What is Paging in simple terms?
Paging is the F1AP non-UE-associated message the gNB-CU sends to the gNB-DU to request radio paging of a UE over the NR air interface.
Initiates radio paging toward a UE, enables CU-controlled UE reachability, supports idle and inactive UE handling, instructs the DU which cells should transmit paging, and bridges core-network or CU-triggered reachability events with radio-layer paging.
Why this message matters
F1AP Paging is the CU's instruction to the DU to transmit radio paging. Check the UE identity, paging index, cell list, and DRX/eDRX fields before deciding why a UE did or did not respond.
ASN.1 for Paging
Paging ::= SEQUENCE {
protocolIEs ProtocolIE-Container { {PagingIEs} },
...
}
PagingIEs F1AP-PROTOCOL-IES ::= {
{ Message Type, mandatory } |
{ UE Identity Index Value, mandatory } |
{ Paging Identity, mandatory } |
{ Paging DRX, optional } |
{ Paging Priority, optional } |
{ Paging Cell List, mandatory } |
{ Paging Origin, optional } |
{ RAN UE Paging DRX, optional } |
{ CN UE Paging DRX, optional } |
{ NR Paging eDRX Information, optional } |
{ NR Paging eDRX Information for RRC INACTIVE, optional } |
{ Paging Cause, optional } |
{ PEIPS Assistance Information, optional } |
{ UE Paging Capability, optional } |
{ Extended UE Identity Index Value, optional } |
{ Hashed UE Identity Index Value, optional } |
{ MT-SDT Information, optional } |
{ NR Paging Long eDRX Information for RRC INACTIVE, optional },
...
}
How to read this ASN.1
This page follows the Release 18 message table in TS 38.473. The everyday decode path is UE Identity Index Value, Paging Identity, Paging Cell List, then optional DRX/eDRX, priority, cause, PEI, capability, or MT-SDT fields.
Detailed field explanation
Message Type
Identifies the F1AP PDU as PAGING.
Presence: Mandatory
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.
UE Identity Index Value
Mandatory value used by the gNB-DU to calculate the paging frame and paging occasion.
Presence: Mandatory
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.
Paging Identity
Mandatory CHOICE identifying the UE being paged, either RAN UE Paging Identity or CN UE Paging Identity.
Presence: Mandatory
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.
Paging Cell List
Mandatory list of NR cells where the gNB-DU should perform paging. Each item includes NR CGI and may include last-used-cell, PEI subgrouping support, or recommended SSB beam information.
Presence: Mandatory
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.
Paging DRX
Optional paging DRX value used to determine the paging cycle for the UE.
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.
Paging Priority
Optional priority indication for the paging request.
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.
Paging Origin
Optional paging origin that the gNB-DU transfers to the UE 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.
RAN UE Paging DRX
Optional RAN paging cycle information as defined in TS 38.304.
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.
CN UE Paging DRX
Optional core-network UE-specific paging cycle information as defined in TS 38.304.
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.
NR Paging eDRX Information
Optional eDRX paging information for extended paging behavior.
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.
NR Paging eDRX Information for RRC INACTIVE
Optional eDRX paging information for RRC inactive UEs.
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.
Paging Cause
Optional paging cause, such as IMS voice, that the DU may send to the UE when supported.
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.
PEIPS Assistance Information
Optional paging early indication and paging subgrouping assistance information.
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.
UE Paging Capability
Optional UE paging capability information, including RedCap-related behavior 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.
Extended UE Identity Index Value
Optional extended index value used for eDRX and subgrouping behavior.
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.
Hashed UE Identity Index Value
Optional hashed identity index value used by paging calculations and subgrouping behavior.
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.
MT-SDT Information
Optional mobile-terminated small data transmission paging information.
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.
NR Paging Long eDRX Information for RRC INACTIVE
Optional long-eDRX paging information for RRC inactive reachability.
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.
