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

・A hands-on resource for mobile engineers

1. 2023/12/24 Sunday 1
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3GPP 5G Standalone Handover Call Flow Rev4.13
August 25, 2022
Centimanicorporation（Technical advisorRYUICHI YASUNAGA）
3GPP 5G SA for field Engineer
English version

2. CONTENTS
１．What is 5G SA Handover?
２．Inter gNBDU handover / Intra gNB CU handover Call flow
３．Inter gNBXnHandover Call flow
４．Inter gNBN2 Handover Call flow
Appendix

3. １．WHAT IS 5G SA HANDOVER?

4. 5G SA Handover
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・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.
①Intra 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
(Xnor N2 based handover)

5. Intra gNB-DU handover
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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.

6. Inter gNBDU handover/Intra gNBCU handover
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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.

7. ２．INTER GNBDU HANDOVER /
INTRA GNBCU HANDOVER
CALL FLOW

8. Inter gNBDU handover / Intra gNBCU handover
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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

9. Inter gNBDU handover / Intra gNBCU handover Call flow
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Prerequisites
Source5GUE
UE gNBDU
gNBCU
gNBCUUPgNBCUCP
Prerequisites：UE is registered and RRC has active DL and UL data sessions
Target
gNBDU
①UE is in RRC_CONNECTED state.
➁Send and receive uplink and downlink data at the source gNB.
➂The above ①to ➂are satisfied and the UE is moving towards the target gNB .
1:Measurement Report
・The UE sends a MeasurementReportmessage containing the signal
strength of the serving cell and neighboring cells to the source gNB -DU.
MeasID：XX
MeasResultServingCell
RSRPXX（- XXX）
RSRQOption
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 MeasurementReportinformation.
gNB-DU-UE-F1AP-ID：XX
gNB-CU UE-F1AP-ID：XXX
RRC Container
(MeasID:XServeCellIDXX
MeasResultServingCell, RSRPXX, MeasResultBestNeighborCellphyCellIDXX,
RSRP XX )
MeasResultBestNeighborCell
PhyCellIDXX
RSRPXX（- XXX）
RSRQOption
SINR Option
Handover preparation

10. Inter gNBDU handover / Intra gNBCU handover Call flow
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Handover execution
Source5GUE
UE gNBDU
gNBCU
gNBCUUPgNBCUCP
Target
gNBDU
・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.
gNB-CU UE-F1AP-ID：XXX
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-CU UE-F1AP-ID：XXX
gNB-DU UE-F1AP-ID：X
DUtoCURRCInformation： C-RNTI
SRBs-Setup-List
DRBs-Setup-List

11. Inter gNBDU handover / Intra gNBCU handover Call flow
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Handover execution
Source5GUE
UE gNBDU
gNBCU
gNBCUUPgNBCUCP
Target
gNBDU
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 RRCReconfigurationmessage, 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 RequestgNB-DU-UE-F1AP-ID：X
gNB-CU-UE-F1AP-ID：XXX
RRC Container：RRCReconfigration （HO Command ）

12. Inter gNBDU handover / Intra gNBCU handover Call flow
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Handover execution
Source5GUE
UE gNBDU
gNBCU
gNBCUUPgNBCUCP
Target
gNBDU
9:RRC Reconfigration
gNB-DU-UE-F1AP-ID：X
gNB-CU-UE-F1AP-ID：XXX
RRC Container：RRCReconfigration
（HO Command ）
・The source gNB-DU responds to the gNB-CUCP with a UE CONTEXT MODIFICATIONRESPONSE
message indicating that RRCReconfigurationhas been sent to the UE.
11: F1AP：UE Context Modification Response
gNB-CU-UE-F1AP-ID：XXX
gNB-DU-UE-F1AP-ID：X
10:Downlink Data Delivery Status
・The source gNB-DU forwards the received RRCReconfiguration
message containing the HO command to the UE.

13. Inter gNBDU handover / Intra gNBCU handover Call flow
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Handover execution
Source5GUE
UE gNBDU
gNBCU
gNBCUUPgNBCUCP
Target
gNBDU
14:RRC ReconfigrationComplete
・The UE responds to the target gNB-DU with an
RRCReconfigurationCompletemessage.
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

14. Inter gNBDU handover / Intra gNBCU handover Call flow
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Handover execution
Source5GUE
UE gNBDU
gNBCU
gNBCUUPgNBCUCP
Target
gNBDU
17:Downlink Data
gNB-DU-UE-F1AP-ID：X
gNB-CU-UE-F1AP-ID：XXX
RRC Container：
RRCReconfigration Complete
16:F1AP:UL RRC Message Transfer
・The target gNB-DU notifies the received RRCReconfigurationCompletemessage 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

15. Inter gNBDU handover / Intra gNBCU handover Call flow
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Post-handover processing
Source5GUE
UE gNBDU
gNBCU
gNBCUUPgNBCUCP
Target
gNBDU
gNB-CU-UE-F1AP-ID：XXX
gNB-DU-UE-F1AP-ID：X
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.
gNB-CU-UE-F1AP-ID：XXX
gNB-DU-UE-F1AP-ID：X
・The source gNB-DU releases the UE context and responds to the
gNB-CUCP with a UE CONTEXT RELEASECOMPLETE message.

16. ３．INTER GNBXNHANDOVER
CALL FLOW

17. Inter gNBXnHandover Call flow
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・Xnhandover is basically the same as 4GLTE's X2 handover.
・The prerequisite for Xnhandover is that the Xninterface must be linked up between
the source gNBand target gNB.
※Applicable only to intra-AMF mobility. (Xn handover cannot be used if the source
and target gNBsare connected to different AMFs.)
・Xnhandover can be intra-frequency HO and inter-frequency HO.
・Source and target gNBscan be connected to two different UPFs.
・If the source gNBand target gNBbelong to different tracking areas (TACs), re-
registration is required after the handover is successful.
※Xnhandover 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.

18. Inter gNB Xn handover overview
2023/12/24 Sunday 18
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The user plane between two gNBsis
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 XnAPprotocol 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 XnAPprotocol 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

19. Inter gNBXnHandover Call flow
2023/12/24 Sunday19
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Prerequisites
Source5GUE
UE gNB#1
5G Core NW
UPFAMF
Prerequisites: UE is registered and RRC has DL and UL data sessions active
Target
gNB#3
0:XnAP：XnSetupRequest
0:XnAP：XnSetupResponse
①Source and target gNBshave active Xnconnections.
➁UE is in RRC_CONNECTED state.
➂Send and receive uplink and downlink data at the
source gNB.
※The above conditions ①to ➂are satisfied, and the
UE is moving toward the target gNB.

20. Inter gNBXnHandover Call flow
2023/12/24 Sunday20
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Handover decision
Source5GUE
UE gNB#1
5G Core NW
UPFAMF
1:Measurement Report
Target
gNB#3
2:Handover Decision
・The UE sends a MeasurementReportmessage containing the signal
strength of the serving cell and neighboring cells to the source gNB#1.
MeasID：XX
MeasResultServingCell
RSRPXX（- XXX）
RSRQOption
SINR Option
MeasResultBestNeighborCell
PhyCellIDXX
RSRPXX（- XXX）
RSRQOption
SINR Option
・Based on the measurement report and other information,
the source gNBdecides 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.)

21. Inter gNBXnHandover Call flow
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Handover preparation
Source5GUE
UE gNB#1
5G Core NW
UPFAMF
Target
gNB#3
3:XnAP：Handover Request
・The source gNBsends an XnAPhandover 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 gNBconsiders the information
received in the RRC container in step 2
and allocates resources to the UE.
・If the target gNBdecides to allow the handover, the target gNBsends 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

22. Inter gNBXnHandover Call flow
2023/12/24 Sunday22
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Handover execution
Source5GUE
UE gNB#1
5G Core NW
UPFAMF
Target
gNB#3
6:RRCReconfigration（HO Command ）
・The source gNBtriggers 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：SNStatusTransfer
9:Dawnlink Data Forwarding
・After the source gNBsends the SN status transfer
message to the target gNB , it transfers the uplink
and downlink PDCP SN, hyperframenumber (HFN)
status.
・The source gNBstarts buffering the DL data from
the UPF and forwards it to the target gNB.
sourceNG-RANnodeUEXnAPID
targeting-RANnodeUEXnAPID
DRBsSubjectToStatusTransfer- List

23. Inter gNBXnHandover Call flow
2023/12/24 Sunday23
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Handover execution
Source5GUE
UE gNB#1
5G Core NW
UPFAMF
Target
gNB#3
10：Execute RACH procedure from UE to target gNB
11:RRCReconfigration Complete
12:Uplink Data Foerwarding
13:NGAP：PathSwitchRequest
・A random access procedure is
performed at the target gNBtaking into
account the rach-ConfigDedicatedpart of
the information received in step 6.
・After the UE successfully connects to the target cell, it
completes the handover procedure by sending an
RRCReconfigurationCompletemessage to the target gNB.
・The target gNBsends 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.

24. Inter gNBXnHandover Call flow
2023/12/24 Sunday24
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Handover execution
Source5GUE
UE gNB#1
5G Core NW
UPFAMF
Target
gNB#3
15:NGAP：PathSwitchAcknowledge
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 gNBsends an end
marker to the target gNB.
16:Downlink Data Foerwarding
・5GC switches downlink data path to target gNB.
・AMF acknowledges the PathSwitchRequestmessage using
the Path Switch Request AcknowledgeNGAPmessage.
※This message includes a list of switched PDU sessions and
a list of PDU sessions to be released.

25. Inter gNBXnHandover Call flow
2023/12/24 Sunday25
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Post-handover processing
Source5GUE
UE gNB#1
5G Core NW
UPFAMF
Target
gNB#3
sourceNG-RANnodeUEXnAPID
targetNG-RANnodeUEXnAPID
17:XnAP：UE Context Release
・Upon receiving the NGAP Path Switch Request Confirm
message from the AMF, the target gNBsends an XnAP
UE Context Release message to the source gNB, and the
source gNBreleases the resources associated with the UE.
Downlink / Uplink Data Forwarding
Downlink / Uplink Data Forwarding

26. ４．INTER GNBN2 HANDOVER
CALL FLOW

27. Inter gNBN2 Handoverとは
2023/12/24 Sunday27
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・N2 handover does not depend on the availability of the Xninterface.
・If two gNBswith Xninterfaces support a specified N2 handover, their XnHOis not
allowed in the configuration.
※Applicable to intra-AMF or inter-AMF mobility.
(Source and target gNBscan be connected to the same AMF or different AMFs)
・N2 handover is applied to intra-frequency HO and inter-frequency HO.
・Source and target gNBscan be connected to two different UPFs.
・If the source gNBand target gNBbelong to different tracking areas (TACs), tracking
area re-registration is required after a successful handover.
※N2 handover takes longer than XnHO.

28. Inter gNB N2 Handover Overview
2023/12/24 Sunday 28
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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

29. Inter gNBN2 Handover Call flow
2023/12/24 Sunday29
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Prerequisites
Source5GUE
UE gNB
5G Core NW
UPFAMF
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 .
1:Measurement Report
2:Handover Decision
・The UE sends a MeasurementReportmessage
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
MeasResultServingCell
RSRPXX（- XXX）
RSRQOption
SINR Option
MeasResultBestNeighborCell
PhyCellIDXX
RSRPXX（- XXX）
RSRQOption
SINR Option
Handover preparation

30. Inter gNBN2 Handover Call flow
2023/12/24 Sunday30
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Handover preparation
Source5GUE
UE gNB
5G Core NW
UPFAMF
Target
gNB
3:NGAP:Handover Required
・Trigger N2 handover based on the decision of the source gNBHO 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 gNBID, 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-desirableradio 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
gNBfrom there and sends a handover request containing the UE security
context, UE capabilities, PDU session information, source-to-target
transparent container, GUAMI, etc.
AMF-UE-NGAP-ID:XXXXXXXXXXX
Handover Type:Intra5gs Cause:Ho-desirableradio condition
UEAggregateMaximumBitRate UESecurityCapabilities SecurityContext
PDUSessionResourceSetupListHOReq
AllowedNSSAI
SoueceToTarget-TransparentContainer
MobilityRestrictionList
GUAMI

31. Inter gNBN2 Handover Call flow
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Handover decision
Source5GUE
UE gNB
5G Core NW
UPFAMF
Target
gNB
6:NGAP:Handover Request Acknowledge
・After receiving the handover request from the AMF, the target gNBmay decide
whether to admit the UE based on the available resources.
・If the target gNBis 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.
AMF-UE-NGAP-ID:XXXXXXXXXXX
RAN-UE-NGAP-ID:XXX
PDUSessionResourceAdmittedList
Target To Source-TransparentContainer
AMF-UE-NGAP-ID:XXXXXXXXXXX
RAN-UE-NGAP-ID:XXX
Handover Type:Intra5gs
Cause:Ho-desirableradio condition
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 gNBcan 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.)

32. Inter gNBN2 Handover Call flow
2023/12/24 Sunday32
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Handover execution
Source5GUE
UE gNB
5G Core NW
UPFAMF
Target
gNB
8:RRC Reconfigration（HO Command ）
・The source gNBtriggers 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
gNBsecurity algorithm identifier.
AMF-UE-NGAP-ID:XXXXXXXXXXX
RAN-UE-NGAP-ID:XXX
RANStatusTransfer- TransparentContainer
9:NGAP:Uplink RAN Status Transfer
・After sending the Handover command, send UPlinkRANStatusTransferto AMF.
※This includes both RAN and AMF UE- NGAP-ID and RANStatusTransfer-
TransparentContainer.
※RANStatusTransfer- TransparentContainerhas information about all DRB
PDCPSNs provided by the source gNB.
10:Data転送

33. Inter gNBN2 Handover Call flow
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Handover execution
Source5GUE
UE gNB
5G Core NW
UPFAMF
Target
gNB
11:NGAP:Downlink RAN Status Transfer
・The AMF forwards the received UplinkRANStatusTransferinformation
to the target gNB using the DownlinkRANStatusTransfermessage.
AMF-UE-NGAP-ID:XXXXXXXXXXX
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 UPlinkRANStatusTransferto the AMF.
※This includes both RAN and AMF UE- NGAP-ID and RANStatusTransfer-
TransparentContainer.
※RANStatusTransfer- TransparentContainerhas information about all DRB
PDCPSNs provided by the source gNB.

34. Inter gNBN2 Handover Call flow
2023/12/24 Sunday34
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Handover execution
Source5GUE
UE gNB
5G Core NW
UPFAMF
Target
gNB
14:RRC ReconfigrationComplete
・After the UE successfully connects to the target cell, it completes the handover
procedure by sending an RCReconfigurationCompletemessage to the target gNB.
※The UE initiates uplink data transfer to the target gNB.
AMF-UE-NGAP-ID:XXXXXXXXXXX
RAN-UE-NGAP-ID:XXX
UserLocationInformation
15:NGAP:Handover Notify
・The target gNBsends 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
gNBtaking into account the information received in step 8
as part of prach -Config Dedicated.

35. Inter gNBN2 Handover Call flow
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Data transfer
Source5GUE
UE gNB
5G Core NW
UPFAMF
Target
gNB
16:Downlink Data
20:N3 End Marker
17:Uplink Data
18:Uplink Data
19:N3 End Marker

36. Inter gNBN2 Handover Call flow
2023/12/24 Sunday36
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Post-handover processing
Source5GUE
UE gNB
5G Core NW
UPFAMF
Target
gNB
21:NGAP:UE Context Release Command
・UEContextReleaseCommand AMF instructs the source gNBto 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
AMF-UE-NGAP-ID:XXXXXXXXXXX
RAN-UE-NGAP-ID:XXX
Cause:successful-handover
22:NGAP:UE Context Release Complete
・When the source gNBsuccessfully deletes the UE context, it sends
UEConextReleaseCompleteand releases all resources associated with the UE.
Downlink / Uplink Data Forwarding
Downlink / Uplink Data Forwarding

37. APPENDIX

38. 5G gNBarchitecture
2023/12/24 Sunday38
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Remote
Radio Head
AMF UPF
PHYLow
PHYHigh
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

39. 用語・略語
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NGAP（NG Application Protocol） ・NGAP is at the N2 reference point between gNBand 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（XnApplication Protocol） ・XnAPis a control protocol used between gNBsto 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 gNBprovides the target gNBwith all
necessary information to handle the subscriber, such as security and user
plane connectivity information.
※For dual connectivity, the master gNBsets up secondary gNBsusing XnAP,
and each gNBserves users at the same time.

40. 用語・略語
2023/12/24 Sunday 40
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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（SignallingRadio 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 (SignallingRadio Bearers).
DRB（Data Radio Bearer） ・DRB is used to transmit data.

41. 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.Pleaseuse it to help readers
consolidate their knowledge and improve their work
productivity.
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