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5G Troubleshooting

PDU Session Establishment Failure

PDU session establishment can fail at several different layers. A UE may receive an immediate PDU SESSION ESTABLISHMENT REJECT, the core may accept the NAS request but the NG-RAN may fail the PDU SESSION RESOURCE SETUP REQUEST, or the session may be accepted while the radio and user-plane configuration still fails to produce a working data path.

This page is designed as a fault-isolation guide. It first separates the failure stage, then maps each branch to the correct NAS, NGAP, and RRC checkpoints. It also includes call flows and example scenarios so the reader can distinguish reject-path failures from resource-setup failures and post-accept connectivity failures.

PDU Session Failure Ladder

Start by identifying which layer produces the first hard failure object: 5GSM reject, NGAP failed setup item, or post-accept bearer or path loss.

Step 1: UE sends PDU SESSION ESTABLISHMENT REQUEST

  • DNN
  • S-NSSAI or requested NSSAI context
  • PDU session type
  • SSC mode
  • EPCO, EAP, or service-level-AA related content if relevant

Step 2: Core decides whether to accept or reject

  • SMF selection
  • subscription and policy checks
  • DNN and slice validation
  • session type and SSC validation
  • resource and UPF checks

Step 3: NAS result

  • PDU SESSION ESTABLISHMENT ACCEPT
  • PDU SESSION ESTABLISHMENT REJECT
  • back-off timer and re-attempt interpretation

Step 4: N2 resource setup

  • AMF sends PDU Session Resource Setup Request
  • NG-RAN tries to set up requested session resources
  • NG-RAN reports success and failure in the Response

Step 5: Radio and bearer continuation

  • RRCReconfiguration
  • DRB, SDAP, and cell-group application
  • completion or later release

Step 6: Working user plane

  • uplink and downlink data path active
  • QoS flows mapped correctly
  • no hidden partial failure

Symptoms and Fast Triage

Use the symptom first, then decide whether you are dealing with NAS rejection, N2 setup failure, or a later data-path problem.

UE gets immediate reject

Likely scope
  • NAS, SMF, subscription, or policy issue
  • DNN, slice, SSC, session type, authentication, or resource rejection

UE gets accept, but session never becomes usable

Likely scope
  • NGAP PDU Session Resource Setup failure
  • radio bearer setup issue
  • user-plane path not created end-to-end

Some PDU sessions succeed, one fails

Likely scope
  • partial failure in PDU Session Resource Setup Response
  • per-session or per-QoS-flow issue rather than global UE failure

Session comes up, but traffic still fails

Likely scope
  • bearer mapping or QoS rule problem
  • DRB, SDAP, N3, or UPF path issue
  • policy or transport misalignment after nominal setup

Call Flows

Read the normal baseline first, then compare it with the reject-path, N2 resource-failure, RRC continuation, and no-data-path branches.

Normal PDU session establishment

Successful PDU session establishment call flow Sequence from PDU SESSION ESTABLISHMENT REQUEST to Accept, PDU Session Resource Setup, RRCReconfiguration, and a usable user-plane path. UE gNB / NG-RAN AMF / SMF 1. PDU SESSION ESTABLISHMENT REQUEST NAS transport and core procedure 2. PDU SESSION ESTABLISHMENT ACCEPT 3. PDU Session Resource Setup Request 4. RRCReconfiguration 5. RRCReconfigurationComplete 6. PDU Session Resource Setup Response

Immediate NAS reject

Immediate PDU SESSION ESTABLISHMENT REJECT The UE sends a request, the core validates DNN, slice, session type, SSC, auth, or resources, and returns a reject with a cause and possibly back-off behavior. UE gNB / NG-RAN AMF / SMF 1. PDU SESSION ESTABLISHMENT REQUEST 2. Validate DNN, type, SSC, auth, or resource 3. PDU SESSION ESTABLISHMENT REJECT cause plus optional back-off timer and re-attempt behavior

NAS accept but N2 resource setup fails

NAS accept followed by NGAP PDU Session Resource Setup failure The session is accepted at NAS level, but the NG-RAN reports failed items during PDU Session Resource Setup Response. UE gNB / NG-RAN AMF / SMF 1. PDU SESSION ESTABLISHMENT ACCEPT 2. PDU Session Resource Setup Request 3. DRB, QoS, or tunnel setup fails for one or more items 4. Response carries failed item(s)

Accept and N2 setup succeed, but radio continuation fails

PDU session accept followed by RRC-side continuation failure The session is accepted and resource setup starts, but the UE cannot apply DRB or QoS setup during RRC continuation. UE gNB / NG-RAN AMF / SMF 1. PDU SESSION ESTABLISHMENT ACCEPT 2. PDU Session Resource Setup Request 3. RRCReconfiguration 4. UE cannot apply DRB or QoS setup and later continuity fails

Session exists but traffic still fails

Established session with unusable data path The session appears active and resource setup completed, but user-plane traffic fails because of N3, QoS, DRB, or UPF-side issues. UE gNB / NG-RAN AMF / SMF / UPF 1. PDU SESSION ESTABLISHMENT ACCEPT 2. PDU Session Resource Setup Response 3. Session appears active, but user-plane traffic fails because of N3, QoS, DRB, or UPF path problems

NAS Reject Path

Use this branch only when the first hard failure object is a 5GSM reject, not when NAS accept already happened and the break occurs later.

What to inspect first

  • exact 5GSM cause value
  • whether Back-off timer value IE is present
  • whether Re-attempt indicator IE is present
  • whether reject is tied to DNN, slice, PDU session type, SSC mode, authentication, or resources

Common reject buckets

  • DNN or subscription mismatch
  • unsupported or wrong PDU session type
  • unsupported SSC mode
  • authentication or authorization failure
  • LADN area restriction
  • user-plane resource shortage
  • policy or service-option restriction

Example: Unsupported session type or SSC mode

Observed behavior
- UE sends PDU SESSION ESTABLISHMENT REQUEST
- UE receives PDU SESSION ESTABLISHMENT REJECT

Typical causes
- #28 unknown PDU session type
- #50 / #51 / #57 / #58 / #61 only-this-type-allowed
- #68 not supported SSC mode

Interpretation
- check requested PDU session type
- check requested SSC mode
- compare against DNN subscription and SMF policy

Example: Resource or policy rejection

Observed behavior
- request is syntactically valid
- network rejects with resource or policy style cause

Typical causes
- #26 insufficient resources
- #29 user authentication or authorization failed
- #33 requested service option not subscribed
- #46 out of LADN service area
- #67 insufficient resources for specific slice and DNN
- #69 insufficient resources for specific slice
- #82 maximum data rate per UE for user-plane integrity protection is too low

Interpretation
- separate UPF or resource shortage from subscription or policy denial
- check whether back-off timer is present and whether re-attempt is allowed

NGAP PDU Session Resource Setup Failure Path

A NAS accept does not guarantee a usable session. This branch is about N2 user-plane and session-resource establishment after the core accepted the session.

What to check

  • which PDU Session IDs were requested
  • whether duplicate PDU Session IDs were present
  • whether required QoS and AMBR fields were included
  • whether the response contains a failed list
  • whether the failure is total or partial

NGAP abnormal condition checks

  • same PDU Session ID repeated in the request list
  • PDU Session ID already active in the NG-RAN context
  • Non-GBR QoS flow present but PDU Session Aggregate Maximum Bit Rate missing
  • GBR QoS flow present but GBR QoS Flow Information missing
  • delay-critical QoS requested without required Maximum Data Burst Volume
  • Ethernet PDU session header-handling implications
  • unstructured PDU session must not use header compression

Interpretation

This is not a NAS reject case. The session can be accepted at 5GSM level while the NG-RAN still reports failure for one or more requested resources in the PDU Session Resource Setup Response.

RRC and Bearer Setup Continuation Failures

A session can clear NAS and NGAP but still fail practically because the UE cannot complete the radio-side bearer and QoS continuation.

RRCReconfiguration checkpoint

  • radioBearerConfig present
  • masterCellGroup or cell-group content present if expected
  • dedicatedNAS-MessageList forwarded correctly if present
  • UE applies bearer and cell-group configuration successfully

Why this stage matters

Treat this separately from pure NAS reject analysis. If the session gets through NAS and NGAP but the UE cannot complete bearer setup or DRB and QoS mapping carried by RRCReconfiguration, the failure is already in radio continuation rather than core-side session decision.

Session Accepted but No Data Path

This is the branch where the session looks established on paper, but the user-plane never becomes usable or only works in one direction.

Practical symptoms

  • UE shows the session as established but no IP traffic works
  • one QoS flow is missing or mis-mapped
  • uplink works but downlink fails, or vice versa
  • N3 or UP tunnel is missing or inconsistent
  • SDAP or DRB mapping looks wrong after configuration

What to correlate

  • NAS accept contents
  • requested versus established QoS flows
  • N2 setup response and failed items
  • DRB and SDAP mapping
  • user-plane tunnel endpoint information
  • first packet traces on N3, gNB, and UE side

Practical Troubleshooting Workflow

Keep the workflow aligned to the first hard failure stage, then map that branch across NAS, NGAP, RRC, and the user plane.

1. Identify the first hard failure

  • PDU SESSION ESTABLISHMENT REJECT
  • PDU Session Resource Setup failure in NGAP
  • RRC continuation failure
  • data-path failure after nominal setup

2. Decode the exact failure object

  • 5GSM cause IE
  • Back-off timer or Re-attempt indicator
  • failed PDU session list in NGAP response
  • missing or failed QoS flow entries
  • RRC configuration application failure

3. Separate subscription or policy from transport or resource

  • DNN, slice, type, SSC, or auth mismatch points to policy or subscription
  • NG-RAN failed items point to radio or user-plane resource setup
  • session exists but traffic is dead points to transport, QoS, DRB, or UPF path

4. Build the cross-layer correlation

  • UE NAS log
  • gNB NGAP trace
  • gNB RRC trace
  • AMF, SMF, and UPF logs
  • first packet attempts on the user plane

Evidence Checklist for Escalation

Escalation should clearly show whether the break is a 5GSM reject, an NGAP failed setup item, or a later bearer or data-path failure.

Minimum NAS evidence

  • PDU SESSION ESTABLISHMENT REQUEST
  • PDU SESSION ESTABLISHMENT ACCEPT or REJECT
  • exact 5GSM cause if rejected
  • Back-off timer and Re-attempt indicator if present

Minimum NGAP evidence

  • PDU SESSION RESOURCE SETUP REQUEST
  • PDU SESSION RESOURCE SETUP RESPONSE
  • failed list and unsuccessful transfer details if any

Minimum RRC evidence

  • RRCReconfiguration
  • RRCReconfigurationComplete or failure symptom
  • DRB, SDAP, and security configuration snapshot if available

Minimum context identifiers

  • SUPI or 5G-GUTI if available
  • PDU Session ID
  • DNN
  • S-NSSAI
  • requested PDU session type
  • requested SSC mode
  • affected QoS flow IDs

Specification Map

  • TS 24.501: UE-requested PDU session establishment and reject handling
  • TS 23.502: end-to-end PDU session establishment procedure context
  • TS 38.413: PDU Session Resource Setup Request, Response, and abnormal conditions
  • TS 38.331: RRCReconfiguration bearer and NAS-delivery checkpoints

FAQ

What is the difference between PDU SESSION ESTABLISHMENT REJECT and PDU Session Resource Setup failure?

PDU SESSION ESTABLISHMENT REJECT is a 5GSM NAS outcome from the core-side session decision path. PDU Session Resource Setup failure happens later during N2 and NG-RAN resource establishment, even if the NAS session phase already looked successful.

Can a PDU session be accepted at NAS level but still fail in NG-RAN?

Yes. TS 38.413 allows the NG-RAN to report failed establishment for one or more requested session resources inside PDU Session Resource Setup Response, so a NAS accept does not guarantee a usable session.

Which cause values usually point to DNN, slice, or session-type problems?

Common high-value 5GSM causes include #28 for unknown PDU session type, #68 for unsupported SSC mode, #33 for requested service option not subscribed, and #67 or #69 for slice-and-resource style denial. Those should be decoded before blaming radio setup.

What should I check if the session is accepted but traffic still does not flow?

Correlate the NAS accept with the NGAP setup response, DRB and SDAP mapping, tunnel endpoint information, QoS flow setup, and the first packet attempts on N3, gNB, and UE-side traces.

How do I distinguish a pure core rejection from a radio bearer setup failure?

A pure core rejection stops at PDU SESSION ESTABLISHMENT REJECT and a 5GSM cause. A radio bearer setup failure happens after accept, when NGAP resource setup, RRCReconfiguration, or DRB and QoS continuation fails.

When should I analyze Back-off timer and Re-attempt indicator behavior?

Analyze them whenever the network rejects the PDU session at NAS level. They help you decide whether the UE should retry immediately, wait, or treat the denial as a policy-driven restriction rather than a transient radio issue.

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