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LTE DL / UL Information Transfer Procedure Call Flow

call-flowLTE | E-UTRAN | RRC | NAS | S1AP

LTE DL / UL Information Transfer is the connected-mode RRC transport path used to carry NAS payloads and related dedicated information between the UE and the network after the basic radio connection already exists. It is built around DL Information Transfer and UL Information Transfer.

This procedure matters because many LTE NAS messages are only visible to the UE over these RRC wrappers once connected signaling is already active.

Introduction

The LTE DL / UL Information Transfer procedure is not a standalone registration or bearer setup flow. It is the transport layer inside connected mode that lets NAS continuation cross the radio interface after RRC setup or resume has already succeeded.

The main nodes are UE, eNB, and MME. The eNB bridges the RRC wrappers on the radio side to S1AP and NAS on the core side.

What Is LTE DL / UL Information Transfer in Simple Terms?

What starts the procedure: A NAS payload needs to cross the radio interface after connected signaling already exists.
What the UE and network want to achieve: Carry later NAS messages reliably over the dedicated RRC path.
What success looks like: The expected DL or UL RRC wrapper appears and the matching NAS continuation is visible on the other side.
What failure means: NAS seems to disappear between radio and core traces, or the wrapper appears without the expected later NAS handling.

Why this procedure matters

This is one of the most important correlation points between LTE air traces and EPC-side NAS traces after connected mode already exists.

Quick Fact Sheet

Procedure name	LTE DL / UL Information Transfer Procedure
Domain	Connected-mode NAS transport over RRC
Main trigger	Later NAS payload needs radio transport in connected mode
Start state	UE already has a connected signaling path
End state	NAS payload crosses the radio interface and continues toward its next endpoint
Main nodes	UE, eNB, MME
Main protocols	RRC, NAS, S1AP
Main success outcome	Air and core traces line up around the same NAS continuation
Main failure outcome	Radio-side wrapper and core-side NAS continuation do not align
Most important messages	DL Information Transfer, UL Information Transfer
Main specs	TS 36.331, TS 24.301, TS 36.413

LTE DL and UL Information Transfer flow across UE, eNB, and MME — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is already in connected signaling.
A NAS message needs to be sent downlink or uplink through the radio path.
The eNB has the matching S1AP context toward the MME.

Nodes involved

Node	Role in this procedure
UE	Sends or receives the RRC wrapper carrying the NAS payload.
eNB	Bridges the RRC transfer message to or from the S1AP/NAS side.
MME	Owns the NAS message being transported toward or from the UE.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the DL and UL RRC transfer wrappers.
S1-MME	eNB <-> MME	Carries the corresponding NAS transport toward the core.

End-to-end call flow

UE               eNB               MME
|                 |                 |
|<- DL Information Transfer ------- |
|                 |                 |
|-- UL Information Transfer ------> |
|                 |                 |
|   matching NAS continues over S1  |

Major phases

Phase	What happens
1. NAS payload exists	A later NAS message needs radio transport.
2. RRC wrapper transfer	The payload is carried through DL or UL Information Transfer.
3. Core-side continuation	The eNB bridges the payload to the matching S1AP or NAS leg.

Step-by-step breakdown

Step 1: Downlink transfer when the network sends NAS

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

Message(s): DL Information Transfer

Purpose: Carry NAS payload toward the UE over the connected radio path.

State or context change: The UE receives the later NAS message inside the RRC wrapper.

Note: The real content to inspect is often the NAS payload inside the wrapper.

Step 2: Uplink transfer when the UE sends NAS

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

Message(s): UL Information Transfer

Purpose: Carry NAS payload from the UE toward the core over the connected radio path.

State or context change: The MME receives the later uplink NAS continuation.

Note: This message is often the cleanest radio-side proof that the NAS uplink really left the UE.

Important messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
DL Information Transfer	RRC	eNB -> UE	Carries downlink NAS payload to the UE.	Which NAS message is inside and whether it matches the expected scenario.
UL Information Transfer	RRC	UE -> eNB	Carries uplink NAS payload from the UE.	Which NAS message is inside and whether the MME later receives it.

Important parameters to inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Embedded NAS message	The real NAS payload carried inside the RRC wrapper.	DL or UL Information Transfer	Defines the actual meaning of the transfer.	Wrapper decoded but NAS payload overlooked.
Direction	Whether the payload is traveling downlink or uplink.	DL vs UL Information Transfer	Helps match the radio event to the correct S1AP or NAS continuation.	Wrong side correlated in multi-layer traces.
Timing relation	The time gap between radio-side transfer and core-side continuation.	Radio trace plus S1-MME trace	Shows whether transport and correlation are healthy.	Partial capture, timing drift, trace merge issues.
Connected context continuity	The active RRC context carrying the transfer.	Around the transfer messages	Explains whether the payload was sent on a stable connected path.	Connection release, resume, or radio instability near the transfer.

Successful completion of the procedure

Success means the DL or UL RRC wrapper appears and the corresponding NAS continuation is visible on the other side of the radio-core boundary.

Common failures in LTE DL / UL Information Transfer

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
NAS exists on S1, but not on air trace	Partial air capture or wrong radio-side correlation.	Connected radio context around the expected transfer.	DL / UL Information Transfer	LTE Uu, S1-MME	Rebuild correlation before assuming payload loss.
UL Information Transfer is present, but MME seems not to receive NAS	Bridge problem between eNB and MME or missing S1 trace visibility.	Radio-core timing alignment.	UL Information Transfer	LTE Uu, S1-MME	Check whether the issue is true transport loss or only trace incompleteness.
DL wrapper appears, but UE handling still looks wrong	The NAS content inside the wrapper does not match the expected scenario.	Embedded NAS message and its procedure meaning.	DL Information Transfer	LTE Uu	Decode the payload first, not only the wrapper.

What to check in logs and traces

Always decode the NAS payload inside the RRC wrapper.
Correlate DL and UL transfer timing with matching S1AP or NAS events.
Check whether the connected radio context was stable when the transfer happened.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

The wrapper is not the whole meaning of the procedure. The real interpretation comes from the NAS message carried inside it.

FAQ

What is LTE DL / UL Information Transfer?

It is the connected-mode RRC wrapper procedure used to transport NAS payloads over the LTE radio interface.

Why is it important?

Because it links the air trace and the core NAS trace after connected mode already exists.

What should I inspect first?

Start with the embedded NAS message, not only the wrapper name.