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NASLTEUE to MME3GPP TS 24.301
LTE Identity Response
Identity Response is the EPS NAS message the UE sends after Identity Request to return the specific identity the MME asked for so the LTE NAS procedure can continue.
Message Fact Sheet
Protocol
nas
Network
lte
Spec
3GPP TS 24.301
Spec Section
5.4.4.3, 8.2.19
Direction
UE to MME
Message Type
EMM signaling
Full message name
LTE Identity Response
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 after the UE receives Identity Request during attach, TAU, or service-request continuation
Typical trigger
Sent after the UE receives Identity Request and can return the specific requested identity type.
Main purpose
Carries the requested UE identity, such as IMSI, IMEI, or IMEISV, when the network needs stronger identification before continuing attach, tracking area update, or service restoration.
Main specification
3GPP TS 24.301, 5.4.4.3, 8.2.19
Release added
Release 8
Procedures where used
EPS Identification Procedure, LTE Attach Procedure, Tracking Area Updating Procedure, LTE Service Request Procedure
Identity Response is the EPS NAS message the UE sends after Identity Request to return the specific identity the MME asked for so the LTE NAS procedure can continue.
Carries the requested UE identity, such as IMSI, IMEI, or IMEISV, when the network needs stronger identification before continuing attach, tracking area update, or service restoration.
Why this message matters
Identity Response is the UE answering the network's identity question so the LTE/EPS procedure can continue.
Where this message appears in the call flow
Initial LTE attach identity response
In the attach path, Identity Response is the UE answer that lets the MME continue into authentication or later attach handling.
Call flow position: UE reply sent after Identity Request in the attach procedure.
Typical state: The attach path is waiting for stronger UE identity before the MME can continue into authentication, accept, or reject handling.
Preconditions:
The UE received Identity Request during attach.
The UE can return the requested identity.
Next likely message: Authentication Request, Attach Accept, or Attach Reject
Tracking area update identity response
In the TAU path, Identity Response shows that the UE answered the explicit identity query before the network resumed the mobility branch.
Call flow position: UE reply sent after Identity Request in the TAU path.
Typical state: The UE is trying to preserve existing EPS context, but the network first required explicit UE identity before continuing the mobility branch.
Preconditions:
The UE received Identity Request during tracking area update.
The UE can encode the requested identity or fall back to no identity when needed.
Next likely message: Authentication Request, Tracking Area Update Accept, or Tracking Area Update Reject
Service restoration identity response
In the service-restoration path, Identity Response confirms that the UE returned the requested identity before the MME decided whether service could continue.
Call flow position: UE reply sent after Identity Request in the service-request path.
Typical state: The UE is trying to restore service, but the MME required stronger identification before deciding whether service may continue.
Preconditions:
The UE received Identity Request after Service Request.
The UE can return the requested identity to the network.
Next likely message: Authentication Request, Service Accept, or Service Reject
Interface: N1 over LTE access / S1-MME control path
Domain: Core-side EPS mobility management signaling used when the network asked the UE for a more explicit identity before later continuation
Signaling bearer: NAS signaling
Logical channel: Commonly carried in uplink NAS transport after the UE receives Identity Request during attach, TAU, or service-request continuation
Transport / encapsulation: EPS NAS message sent by the UE and delivered to the MME through the eNodeB as part of the EPS identification procedure
Security context: Identity Response is part of the common NAS identification branch, so its protection state depends on the wider procedure and whether earlier security context was already usable.
Message Structure Overview
Identity Response is an EPS mobility-management message rather than an ASN.1 LTE RRC structure.
The practical reading path starts with the triggering NAS procedure, then the returned Mobile identity value.
In real traces, this message confirms that the UE answered the identification branch and lets you follow what the MME did next.
ASN.1 Message Syntax for LTE Identity Response
Identity Response
Mobile identity
How to read this message syntax
Identity Response is a NAS layer-3 message, not an ASN.1 LTE RRC message. The main practical question is whether the UE returned the identity the MME asked for and whether the procedure then continued cleanly.
Start with the higher-level procedure: attach, TAU, or service restoration.
The returned Mobile identity is the main field because it shows what the UE actually gave back to the MME.
After this message, move quickly to the next network step to see whether identification was accepted.
Important Information Elements
IE
Required
Description
Mobile identity
Yes
Carries the UE identity requested by the network, such as IMSI, IMEI, IMEISV, or no identity when the requested value cannot be encoded.
Detailed field explanation
Mobile identity
Carries the UE identity requested by the network, such as IMSI, IMEI, IMEISV, or no identity when the requested value cannot be encoded.
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 which higher-level NAS procedure triggered Identity Response.
Correlate it with the exact Identity Request that came before it.
Check whether the returned Mobile identity matches the requested identity type.
Follow the trace into Authentication Request, later accept handling, or reject handling.
If the procedure restarts, check whether lower-layer failure or no identity handling explains the branch.
Common Issues and Troubleshooting
Attach or TAU detours through identity exchange before continuing.
Likely cause: The MME required stronger identification and the UE answered with Identity Response before later authentication or accept handling.
What to inspect: Check Identity Request, the requested identity type, Identity Response, and the next MME message as one sequence.
Next step: Treat the trace as an identification checkpoint before blaming later authentication or reject handling.
Identity exchange completes, but the procedure still fails or restarts.
Likely cause: The UE may have returned no identity, the returned identity may not satisfy the network, or the branch may fail later in authentication or mobility handling.
What to inspect: Compare the returned Mobile identity with the request and the next Authentication Request, accept, or reject message.
Next step: Decide first whether the issue is identity quality, lower-layer delivery, or a later procedure failure.
Service restoration does not continue even after the UE answered Identity Request.
Likely cause: Identity Response may have completed identification, but the MME may still reject or redirect the branch based on the restored context.
What to inspect: Check Service Request, Identity Request, Identity Response, and the following Authentication Request, Service Accept, or Service Reject.
Next step: Read the branch as service restoration plus identification rather than as a single-message fault.
LTE / 5G / Variant Comparison
Compared with Identity Request
Identity Request is the network prompt. Identity Response is the UE answer carrying the requested identity.
Compared with Authentication Request
Identity Response returns subscriber or equipment identity. Authentication Request comes later when the network challenges the UE to prove that identity.
Compared with Attach Request
Attach Request starts a larger procedure with available identity context. Identity Response is a later UE reply sent only when the network explicitly asks for stronger identity.
FAQ
What is Identity Response in LTE?
It is the EPS NAS message the UE sends after Identity Request to return the specific identity the network asked for.
What should I inspect first in Identity Response?
Start with the triggering NAS procedure and the returned Mobile identity value, then move to the next network message.
What usually comes after Identity Response?
The network often continues with Authentication Request, later accept handling, or a reject branch depending on the wider context.
Why is Identity Response important in troubleshooting?
Because it confirms that the UE answered the identification branch, which helps separate identity-exchange issues from later authentication, mobility, or service problems.
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.
