NAS 5G SMF to UE via AMF 3GPP TS 24.501

5G NAS - PDU Session Establishment Accept

PDU Session Establishment Accept is the 5GSM message the network sends when it accepts a UE request to create a new PDU session and returns the address, QoS, and session configuration the UE should use.

Message Fact Sheet

Protocol	nas	Network	5g
Spec	3GPP TS 24.501	Spec Section	8.3.2
Direction	SMF to UE via AMF	Message Type	5GSM signaling

Full message name	5G NAS - PDU Session Establishment Accept
Protocol	NAS
Technology	5G
Direction	SMF to UE via AMF
Interface	N1
Signaling bearer / channel	NAS signaling / Usually carried inside DL NAS Transport on the access side
Typical trigger	The network has accepted the UE's PDU Session Establishment Request and finished the main session selection, policy, address, and user-plane preparation steps.
Main purpose	Confirms successful session establishment and delivers the operational session context such as PDU address, authorized QoS flows, session AMBR, DNN-related configuration, and optional protocol configuration.
Main specification	3GPP TS 24.501, 8.3.2
Release added	Release 15
Procedures where used	5G PDU Session Establishment, VoNR Registration, IMS or enterprise session bring-up
Related timers	T3580
Related cause values	PDU Session Establishment Accept is a success message and does not carry reject causes, Session issues are inferred from returned context, missing fields, or later service behavior

What is PDU Session Establishment Accept in simple terms?

Confirms successful session establishment and delivers the operational session context such as PDU address, authorized QoS flows, session AMBR, DNN-related configuration, and optional protocol configuration.

Why this message matters

PDU Session Establishment Accept is the network saying the data session was created and returning the actual address and service settings the UE can now use.

Where this message appears in the call flow

5G PDU Session Establishment

Call flow position: Successful 5GSM response returned after the network accepts the UE's session request.

Typical state: The session is being finalized and the UE is learning the usable session parameters.

Preconditions:

The UE request reached session-management handling successfully.
The requested DNN, slice, and policy combination was accepted.
The network completed the main address and QoS decision for the session.

Next likely message: User-plane data usage or later session modification procedures

VoNR Registration

Call flow position: Often returned when the IMS session request for voice service is accepted.

Typical state: UE has registered already and is now receiving the IMS session context required for voice over NR service.

Preconditions:

Registration completed successfully.
The requested IMS DNN and slice combination is allowed.

Next likely message: IMS signaling over the newly established session

Call flow position

Previous message(s): PDU Session Establishment Request, DL NAS Transport, SMF session selection and policy evaluation

Next message(s): QoS flow setup handling, Application data transfer, PDU Session Modification Request

Message direction and transport

Sender and receiver: SMF to UE via AMF

Interface: N1

Domain: Core-side session management response with access-side NAS delivery dependency

Signaling bearer: NAS signaling

Logical channel: Usually carried inside DL NAS Transport on the access side

Transport / encapsulation: 5GSM NAS message transported end-to-end from SMF to UE through AMF mediation

Security context: Normally sent after NAS security has already been established, so engineers usually expect protected downlink NAS delivery.

Message Structure Overview

PDU Session Establishment Accept is a 5GSM success response, so engineers read it as a TS 24.501 information-element structure rather than ASN.1.
Operationally, the message tells the UE what session the network actually created and what address, QoS, and service context came with it.
In traces, the most useful first checks are PDU Session ID, PTI, PDU address, selected SSC mode, session AMBR, authorized QoS flows, DNN, and S-NSSAI.

Structure Syntax

This message is not typically analyzed as ASN.1 on the wire. Engineers usually inspect it as a NAS or protocol field decode rather than an ASN.1 tree.

This is a NAS 5GSM message defined by TS 24.501 information elements rather than ASN.1 syntax.

Decoded Message Dump

PDU Session Establishment Accept
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 1
  Message Type: PDU Session Establishment Accept
  Selected SSC Mode: SSC mode 1
  Authorized QoS Flow Descriptions:
    QFI: 9
    5QI: 9
    GFBR: not present
    MFBR: not present
  Session AMBR:
    DL: 200 Mbps
    UL: 100 Mbps
  PDU Address: IPv4v6
    IPv4: 10.145.23.14
    IPv6: 2001:db8:100::14
  S-NSSAI:
    SST: 1
    SD: 0x112233
  DNN: ims
  Extended Protocol Configuration Options:
    DNS IPv4 address: 10.1.1.1
    DNS IPv6 address: 2001:db8::53

How to read this dump

Start with PDU Session ID and PTI so you know which request this accept belongs to.
Check PDU Address early because it immediately tells you whether the UE received usable addressing for the requested session type.
Authorized QoS flow descriptions and Session AMBR are the fastest way to understand the network's accepted service profile.
DNN and S-NSSAI should be compared against the original request and earlier registration context if the session behavior looks unexpected.

Important Information Elements

IE	Required	Description
`PDU Session ID`	Yes	Confirms which session request was accepted and is used to correlate later modify, release, and error procedures.
`PTI`	Yes	Lets engineers match the accept to the original 5GSM transaction.
`Selected SSC mode`	Yes	Shows the continuity model that the network actually accepted for the session.
`PDU address`	Optional	Carries the IP or other session address context the UE will use for service traffic.
`Session AMBR`	Optional	Defines aggregate bit-rate limits and is often inspected when throughput looks lower than expected from the start.
`Authorized QoS flow descriptions`	Optional	Shows which QoS flows were accepted and what operational QoS characteristics were assigned.
`S-NSSAI`	Optional	Confirms the slice context actually used by the established session when slice selection matters.
`DNN`	Optional	Helps verify the data network the session was actually built toward.
`Extended protocol configuration options`	Optional	Carries optional session-side configuration such as DNS or other protocol parameters that often explain later service behavior.

Detailed field explanation

`PDU Session ID`

Confirms which session request was accepted and is used to correlate later modify, release, and error procedures.

Presence: Required

`PTI`

Lets engineers match the accept to the original 5GSM transaction.

Presence: Required

`Selected SSC mode`

Shows the continuity model that the network actually accepted for the session.

Presence: Required

`PDU address`

Carries the IP or other session address context the UE will use for service traffic.

Presence: Optional

`Session AMBR`

Defines aggregate bit-rate limits and is often inspected when throughput looks lower than expected from the start.

Presence: Optional

`Authorized QoS flow descriptions`

Shows which QoS flows were accepted and what operational QoS characteristics were assigned.

Presence: Optional

`S-NSSAI`

Confirms the slice context actually used by the established session when slice selection matters.

Presence: Optional

`DNN`

Helps verify the data network the session was actually built toward.

Presence: Optional

`Extended protocol configuration options`

Carries optional session-side configuration such as DNS or other protocol parameters that often explain later service behavior.

Presence: Optional

What to check in logs and traces

Confirm the accept matches the correct request using PDU Session ID and PTI.
Verify the selected SSC mode, DNN, and S-NSSAI are what the UE expected.
Inspect PDU address allocation carefully, especially in IMS, enterprise, or dual-stack scenarios.
Check the authorized QoS flow descriptions and session AMBR before starting throughput or policy troubleshooting.
Compare the EPCO content with later DNS, MTU, or application-connectivity symptoms.
Correlate the accept with the next data-plane or QoS-activation behavior instead of stopping analysis at the NAS layer.

Common Issues and Troubleshooting

The session is accepted but applications still cannot connect.

Likely cause: The session may have come up with incomplete address or protocol configuration, wrong DNN, or unexpected QoS context.

What to inspect: Check PDU address, DNN, EPCO values, and authorized QoS flow descriptions.

Next step: Continue into user-plane tunnel and application-side verification.

The session is accepted but throughput is poor from the start.

Likely cause: The returned QoS profile, session AMBR, or slice context may be more restrictive than expected.

What to inspect: Check Session AMBR, QoS flow descriptions, QFI allocation, and any slice-specific policy context.

Next step: Correlate NAS session output with QoS flow and radio-layer behavior.

The accept arrives but later modification or service behavior uses unexpected values.

Likely cause: The network may have normalized or changed the session parameters from what the UE originally requested.

What to inspect: Compare this accept against the original request, especially DNN, S-NSSAI, SSC mode, and session type.

Next step: Use this accept as the baseline for later modification or policy troubleshooting.

Detailed explanation

5G NAS - PDU Session Establishment Accept Explained

PDU Session Establishment Accept is the 5GSM message the network sends when it accepts the UE’s request to create a new PDU session. In simple terms, it means the session was created and the network is now returning the actual address, QoS, and service settings the UE should use.

For beginners, the simple meaning is: the network is saying “your data session is ready.”
For experienced engineers, this is one of the most useful 5GSM messages for verifying the returned PDU address, authorized QoS flows, Session AMBR, DNN, S-NSSAI, and the final session characteristics the network actually accepted.

What is PDU Session Establishment Accept in simple terms?

The UE asked for a new data session. The network evaluated the request, selected the right policy and session context, and decided to allow it.

This message tells the UE:

the session was accepted
what session identity the network is using
what address the UE should use
what QoS profile the session received
which DNN and slice context apply in the final accepted session

Without this response, the UE does not have the full session parameters needed for normal data use.

Why this message matters

This message matters because successful session setup is not just about getting an Accept. It is about what the network actually returned.

Engineers inspect this message to understand:

whether the UE got the expected IP or Ethernet addressing
whether the correct DNN and slice were used
whether the QoS profile matches the intended service
whether the session is likely to support IMS, enterprise, or internet traffic correctly

Many service issues that look like user-plane or radio problems actually begin with the values returned here.

Where this message appears in the call flow

A simplified successful session path is:

UE sends PDU Session Establishment Request
Network processes policy, DNN, slice, and user-plane preparation
Network sends PDU Session Establishment Accept
UE begins using the established session

Mini sequence:

UE                              gNB / AMF / SMF
|                                   |
|--- UL NAS Transport ------------->|
|    PDU Session Establishment Req  |
|<-- DL NAS Transport --------------|
|    PDU Session Establishment Acc  |
|========== user data later =======>|

In practical traces, the session is usually considered successful only after this accept can be correlated with usable address and QoS context.

Transport characteristics

sender: network / SMF side
receiver: UE
interface: N1
access-side transport: usually inside DL NAS Transport
signaling class: 5GSM
security expectation: normally delivered over an already protected NAS path

This is still a NAS session-management message, even though the access network and core transport layers carry it for delivery.

What PDU Session Establishment Accept means operationally

Operationally, PDU Session Establishment Accept is the network’s statement of what session was really created, not just whether the request succeeded.

This is where engineers verify:

which PDU Session ID was accepted
whether the PTI matches the original request
what PDU address was assigned
what Session AMBR and QoS flows the network returned
which DNN and S-NSSAI actually ended up in the session
whether the session looks suitable for IMS, internet, or enterprise use

That is why this message often becomes the baseline for all later QoS, throughput, and application troubleshooting.

Important Information Elements

IE	Why it matters
`PDU Session ID`	Confirms which requested session was accepted.
`PTI`	Matches the response to the original transaction.
`Selected SSC Mode`	Shows what continuity model the network actually accepted.
`PDU Address`	Defines the usable address context for the session.
`Session AMBR`	Shows session-level rate limits returned by the network.
`Authorized QoS Flow Descriptions`	Shows what QoS flows and service profile the session received.
`DNN`	Confirms which data network the session is attached to.
`S-NSSAI`	Confirms the slice context used by the session.
`EPCO`	Often explains DNS and service-configuration behavior.

Example message dump

PDU Session Establishment Accept
  Extended Protocol Discriminator: 5G Session Management
  PDU Session ID: 10
  PTI: 1
  Message Type: PDU Session Establishment Accept
  Selected SSC Mode: SSC mode 1
  Authorized QoS Flow Descriptions:
    QFI: 9
    5QI: 9
    GFBR: not present
    MFBR: not present
  Session AMBR:
    DL: 200 Mbps
    UL: 100 Mbps
  PDU Address: IPv4v6
    IPv4: 10.145.23.14
    IPv6: 2001:db8:100::14
  S-NSSAI:
    SST: 1
    SD: 0x112233
  DNN: ims
  Extended Protocol Configuration Options:
    DNS IPv4 address: 10.1.1.1
    DNS IPv6 address: 2001:db8::53

How to read this dump

Start with PDU Session ID and PTI to verify you are reading the right session response.
Then inspect PDU Address because it immediately tells you whether the session has usable addressing.
Check Authorized QoS Flow Descriptions and Session AMBR next to understand the service profile the network actually granted.
Compare DNN and S-NSSAI with the original request if the outcome looks different from what the UE expected.

What to check in logs

confirm the accept matches the expected request transaction
inspect the returned PDU Address
check DNN, S-NSSAI, and Selected SSC Mode
verify QoS flow and Session AMBR values before blaming the radio layer
inspect EPCO when DNS or application configuration looks incomplete
correlate the accept with later user-plane and application behavior

FAQ

What is PDU Session Establishment Accept in 5G?

It is the 5GSM NAS success message that tells the UE its requested session was accepted and returns the session configuration.

Who sends PDU Session Establishment Accept?

The network sends it toward the UE as the successful response to a UE-requested PDU session establishment.

What is the most important field in this message?

There is no single field, but engineers usually start with PDU Session ID, PTI, PDU address, DNN, S-NSSAI, and authorized QoS flow descriptions.

Does this message mean the user plane is fully usable?

It usually means the session was accepted, but engineers still need to verify user-plane tunnel setup, address usability, QoS, and application behavior.

What comes after PDU Session Establishment Accept?

Usually the UE proceeds with traffic or service signaling over the established session, and later changes may trigger modification procedures.

Summary

Advanced Engineer Notes

Many engineers look only at whether the message exists, but the real value is in the returned QoS, address, and policy context.
When session setup appears successful but IMS or enterprise service still fails, this message is often the first place where the mismatch is visible.
Session AMBR and authorized QoS flow descriptions are especially important when the complaint sounds like a radio problem but actually starts in session policy.

LTE / 5G / Variant Comparison

Compared with PDU Session Establishment Reject

Establishment Accept confirms the network created the session and returns operational parameters, while Establishment Reject explains why the session could not be created.

Related message pages

Related Procedures

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.