LTE Protocol Family Cards
LTE RRC MessagesUE-to-eNodeB radio control messages for system information, RRC connection setup, security, reconfiguration, measurements, mobility, reestablishment, release, MBMS, sidelink, and UE capability.3GPP TS 36.331LTE NAS MessagesUE-to-EPC NAS messages for attach, authentication, security mode, tracking area update, service request, detach, ESM bearer management, and EPS session handling.3GPP TS 24.301S1AP MessagesS1-MME signalling messages between eNodeB and MME for UE context, NAS transport, initial context setup, E-RAB setup, paging, handover, reset, and overload.3GPP TS 36.413IMS SIP MessagesSIP requests and responses used in VoLTE registration, call setup, early media, session update, supplementary services, and call release.3GPP TS 24.229 / RFC 3261X2AP Messages
X2 signalling messages between eNodeBs for X2 setup, LTE handover, SN status transfer, UE context release, load indication, and inter-eNodeB coordination.
3GPP TS 36.423
Browse By Signalling Area
Radio Access / RRC
System information, RRC connection setup, radio bearer configuration, measurements, mobility, reestablishment, and release.
Core Network / NAS
Attach, detach, tracking area update, authentication, security, service request, and EPS bearer/session management.
RAN-Core Interface
S1 signalling between eNodeB and MME for NAS transport, UE context, E-RAB handling, paging, handover, and reset.
Inter-eNodeB Mobility
X2 signalling for handover preparation, status transfer, context release, and inter-eNodeB coordination.
X2AP Messages
VoLTE / IMS
IMS registration, SIP INVITE call setup, 183 Session Progress, PRACK, 200 OK, ACK, BYE, and VoLTE troubleshooting.
Most Used LTE Message Hubs
Common LTE Procedure Paths
LTE Cell Access
MIB -> SIB1 -> System Information -> RRC Connection Request
LTE RRC Connection Setup
RRC Connection Request -> RRC Connection Setup / Reject -> RRC Connection Setup Complete
LTE Attach
RRC Connection Setup -> Initial NAS Attach Request -> Authentication -> Security Mode -> Attach Accept -> Attach Complete
EPS Bearer Setup
NAS ESM request/accept -> S1AP E-RAB Setup -> RRC Connection Reconfiguration
LTE Handover
Measurement Report -> X2AP/S1AP Handover Preparation -> RRC Connection Reconfiguration -> UE Context Release
VoLTE Registration
LTE Attach -> IMS REGISTER -> 401 Unauthorized -> REGISTER with Authorization -> 200 OK
VoLTE Call Setup
INVITE -> 183 Session Progress / 180 Ringing -> PRACK -> 200 OK -> ACK
FAQ
What are LTE messages?
LTE messages are signalling messages used between UE, eNodeB, EPC, neighbouring eNodeBs, and IMS network functions to support access, attach, authentication, bearer setup, mobility, paging, release, and VoLTE services.
Which LTE protocol message families are important?
The main LTE message families are LTE RRC, LTE NAS, S1AP, X2AP, and IMS SIP for VoLTE.
What is the difference between LTE RRC and LTE NAS?
LTE RRC runs between UE and eNodeB for radio connection control, while LTE NAS runs between UE and EPC/MME for attach, authentication, mobility management, and EPS bearer/session control.
What is S1AP used for in LTE?
S1AP is used between eNodeB and MME over S1-MME for NAS transport, UE context handling, E-RAB management, paging, handover, reset, and overload procedures.
What is X2AP used for in LTE?
X2AP is used between neighbouring eNodeBs for X2 setup, handover preparation, SN status transfer, UE context release, and inter-eNodeB coordination.
Which specification defines LTE RRC?
LTE RRC is defined in 3GPP TS 36.331.
Where are VoLTE messages documented?
VoLTE call signalling uses IMS SIP messages, available under the IMS SIP message hub.