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NASLTEUE to MME3GPP TS 24.301
LTE Uplink NAS Transport
Uplink NAS Transport is the EPS mobility-management message the UE sends to carry an encapsulated SMS NAS payload toward the network over LTE NAS signaling.
Message Fact Sheet
Protocol
nas
Network
lte
Spec
3GPP TS 24.301
Spec Section
5.6.3.2, 8.2.30
Direction
UE to MME
Message Type
EMM signaling
Full message name
LTE Uplink NAS Transport
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 during registered LTE operation when the UE sends SMS over NAS.
Typical trigger
Sent when the UE SMS entity requests delivery of an SMS message over EPS NAS transport.
Main purpose
Lets the UE deliver SMS content through EPS NAS transport when the SMS entity needs network delivery without switching to a separate legacy signaling path.
Main specification
3GPP TS 24.301, 5.6.3.2, 8.2.30
Release added
Release 8
Procedures where used
Transport of NAS Messages Procedure, SMS over NAS, Registered-State SMS Delivery, Control Plane CIoT SMS Transport
Uplink NAS Transport is the EPS mobility-management message the UE sends to carry an encapsulated SMS NAS payload toward the network over LTE NAS signaling.
Lets the UE deliver SMS content through EPS NAS transport when the SMS entity needs network delivery without switching to a separate legacy signaling path.
Why this message matters
Uplink NAS Transport is the UE sending SMS content toward the network inside an EPS NAS message.
Where this message appears in the call flow
SMS over NAS during registered LTE operation
During normal registered LTE operation, Uplink NAS Transport carries the encapsulated SMS payload from the UE toward the network.
Call flow position: UE-originated EMM transport message sent when SMS is carried over EPS NAS while the UE already has usable registered context.
Typical state: The UE is registered and is using NAS signaling as the carrier for the encapsulated SMS payload.
Preconditions:
A valid EMM context already exists.
The SMS entity requested uplink transport of an SMS message.
Next likely message: Downlink NAS Transport or normal SMS transfer continuation
SMS over NAS after service restoration
After service access is restored, the UE may send Uplink NAS Transport to continue the SMS-over-NAS branch.
Call flow position: Uplink NAS transport message sent after service access has been restored and the UE can now deliver the encapsulated SMS payload.
Typical state: The UE already recovered the signaling path and is using the restored context for SMS transfer.
Preconditions:
The UE has a usable signaling path after service restoration.
The SMS entity still needs to send the message.
Next likely message: Downlink NAS Transport or registered-state continuation
Control plane CIoT SMS transport
In control plane CIoT EPS optimization, Uplink NAS Transport may still carry the encapsulated SMS payload without the usual PDN assumptions.
Call flow position: UE-originated NAS transport message sent when control plane CIoT EPS optimization allows SMS transport without the usual PDN assumptions.
Typical state: The UE is using the control-plane transport path for SMS delivery instead of depending on normal user-plane service context.
Preconditions:
The UE supports the relevant CIoT optimization context.
The SMS entity requested delivery through NAS transport.
Next likely message: Downlink NAS Transport or SMS transport continuation
Interface: N1 over LTE access / S1-MME control path
Domain: Core-side EPS mobility management signaling used for transport of encapsulated NAS messages, especially SMS.
Signaling bearer: NAS signaling
Logical channel: Commonly carried in uplink NAS transport during registered LTE operation when the UE sends SMS over NAS.
Transport / encapsulation: EPS NAS EMM message sent by the UE and delivered to the MME through the eNodeB with the SMS payload inside NAS message container.
Security context: Uplink NAS Transport is sent during an established EMM context, so it normally appears in protected NAS continuation rather than in early unauthenticated access.
Message Structure Overview
Uplink NAS Transport is an EPS mobility-management message rather than an ASN.1 LTE RRC structure.
The practical reading path starts with the procedure context, then the presence of the NAS message container carrying the SMS payload.
In real traces, this message matters because it shows the UE chose NAS transport for SMS delivery and because timers like T3346 or T3447 may block it.
ASN.1 Message Syntax for LTE Uplink NAS Transport
Uplink NAS Transport
NAS message container
How to read this message syntax
Uplink NAS Transport is a NAS layer-3 message, not an ASN.1 LTE RRC message. The main structural value is the NAS message container IE because it carries the encapsulated SMS payload.
LTE Uplink NAS Transport - Example Dump
Uplink NAS Transport
Protocol discriminator: EPS mobility management
Security header type: Integrity protected and ciphered
Message type: Uplink NAS Transport
NAS message container: encapsulated SMS payload
How to read this dump
Start by confirming that the message belongs to the SMS-over-NAS branch rather than a service request or mobility procedure.
The highest-value field is the NAS message container because it proves the UE is transporting encapsulated SMS content.
If the message is missing, check T3346 and T3447 restrictions before assuming the SMS entity never tried to send it.
Important Information Elements
IE
Required
Description
NAS message container
Yes
Carries the encapsulated SMS payload that the UE wants to transport through EPS NAS signaling.
Detailed field explanation
NAS message container
Carries the encapsulated SMS payload that the UE wants to transport through EPS NAS signaling.
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 the UE already had an established EMM context when it sent the message.
Check that the NAS message container is present.
Decide whether the branch is normal registered-state SMS delivery, post-service-restoration delivery, or CIoT control-plane transport.
If missing, inspect T3346 and T3447 restrictions first.
Correlate the message with later Downlink NAS Transport or higher-layer SMS completion.
Common Issues and Troubleshooting
SMS over LTE NAS is not sent even though the UE is registered.
Likely cause: The UE may be blocked by T3346 or T3447, or the signaling path may not yet be usable for the SMS transport branch.
What to inspect: Check timer state, registered-state context, and whether service restoration happened before the SMS attempt.
Next step: Treat it as a transport-permission problem before assuming the SMS payload itself is wrong.
SMS works in some registered cases but not after attach without PDN connection.
Likely cause: T3447 restrictions may block Uplink NAS Transport in that specific context unless the required exception applies.
What to inspect: Check whether the UE attached without PDN connection and whether any network-initiated signaling message was received.
Next step: Compare the failing case against a working case with the same EMM context assumptions.
CIoT control-plane SMS behavior differs from normal LTE cases.
Likely cause: The UE may be following the CIoT optimization path, where NAS transport can be used with different service assumptions.
What to inspect: Check whether control plane CIoT EPS optimization is in use and whether the transport path fits that context.
Next step: Read the message in its CIoT procedure context instead of forcing a normal PDN-connected interpretation.
LTE / 5G / Variant Comparison
Compared with Downlink NAS Transport
Uplink NAS Transport carries encapsulated SMS content from the UE toward the network. Downlink NAS Transport carries encapsulated SMS content from the network toward the UE.
Compared with Service Request
Service Request restores a usable signaling or service path. Uplink NAS Transport uses that path to carry the encapsulated SMS payload.
Compared with Attach Accept
Attach Accept grants or refreshes EPS registration context. Uplink NAS Transport uses an already established EMM context to carry SMS content.
FAQ
What is Uplink NAS Transport in LTE?
It is the EPS NAS message the UE sends to carry an encapsulated SMS payload toward the network.
What should I inspect first in Uplink NAS Transport?
Start with the procedure context and the NAS message container, then check whether timer restrictions such as T3346 or T3447 blocked the branch.
Why is this message important in troubleshooting?
Because it shows whether the UE actually tried to send SMS over NAS and whether the EMM context allowed that transport path.
What usually comes after Uplink NAS Transport?
The SMS branch normally continues with Downlink NAS Transport or later higher-layer SMS handling.
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.
