【English version】3GPP 5G Standalone Handover Call flow_Rev4.13_20231224.pdf

2023/12/24 Sunday 1
COPYRIGHT © 2022 Centimani CO. CONFIDENTIAL. ALL RIGHTS RESERVED
3GPP 5G Standalone Handover Call Flow Rev4.13
August 25, 2022
Centimani corporation（Technical advisor RYUICHI YASUNAGA）
3GPP 5G SA for field Engineer
English version

CONTENTS
１．What is 5G SA Handover?
２．Inter gNB DU handover / Intra gNB CU handover Call flow
３．Inter gNB Xn Handover Call flow
４．Inter gNB N2 Handover Call flow
Appendix

5G SA Handover
2023/12/24 Sunday 4
・The 3GPP specifications define 5GNR-to-5GNR handover, similar to LTE.
・5G has defined handover types based on the newly introduced disaggregated gNB-
DU and gNB-CU architecture and 5G core.
① Intra gNB-DU handover
➁Inter gNB-DU and Intra gNB-CU handover
(source and target gNB-DU connected to common gNB-CU)
➂Inter gNB-CU handover
(Xn or N2 based handover)

Intra gNB-DU handover
2023/12/24 Sunday 5
This procedure is used when the UE moves from one cell to another within the same gNB-DU
or when an intra-cell handover is performed during NR operation and supported by the UE
context change (gNB-CU initiated) procedure. used for
be done.
※As specified in TS 38.473.
When intra-cell handover is performed...
・gNB-CU provides new UL GTP TEID to gNB-DU.
・gNB-DU provides new DL GTPTEID to gNB-CU.
・The gNB-DU continues to send ULPDCP PDUs to the gNB-CU using the previous UL
GTPTEID until it re-establishes RLC.
・Use new UL GTPTEID after re-establishing RLC.
・The gNB-CU continues to send DLPDCP PDUs to the gNB-DU using the previous DL
GTPTEID until it performs PDCP re-establishment or PDCP data recovery.
・Use new DL GTP TEID upon PDCP re-establishment or data recovery.

Inter gNB DU handover/Intra gNB CU handover
2023/12/24 Sunday 6
This procedure is used when the UE moves from one cell to another within the same
gNB-DU or when an intra-cell handover is performed during NR operation and supported
by the UE context change (gNB-CU initiated) procedure.
※As specified in TS 38.473.
When intra-cell handover is performed...
・gNB-CU provides new UL GTP TEID to gNB-DU.
・gNB-DU provides new DL GTPTEID to gNB-CU.
・The gNB-DU continues to send ULPDCP PDUs to the gNB-CU using the previous UL
GTPTEID until it re-establishes RLC.
・Use new UL GTPTEID after re-establishing RLC.
・The gNB-CU continues to send DLPDCP PDUs to the gNB-DU using the previous DL
GTPTEID until it performs PDCP re-establishment or PDCP data recovery.
・Use new DL GTP TEID after PDCP re-establishment or data recovery.

２．INTER GNB DU HANDOVER /
INTRA GNB CU HANDOVER
CALL FLOW

Inter gNB DU handover / Intra gNB CU handover
2023/12/24 Sunday 8
All control plane and user plane contexts are
managed by the CUs (CUCP and CUUP), so there
is no signaling exchange to the 5G core.
※Both source and target cells are under the
same CU.
This handover procedure involves messaging
over the F1 interface using the F1AP protocol
and the E1 interface using the E1AP protocol.
Cell 1
Cell 3
5G NR UE
gNB
DU #1
gNB
CU CP #1
F1-C
F1-U
E1
gNB
DU #3
gNB
CU UP #1
F1-C
F1-U
E1
Xn-C
Source
Target
Source
PCI:XXX
NR ARFCN：
NR CGI：
Target
PCI:YYY
NR ARFCN：
NR CGI：
gNB＃1
gNB＃3

Inter gNB DU handover / Intra gNB CU handover Call flow
2023/12/24 Sunday 9
Prerequisites
Source
5G UE
UE gNB DU
gNB CU
gNB CUUP
gNB CUCP
Prerequisites：UE is registered and RRC has active DL and UL data sessions
Target
gNB DU
①UE is in RRC_CONNECTED state.
➁ Send and receive uplink and downlink data at the source gNB.
➂The above ① to ➂ are satisfied and the UE is moving towards the target gNB.
1:Measurement Report
・The UE sends a MeasurementReport message containing the signal
strength of the serving cell and neighboring cells to the source gNB-DU.
MeasID：XX
MeasResultServingCell
RSRP XX（-XXX）
RSRQ Option
SINR Option 2:F1AP：UL RRC Message Transfer
・The source gNB-DU sends a UL RRC MESSAGE
TRANSFER message to the gNB-CUCP.
※The message includes MeasurementReport information.
gNB-DU-UE-F1AP-ID：XX
gNB-CU UE-F1AP-ID：XXX
RRC Container
(MeasID:X ServeCellID XX
MeasResultServingCell, RSRPXX,
MeasResultBestNeighborCell phyCellID XX,
RSRP XX )
MeasResultBestNeighborCell
PhyCellID XX
RSRP XX（-XXX）
RSRQ Option
SINR Option
Handover preparation

2023/12/24 Sunday 10
Handover execution
Source
5G UE
UE gNB DU
gNB CU
gNB CUUP
gNB CUCP
Target
gNB DU
・Based on the measurement report and other
information, CUCP decides on handover and
selects handover target gNB-DU.
3:Handover Decision
4:F1AP：UE Context Setup Request
・The gNB-CUCP sends a UE CONTEXT SETUP
REQUEST message to the target gNB-DU to
create a UE context and set up SRB and DRB
bearers.
SpCell-ID：XXXXX
ServeCellIndex
CUtoDURRCInformation：
CellGroupConfig,
SpCellConfig
SRBsToBESetupList
DRBsToBeSetupList
5:F1AP：UE Context Setup Response
・The target gNB-DU responds to the gNB-CUCP
with a UE CONTEXT SETUP RESPONSE message.
*This message is a setup list of newly assigned
C-RNTI, SRB, and DRB.
gNB-DU UE-F1AP-ID：X
DUtoCURRCInformation：
C-RNTI
SRBs-Setup-List
DRBs-Setup-List

2023/12/24 Sunday 11
Handover execution
Source
5G UE
UE gNB DU
gNB CU
gNB CUUP
gNB CUCP
Target
gNB DU
6:E1AP：Bearer Modification Request
・ Respond with gNB-CUUPMODIFICATIONRESPONSE.
gNB-DU-UE-F1AP-ID：X
gNB-CU-UE-F1AP-ID：XXX
RRC Container：RRCReconfigration
（HO Command）
7: E1AP：Bearer Modification Response
・ The gNB-CUCP sends a BEARER CONTEXT
MODIFICATION REQUEST containing the list
of DRBs to be modified to the gNB-CUUP.
・ The gNB-CUCP sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU.
※This includes the generated RRCReconfiguration message, indicating that the UE will stop
transmitting data.
・The source gNB-DU also sends a downlink data delivery status frame to inform the gNB-CUUP
about downlink data that was not sent to the UE.
8: F1AP：UE Context Modification Request
（HO Command）

2023/12/24 Sunday 12
Handover execution
Source
5G UE
UE gNB DU
gNB CU
gNB CUUP
gNB CUCP
Target
gNB DU
9:RRC Reconfigration
（HO Command）
・The source gNB-DU responds to the gNB-CUCP with a UE CONTEXT MODIFICATIONRESPONSE
message indicating that RRCReconfiguration has been sent to the UE.
11: F1AP：UE Context Modification Response
10:Downlink Data Delivery Status
・The source gNB-DU forwards the received RRCReconfiguration
message containing the HO command to the UE.

2023/12/24 Sunday 13
Handover execution
Source
5G UE
UE gNB DU
gNB CU
gNB CUUP
gNB CUCP
Target
gNB DU
14:RRC Reconfigration Complete
・The UE responds to the target gNB-DU with an
RRCReconfigurationComplete message.
12:RACH procedure (target)
・The target gNB-DU sends a downlink data delivery status frame to notify the gNB-CU.
・Downlink packets that may include PDCPPDUs that were not successfully transmitted in the
source gNB-DU are sent from the gNB-CU to the target gNB-DU.
※In the downlink data distribution status, whether to start sending DL user data to gNB-DU
before or after reception depends on the implementation of gNB-CU.
13:Downlink Data Delivery Status
15:Downlink Data

2023/12/24 Sunday 14
Handover execution
Source
5G UE
UE gNB DU
gNB CU
gNB CUUP
gNB CUCP
Target
gNB DU
17:Downlink Data
RRC Container：
RRCReconfigration Complete
16:F1AP:UL RRC Message Transfer
・The target gNB-DU notifies the received RRCReconfigurationComplete message after
transmitting the UL RRC MESSAGE TRANSFER message to the gNB-CU.
・Downlink packets are sent to the UE.
・Uplink packets are sent from the UE and forwarded to the gNB-CU via the target gNB-DU.
18:Uplink Data

2023/12/24 Sunday 15
Post-handover processing
Source
5G UE
UE gNB DU
gNB CU
gNB CUUP
gNB CUCP
Target
gNB DU
Cause
19:F1AP:UE Context Release Command
20:F1AP:UE Context Release Complete
・The gNB-CUCP sends a UE CONTEXT RELEASECOMMAND message
to the source gNB-DU with an appropriate cause value to release
the UE context.
・The source gNB-DU releases the UE context and responds to the
gNB-CUCP with a UE CONTEXT RELEASECOMPLETE message.

３．INTER GNB XN HANDOVER
CALL FLOW

Inter gNB Xn Handover Call flow
2023/12/24 Sunday 17
・Xn handover is basically the same as 4GLTE's X2 handover.
・The prerequisite for Xn handover is that the Xn interface must be linked up between
the source gNB and target gNB.
※Applicable only to intra-AMF mobility. (Xn handover cannot be used if the source
and target gNBs are connected to different AMFs.)
・Xn handover can be intra-frequency HO and inter-frequency HO.
・Source and target gNBs can be connected to two different UPFs.
・If the source gNB and target gNB belong to different tracking areas (TACs), re-
registration is required after the handover is successful.
※Xn handover is faster compared to N2/NGAP handover due to the short signaling
path and the 5G core is only involved in switching the PDU session path.

Inter gNB Xn handover overview
2023/12/24 Sunday 18
The user plane between two gNBs is
managed by their respective CUUP
functions using the GTP-U protocol.
Connecting to gNB-CUCP #2 via the Xn
interface for control plane signaling using
the XnAP protocol considering the control
plane and user plane split architecture of
gNB-CUCP #1.
In this handover procedure, signaling
includes messaging over the Xn interface
using the XnAP protocol and the N2
interface using the NGAP protocol.
Cell
1
Cell
3
5G NR
UE
gNB
DU #1
gNB
CU CP #1
gNB
CU UP #1
F1-C
F1-U
E1
gNB
DU #3
gNB
CU CP #3
gNB
CU UP #3
F1-C
F1-U
E1
Xn-U
AMF
UPF
Xn-C
ソースgNB
ターゲットgNB
PCI:
NR ARFCN：
NR CGI：
PCI:
NR ARFCN：
NR CGI：
The figure on the right shows a mobility
scenario in which the UE is connected to
source cell 1 of gNB#1 on PCI (source)
and is moving towards gNB#3 and target
cell #3 on PCI (target).
gNB＃1
gNB＃3

2023/12/24 Sunday 19
Prerequisites
Source
5G UE
UE gNB #1
5G Core NW
UPF
AMF
Prerequisites: UE is registered and RRC has DL and UL data sessions active
Target
gNB #3
0:XnAP：XnSetupRequest
0:XnAP：XnSetupResponse
① Source and target gNBs have active Xn connections.
➁UE is in RRC_CONNECTED state.
➂ Send and receive uplink and downlink data at the
source gNB.
※The above conditions ① to ➂ are satisfied, and the
UE is moving toward the target gNB.

2023/12/24 Sunday 20
Handover decision
Source
5G UE
UE gNB #1
5G Core NW
UPF
AMF
Target
gNB #3
2:Handover Decision
・The UE sends a MeasurementReport message containing the signal
strength of the serving cell and neighboring cells to the source gNB#1.
MeasID：XX
RSRP XX（-XXX）
RSRQ Option
SINR Option
PhyCellID XX
RSRP XX（-XXX）
RSRQ Option
SINR Option
・Based on the measurement report and other information,
the source gNB decides to handover the UE and selects the
handover target gNB.
・Other information (e.g. taking into account cell loading
and UE mobility limitations and radio capabilities, etc.)

2023/12/24 Sunday 21
Source
5G UE
UE gNB #1
5G Core NW
UPF
AMF
Target
gNB #3
3:XnAP：Handover Request
・The source gNB sends an XnAP handover request message to the target gNB
via the Xn interface.
・This message also carries a transparent RRC container containing handover
preparation information, RRC messages, along with target cell ID, list of PDU
sessions, and other information.
sourceNG-RANnodeUEXnAPIDCause
targetCellGlobalID
GUAMI
UEContextInfoHORequest
UEHistoryInformation
4:Handover Admission
5:XnAP：Handover Request Acknowledge
・The target gNB considers the information
received in the RRC container in step 2
and allocates resources to the UE.
・If the target gNB decides to allow the handover, the target gNB sends a
handover request confirmation message to the source gNB.
※This includes a transparent container sent to the UE as an RRC message
to perform the handover and a list of allowed and disallowed PDU sessions.
sourceNG-RANnodeUEXnAPID
targetiNG-RANnodeUEXｎAPID
PDUSessionResourcesAdmitted-List
Target（番号）SourcesNG-RANnodeTranspContainer

2023/12/24 Sunday 22
Handover execution
Source
5G UE
UE gNB #1
5G Core NW
UPF
AMF
Target
gNB #3
6:RRCReconfigration（HO Command）
・The source gNB triggers the handover by sending the RRCReconfiguration
message containing the information necessary to access the target cell received
in step 5 to the UE.
※This message includes at least the target cell ID, new C-RNTI, and target gNB
security algorithm identifier.
sourceNG-RANnodeUEXnAPIDCause
targetCellGlobalID
GUAMI
UEContextInfoHORequest
UEHistoryInformation
8:Deliver buffered data
7:XnAP：SNStatus Transfer
9:Dawnlink Data Forwarding
・After the source gNB sends the SN status transfer
message to the target gNB, it transfers the uplink
and downlink PDCP SN, hyperframe number (HFN)
status.
・The source gNB starts buffering the DL data from
the UPF and forwards it to the target gNB.
targeting-RANnodeUEXnAPID
DRBsSubjectToStatusTransfer-List

2023/12/24 Sunday 23
Handover execution
Source
5G UE
UE gNB #1
5G Core NW
UPF
AMF
Target
gNB #3
10：Execute RACH procedure from UE to target gNB
11:RRCReconfigration Complete
12:Uplink Data Foerwarding
13:NGAP：PathSwitch Request
・A random access procedure is
performed at the target gNB taking into
account the rach-ConfigDedicated part of
the information received in step 6.
・After the UE successfully connects to the target cell, it
completes the handover procedure by sending an
RRCReconfigurationComplete message to the target gNB.
・The target gNB sends an NGAP path switching request message to the AMF via the NG interface, triggers 5GC,
switches the downlink data path to the target gNB, and establishes an NG-C interface instance towards the
target gNB.
※This message also includes a list of PDU sessions to be switched and a list of PDU sessions that failed to set up
on the target gNB.

2023/12/24 Sunday 24
Handover execution
Source
5G UE
UE gNB #1
5G Core NW
UPF
AMF
Target
gNB #3
15:NGAP：PathSwitch Acknowledge
14:End Marker
・The UPF sends an end marker to the source gNB
to indicate that the data flow is terminated to the
source gNB, and the source gNB sends an end
marker to the target gNB.
16:Downlink Data Foerwarding
・5GC switches downlink data path to target gNB.
・AMF acknowledges the PathSwitchRequest message using
the Path Switch Request AcknowledgeNGAP message.
※This message includes a list of switched PDU sessions and
a list of PDU sessions to be released.

2023/12/24 Sunday 25
Source
5G UE
UE gNB #1
5G Core NW
UPF
AMF
Target
gNB #3
targetNG-RANnodeUEXnAPID
17:XnAP：UE Context Release
・Upon receiving the NGAP Path Switch Request Confirm
message from the AMF, the target gNB sends an XnAP
UE Context Release message to the source gNB, and the
source gNB releases the resources associated with the UE.
Downlink / Uplink Data Forwarding

４．INTER GNB N2 HANDOVER
CALL FLOW

Inter gNB N2 Handoverとは
2023/12/24 Sunday 27
・N2 handover does not depend on the availability of the Xn interface.
・If two gNBs with Xn interfaces support a specified N2 handover, their XnHO is not
allowed in the configuration.
※Applicable to intra-AMF or inter-AMF mobility.
(Source and target gNBs can be connected to the same AMF or different AMFs)
・N2 handover is applied to intra-frequency HO and inter-frequency HO.
・Source and target gNBs can be connected to two different UPFs.
・If the source gNB and target gNB belong to different tracking areas (TACs), tracking
area re-registration is required after a successful handover.
※N2 handover takes longer than XnHO.

Inter gNB N2 Handover Overview
2023/12/24 Sunday 28
This handover procedure involves
messaging over the N2 interface using the
NGAP protocol for signaling.
The figure shows a mobility scenario in
which the UE is connected to source cell #1
of gNB #1 on PCI (source) and is moving
towards target cell #3 of gNB #3 on PCI
(target).
Cell
1
Cell
3
5G NR
UE
gNB
DU #1
gNB
CU CP #1
gNB
CU UP #1
F1-C
F1-U
E1
gNB
DU #3
gNB
CU CP #3
gNB
CU UP #3
F1-C
F1-U
E1
AMF
UPF
source gNB
Target gNB
PCI:
NR ARFCN：
NR CGI：
PCI:
NR ARFCN：
NR CGI：
N2
N2
N3
N3
gNB＃1
gNB＃3

Inter gNB N2 Handover Call flow
2023/12/24 Sunday 29
Prerequisites
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Prerequisites：UE is registered and RRC has DL and UL data sessions active
Target
gNB
・Based on the measurement report and other
information, the source gNB decides to handover
the UE and selects a handover target gNB.
2:Handover Decision
・The UE sends a MeasurementReport message
containing the signal strength of the serving
cell and neighboring cells to the source gNB#1.
・Other information
(Example: Considering cell load, UE mobility
restrictions and radio functions, etc.)
MeasID：XX
RSRP XX（-XXX）
RSRQ Option
SINR Option
PhyCellID XX
RSRP XX（-XXX）
RSRQ Option
SINR Option

2023/12/24 Sunday 30
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Target
gNB
3:NGAP:Handover Required
・Trigger N2 handover based on the decision of the source gNB HO and send a
message to AMF that N2 handover is required.
・The message includes information about the UE RAN-NGAP-ID, AMF-NGAP-ID,
target gNB ID, handover type, handover reason, and the PDU session being
handed over.
AMF-UE-NGAP-ID:XXXXXXXXXXX
RAN-UE-NGAP-ID:XXX
Handover Type:Intra5gs
Cause:Ho-desirable radio condition
TargetID,PLMN,CellID
DirectForwardingPathAvailability:No
PDUSessionResourceListHORqd
Source To Target-
TransparentContainer
4:NGAP:Handover Request
5:Handover Admission
・Once the AMF receives the required handover, it identifies the target
gNB from there and sends a handover request containing the UE security
context, UE capabilities, PDU session information, source-to-target
transparent container, GUAMI, etc.
UEAggregateMaximumBitRate
UESecurityCapabilities
SecurityContext
PDUSessionResourceSetupListHOReq
AllowedNSSAI
SoueceToTarget-TransparentContainer
MobilityRestrictionList
GUAMI

2023/12/24 Sunday 31
Handover decision
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Target
gNB
6:NGAP:Handover Request Acknowledge
・After receiving the handover request from the AMF, the target gNB may decide
whether to admit the UE based on the available resources.
・If the target gNB is able to authorize all PDU sessions with the respective data
bearer, it can respond to the AMF using HandoverRequestAcknowledge.
※The HandoverRequestAcknowledge message includes the UE-NGAP-ID, the list of
authorized PDU sessions, and the TargetToSource-TansparentContainer.
RAN-UE-NGAP-ID:XXX
PDUSessionResourceAdmittedList
Target To Source-TransparentContainer
RAN-UE-NGAP-ID:XXX
PDUSessionResourceHandoverList
TargetToSource-TransparentContainer
7:NGAP:Handover Command
・AMF sends a handover command to the source gNB.
・This message includes the information received in step 6.
※The source gNB can now send the handover command to the UE in step #6.
(After receiving the handover command, the UE leaves the source cell and
starts connecting to the target cell.)

2023/12/24 Sunday 32
Handover execution
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Target
gNB
8:RRC Reconfigration（HO Command）・The source gNB triggers the handover by sending the RRCReconfiguration
message containing the information necessary to access the target cell
received in step 7 to the UE.
※This message includes at least the target cell ID, new C-RNTI, and target
gNB security algorithm identifier.
RAN-UE-NGAP-ID:XXX
RANStatusTransfer-TransparentContainer
9:NGAP:Uplink RAN Status Transfer
・After sending the Handover command, send UPlinkRANStatusTransfer to AMF.
※This includes both RAN and AMF UE-NGAP-ID and RANStatusTransfer-
TransparentContainer.
※RANStatusTransfer-TransparentContainer has information about all DRB
PDCPSNs provided by the source gNB.
10:Data転送

2023/12/24 Sunday 33
Handover execution
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Target
gNB
11:NGAP:Downlink RAN Status Transfer
・The AMF forwards the received UplinkRANStatusTransfer information
to the target gNB using the DownlinkRANStatusTransfer message.
RAN-UE-NGAP-ID:XXX
RANStatusTransfer-TransparentContainer
12:NGAP:Uplink RAN Status Transfer
・After sending the Handover command through the air interface source
gNB, send the UPlinkRANStatusTransfer to the AMF.
※This includes both RAN and AMF UE-NGAP-ID and RANStatusTransfer-
TransparentContainer.
※RANStatusTransfer-TransparentContainer has information about all DRB
PDCPSNs provided by the source gNB.

2023/12/24 Sunday 34
Handover execution
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Target
gNB
14:RRC Reconfigration Complete
・After the UE successfully connects to the target cell, it completes the handover
procedure by sending an RCReconfigurationComplete message to the target gNB.
※The UE initiates uplink data transfer to the target gNB.
RAN-UE-NGAP-ID:XXX
UserLocationInformation
15:NGAP:Handover Notify
・The target gNB sends a handover notification to the AMF.
※この通知で、ターゲットgNBはハンドオーバー成功と見なす。
13:RACH（target）
・A random access procedure is performed at the target
gNB taking into account the information received in step 8
as part of prach-Config Dedicated.

2023/12/24 Sunday 35
Data transfer
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Target
gNB
16:Downlink Data
20:N3 End Marker
17:Uplink Data
18:Uplink Data
19:N3 End Marker

2023/12/24 Sunday 36
Source
5G UE
UE gNB
5G Core NW
UPF
AMF
Target
gNB
21:NGAP:UE Context Release Command
・UEContextReleaseCommand AMF instructs the source gNB to release
resources associated with this UE.
・The message includes the UE-NGAP-ID, which identifies the UE context
and serves as a cause value indicating the success of the handover.
UE-NGAP-IDs
RAN-UE-NGAP-ID:XXX
Cause:successful-handover
22:NGAP:UE Context Release Complete
・When the source gNB successfully deletes the UE context, it sends
UEConextReleaseComplete and releases all resources associated with the UE.

5G gNB architecture
2023/12/24 Sunday 38
Remote
Radio Head
AMF UPF
PHY Low
PHY High
MAC
RLC
DU
RRH
CU
Distributed
Unit
Central
Unit
RRC
PDCP-C
CU-CP
SDAP
PDCP-U
CU-UP
gNB
F2-C F2-U
F1-C F1-U
E1
NG-C NG-U
Xn-C
Xn-U
Lower Layer Split
Higher Layer Split
別のgNB
New architecture
①Central Unit
➁Distributed Unit
➂Remote Radio Head

用語・略語
2023/12/24 Sunday 39
NGAP（NG Application Protocol）・NGAP is at the N2 reference point between gNB and AMF (Core Access and
Mobility and Management Functions) to support services related to both UE
and non-UE.
・This includes operations such as configuration updates, UE context transfer,
PDU session resource management, and support for mobility procedures.
・NGAP is also used to carry downlink and uplink NAS (Non Access Stratum)
messages as payloads, supporting CM idle and CM connection operations such
as paging and UE context release.
N2（N2 Reference Point）・The N2 reference point is between (R)AN and AMF (Access and Mobility
Management Function).
XnAP（Xn Application Protocol）・XnAP is a control protocol used between gNBs to support various RAN-related
procedures such as dual connectivity establishment, Xn-based handover
coordination, data transfer, and RAN paging.
・During the handover process, the source gNB provides the target gNB with all
necessary information to handle the subscriber, such as security and user
plane connectivity information.
※For dual connectivity, the master gNB sets up secondary gNBs using XnAP,
and each gNB serves users at the same time.

用語・略語
2023/12/24 Sunday 40
TEID（Tunnel Endpoint Identifier）・Unambiguously identifies the tunnel endpoint of the receiving GTP-U (GPRS
Tunneling Protocol – User) or GTP-C (GPRS Tunneling Protocol – Control)
protocol entity.
・The receiver of a GTP tunnel locally assigns a TEID value for use by the sender.
・TEID values are exchanged between tunnel endpoints using RANAP (Radio
Access Network Application Part) of GTP-C messages (or UTRAN).
C-RNTI（Cell Radio Network Temporary
Identifier）
・C-RNTI may be used and assigned to a common transport channel for
optimization purposes.
・The decision whether to use C-RNTI is made by the CRNC (Controlling Radio
Network Controller).
SRB（Signalling Radio Bearer）・The RRC (Radio Resource Control) connection process establishes an RB (Radio
Bearer) that can be used in DCCH (Dedicated Control Channel) and CCCH
(Common Control Channel) RRC messages.
・These radio bearers are identified as SRBs (Signalling Radio Bearers).
DRB（Data Radio Bearer）・DRB is used to transmit data.

COPYRIGHT © 2022 Centimani CO. CONFIDENTIAL. ALL RIGHTS RESERVED 2023/12/24 Sunday 41
It is difficult to understand the 3GPP standardization
technical specifications simply by translating and reading
them.
To understand this, it is essential to check consistency with
actual communication logs based on actual call flow
scenarios, but currently such materials are not widely
shared in Japan.
This document was created within our company to
improve understanding and knowledge of 3GPP technical
specifications.
The content is created to exclude vendor/carrier
dependence as much as possible, but it may contain
descriptions that are not consistent with systems that have
been standardized or put into practical use.
This material is royalty-free.Please use it to help readers
consolidate their knowledge and improve their work
productivity.
THANK YOU FOR
VIEWS
ryuichi@centimani.biz

【English version】3GPP 5G Standalone Handover Call flow_Rev4.13_20231224.pdf

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【English version】3GPP 5G Standalone Handover Call flow_Rev4.13_20231224.pdf