Understanding 4G Handover in Mobile Networks

Handover
 
4G Handover
 
交遞型式
eNodeB
基站之間
 
Inter-site
)的交遞型式,因其組態不同又可區
分為
X2-Handover
S1-Handover
X2-Handover
適用同屬一個
MME
且彼此之間有
X2
介面連接的兩個
eNodeB
S1-Handover
適用於無
X2
介接的兩個
eNodeB
或者是跨
MME
間的交
 
 
X2-Handover
程序
5
Handover Preparation
X2 Handover (1/4)
Resource: Nokia, TTC redraw
6
1.
原服務基地台(
Source eNB
)發送量測控制
Measurement Control
)訊息,指示
UE
量測的門檻值、
鄰細胞等相關資訊。
2.
UE
根據系統參數指示啟動量測,當符合回報條件時,
UE
送出
Measurement Report
Source eNB
3.
Source eNB
收到
Measurement Report
後,根據回報數據決
定是否觸發交遞(
Handover
)。
4.
Source eNB
決定觸發交遞(
Handover
)後,
Source eNB
藉由
X2-AP
發送
HANDOVER REQUEST
訊息給目標基地
台(
Target eNB
),包含
Source eNB
分配的
Old eNB UE X2-AP ID
MME
分配的
MME UE S1-AP ID
需要建立的
EPS
承載清單以及每個
EPS
承載對應的核心網的資
料傳送之位址
5.
目標基地台(
Target eNB
)收到交遞請求
HANDOVER REQUEST
)後,開始對要交遞的
ERABs 
啟動允入控制(
Admission Control
)機制,以
評估是否同意此次交遞(
Handover
)的請求。
6.
目標基地台(
Target eNB
)同意接受交遞(
Handover
後,
Target eNB
Source eNB
發送
HANDOVER
REQUEST ACKNOWLEDGE 
訊息,包含
New eNB UE
X2-AP ID
Old eNB UE X2AP ID
、新建
EPS
承載對應
上下行資料傳送的位址及目標側分配的專用接入簽名
等參數。
7.
源基地台(
Source eNB
)收到來自
Target eNB
同意
Handover
請求後,向
UE
下達交遞命令(
Handover
Command
)(透過
RRC CONNECTION
RECONFIGURATION
),將分配的專用接入簽名配置
UE
X2 Handover (2/4)
Handover Execution
Resource: Nokia, TTC redraw
9
8.
源基地台(
Source eNB
)將上下行
PDCP
的序號透過
SN STATUS TRANSFER 
訊息發送給目標基地台
Target eNB
)。同時,
Handover
期間的訊務資料開始
Source eNB 
轉發給
Target eNB
9.
---
10.
---
11.
UE
在接入
Target eNB
後,發送
RRC CONNECTION
RECONFIGURATION COMPLETE
Handover Confirm
訊息給
Target eNB
,表示
UE
已經交遞(
Handover
)到
Target eNB
X2 Handover (3/4)
Resource: Nokia, TTC redraw
Handover Completion
11
12.
Target eNB
MME
發送
PATH SWITCH REQUEST
訊息,通
MME
切換訊務資料的傳送路徑,從
Source eNB
變更為
Target eNB
,訊息中包含
原來的
MME UE S1-AP ID
新分配的
eNB UE S1-AP ID
EPS
承載在
Target eNB
將使用的下行位
址。
13.
---
14.
---
15.
---
16.
MME
回覆
PATH SWITCH REQUEST ACKNOWLEDGE
訊息,
表明
Target eNB
下行位址接續已經完成,
Target eNB
保存訊
息中的
MME UE S1-AP ID
17.
---
18.
Target eNB
透過
X2
介面傳送
UE Context Release
訊息,釋放掉
Source eNB
的資源。
X2 Handover (4/4)
 
13
Handover (source eNB)
RRC
RRM
RRC_MEAS_REPORT_IND
(
tMeasReportIndMsg
)
RRC_HO_INFO_REQ
(
tRrmInfo
)
UE
MeasurementReport (A3)
RRC_HO_INFO_CFM
(
tHoInfoCfm
 + 
HoPrepare ASN.1 msg
)
RRC_HO_CMD_REQ
(tCrntiMsg + 
HandoverCommand ASN.1msg)
RRC_HO_CMD_CFM
(
tHoCmdCfm
)
RRCConnectionReconfiguration
(Handover)
RRC_CON_REL_REQ
RRC_CON_REL_CFM
Including DRB
PDCP SN status
Request RRC to encode
HoPrepare msg
Target RRM
1
4
5
8
Handover (target eNB)
RRC
RRM
RRC_HO_PREPAR_REQ
(
tHoPrpReq 
+ 
HoPrepare ASN.1 msg
)
UE
RRC_HO_PREPAR_CFM
(
tHoPrpCfm
)
      RRC_HO_RSV_REQ
(
tHandoverCommandMaterial
)
                    RRC_HO_RSV_CFM
(
tHoRsvCfm
 + 
HandoverCommand ASN.1msg
)
RRCConnectionReconfigurationComplete
RRC_HO_PS_IND
(
tSecCnxInfo
)
RRC_HO_CFM_IND
(
tUeCfmMsg
)
RRC_STATUS_IND
(
tStatusInd
)
DRB PDCP SN status
Reserve ue radio resource in
target eNB
and prepare handover command
UE handover to target eNB
Request RRC to decode
HoPrepare msg
3
6
7
2
S1
 
Handover
 程序
16
Handover Preparation
S1 Handover (1/4)
Resource: Nokia, TTC redraw
17
1.
原服務基地台(
Source eNB
)發送量測控制(
Measurement
Control
)訊息,指示
UE
量測的門檻值、鄰細胞等相關資訊。
2.
eNB
收到
Measurement Report
,且觸發
Handover
決定後,進
eNB
間透過
S1 
介面(
Interface
)的交遞(
Handover
)。
3.
---
4.
Source eNB
透過
S1
介面向
MME
發送交遞請求 (
Handover
Required
)訊息,訊息中包含
MME UE S1-AP ID
原分配的
eNB UE S1-AP ID
等資訊
5.
---
6.
MME
向目標基地台(
Target eNB
)發送
Handover Request
訊息,
訊息中包括
MME
分配的
MME UE S1-AP ID
需要建立的
EPS
清單以及每個
EPS
承載對應的核心網端資料傳送的位
址等參數。
7.
Target eNB
分配相關的資源後,進行
Handover
承載的允
入處理(
Admission Control
) ,並向
MME
發送
Handover Request Ack
訊息,包含
Target eNB
分配的
eNB
UE S1-AP ID
,允入成功的
EPS
承載對應的
eNodeB
端資
料傳送的位址等參數。
8.
MME
收到來自
Target Cell
Handover Request Ack
訊息後,
S-GW
發送
Create Indirect Data Forwarding Tunnel
Request
訊息,請求
S-GW
建立
Indirect Tunnel
以供
UE
Handover
後可以傳送封包。
9.
S-GW
在分配
S1 S-GW TEID
後,透過
Create Indirect Data
Forwarding Tunnel Response
訊息告知
MME
已建立連結
S-
GW
Indirect Tunnel
10.
MME
下達交遞命令(
Handover Command
)訊息給
Source eNB
,該訊息中包含
S1 S-GW TEID
S1 Handover (2/4)
Handover Execution
Resource: Nokia, TTC redraw
20
10.
MME
下達交遞命令(
Handover Command
)訊息給
Source
eNB
,該訊息中包含
S1 S-GW TEID
11.
---
12.
---
13.
Source eNB
發送
eNB Status Transfer
MME
,包含
DL Count
UL Count
藉以告知
Target Cell Hyper Frame Number
PDCP
Sequence Number
MME
接著藉由
MME Status Transfer
轉發給
Target eNB
Target eNB
知道該
UE 
交遞(
Handover
)後,接
續傳送封包的順序。
14.
Source eNB
UE
發送
RRCConnectionReconfiguration
訊息,指
UE
相關訊息,包含
C-RNTI, DRB ID
等並要求
UE
Source
eNB
交遞至 
Target eNB
15.
UE
在接入
Target eNB
後,發送
RRC Connection
Reconfiguration Complete 
Handover Confirm
)訊息。表示
UE
已經
Handover
Target eNB
Target eNB
在收到來自該
UE
Handover Confirm
後,開始續傳原先儲存在
Target eNB
的封
包。
S1 Handover (3/4)
Resource: Nokia, TTC redraw
Handover Completion
22
16.
Target eNB
發送
Handover Notify
MME
告知該
UE
已經成
功完成
Handover
17.
MME
透過
Modify Bearer Request
訊息轉發
Target eNB
配的
S1 TEID
要求
S-GW
修改
S1 Bearer
S-GW
會依據要
求建立連接
Target eNB
DL S1 Bearer
並以
Modify
Response Response
回覆
MME
開始以新的
S1 Bearer
傳送新
的封包。
18.
MME
發出
UE Contxt Realease Command
要求
Source eNB
釋放
UE Context
S1
資源。同時,
MME
S-GW
發出
Delete Indirect Data Forwarding Tunnel Requeat
要求釋放
原有的
Indirect Tunnel
S-GW
據此以
Delete Indirect Data
Forwarding Tunnel Response
告知
MME
釋放完成。
S1 Handover (4/4)
 
24
5G Handover
 
 
 
Xn based inter NG-RAN handover
27
AMF
 
/ UPF
NG
AMF
 
/ UPF
NG
NG
NG
NG
NG
NG
NG
Xn
Xn
Xn
Xn
Xn
Without Xn interface
Cannot do Xn based
handover
Xn based inter NG-RAN handover
Without UPF re-allocation
28
Hand over a UE from
a source NG-RAN to
target NG-RAN
Source NG-RAN have
Xn connectivity to the
target NG-RAN
AMF is unchanged
SMF decides to keep
the existing UPF and
I-UPF
UPF
Source 
NG-RAN
Target 
NG-RAN
DN
I-UPF
AMF
Handover
SMF
Xn Connectivity 
Xn based inter NG-RAN handover
Without UPF re-allocation
29
Xn based inter NG-RAN handover
Without UPF re-allocation (Step 1)
30
N2 Path Switch Request
List of PDU Sessions To Be Switched with N2 SM Information
Include the list of accepted QoS Flows
List of PDU Sessions Rejected with for each PDU Session N2 SM Information including a rejection Cause
None of the QoS Flows of a PDU Session are accepted by the Target gNB
Corresponding network slice is not supported in the Target gNB
UE Location Information
Selected PLMN ID
S-NSSAI is introduced as part of the PDU session information
Xn based inter NG-RAN handover
Without UPF re-allocation (Step 2)
31
SMF Services
Nsmf_PDUSession_UpdateSMContext Request
Inputs
PDU Session To Be Switched with N2 SM Information (Secondary RAT usage data, Handover Flag)
PDU Sessions Rejected with a rejection Cause
UE Location Information
UE presence in LADN service area
Xn based inter NG-RAN handover
Without UPF re-allocation (Step 3)
32
Modify N3 tunnel (gNB <-> UPF)
N4 Session Modification Request
AN Tunnel Info
CN Tunnel Info
UPF used for connection to Target gNB and connection to Source gNB
CN Tunnel Info (on N3) of UPF need be re-allocated
N4 Session Modification Response
CN Tunnel Info
N3 (R)AN tunnel information is released
Xn based inter NG-RAN handover
Without UPF re-allocation (Step 4)
33
UPF sends one or more "end marker" packets for each N3 tunnel on the old path immediately after
switching the path
The UPF starts sending downlink packets to the Target gNB
Xn based inter NG-RAN handover
Without UPF re-allocation (Step 5)
34
SMF Services
Nsmf_PDUSession_UpdateSMContext Response 
Outputs
CN Tunnel Info
Used to setup N3 tunnel
Without including the CN Tunnel Info which user
plane resources are deactivated or released
Xn based inter NG-RAN handover
Without UPF re-allocation (Step 6)
35
N2 Path Switch Response
N2 SM Information
Failed PDU Sessions
Xn based inter NG-RAN handover
Without UPF re-allocation (Step 7)
36
Target gNB confirms success of the handover
Target gNB triggers the release of resources with the Source gNB
Xn based inter NG-RAN handover
With insertion of intermediate UPF
37
Hand over a UE from
a source NG-RAN to
target NG-RAN
Source NG-RAN have
Xn connectivity to the
target NG-RAN
AMF is unchanged
SMF decides that
insertion of a new
additional I-UPF is
needed
UPF
(PSA)
Source 
NG-RAN
Target 
NG-RAN
DN
I-UPF
AMF
Handover
SMF
Xn Connectivity 
Xn based inter NG-RAN handover
With insertion of intermediate UPF
38
SMF
UE
Source
gNB
AMF
1. Path Switch Request
UPF
(PSA)
Target
gNB
2. PDU Session Update Request
8. PDU Session Update Response
7a . End marker
6. Session Modification
Handover Preparation
Handover Execution
Forwarding of data
7b . End marker
9. Path Switch Response
10. Release Resources
3. I-UPF selection
new
I-UPF
5. Session Establishment
4. Session Modification
Xn based inter NG-RAN handover
With insertion of intermediate UPF (Step 3)
39
SMF
UE
Source
gNB
AMF
1. Path Switch Request
UPF
(PSA)
Target
gNB
2. PDU Session Update Request
8. PDU Session Update Response
7a . End marker
6. Session Modification
Handover Preparation
Handover Execution
Forwarding of data
7b . End marker
9. Path Switch Response
10. Release Resources
3. I-UPF selection
new
I-UPF
5. Session Establishment
4. Session Modification
SMF selects a new UPF to act as intermediate UPF for the PDU Session
Xn based inter NG-RAN handover
With insertion of intermediate UPF (Step 4)
40
SMF
UE
Source
gNB
AMF
1. Path Switch Request
UPF
(PSA)
Target
gNB
2. PDU Session Update Request
8. PDU Session Update Response
7a . End marker
6. Session Modification
Handover Preparation
Handover Execution
Forwarding of data
7b . End marker
9. Path Switch Response
10. Release Resources
3. I-UPF selection
new
I-UPF
5. Session Establishment
4. Session Modification
Conditional
Different CN Tunnel Info need be used
CN Tunnel Info is allocated by the UPF
Modify and Get N9 tunnel  by UPF (UPF <-> UPF)
Uplink
N4 Session Modification Request
CN Tunnel
N4 Session Modification Response
CN Tunnel Info
UPF (PSA) allocates CN Tunnel Info (on N9) of UPF (PSA)
Provides CN Tunnel Info (on N9) to the SMF
Xn based inter NG-RAN handover
With insertion of intermediate UPF (Step 5)
41
SMF
UE
Source
gNB
AMF
1. Path Switch Request
UPF
(PSA)
Target
gNB
2. PDU Session Update Request
8. PDU Session Update Response
7a . End marker
6. Session Modification
Handover Preparation
Handover Execution
Forwarding of data
7b . End marker
9. Path Switch Response
10. Release Resources
3. I-UPF selection
new
I-UPF
5. Session Establishment
4. Session Modification
Establish N9 tunnel by UPF(UPF <-> UPF)
Uplink and Downlink
N4 Session Establishment Request
Target gNB Tunnel Info
CN Tunnel Info of the PDU Session Anchor (PSA)
Used to setup N9 tunnel
If the CN Tunnel Info of the I-UPF is allocated by the SMF,
provides the UL and DL CN Tunnel Info of I-UPF to the I-UPF
N4 Session Establishment Response
CN Tunnel
If the CN Tunnel Info of the I-UPF is allocated by the UPF, the
UL and DL CN Tunnel Info of I-UPF is sent to the SMF
Xn based inter NG-RAN handover
With insertion of intermediate UPF (Step 6)
42
SMF
UE
Source
gNB
AMF
1. Path Switch Request
UPF
(PSA)
Target
gNB
2. PDU Session Update Request
8. PDU Session Update Response
7a . End marker
6. Session Modification
Handover Preparation
Handover Execution
Forwarding of data
7b . End marker
9. Path Switch Response
10. Release Resources
3. I-UPF selection
new
I-UPF
5. Session Establishment
4. Session Modification
Modify N9 tunnel by SMF (UPF <-> UPF)
Downlink
Uplink and Downlink
If  CN Tunnel Info is allocated by the SMF (without Step 4)
N4 Session Modification Request
DL CN Tunnel Info of the I-UPF
UL CN Tunnel Info
N4 Session Modification Response
PDU Session Anchor starts sending downlink packets to the Target gNB
via I-UPF
Xn based inter NG-RAN handover
With re-allocation of intermediate UPF
43
Hand over a UE from
a source NG-RAN to
target NG-RAN
Source NG-RAN have
Xn connectivity to the
target NG-RAN
AMF is unchanged
SMF decides that the
I-UPF is to be changed
Source
I-UPF
Source 
NG-RAN
Target 
NG-RAN
DN
Target
I-UPF
AMF
Handover
SMF
Xn Connectivity 
UPF
(PSA)
Xn based inter NG-RAN handover
With re-allocation of intermediate UPF
44
Xn based inter NG-RAN handover
With re-allocation of intermediate UPF (Step 6, 7)
45
Re-allocation of UPF (Non-roaming and Roaming with Local
Breakout )
Do step 6 and skip step 7
SMF send 
Session Modification 
to modify the PDU
Session
Modify  N9 tunnel
Re-allocation of vUPF (In case of home routed roaming)
Do step 7 and skip step 6
UPF (PSA) allocate at HPLMN
vSMF sent 
PDU Session Update 
to notify the hSMF to
modify PDU Session
Xn based inter NG-RAN handover
With re-allocation of intermediate UPF (Step 13)
46
The timer is started in step 5 if the source 
UPF is not the
PSA UPF
When this timer is expired, the SMF initiates Source UPF
Release procedure
N4 Session Release Request
Release Cause
N4 Session Release Response
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Explore the complexities of 4G handover procedures, including eNodeB interactions, X2 handovers, preparation, execution, completion, and more. Learn about the roles of MME, UE, and the core network in ensuring seamless handover experiences for mobile users. Dive into detailed illustrations and descriptions to enhance your understanding of this critical aspect of mobile communication networks.


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  1. Handover

  2. 4G Handover

  3. eNodeB Inter-site X2-Handover S1-Handover X2-Handover MME X2 eNodeB S1-Handover X2 eNodeB MME

  4. X2-Handover 5

  5. X2 Handover (1/4) Handover Preparation 6 Resource: Nokia, TTC redraw

  6. 1. Source eNB Measurement Control UE 2. UE UE Measurement Report Source eNB 3. Source eNB Measurement Report Handover 4. Source eNB Handover Source eNB X2-AP HANDOVER REQUEST Target eNB Source eNB Old eNB UE X2-AP ID MME MME UE S1-AP ID EPS EPS

  7. 5. Target eNB HANDOVER REQUEST ERABs Admission Control Handover 6. Target eNB Handover Target eNB Source eNB HANDOVER REQUEST ACKNOWLEDGE New eNB UE X2-AP ID Old eNB UE X2AP ID EPS 7. Source eNB Target eNB Handover UE Handover Command RRC CONNECTION RECONFIGURATION UE

  8. X2 Handover (2/4) Handover Execution 9 Resource: Nokia, TTC redraw

  9. 8. Source eNBPDCP SN STATUS TRANSFER Target eNB Handover Source eNB Target eNB 9. --- 10. --- 11. UE Target eNB RRC CONNECTION RECONFIGURATION COMPLETE Handover Confirm Target eNB UE Handover Target eNB

  10. X2 Handover (3/4) Handover Completion 11 Resource: Nokia, TTC redraw

  11. 12. Target eNBMMEPATH SWITCH REQUEST MME Source eNB Target eNB MME UE S1-AP ID eNB UE S1-AP ID EPS Target eNB 13. --- 14. --- 15. --- 16. MME PATH SWITCH REQUEST ACKNOWLEDGE Target eNB Target eNB MME UE S1-AP ID 17. --- 18. Target eNB X2 UE Context Release Source eNB

  12. X2 Handover (4/4) 13

  13. Handover (source eNB) UE Target RRM RRC RRM MeasurementReport (A3) RRC_MEAS_REPORT_IND (tMeasReportIndMsg) Request RRC to encode HoPrepare msg RRC_HO_INFO_REQ (tRrmInfo) RRC_HO_INFO_CFM (tHoInfoCfm + HoPrepare ASN.1 msg) 1 4 RRCConnectionReconfiguration (Handover) RRC_HO_CMD_REQ (tCrntiMsg + HandoverCommand ASN.1msg) Including DRB PDCP SN status RRC_HO_CMD_CFM (tHoCmdCfm) 5 8 RRC_CON_REL_REQ RRC_CON_REL_CFM

  14. Handover (target eNB) RRC RRM UE Request RRC to decode HoPrepare msg 2 RRC_HO_PREPAR_REQ (tHoPrpReq + HoPrepare ASN.1 msg) RRC_HO_PREPAR_CFM (tHoPrpCfm) Reserve ue radio resource in target eNB and prepare handover command RRC_HO_RSV_REQ (tHandoverCommandMaterial) RRC_HO_RSV_CFM DRB PDCP SN status (tHoRsvCfm + HandoverCommand ASN.1msg) 3 6 RRC_STATUS_IND (tStatusInd) RRCConnectionReconfigurationComplete RRC_HO_CFM_IND (tUeCfmMsg) UE handover to target eNB RRC_HO_PS_IND (tSecCnxInfo) 7

  15. S1 Handover 16

  16. S1 Handover (1/4) Handover Preparation 17 Resource: Nokia, TTC redraw

  17. 1. Source eNBMeasurement Control UE 2. eNB Measurement Report Handover eNB S1 Interface Handover 3. --- 4. Source eNB S1 MME Handover Required MME UE S1-AP ID eNB UE S1-AP ID 5. --- 6. MME Target eNB Handover Request MME MME UE S1-AP ID EPS EPS

  18. 7. Target eNBHandover Admission Control MME Handover Request Ack Target eNB eNB UE S1-AP ID EPS eNodeB 8. MME Target Cell Handover Request Ack S-GW Create Indirect Data Forwarding Tunnel Request S-GW Indirect Tunnel UE Handover 9. S-GW S1 S-GW TEID Create Indirect Data Forwarding Tunnel Response MME S- GW Indirect Tunnel 10. MME Handover Command Source eNB S1 S-GW TEID

  19. S1 Handover (2/4) Handover Execution 20 Resource: Nokia, TTC redraw

  20. 10. MMEHandover CommandSource eNB S1 S-GW TEID 11. --- 12. --- 13. Source eNB eNB Status Transfer MME DL Count UL Count Target Cell Hyper Frame Number PDCP Sequence Number MME MME Status Transfer Target eNB Target eNB UE Handover 14. Source eNB UE RRCConnectionReconfiguration UE C-RNTI, DRB ID UE Source eNB Target eNB 15. UE Target eNB RRC Connection Reconfiguration Complete Handover Confirm UE Handover Target eNB Target eNB UE Handover Confirm Target eNB

  21. S1 Handover (3/4) Handover Completion 22 Resource: Nokia, TTC redraw

  22. 16. Target eNBHandover NotifyMMEUE Handover 17. MME Modify Bearer Request Target eNB S1 TEID S-GW S1 Bearer S-GW Target eNB DL S1 Bearer Modify Response Response MME S1 Bearer 18. MME UE Contxt Realease Command Source eNB UE Context S1 MME S-GW Delete Indirect Data Forwarding Tunnel Requeat Indirect Tunnel S-GW Delete Indirect Data Forwarding Tunnel Response MME

  23. S1 Handover (4/4) 24

  24. 5G Handover

  25. Xn based inter NG-RAN handover AMF / UPF AMF / UPF NG NG NG NG NG NG NG NG Xn Xn Xn Without Xn interface Xn Cannot do Xn based handover Xn 27

  26. Xn based inter NG-RAN handover Without UPF re-allocation SMF Hand over a UE from a source NG-RAN to target NG-RAN DN I-UPF UPF AMF Source NG-RAN have Xn connectivity to the target NG-RAN Xn Connectivity AMF is unchanged Target NG-RAN SMF decides to keep the existing UPF and I-UPF Source NG-RAN Handover 28

  27. Xn based inter NG-RAN handover Without UPF re-allocation Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. Session Modification 4a . End marker 4b . End marker 5. PDU Session Update Response 6. Path Switch Response 7. Release Resources 29

  28. Xn based inter NG-RAN handover Without UPF re-allocation (Step 1) Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request N2 Path Switch Request List of PDU Sessions To Be Switched with N2 SM Information Include the list of accepted QoS Flows List of PDU Sessions Rejected with for each PDU Session N2 SM Information including a rejection Cause None of the QoSFlows of a PDU Session are accepted by the Target gNB 3. Session Modification 4a . End marker 4b . End marker Corresponding network slice is not supported in the Target gNB UE Location Information Selected PLMN ID 5. PDU Session Update Response S-NSSAI is introduced as part of the PDU session information 6. Path Switch Response 7. Release Resources 30

  29. Xn based inter NG-RAN handover Without UPF re-allocation (Step 2) Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. Session Modification SMF Services Nsmf_PDUSession_UpdateSMContext Request Inputs PDU Session To Be Switched with N2 SM Information (Secondary RAT usage data, Handover Flag) PDU Sessions Rejected with a rejection Cause UE Location Information UE presence in LADN service area 4a . End marker 4b . End marker 5. PDU Session Update Response 6. Path Switch Response 7. Release Resources 31

  30. Xn based inter NG-RAN handover Without UPF re-allocation (Step 3) Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. Session Modification 4a . End marker Modify N3 tunnel (gNB <-> UPF) 4b . End marker N4 Session Modification Request AN Tunnel Info CN Tunnel Info UPF used for connection to Target gNB and connection to Source gNB CN Tunnel Info (on N3) of UPF need be re-allocated N4 Session Modification Response 5. PDU Session Update Response 6. Path Switch Response 32 7. Release Resources CN Tunnel Info N3 (R)AN tunnel information is released

  31. Xn based inter NG-RAN handover Without UPF re-allocation (Step 4) Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. Session Modification 4a . End marker 4b . End marker 5. PDU Session Update Response UPF sends one or more "end marker" packets for each N3 tunnel on the old path immediately after switching the path 6. Path Switch Response 7. Release Resources The UPF starts sending downlink packets to the Target gNB 33

  32. Xn based inter NG-RAN handover Without UPF re-allocation (Step 5) Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. Session Modification 4a . End marker 4b . End marker SMF Services Nsmf_PDUSession_UpdateSMContext Response 5. PDU Session Update Response Outputs CN Tunnel Info 6. Path Switch Response Used to setup N3 tunnel 7. Release Resources Without including the CN Tunnel Info which user plane resources are deactivated or released 34

  33. Xn based inter NG-RAN handover Without UPF re-allocation (Step 6) Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. Session Modification 4a . End marker 4b . End marker 5. PDU Session Update Response 6. Path Switch Response N2 Path Switch Response N2 SM Information Failed PDU Sessions 7. Release Resources 35

  34. Xn based inter NG-RAN handover Without UPF re-allocation (Step 7) Source gNB Target gNB UPF UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. Session Modification 4a . End marker 4b . End marker 5. PDU Session Update Response 6. Path Switch Response Target gNB confirms success of the handover Target gNB triggers the release of resources with the Source gNB 7. Release Resources 36

  35. Xn based inter NG-RAN handover With insertion of intermediate UPF UPF (PSA) DN Hand over a UE from a source NG-RAN to target NG-RAN SMF I-UPF AMF Source NG-RAN have Xn connectivity to the target NG-RAN AMF is unchanged Xn Connectivity SMF decides that insertion of a new additional I-UPF is needed Target NG-RAN Source NG-RAN Handover 37

  36. Xn based inter NG-RAN handover With insertion of intermediate UPF UPF (PSA) new I-UPF Source gNB Target gNB UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification 5. Session Establishment 6. Session Modification 7a . End marker 7b . End marker 8. PDU Session Update Response 9. Path Switch Response 10. Release Resources 38

  37. Xn based inter NG-RAN handover With insertion of intermediate UPF (Step 3) UPF (PSA) new I-UPF Source gNB Target gNB UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification SMF selects a new UPF to act as intermediate UPF for the PDU Session 5. Session Establishment 6. Session Modification 7a . End marker 7b . End marker 8. PDU Session Update Response 9. Path Switch Response 10. Release Resources 39

  38. Xn based inter NG-RAN handover With insertion of intermediate UPF (Step 4) UPF (PSA) new I-UPF Source gNB Target gNB UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification Conditional Different CN Tunnel Info need be used CN Tunnel Info is allocated by the UPF 5. Session Establishment 6. Session Modification Modify and Get N9 tunnel by UPF (UPF <-> UPF) Uplink 7a . End marker N4 Session Modification Request CN Tunnel N4 Session Modification Response CN Tunnel Info UPF (PSA) allocates CN Tunnel Info (on N9) of UPF (PSA) 7b . End marker 8. PDU Session Update Response 9. Path Switch Response 10. Release Resources Provides CN Tunnel Info (on N9) to the SMF 40

  39. Xn based inter NG-RAN handover With insertion of intermediate UPF (Step 5) UPF (PSA) new I-UPF Source gNB Target gNB UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification Establish N9 tunnel by UPF(UPF <-> UPF) Uplink and Downlink 5. Session Establishment N4 Session Establishment Request Target gNB Tunnel Info CN Tunnel Info of the PDU Session Anchor (PSA) Used to setup N9 tunnel 6. Session Modification 7a . End marker 7b . End marker If the CN Tunnel Info of the I-UPF is allocated by the SMF, provides the UL and DL CN Tunnel Info of I-UPF to the I-UPF N4 Session Establishment Response CN Tunnel If the CN Tunnel Info of the I-UPF is allocated by the UPF, the UL and DL CN Tunnel Info of I-UPF is sent to the SMF 8. PDU Session Update Response 9. Path Switch Response 10. Release Resources 41

  40. Xn based inter NG-RAN handover With insertion of intermediate UPF (Step 6) UPF (PSA) new I-UPF Source gNB Target gNB UE AMF SMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification Modify N9 tunnel by SMF (UPF <-> UPF) Downlink Uplink and Downlink If CN Tunnel Info is allocated by the SMF (without Step 4) 5. Session Establishment 6. Session Modification N4 Session Modification Request DL CN Tunnel Info of the I-UPF UL CN Tunnel Info N4 Session Modification Response 7a . End marker 7b . End marker 8. PDU Session Update Response 9. Path Switch Response PDU Session Anchor starts sending downlink packets to the Target gNB via I-UPF 10. Release Resources 42

  41. Xn based inter NG-RAN handover With re-allocation of intermediate UPF Source I-UPF Hand over a UE from a source NG-RAN to target NG-RAN SMF UPF (PSA) DN Target I-UPF AMF Source NG-RAN have Xn connectivity to the target NG-RAN AMF is unchanged Xn Connectivity SMF decides that the I-UPF is to be changed Target NG-RAN Source NG-RAN Handover 43

  42. Xn based inter NG-RAN handover With re-allocation of intermediate UPF Source I-UPF UPF (PSA) Target I-UPF SMF / vSMF Source gNB Target gNB hSMF UE AMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification 5. Session Establishment 6. Session Modification 7. PDU Session Update 8a . End marker 9b . End marker 10. PDU Session Update Response 11. Path Switch Response 12. Release Resources 13. Session Release 44

  43. Xn based inter NG-RAN handover With re-allocation of intermediate UPF (Step 6, 7) Source I-UPF UPF (PSA) Target I-UPF SMF / vSMF Source gNB Target gNB hSMF UE AMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification 5. Session Establishment Re-allocation of UPF (Non-roaming and Roaming with Local Breakout ) Do step 6 and skip step 7 SMF send Session Modification to modify the PDU Session Modify N9 tunnel Re-allocation of vUPF (In case of home routed roaming) Do step 7 and skip step 6 UPF (PSA) allocate at HPLMN 6. Session Modification 7. PDU Session Update 8a . End marker 9b . End marker 10. PDU Session Update Response 11. Path Switch Response 12. Release Resources vSMF sent PDU Session Update to notify the hSMF to modify PDU Session 13. Session Release 45

  44. Xn based inter NG-RAN handover With re-allocation of intermediate UPF (Step 13) Source I-UPF UPF (PSA) Target I-UPF SMF / vSMF Source gNB Target gNB hSMF UE AMF Handover Preparation Handover Execution Forwarding of data 1. Path Switch Request 2. PDU Session Update Request 3. I-UPF selection 4. Session Modification 5. Session Establishment 6. Session Modification 7. PDU Session Update The timer is started in step 5 if the source UPF is not the PSA UPF When this timer is expired, the SMF initiates Source UPF Release procedure 8a . End marker 9b . End marker 10. PDU Session Update Response 11. Path Switch Response N4 Session Release Request 12. Release Resources Release Cause N4 Session Release Response 13. Session Release 46

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