LTE E-RAB Modify Procedure Call Flow
LTE E-RAB Modify Procedure changes the radio-access side treatment of an active bearer without releasing the whole bearer relation. It is used when the MME needs the eNB to update an active E-RAB because the bearer profile, transport handling, or related access-side context has changed.
The main S1AP pair is E-RAB Modify Request and E-RAB Modify Response.
Introduction
The E-RAB modify path is the access-side partner of bearer change handling. It does not create a brand-new bearer and does not represent initial attach. Instead, it refreshes the live eNB-side bearer view for an already active session.
The main nodes are the MME and eNB, with the UE affected through the updated radio-bearer behavior.
What Is LTE E-RAB Modify Procedure in Simple Terms?
- What starts the procedure: The network needs to change an already active E-RAB at the access side.
- What the UE and network want to achieve: Keep the bearer active while updating radio-side handling.
- What success looks like: The eNB receives E-RAB Modify Request and returns E-RAB Modify Response.
- What failure means: The access side stays on the old configuration or the bearer enters a partially updated state.
Why this procedure matters
This procedure explains cases where bearer behavior changes at the access side even though the session was already active before the update started.
Quick Fact Sheet
| Procedure name | LTE E-RAB Modify Procedure |
|---|---|
| Domain | Active radio bearer update |
| Main trigger | Change in bearer-related access context |
| Start state | Active E-RAB already exists at the eNB |
| End state | Existing E-RAB remains active with updated parameters |
| Main nodes | MME, eNB, UE |
| Main protocols | S1AP |
| Main success outcome | eNB applies the updated E-RAB definition |
| Main failure outcome | E-RAB update fails or remains only partially applied |
| Most important messages | E-RAB Modify Request, E-RAB Modify Response |
| Main specs | TS 36.413, TS 23.401 |
Preconditions
- An E-RAB is already active at the eNB.
- The MME has an updated bearer-related access-side instruction.
- The eNB can still identify the live E-RAB and UE context cleanly.
Nodes and Interfaces
Nodes involved
| Node | Role in this procedure |
|---|---|
| MME | Starts the E-RAB modification toward the eNB. |
| eNB | Applies the requested change to the active E-RAB. |
| UE | Uses the updated radio path after the eNB applies the change. |
Interfaces used
| Interface | Path | Role |
|---|---|---|
| S1-MME | MME <-> eNB | Carries E-RAB Modify Request and Response. |
| LTE Uu | UE <-> eNB | Reflects the later result of the updated radio-bearer behavior. |
End-to-End Call Flow
UE eNB MME
| |<-- E-RAB Modify Req --|
| |-- radio bearer update |
| |--- E-RAB Modify Resp ->|
| updated E-RAB behavior becomes live |Major Phases
| Phase | What happens |
|---|---|
| 1. Access-side update decision | The MME decides the eNB must refresh an active E-RAB. |
| 2. E-RAB modification | The eNB receives the change request and updates its live context. |
| 3. Result reporting | The eNB confirms success or partial failure back to the MME. |
Step-by-Step Breakdown
Step 1: E-RAB Modify Request
Sender -> receiver: MME -> eNB
Message(s): E-RAB Modify Request
Purpose: Ask the eNB to refresh one or more active E-RAB definitions.
State or context change: The eNB begins updating live bearer-specific access context.
Note: E-RAB ID and QoS-related fields are the first checks.
Step 2: E-RAB Modify Response
Sender -> receiver: eNB -> MME
Message(s): E-RAB Modify Response
Purpose: Confirm which E-RABs were updated successfully.
State or context change: The MME now knows whether the requested access-side change took effect.
Note: Partial success cases are important in multi-bearer traces.
Important Messages in This Flow
| Message | Protocol | Direction | Purpose in this procedure | What to inspect briefly |
|---|---|---|---|---|
| E-RAB Modify Request | S1AP | MME -> eNB | Requests a live E-RAB update at the access side. | E-RAB IDs, QoS-related fields, and whether the right UE context is targeted. |
| E-RAB Modify Response | S1AP | eNB -> MME | Reports the result of the requested E-RAB changes. | Success list, failed items, and partial completion details. |
Important Parameters to Inspect
| Parameter | What it is | Where it appears | Why it matters | Common issues |
|---|---|---|---|---|
| E-RAB ID | The identifier of the radio access bearer being changed. | Modify Request and Response | Keeps the change tied to the right active bearer. | Wrong bearer identity or missing item mapping. |
| QoS information | The bearer treatment profile requested at the access side. | Modify Request | Shows why the eNB must change handling. | Unexpected values or inconsistent bearer profile. |
| S1AP UE IDs | The UE identifiers used on S1-MME. | Request and Response | Ensure the correct UE context is being updated. | Context mismatch or stale UE ID mapping. |
Successful Completion
Success is confirmed when the eNB returns E-RAB Modify Response and the requested E-RAB changes are visible as applied at the access side.
Common Failures and Troubleshooting
| Symptom | Likely cause | Where to inspect | Relevant message(s) | Relevant interface(s) | Likely next step |
|---|---|---|---|---|---|
| Modify response shows partial success | Only some bearer items could be changed at the eNB. | Per-item success and failure details. | E-RAB Modify Request, E-RAB Modify Response | S1-MME | Check which E-RAB IDs failed and why. |
| Access side stays on the old behavior | The modify branch was not fully applied or mapped to the wrong UE context. | UE IDs and E-RAB IDs in the request. | Modify Request and Response | S1-MME, LTE Uu | Verify access-side context continuity first. |
What to Check in Logs and Traces
- Check that the request targets the correct UE context and E-RAB IDs.
- Compare the requested access-side changes with the response result list.
- If the bearer change began in EPC signaling, correlate the S1AP modify branch with the earlier bearer update.
Related Pages
Related sub-procedures
Related message reference pages
- LTE Initial Context Setup Procedure — common earlier E-RAB setup context
Related troubleshooting pages
Notes
E-RAB modify is an access-side bearer update. It changes a live radio-access bearer relation rather than creating a new one from scratch.
FAQ
What is LTE E-RAB Modify Procedure?
It is the S1AP procedure used to change an already active E-RAB at the access side.
What is the main success message?
E-RAB Modify Response is the clearest confirmation that the eNB finished the requested update branch.