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NASLTEMME to UE3GPP TS 24.301
LTE CS Service Notification
CS Service Notification is the LTE NAS message the network sends when it needs to notify the UE about an incoming CS-related service branch such as CS fallback.
Message Fact Sheet
Protocol
nas
Network
lte
Spec
3GPP TS 24.301
Spec Section
Release 18 message definition for CS Service Notification
Direction
MME to UE
Message Type
EMM signaling
Full message name
LTE CS Service Notification
Protocol
NAS
Technology
LTE
Direction
MME to UE
Interface
N1 over LTE access / S1-MME control path
Signaling bearer / channel
NAS signaling / Sent while the UE is EPS-registered and the network needs to notify a CS-related incoming service condition.
Typical trigger
Triggered when the network needs to notify the UE about incoming CS fallback or related service handling.
Main purpose
Alerts the UE that a CS-oriented service path is pending so the UE can continue the correct extended or fallback service branch.
Main specification
3GPP TS 24.301, Release 18 message definition for CS Service Notification
Release added
Release 8
Procedures where used
CS Fallback, Service Notification Handling
What is LTE CS Service Notification in simple terms?
CS Service Notification is the LTE NAS message the network sends when it needs to notify the UE about an incoming CS-related service branch such as CS fallback.
Alerts the UE that a CS-oriented service path is pending so the UE can continue the correct extended or fallback service branch.
Why this message matters
CS Service Notification tells the UE that a CS-related service branch is pending.
Where this message appears in the call flow
Incoming CS fallback notification
When a CS-related incoming service event exists, the network can notify the UE before the fallback branch continues.
Call flow position: Network notification message sent before the UE continues a CS-oriented restore branch.
Typical state: The UE is already registered and is being notified about a pending CS-related service condition.
Preconditions:
A CS-related incoming service event exists.
Next likely message: Extended Service Request
Notification after paging
The notification can appear as part of the wider reachability and paging branch before the UE continues with the correct service response.
Interface: N1 over LTE access / S1-MME control path
Domain: Core-side EPS mobility-management signaling for CS-related service notification.
Signaling bearer: NAS signaling
Logical channel: Sent while the UE is EPS-registered and the network needs to notify a CS-related incoming service condition.
Transport / encapsulation: EPS NAS EMM message sent by the MME before or around CS fallback-related continuation.
Security context: Usually protected because it appears in an existing EMM context.
Message Structure Overview
CS Service Notification tells the UE that a CS-related service branch is pending.
ASN.1 Message Syntax for LTE CS Service Notification
CS Service Notification
Paging identity OPTIONAL
CLI OPTIONAL
SS code OPTIONAL
LCS indicator OPTIONAL
How to read this message syntax
Read this message from the wider paging and CS fallback context first.
LTE CS Service Notification - Example Dump
CS Service Notification
Protocol discriminator: EPS mobility management
Security header type: Integrity protected and ciphered
Message type: CS Service Notification
CLI: incoming CS service indication
How to read this dump
The main value is procedural: this message explains why the UE later enters a CS-related branch.
Important Information Elements
IE
Required
Description
Paging identity
Optional
Can help tie the notification to the surrounding paging and service context.
CLI
Optional
May carry calling-line or related CS notification detail when present.
Detailed field explanation
Paging identity
Can help tie the notification to the surrounding paging and service context.
Presence: Optional
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.
CLI
May carry calling-line or related CS notification detail when present.
Presence: Optional
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
Check whether paging or service notification context appears immediately before this message.
Follow the branch into Extended Service Request or other fallback continuation.
Common Issues and Troubleshooting
The UE suddenly enters an extended service or CS fallback branch.
Likely cause: CS Service Notification may have signaled an incoming CS-related service condition.
What to inspect: Check paging, CS Service Notification, and the later Extended Service Request.
Next step: Treat the branch as service notification handling before blaming random service-request behavior.
FAQ
What is CS Service Notification in LTE?
It is the LTE NAS message the network sends to notify the UE about a pending CS-related service branch.
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.
