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NASLTEUE to MME3GPP TS 24.301
LTE Attach Complete
Attach Complete is the EPS NAS message the UE sends after Attach Accept to confirm that the granted attach context was received and processed.
Message Fact Sheet
Protocol
nas
Network
lte
Spec
3GPP TS 24.301
Spec Section
5.5.1, 8.2.2
Direction
UE to MME
Message Type
EMM signaling
Full message name
LTE Attach Complete
Protocol
NAS
Technology
LTE
Direction
UE to MME
Interface
N1 over LTE access / S1-MME control path
Signaling bearer / channel
NAS signaling / Commonly carried in uplink NAS transport immediately after Attach Accept, often together with Activate default EPS bearer context accept or reject
Typical trigger
Sent after the UE receives Attach Accept and finishes the local processing needed to acknowledge the granted EPS mobility context.
Main purpose
Confirms successful processing of Attach Accept, completes the UE side of the EPS attach grant, and commonly carries the embedded default bearer activation response needed for service continuation.
Main specification
3GPP TS 24.301, 5.5.1, 8.2.2
Release added
Release 8
Procedures where used
LTE Attach Procedure, Initial Access, Combined EPS Attach, Re-attach after Context Loss
Attach Complete is the EPS NAS message the UE sends after Attach Accept to confirm that the granted attach context was received and processed.
Confirms successful processing of Attach Accept, completes the UE side of the EPS attach grant, and commonly carries the embedded default bearer activation response needed for service continuation.
Why this message matters
Attach Complete is the UE telling the network that the granted LTE attach context was received and processed.
Where this message appears in the call flow
Initial LTE attach completion
In the initial LTE attach path, Attach Complete is the UE confirmation sent after Attach Accept, commonly together with the default bearer activation response in the ESM message container.
Call flow position: UE confirmation message sent after Attach Accept granted attach and returned the default bearer continuation context.
Typical state: UE has accepted the returned EPS mobility context and is confirming attach completion toward the MME.
Preconditions:
The MME sent Attach Accept.
The UE processed the granted attach context.
Next likely message: Registered service continuation
Combined EPS / IMSI attach completion
In a combined EPS / IMSI attach scenario, Attach Complete confirms that the UE processed the granted combined mobility context and completed the attach acceptance path.
Call flow position: UE completion message sent after the combined attach grant is processed.
Typical state: UE is confirming the granted combined mobility context rather than only a basic EPS attach result.
Preconditions:
The combined attach path was accepted.
The UE has processed the returned mobility context.
Next likely message: Registered service continuation
Re-attach after context loss
When older EPS context is no longer usable, Attach Complete closes the fresh attach path after the rebuilt context was granted in Attach Accept.
Call flow position: UE completion message sent after a fresh attach was granted because older context could not be reused.
Typical state: UE is confirming a rebuilt EPS context after the network accepted the fresh attach path.
Preconditions:
A fresh attach path replaced older unusable context.
The new Attach Accept was processed successfully.
Next likely message: Registered service continuation
Interface: N1 over LTE access / S1-MME control path
Domain: Core-side EPS mobility management signaling that finalizes the attach grant and hands the procedure toward registered service continuity
Signaling bearer: NAS signaling
Logical channel: Commonly carried in uplink NAS transport immediately after Attach Accept, often together with Activate default EPS bearer context accept or reject
Transport / encapsulation: EPS NAS message sent by the UE and delivered to the MME through the eNodeB after attach has been granted
Security context: Normally sent after NAS security is active for the attach procedure, so the message is commonly integrity protected and ciphered.
Message Structure Overview
Attach Complete is an EPS mobility-management message rather than an ASN.1 LTE RRC structure.
The practical reading path starts with whether the message appears at all after Attach Accept and what ESM response the container carries.
In real traces, this is the UE-side confirmation that closes the attach acceptance loop.
ASN.1 Message Syntax for LTE Attach Complete
Attach Complete
ESM message container OPTIONAL
How to read this message syntax
Attach Complete is a NAS layer-3 message with a very small structure. The main practical check is whether it appears after Attach Accept and what ESM response, if any, is carried in the message container.
The first useful question is whether Attach Complete appears promptly after Attach Accept.
The ESM message container is the high-value field because it often shows whether the default bearer continuation was accepted or rejected.
If the attach grant looks clean but the MME keeps retransmitting Attach Accept, this message is one of the first checkpoints to verify.
Important Information Elements
IE
Required
Description
ESM message container
Yes
Carries the embedded default bearer activation response or related ESM continuation returned together with attach completion.
Detailed field explanation
ESM message container
Carries the embedded default bearer activation response or related ESM continuation returned together with attach completion.
Presence: Required
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
What to check in logs and traces
Confirm Attach Complete appears after Attach Accept.
Inspect the ESM message container to see whether the default bearer activation was accepted or rejected.
Correlate the message with the end of timer T3450 behavior on the network side.
Check whether the UE then moves into stable registered service behavior.
If Attach Complete is missing, compare retransmissions or attach restarts around the same interval.
Common Issues and Troubleshooting
Attach Accept is seen, but the attach procedure does not finalize cleanly.
Likely cause: Attach Complete may be missing, delayed, or carrying an embedded ESM rejection that changes the service outcome.
What to inspect: Check whether Attach Complete was sent and inspect the ESM message container.
Next step: Read Attach Accept and Attach Complete as one completion pair rather than treating the accept alone as the full success indicator.
The network retransmits Attach Accept.
Likely cause: The MME may still be waiting for Attach Complete and timer T3450 may still be running.
What to inspect: Check whether the UE actually transmitted Attach Complete and whether the uplink NAS path delivered it.
Next step: Correlate the retransmission timing with T3450-related behavior and uplink NAS transport.
Attach succeeds differently after restart or context rebuild.
Likely cause: The rebuilt attach may reach Attach Complete with different embedded ESM continuation from an earlier successful case.
What to inspect: Compare the Attach Complete ESM container and the preceding Attach Accept across the working and failing cases.
Next step: Treat Attach Complete as the final attach-closeout checkpoint before assuming the UE entered healthy registered service.
LTE / 5G / Variant Comparison
Compared with Attach Accept
Attach Accept grants the attach and returns EPS context. Attach Complete is the UE confirmation that closes that grant path.
Compared with Attach Request
Attach Request starts the EPS attach procedure. Attach Complete is the UE-side confirmation near the end of a successful attach.
Compared with LTE NAS Service Request
Attach Complete closes a fresh attach procedure. Service Request restores active service using already valid EPS context.
FAQ
What is Attach Complete in LTE?
It is the EPS NAS message the UE sends after Attach Accept to confirm that the granted attach context was received and processed.
What usually comes after Attach Complete?
The attach procedure is normally considered closed and the UE continues in registered EPS service, subject to the embedded ESM continuation.
What should I inspect first in Attach Complete?
Start by checking whether it appears at all after Attach Accept, then inspect the ESM message container.
Why is Attach Complete important in troubleshooting?
Because Attach Accept alone does not fully close the attach path; Attach Complete is the UE confirmation that lets the network stop waiting for attach completion.
Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.
