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LTE Paging Procedure Call Flow

call-flowLTE | RRC | NAS | S1AP

LTE Paging is the idle-reachability procedure used when the network needs to contact a UE that is not currently in active connected signaling. It is the boundary between idle monitoring and the next access or service-restoration step.

Introduction

The LTE Paging Procedure lets the network reach an idle UE for downlink data, signaling, voice, or system-information change. The page itself is only the alert. The real success check is the next UE action after the page is received.

What Is LTE Paging in Simple Terms?

What starts the procedure: The network needs an idle UE to return to active processing.
What the UE and network want to achieve: Alert the UE and move it into the correct next step.
What success looks like: The UE detects the page and starts fresh access or the right next procedure.
What failure means: The UE misses the page, the identity does not match, or the following access step fails.

Why this procedure matters

Paging is one of the main idle-side checkpoints in LTE because many apparent paging failures are really problems in broadcast configuration, identity matching, or the access step that follows.

Quick Fact Sheet

Main trigger	Pending downlink data, signaling, voice, or system-information change
Start state	UE is idle and monitoring paging occasions
End state	UE either returns to active signaling or remains idle if the page is missed or does not match
Most important messages	Paging, RRC Connection Request, Service Request

LTE Paging procedure flow across UE, eNB, and MME from paging trigger to return access — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is in idle monitoring state and has valid paging configuration.
The network knows the UE paging area and identity.
The serving cell can broadcast the page in the right paging occasion.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
MME	Decides that the idle UE must be contacted.
eNB	Broadcasts the paging message over the radio side.
UE	Monitors paging occasions and reacts if the page matches its identity.

Interfaces used

Interface	Endpoints	Role
S1-MME	MME <-> eNB	Carries the paging trigger toward the eNB.
LTE Uu	eNB -> UE	Carries the broadcast paging message.

End-to-End Call Flow

UE               eNB               MME
|                 |                 |
|  idle, monitoring paging occasion |
|                 |<--S1AP Paging---|
|<--Paging--------|                 |
|--RRC Conn Req-->|                 |
|<-RRC Conn Setup-|                 |
|--RRC Setup Complete + Service Request---------->|
|                 |--Initial UE Message---------->|
|                 |<--Downlink NAS / context------|
|<--next access or service continuation-----------|

Major Phases

1. Paging decision

The network decides the idle UE must be reached.

2. Broadcast page

The eNB transmits the page in the configured paging occasion.

3. UE reaction

If the page matches, the UE starts fresh access and later NAS continuation.

Step-by-Step Breakdown

Paging trigger

Sender -> receiver: Network internal trigger, then MME -> eNB

Message(s): Paging context toward the radio side

Purpose: Decide that the idle UE must return.

State or context change: The UE is targeted for idle-to-connected return.

Note: This stage does not yet prove the UE saw the page.

Paging broadcast

Sender -> receiver: eNB -> UE

Message(s): Paging

Purpose: Alert the idle UE.

State or context change: UE compares the page with its stored identity and timing.

Note: Identity matching and paging occasion timing are the first checks.

Access and continuation

Sender -> receiver: UE -> eNB -> MME

Message(s): RRC Connection Request and later NAS continuation such as Service Request

Purpose: Return the UE to active signaling.

State or context change: Idle monitoring ends and connected signaling begins.

Note: A correct page with a failed later access is not a paging-decode failure.

Important Messages

Message	Protocol	Sender -> Receiver	Purpose in this procedure	What to inspect briefly
Paging	RRC	eNB -> UE	Alerts the idle UE that the network needs it to return.	Paging record identity, occasion timing, and system-information-change indicators.
RRC Connection Request	RRC	UE -> eNB	Starts the fresh access leg after the page is detected.	Whether the UE actually answered the page.
Service Request	NAS	UE -> MME	Common NAS continuation after paging return.	Whether the page led to a real service-restoration attempt.

Important Parameters to Inspect

Item	What it is	Where it appears	Why it matters	Common issues
Paging identity	The UE identity placed in the page.	Paging	Determines whether the UE should react.	Wrong S-TMSI or wrong area.
Paging occasion	The expected time window for monitoring.	Paging configuration and UE behavior	Explains why the UE did or did not see the page.	DRX mismatch, wrong timing assumption.
systemInfoModification	Broadcast-related change indicator in the page.	Paging	Can change the next UE action from service return to broadcast reacquisition.	Wrong next-step interpretation.

Successful Completion

A successful paging path means the UE detects the page and starts the correct next action, usually fresh access and NAS continuation.

Common Failures and Troubleshooting

Paging is transmitted but the UE does not respond

Likely cause: Identity mismatch, timing mismatch, or the following access step fails.

Where to inspect: Paging record, paging occasion, next RRC access attempt.

Relevant message(s): Paging, RRC Connection Request

Relevant interface(s): LTE Uu

Likely next step: Decide whether the page was missed or the return-to-service leg failed after the page.

What to Check in Logs and Traces

Check the paging identity first.
Validate the paging occasion and DRX assumptions.
Always follow the next RRC or NAS message after the page.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Paging only proves that the network tried to reach the UE. To decide whether paging really worked, always follow the next access or NAS continuation after the page.

FAQ

Does Paging itself restore service?

No. Paging only alerts the UE. Service is restored by the access and NAS continuation that follow.

What should I inspect first in a paging issue?

Start with paging identity and paging occasion timing.