedures. Each elementary procedure defines a signaling action between the NG-RAN node and the AMF, along with the messages and rules that belong to that action.

Concept	Meaning	Why it matters
Elementary procedure	One defined NGAP signaling procedure.	Gives the real unit of procedure reading and troubleshooting.
Message	One NGAP PDU used inside an elementary procedure.	Must be read in the context of its procedure role.
Information elements	Fields carried inside the message.	Explain the actual content, identity, and cause context of the procedure step.

elementary procedure
        -> message role
                -> information elements
                        -> procedure meaning

Class 1 and Class 2 procedures

One of the most important first checks is the procedure class. Class 1 and Class 2 procedures behave differently, and that changes what you should expect to see next in a trace.

Procedure class	Structure	Expectation in traces
Class 1	Initiating message with successful outcome or unsuccessful outcome when defined.	Look for paired message progression and check whether the expected outcome arrived.
Class 2	Initiating message only, with no explicit outcome message.	Do not assume the procedure failed only because there is no response message.

Class 1 pattern

Initiating message
        -> Successful outcome
or
Initiating message
        -> Unsuccessful outcome

This is the main pairing model for response-oriented NGAP procedures. Missing outcomes often point to transport break, timeout, peer rejection, or an incomplete trace.

Class 2 pattern

Initiating message
        -> no explicit outcome
        -> follow later steps

This is the main one-way model. The procedure is usually validated by later progression rather than by a paired outcome message.

Message roles

The role of a message inside the procedure matters more than the message name alone. NGAP PDUs are grouped by message role so the receiving side can interpret how the message fits into the procedure.

Message role	Meaning	Typical reading question
Initiating message	Starts a procedure or sends the one-way procedure step.	What triggered this procedure, and what should follow next?
Successful outcome	Confirms the successful result of the initiating message.	Did the requested action complete as expected?
Unsuccessful outcome	Reports failure or rejection of the requested action.	Why did the procedure fail, and which cause-related IEs explain it?

Trace note: The same high-level procedure family may contain multiple messages with similar names. Message role is what tells you whether you are at the start of the procedure, the success path, or the failure path.

NGAP PDU structure

At a high level, an NGAP message is carried in an NGAP PDU that identifies the message role and then carries the procedure code, criticality handling, and the list of information elements for that message.

NGAP-PDU
  -> initiatingMessage
  or successfulOutcome
  or unsuccessfulOutcome
        -> procedureCode
        -> criticality
        -> protocolIEs

PDU part	Purpose	Why it matters
Message role branch	Tells you whether the message is initiating, successful outcome, or unsuccessful outcome.	Fastest way to place the message inside the procedure.
Procedure code	Identifies which elementary procedure is being used.	Links the raw message to the procedure definition and expected message set.
Criticality	Defines handling expectations when parts of the message are problematic.	Important when analyzing invalid or partially rejected content.
Protocol IEs	Carry the actual content such as identities, causes, lists, and context fields.	These determine what the procedure step actually means.

Read PDU example

NGAP-PDU
  -> initiatingMessage
        -> procedureCode: id-InitialContextSetup
        -> criticality: reject
        -> value: InitialContextSetupRequest
              -> AMF-UE-NGAP-ID
              -> RAN-UE-NGAP-ID
              -> NAS-PDU
              -> PDU Session Resource Setup List

Read this from top to bottom. First identify the message role, then the procedure, then the exact message value, and finally the IEs that explain what the message is trying to do.

Information elements

Information elements are where the message becomes operational. A correct decode depends on knowing which IEs are mandatory, which are optional, and which ones are the real decision points for the procedure.

IE category	Typical examples	Reading value
Identity IEs	AMF UE NGAP ID, RAN UE NGAP ID	Used to correlate messages to the correct UE context.
Cause-related IEs	Cause and related failure fields	Used to explain rejection, release, or failure outcomes.
Payload or list IEs	NAS-PDU, PDU session resource lists, setup lists	Used to understand what the procedure is actually trying to carry or configure.
Configuration or context IEs	Procedure-specific setup or modification fields	Used to understand the requested network action and its scope.

Decode workflow

A repeatable decode method helps much more than trying to remember dozens of message names from memory.

1. identify the procedure code
2. identify the message role
3. decide whether an outcome is expected
4. check the key identity IEs
5. check cause or payload IEs
6. place the message in the wider call flow

Step	Main question	Result
Procedure	Which elementary procedure is this?	You know the valid message set and the general purpose.
Role	Is this initiating, successful outcome, or unsuccessful outcome?	You know whether to expect a response or to inspect a failure path.
Identity	Which UE or interface context does it belong to?	You avoid mixing unrelated traces or contexts.
Meaning	Which IEs explain the action or failure?	You move from raw decode to actual procedure understanding.

Procedure examples

The procedure model becomes easier to remember when tied to familiar NGAP examples.

Example	What to notice	Open next
Initial UE Message	A strong initiating-message example that starts access-side NGAP context.	Initial UE Message
Initial Context Setup	Useful for studying request, response, and failure paths together.	Initial Context Setup Request
NG Reset	Useful when separating interface-level procedures from UE-associated procedures.	NG Reset
PDU Session Resource Setup	Useful for message lists, procedure outcomes, and multi-item payload reading.	PDU Session Resource Setup Request
Error Indication	Useful when analyzing abnormal conditions and message-validity problems.	Error Indication

Troubleshooting

Symptom	Procedure-model angle	Next check
Expected response never appears	Check whether the procedure is actually Class 1 and whether an outcome is required.	Confirm the procedure class before treating the trace as incomplete.
Failure message is hard to interpret	Check message role first, then cause-related IEs.	Separate failure structure from procedure trigger.
Trace looks out of sequence	Check whether messages from different procedures or UEs were mixed together.	Verify procedure code and identity IEs before concluding that sequencing is wrong.
Message decode is technically correct but still unclear	Check which procedure stage the message belongs to.	Use the message role and wider call flow rather than the message name alone.

References

3GPP TS 38.413 - NGAP procedure definitions, message roles, and ASN.1 structure
3GPP TS 38.410 - NG general aspects and principles
3GPP TS 38.412 - NG signaling transport context
3GPP TS 23.502 - Procedure context across the 5G System

FAQ

What is an NGAP elementary procedure?

It is one defined signaling procedure in NGAP, made up of one or more messages with clear message roles and rules.

How do I know whether a response is expected?

Check the procedure class and the message role. Class 1 procedures may have outcome messages, while Class 2 procedures are one-way.

Why is the message role so important?

Because it tells you whether the message starts the procedure, confirms success, or reports failure. Without that context, the decode remains incomplete.

What should I inspect first in a failed NGAP procedure?

First identify the procedure and message role, then check the identity IEs and cause-related IEs.

What is the fastest way to read an NGAP decode?

Start with procedure code, then message role, then identity IEs, then the main payload or cause fields, and finally place the message in the wider procedure flow.