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LTE APN / PDN Connectivity Setup Flow

call-flowLTE | EPC | NAS | GTPv2-C

LTE APN / PDN Connectivity Setup is the end-to-end flow that turns a UE request for PDN service into an actual EPC session tied to an APN and a default EPS bearer. The procedure begins with PDN Connectivity Request and reaches the bearer side through default bearer activation.

This page focuses on the session-building path rather than only the final bearer activation step.

Introduction

The APN / PDN connectivity flow links the UE's requested data service to EPC-side session creation at the SGW and PGW. It is the path that answers which APN is used, which PDN type is selected, and whether a usable packet-data session is created at all.

The main nodes are the UE, MME, SGW, and PGW.

What Is LTE APN / PDN Connectivity Setup in Simple Terms?

  • What starts the procedure: The UE asks for PDN connectivity through a NAS request.
  • What the UE and network want to achieve: Create a usable EPC session for the requested APN.
  • What success looks like: PDN Connectivity Request is accepted, EPC session is created, and default bearer activation follows.
  • What failure means: The UE cannot get the APN service it asked for or the EPC session stops before bearer activation.

Why this procedure matters

This flow explains many early data-service failures that are not caused by radio access but by APN policy, PDN type, or session handling in the core.

Quick Fact Sheet

Procedure nameLTE APN / PDN Connectivity Setup Flow
DomainInitial EPC session creation
Main triggerThe UE requests PDN service
Start stateUE needs packet-data session context for an APN
End stateAPN-linked EPC session exists and default bearer activation can proceed
Main nodesUE, MME, SGW, PGW
Main protocolsNAS, GTPv2-C
Main success outcomeRequested APN and PDN service are created successfully
Main failure outcomePDN request is rejected or EPC session creation fails
Most important messagesPDN Connectivity Request, Activate Default EPS Bearer Context Request
Main specsTS 24.301, TS 23.401, TS 29.274
LTE APN and PDN connectivity setup flow across UE, MME, SGW, and PGW
Click the diagram to open the full-size in a new tab.
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Preconditions

  • The UE has already reached the point where NAS session requests can be delivered.
  • The network can evaluate APN permission and PDN type policy for the subscriber.
  • The EPC can create the required session at SGW and PGW.

Nodes and Interfaces

Nodes involved

NodeRole in this procedure
UERequests PDN service for an APN.
MMEProcesses the NAS request and coordinates EPC session creation.
SGWActs as serving gateway for the session path.
PGWProvides the PDN-side service context and addressing.

Interfaces used

InterfacePathRole
NASUE <-> MMECarries PDN Connectivity Request.
S11MME <-> SGWCarries EPC session creation signaling.
S5/S8SGW <-> PGWBuilds the PDN-side session relation.

End-to-End Call Flow

UE               MME               SGW               PGW
|-- PDN Connectivity Request -->|                 |
|                               |-- Create Session ---->|
|                               |<-- Session result -----|
|<- Default bearer activation follows when accepted    |

Major Phases

PhaseWhat happens
1. PDN requestThe UE asks for data service for an APN.
2. Session creationThe EPC creates the session path at SGW and PGW.
3. Bearer activation handoffThe session result moves toward default bearer activation.

Step-by-Step Breakdown

Step 1: PDN Connectivity Request

Sender -> receiver: UE -> MME

Message(s): PDN Connectivity Request

Purpose: Ask the network for PDN service to a selected APN.

State or context change: The network starts session policy and routing checks.

Note: APN, PDN type, and protocol options are the key fields.

Step 2: Create Session

Sender -> receiver: MME -> SGW -> PGW

Message(s): GTPv2-C Create Session Request and Response

Purpose: Build the EPC session for the requested APN service.

State or context change: The core now has the session context needed for default bearer activation.

Note: This is where subscription and APN policy issues often appear.

Step 3: Handoff to default bearer activation

Sender -> receiver: MME -> UE through later NAS delivery

Message(s): Activate Default EPS Bearer Context Request

Purpose: Turn the successful PDN session into an active bearer at the UE side.

State or context change: The flow moves from session creation into bearer activation.

Note: This is the bridge to LTE Default EPS Bearer Establishment.

Important Messages in This Flow

MessageProtocolDirectionPurpose in this procedureWhat to inspect briefly
PDN Connectivity RequestNASUE -> MMEStarts APN and PDN session setup.APN, PDN type, and protocol options.
Activate Default EPS Bearer Context RequestNASMME -> UEBegins the bearer side of the accepted session.Whether the session result really progressed into bearer activation.

Important Parameters to Inspect

ParameterWhat it isWhere it appearsWhy it mattersCommon issues
APNThe service name the UE is asking to reach.PDN Connectivity RequestDefines the target packet-data service.Unknown or blocked APN.
PDN typeThe requested address-family type.PDN Connectivity RequestDefines the IP service model to build.Unsupported IPv4/IPv6 combination.
Protocol optionsConfiguration options requested by the UE.PDN Connectivity Request and later bearer activationExplains DNS and IP-related configuration behavior.Missing or unexpected options.

Successful Completion

Success means the PDN request is accepted, the EPC session is created, and the flow moves into default bearer activation with usable APN service context.

Common Failures and Troubleshooting

SymptomLikely causeWhere to inspectRelevant message(s)Relevant interface(s)Likely next step
PDN request rejected or stallsAPN policy, subscription issue, or PDN type mismatch.PDN Connectivity Request and EPC session result.PDN Connectivity RequestNAS, S11, S5/S8Validate APN and subscriber policy first.
Core session looks fine, but bearer never activatesThe handoff from session creation to bearer activation failed.Timing between create-session result and default bearer activation request.Activate Default EPS Bearer Context RequestNAS, S1-MMEMove into default bearer analysis.
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What to Check in Logs and Traces

  • Start with APN and PDN type in PDN Connectivity Request.
  • Confirm that EPC session creation succeeded before checking bearer activation.
  • Use default bearer activation as the handoff point into the next stage.

Related Pages

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page covers the full session-creation path. The final UE-side bearer activation is covered in the default bearer page in more detail.

FAQ

What is LTE APN / PDN Connectivity Setup?

It is the EPC session-creation flow that turns a UE PDN request into usable APN service context.

What message starts it?

PDN Connectivity Request starts the flow.