NR-NR Dual Connectivity
Feature Description
113/221 04-LZA 701 6017/1 Uen AH20J
Copyright
© Ericsson AB 2021, 2022. All rights reserved. No part of this document may be
reproduced in any form without the written permission of the copyright owner.
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Contents
Contents
1
Overview
1
1.1
NR-DC Deployment
2
2
Dependencies
6
3
Feature Operation
13
3.1
Master Node Criteria for NR-DC Setup
13
3.2
Termination Point Control of QoS Flows in the Master Node
17
3.3
Bearer Type Change in NR-DC
19
3.4
User Plane Management
20
3.5
NR-DC Band Combination Selection
22
3.6
User Activity Supervision
24
3.7
Xn Link Break Handling
25
3.8
3.8.1
3.8.2
3.8.3
3.8.4
Master Node Mobility Support in NR-DC
Intra-gNodeB Handover When NR-DC Is Configured
Inter-gNodeB Handover When NR-DC Is Configured
IRAT Handover to LTE When NR-DC Is Configured
Incoming IRAT Handover from LTE into an NR-DC-Capable
PCell
Scenarios for Master Node Triggered NR-DC Procedures
Reception of an A4 Measurement Report
UE Indicating SCG Radio Link Failure
UE Released to Idle Mode
PDU Session Resource Setup Procedure
PDU Session Resource Modify Procedure
PDU Session Resource Release Procedure
Detection of an Xn Link Break in the Master Node
Outgoing Intra-System Handover from the Source Cell
Incoming Handover into a New PCell
Outgoing IRAT Handover from the Master Node
27
27
28
29
3.9
3.9.1
3.9.2
3.9.3
3.9.4
3.9.5
3.9.6
3.9.7
3.9.8
3.9.9
3.9.10
3.10
3.10.1
3.10.2
3.10.3
3.10.4
3.10.5
3.10.6
Scenarios for Secondary Node Triggered NR-DC Procedures
Secondary Node Detects Radio Link Failure
NR Cell or Sector Carrier Is Locked on the Secondary Node
Random Access Timer Expiry in Secondary Node
PDCP COUNT Threshold Is Reached
High-Band Uplink Radio Link Quality Falls Below Configured
Threshold in the PSCell
High-Band Uplink Radio Link Quality Rises Above Configured
Threshold in the PSCell
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29
30
30
30
31
31
32
34
36
36
36
37
37
37
37
38
38
38
39
NR-NR Dual Connectivity
3.11
3.11.1
3.11.2
3.11.3
3.11.4
3.11.5
3.11.6
NR-DC Procedures
Secondary Node Addition
Master Node Initiated Secondary Node Modification
Secondary Node Initiated Secondary Node Modification
Master Node Initiated UE Context Release
Master Node Initiated Secondary Node Release
Secondary Node Initiated Secondary Node Release
39
39
43
45
46
46
49
4
Network Impact
53
5
Parameters
56
6
Performance
62
7
Activate
68
8
Deactivate
69
9
Engineering Guidelines
70
9.1
Configure IP Connectivity in NR-DC for the Xn-U Interface
70
9.2
TCP Optimization for the Secondary Node Terminated Split
DRB in NR-DC
Configure 5QI and ARP Levels for QoS Flows Allowed to Be
Secondary Node Terminated
Configuration Considerations for Switching of Uplink MCG and
SCG Radio Resources of the Secondary Node Terminated Split
DRB
9.3
9.4
71
72
73
Appendix A: Feature Change History
74
Appendix A.a: 22.Q1: Switching of Uplink MCG and SCG Radio Resources
of the Secondary Node Terminated Split DRB in NR-DC
74
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1
NR-NR Dual Connectivity Overview
The NR-NR Dual Connectivity feature introduces support for Multi-Radio
Dual Connectivity with the 5GC. This allows the UE to be connected to one
gNodeB acting as the Master Node and another gNodeB acting as the
Secondary Node.
Access Type:
NR
Feature Identity:
FAJ 121 5380
Value Package Name:
Peak Rate Evolution
Value Package Identity:
FAJ 801 4005
Node Type:
Baseband Radio Node
Licensing:
Licensed feature. One license is required for each
node. The feature license must be installed on both
the Master Node and the Secondary Node.
Summary
The NR-NR Dual Connectivity feature enables the UE to connect to the 5GC using
a Master Node with Low-Band (FR1) or Mid-Band (FR1) and a Secondary Node
with High-Band (FR2) NR frequencies.
The feature allows the UE to take advantage of the downlink and uplink bit rates
of High-Band frequencies while the UE is connected to the 5GC using Low-Band
or Mid-Band anchor frequencies.
The feature provides the following benefits:
— The UEs utilize the higher bit rates of High-Band frequencies both in
downlink and uplink.
— The packet processing resources are offloaded in gNodeBs with Low-Band or
Mid-Band frequencies by using PDCP resources in gNodeBs with High-Band
frequencies.
RELATED INFORMATION
3GPP TS 37.340; NR; Multi-connectivity; Overall description; Stage-2
3GPP TS 38.331; NR; Radio Resource Control (RRC); Protocol specification
3GPP TS 38.413; NG-RAN; NG Application Protocol (NGAP)
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1
NR-NR Dual Connectivity
3GPP TS 38.423; NG-RAN; Xn Application Protocol (XnAP)
1.1
NR-DC Deployment
The NR-DC-capable UE operating in NR SA mode is always connected to a
gNodeB with Low-Band (FR1) or Mid-Band (FR1) frequencies. This type of
gNodeB acts as the Master Node in NR-DC. The UE can connect to another
gNodeB with High-Band (FR2) frequencies if the UE is in High-Band coverage.
This type of gNodeB acts as the Secondary Node in NR-DC. The UE connects to
the Master Node over the Uu interface. The Uu interface between the UE and the
Secondary Node is only established when the UE is configured with NR-DC.
The Master Node is connected to the 5GC over the NG interface. The NG-C
interface is used for control plane, while the NG-U interface is used for user plane.
The Secondary Node connects to the 5GC over the NG-U interface for user plane.
The Master Node and the Secondary Node are connected to each other over the
Xn interface. The Xn-C interface is used for control plane, while the Xn-U
interface is used for user plane. One Master Node can be connected to several
Secondary Nodes, and one Secondary Node can be connected to several Master
Nodes.
5GC
N
G
-U
NG
-U
N
G
-C
Secondary
Node
Master
Node
Xn-C
Xn-U
Uu
Uu (MCG)
G
(SC
)
PSCell
PCell
Legend
User Plane
Control Plane
Master Node = the master gNodeB providing the MCG
Secondary Node = the secondary gNodeB providing the SCG
High-Band coverage
Low-Band or Mid-Band coverage
L0003062A
Figure 1 Interface Connections in NR-DC
In NR-DC, the UE is in RRC_CONNECTED mode in the Master Node. The control of
the UE is managed through the RRC signaling between the UE and the Master
2
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Node. Information specific to the Secondary Node resources is carried by
containers conveying RRC messages between the Secondary Node and the
Master Node over the Xn-C interface and between the Master Node and the UE
over the Uu interface.
The following cell groups can serve the NR-DC-capable UE:
Master Cell Group (MCG)
The MCG, configured for Low-Band or Mid-Band
frequencies, is provided by the Master Node. The MCG
consists of one PCell if NR-DC is configured.
Secondary Cell Group (SCG)
The SCG, configured for High-Band frequencies, is
provided by the Secondary Node. The SCG consists of one
PSCell and optionally one or several SCells.
When the UE is configured with NR-DC, the following types of PDU sessions are
provided for user data transport between the UE and a data network using the
5GC:
Master Node terminated PDU sessions
This type of PDU session carries QoS flows configured
with specific 5QI profiles and ARP levels that prevent the
QoS flows from being Secondary Node terminated. This is
why all QoS flows within this type of PDU session are
terminated by the Master Node. For such a PDU session,
one user plane tunnel is configured over the NG-U
interface for all its QoS flows between the Master Node
and the 5GC.
Secondary Node terminated PDU sessions
This type of PDU session carries QoS flows configured
with specific 5QI profiles and ARP levels that allow the
QoS flow to be Secondary Node terminated. This is why
all QoS flows within this type of PDU session are
terminated by the Secondary Node. For such a PDU
session, one user plane tunnel is set up over the NG-U
interface for all its QoS flows between the Secondary
Node and the 5GC.
It is possible that a UE connected with NR-DC can use a combination of the two
types of PDU sessions. This way a UE can have some QoS flows that are
Secondary Node terminated, while some other QoS flows for the UE remain to be
Master Node terminated.
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3
NR-NR Dual Connectivity
EPC
5GC
Master Node
terminated
QoS flows
Master Node
terminated
PDU session
Master Node
Secondary Node
terminated
QoS flows
Secondary Node
terminated
PDU session
Secondary Node
Master Node terminated user plane traffic
Secondary Node terminated user plane traffic
L0003099A
Figure 2 Termination of QoS Flows within PDU Sessions
QoS flows are carried by DRBs between the UE, the Master Node, and the
Secondary Node. When the UE is connected with NR-DC, the UE can be
configured with the following bearer types:
Master Node terminated MCG DRBs
The Master Node terminated MCG DRBs use only MCG
radio resources and the packet processing resources of
the Master Node. These DRBs transmit user plane traffic
between the UE and the Master Node. The Master Node
terminated MCG DRBs are ciphered and deciphered in the
Master Node.
Secondary Node terminated split DRBs
The Secondary Node terminated split DRBs use MCG and
SCG radio resources, and the packet processing resources
of the Secondary Node. Split DRBs are configured with
both the MCG and SCG radio resources after Secondary
Node Addition. These DRBs are ciphered and deciphered
in the Secondary Node.
4
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EPC
5GC
Secondary Node
terminated QoS flows
Master Node
terminated QoS flows
NG-U
NG-U
Master Node
Secondary Node
SDAP
SDAP
Master Node
terminated
MCG DRB
Secondary Node
terminated
Split DRB
Xn-U
PDCP
(Master key)
RLC
PDCP
(Secondary key)
RLC
RLC
MAC and Layer 1
MCG radio resources
MAC and Layer 1
SCG radio resources
User Plane (Master Node terminated)
User Plane (Secondary Node terminated)
L0003098A
Figure 3 MCG and SCG Radio Resources for User Plane in NR-DC
RELATED INFORMATION
Termination Point Control of QoS Flows in the Master Node
Bearer Type Change in NR-DC
User Plane Management
Configure 5QI and ARP Levels for QoS Flows Allowed to Be Secondary Node
Terminated
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5
NR-NR Dual Connectivity
2
Dependencies of NR-NR Dual Connectivity
The feature has prerequisites. The UE connected with NR-DC is handled
differently when used with a number of other features. Related features
impact NR-DC performance. The feature comes with requirements on the
5GC, the UE, the number of Baseband units and on the 5QI settings.
Table 1
Feature Dependencies
Feature
Relationship
Description
NR Standalone (FAJ 121 5060)
Prerequisite
The NR Standalone feature must be
operable on the Master Node, so that
the UE is provided with a PCell on
Low-Band or Mid-Band frequencies.
IEEE 1588 Time and Phase
Synchronization (FAJ 121 2022)
Related
The Master Node and the Secondary
Node must be time and phase
synchronized with the same frame
start offset by either using GNSS or
the IEEE 1588 Time and Phase
Synchronization feature. The IEEE
1588 Time and Phase
Synchronization feature is a
prerequisite if GNSS is not used.
Measurement Gap-Aware NR
Scheduling (FAJ 121 5295)
Prerequisite
Measurement gaps are configured in
the Master Node when an A4
measurement for NR-DC setup is
started.
Measurement gaps are removed in
the Master Node when the A4
measurement stops.
NR Air Interface Protection (FAJ 121
5340)
Conflicting
The NR Air Interface Protection
feature is conflicting with NR-NR
Dual Connectivity on node level.
If the NR Air Interface Protection
feature is enabled on the Master
Node, NR-DC setup is allowed for the
UE but user plane integrity protection
cannot continue in the Secondary
Node. This potentially results in user
plane data transmission failure if
integrity protection is activated in the
UE before the NR-DC setup.
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Feature
Relationship
Description
NR Flexible Cell Shaping High-Band
(FAJ 121 5441)
Conflicting
The NR Flexible Cell Shaping HighBand feature is conflicting with NRNR Dual Connectivity on node level.
When NR-DC is configured, the NR
Flexible Cell Shaping High-Band
feature must not be used in
combination with the NR 8CC DL
Carrier Aggregation High-Band
feature.
Fronthaul Sharing (FAJ 121 5213)
Conflicting
The Fronthaul Sharing feature is
conflicting with the NR-NR Dual
Connectivity feature on the cell level.
When NR-DC is configured, the
Fronthaul Sharing feature must not
be used in combination with the NR
8CC DL Carrier Aggregation HighBand feature.
NR DL Carrier Aggregation (FAJ 121
5201)
Related
If configured, SCells on the Master
Node are removed from the MCG at
the Secondary Node Addition
procedure.
Carrier aggregation on the Master
Node is not regained after the
Secondary Node Release procedure.
A new attempt to configure carrier
aggregation on the Master Node is
made after the next handover
performed on the Master Node.
Coverage-Optimized Uplink
Transmission High-Band (FAJ 121
5145)
Related
The Coverage-Optimized Uplink
Transmission High-Band feature is
not supported in the SCG provided by
the Secondary Node for UEs
connected with NR-DC.
Coverage-Optimized Uplink
Scheduling High-Band (FAJ 121
5387)
Related
The Coverage-Optimized Uplink
Scheduling High-Band feature is not
supported in the SCG provided by the
Secondary Node for UEs connected
with NR-DC.
Location Support (FAJ 121 5311)
Related
In NR-DC, the Secondary Node sends
the selected PSCell ID to the Master
Node during the Secondary Node
Addition procedure. The Master Node
stores the received PSCell ID. The
Master Node includes the PSCell ID in
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NR-NR Dual Connectivity
Feature
Relationship
Description
the NGAP LOCATION REPORT message
if requested by the AMF.
Multiple PSCells High-Band (FAJ 121
5341)
Related
Even if multiple PSCells are
configured in the Secondary Node,
only one of them can be used for UEs
configured with NR-DC. The PSCellcapable High-Band frequency used is
the one that has a frequency relation
configured to the PCell on the Master
Node.
Scheduler High-Band (FAJ 121 4904) Related
The improved periodic scheduling
request on PUCCH function of the
Scheduler High-Band feature is not
supported in the SCG provided by the
Secondary Node for UEs connected
with NR-DC.
RRC Inactive State (FAJ 121 5328)
UEs connected with NR-DC are
released to RRC_IDLE mode instead
of RRC_INACTIVE state at user
inactivity.
Related
The criteria for starting A4
measurements to set up NR-DC are
not reevaluated when the UE
resumes RRC_CONNECTED mode after
being in RRC_INACTIVE state.
NR RRC Connection Reestablishment (FAJ 121 5346)
Related
With the NR RRC Connection Reestablishment feature, the UE
configured with NR-DC falls back to
the RRC Connection Establishment
procedure after sending the
RRCReestablishmentRequest
message.
The criteria for starting an A4
measurement to set up NR-DC are
not reevaluated when the UE
performs an RRC Connection Reestablishment procedure.
8
NR Automated Neighbor Relations
(FAJ 121 5218)
Related
The Master Node prevents the UE
with NR-DC connection from
performing NCGI measurements
without checking the relevant UE
capabilities.
Shared NR RAN (FAJ 121 5211)
Related
The Shared NR RAN feature is not
supported to be used with NR-DC.
Neither the MRL, nor the UE-selected
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Feature
Relationship
Description
PLMN is considered in PSCell
selection.
RAN Slicing Framework (FAJ 121
5095)
Related
The supported S-NSSAI list of the
Secondary Node is not considered at
PSCell selection.
NR Mobility (FAJ 121 5041)
Related
When the mobility control at poor
coverage function triggers the UE
with NR-DC connection to perform
inter-frequency or IRAT
measurements, the Secondary Node
is not aware of the measurement
gaps configured in the Master Node
for these measurements. This results
in the Secondary Node continuing to
schedule data in the SCG during the
measurement gaps.
All inter-gNodeB handovers for UEs
connected with NR-DC are performed
using NG-based handover.
LTE-NR Dual Connectivity (FAJ 121
4908)
Related
The LTE-NR Dual Connectivity
feature provides the means of
configuring the following functions
also used in NR-DC:
— The primary path of uplink user
data
— The switching of downlink MCG
and SCG radio resources of the
Secondary Node terminated split
DRB
— The switching of uplink MCG and
SCG radio resources of the
Secondary Node terminated split
DRB
The configurable attributes to control
the functions in EN-DC are also used
in NR-DC.
LTE-NR Downlink Aggregation (FAJ
121 4912)
Related
The LTE-NR Downlink Aggregation
feature provides the means of
configuring the following functions
also used in NR-DC:
— The downlink PDCP aggregation
of the MCG and SCG radio
resources
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9
NR-NR Dual Connectivity
Feature
Relationship
Description
— The flow control mechanism
The configurable attributes to control
the functions in EN-DC are also used
in NR-DC.
LTE-NR Uplink Aggregation (FAJ 121
5091)
Related
The LTE-NR Uplink Aggregation
feature provides the means of
configuring whether uplink user data
is aggregated over both the MCG and
SCG radio resources in NR-DC. The
configurable attribute to control the
function in EN-DC is also used in NRDC.
Uplink PDCP aggregation is disabled
for NR-DC UEs when the primary
data path is MCG. The MCG
aggregation threshold that is
introduced by the coveragecontrolled uplink PDCP aggregation
function is not used for NR-DC UEs.
NR 4CC DL Carrier Aggregation High- Related
Band (FAJ 121 4902)
The NR 4CC DL Carrier Aggregation
High-Band feature enables up to four
downlink component carriers for the
SCG on the Secondary Node.
NR 8CC DL Carrier Aggregation High- Related
Band (FAJ 121 4964)
The NR 8CC DL Carrier Aggregation
High-Band feature enables up to
eight downlink component carriers
for the SCG on the Secondary Node.
NR Uplink Carrier Aggregation HighBand (FAJ 121 5092)
Related
The NR Uplink Carrier Aggregation
High-Band feature enables uplink
carrier aggregation for the SCG on
the Secondary Node.
EPS Fallback for IMS Voice (FAJ 121
5059)
Related
At voice calls, the UE connected with
NR-DC is possible to be redirected or
handed over to LTE.
NR Emergency Fallback to LTE (FAJ
121 5166)
Related
At emergency calls, the UE connected
with NR-DC is possible to be
redirected to LTE.
Basic Voice over NR (FAJ 121 5219)
Related
The Secondary Node Addition and
the Secondary Node Release
procedures can be performed with
minimum user data interruption of
Master Node terminated PDU
sessions. Therefore, the split DRBs for
QoS flows in the Secondary Node
terminated PDU sessions are kept
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Feature
Relationship
Description
during a VoNR call for UEs in HighBand coverage.
NR Robust Header Compression for
Voice (FAJ 121 5400)
Related
VoNR bearers that are set up with the
profile 5QI=1 must not be set up as
Secondary Node terminated split
DRBs. Otherwise, performance
degradation is expected.
Advanced Downlink Single-User
MIMO Mid-Band (FAJ 121 5262)
Related
For UEs configured with NR-DC, most
of the traffic is transmitted through
the SCG, therefore, the benefit of the
enhanced SRS resource allocation
function is less visible.
NR Traffic Steering (FAJ 121 5458)
Related
If the UE is NR-DC-capable and all
preconditions for the NR-DC setup
are fulfilled in the serving cell, the
Capability-Aware PCell Selection
function of NR Traffic Steering is not
triggered for the UE.
The Bandwidth-Triggered InterSystem Handover function of NR
Traffic Steering is triggered if NR-DC
cannot be established for the UE
during the first NR-DC measurement
period. If the UE is configured with
NR-DC, the evaluation of the
Bandwidth-Triggered Inter-System
Handover function is not triggered for
the UE.
Hardware
No support for NR-DC if the Master Node and the Secondary Node are located in
the same Baseband.
Limitations
No known limitations.
Network Requirements
— The UE must be capable of NR-DC and compliant with the 15.6.0 or later
version of the RRC protocol.
— The AMF in the 5GC must support the PDU Session Resource Modify
Indication procedure compliant with the 15.8.0 or later version of the NGAP
protocol.
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NR-NR Dual Connectivity
— The RLC mode for a specific 5QI must be set to the same value in both the
Master Node and in the Secondary Node. In both of the nodes, the RLC mode
for a 5QI is set using the CUCP5qi.rlcMode attribute under the
CUCP5qiTable MO for the corresponding 5QI.
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3
Feature Operation of NR-NR Dual
Connectivity
3.1
Master Node Criteria for NR-DC Setup
An A4 measurement is used to detect that the UE is in High-Band coverage. The
purpose of this inter-frequency measurement is that the UE finds a candidate
PSCell cell on a High-Band frequency to establish NR-DC.
The following conditions must be fulfilled, so that the Master Node configures the
UE to start A4 measurements for the NR-DC setup:
— The NR-NR Dual Connectivity feature is activated and operable on the
Master Node.
— The NrdcMnCellProfileUeCfg.nrdcEnabled attribute is set to true in the
instance of the cell MO that represents the PCell on the Master Node.
— At least one established PDU session is allowed by configuration to be
Secondary Node terminated.
— A measurement object, the SSB on one High-Band frequency, can be selected
with all the following conditions being fulfilled:
— The frequency of the measurement object is represented by an
NRFrequency MO instance.
— At least one NRFreqRelation MO instance must exist from the
NRCellCU MO instance representing the PCell to the NRFrequency MO
instance representing the High-Band frequency of the measurement
object.
Note:
The NRFreqRelation MO instance between the PCell and the
High-Band frequency must be created manually.
— The UE is capable of at least one combination that supports NR-DC of
the following bands:
•
At least one band of the PCell frequency.
•
At least one band of the NRFrequency MO representing the HighBand frequency of the measurement object.
Such band combinations are checked against the list of the allowed band
combinations. If the list exists, it contains the band combinations filtered
by the operating band of the PCell frequency. If not, all bands of the
PCell frequency are considered.
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NR-NR Dual Connectivity
If multiple instances of the NRFrequency MO exist with different High-Band
frequencies, the one with the lowest NR-ARFCN is selected.
If all the preconditions for starting an A4 measurement are met, the Master Node
configures the UE with the A4 measurement in the following cases:
— At Initial Context Setup
— At the PDU Session Resource Setup procedure
— At the PDU Session Resource Modify procedure
— At an incoming handover to the Master Node
— At both the Master Node initiated and the Secondary Node initiated
Secondary Node Release procedures
The A4 measurement is configured for the UE during the RRC Reconfiguration
procedure to have the SSB on one High-Band frequency as the measurement
object. The trigger quantity for the A4 measurement is RSRP, and the
measurement is started and stopped periodically. The measurement time is the
sum of the NrdcMnCellProfileUeCfg.nrdcMeasTime attribute value and the
value set in the timeToTrigger member of the
NrdcMnCellProfileUeCfg.rsrpPSCellCandidate attribute. The measurement
interval, while the measurement is halted, is configured by using the
NrdcMnCellProfileUeCfg.nrdcMeasRestartTime attribute. This configurable
periodicity of the measurement provides a means of limiting UE battery
consumption. The A4 measurement is stopped and removed from the UE
configuration at the Secondary Node Addition procedure. No A4 measurement is
started while the UE is connected with NR-DC. When the Secondary Node
Release procedure is triggered, either by the Master Node or the Secondary Node,
the A4 measurement restarts.
Measurement
time
Measurement
time
Measurement
interval
NR-DC configured
Measurement
interval
Time [sec]
Start
A4 Measurement
1. Initial Context Setup
2. Incoming handover
to Master Node
3. PDU Session Resource Setup
4. PDU Session Resource Modify
Remove
A4 Measurement
1. Master Node receives
A4 measurement report
2. Secondary Node Addition
Master Node or Secondary Node
initiated Secondary Node Release
Remove
A4 Measurement
Start
A4 Measurement
L0003103C
Figure 4 A4 Measurement Handling and Periodicity
A measurement gap pattern is always configured for A4 measurements in the
Master Node and in the UE. For A4 measurements, the 3GPP-defined gap pattern
1 is used with a gap length of 6 ms and a gap repetition period of 80 ms.
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The NrdcMnCellProfileUeCfg.rsrpPSCellCandidate attribute contains the
following configurable members for the A4 measurement:
threshold
If the RSRP of any cells on the measured High-Band
frequency is above this configured threshold, the UE
sends an A4 measurement report to the Master Node.
hysteresis
The hysteresis is used to prevent the retriggering of an A4
measurement report if the measured value fluctuates
close to the configured threshold.
timeToTrigger
The value defines how long the threshold criteria must be
fulfilled before triggering an A4 measurement report.
The UE sends the A4 measurement report to the Master Node in the RRC
MeasurementReport message. The measurement report contains the PCI of a
neighbor cell on the measured High-Band frequency. If the following criteria are
met, the Master Node initiates the Secondary Node Addition procedure to the
gNodeB hosting the PSCell candidate the UE reported:
— The reported PCI can be mapped to an existing ExternalNRCellCU MO
instance representing the PSCell candidate on the measured High-Band
frequency. The MO is a child to the ExternalGNBCUCPFunction MO
representing the Secondary Node in the MO structure of the Master Node.
— One NRCellRelation MO instance exists between the ExternalNRCellCU
MO and the NRCellCU MO instance that represents the serving PCell on the
Master Node.
Note:
The NRCellRelation MO instance between the NRCellCU and
ExternalNRCellCU MOs must be created manually.
— At least one band combination exists in the list of allowed band
combinations that contains the band of the found PSCell candidate.
— A PDU session exists for this UE that is allowed to be Secondary Node
terminated by configuration.
— A connection over the Xn interface exists between the Master Node hosting
the PCell and the Secondary Node hosting the PSCell candidate. The
connection is represented by a TermPointToGNodeB MO instance. The MO is
a child to the ExternalGNBCUCPFunction MO representing the Secondary
Node in the MO structure of the Master Node. For the connection to be
functional, the TermPointToGNodeB.administrativeState attribute must
be set to UNLOCKED and the TermPointToGNodeB.operationalState
attribute must display ENABLED.
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NR-NR Dual Connectivity
Master Node MO Structure
Secondary Node MO Structure
on the Master Node
GNBCUCPFunction
NRCellCU
ExternalGNBCUCPFunction
NRNetwork
A
NRFrequency
ExternalNRCellCU
B
C
NRFreqRelation
NRFrequency
D
E
F
NRCellRelation
TermPointToGNodeB
A
NRCellCU.nRFrequencyRef
D
NRFreqRelation.nRFrequencyRef
B
ExternalNRCellCU.nRFrequencyRef
E
NRCellRelation.nRFreqRelationRef
C
NRCellRelation.nRCellRef
F
NRCellRelation.nRCellRef
Parent-Child Connection
Reference Connection
L0003100A
Figure 5 Master Node MO Structure with Relations Considered for A4
Measurements
RELATED INFORMATION
Reception of an A4 Measurement Report
NR-DC Band Combination Selection
PDU Session Resource Setup Procedure
PDU Session Resource Modify Procedure
Incoming Handover into a New PCell
Secondary Node Addition
Master Node Initiated Secondary Node Release
Secondary Node Initiated Secondary Node Release
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3GPP TS 38.331; NR; Radio Resource Control (RRC); Protocol specification
3.2
Termination Point Control of QoS Flows in the Master
Node
For each QoS flow, the 5GC provides a 5QI value and an ARP value. These values
are compared to a framework of 5QI profiles and the configuration of Secondary
Node termination to determine whether a QoS flow is allowed to be Secondary
Node terminated or not.
GNBCUCPFunction
NrdcSnTermination
CUCP5qiTable
defaultNrdcSnTerm
NrdcSnTerminationUeCfg
nrdcSnTermAllowed
CUCP5qi
nrdcSnTerminationRef
Legend
Reference Connection
Affected MOs and parameters
L0003142A
Figure 6 MO Structure for Controlling Secondary Node Termination of QoS
Flows
The CUCP5qi MO instance under the CUCP5qiTable MO contains the
configuration of a specific 5QI profile. The CUCP5qi.nrdcSnTerminationRef
attribute can contain a reference to an NrdcSnTermination MO instance.
The NrdcSnTermination MO has the NrdcSnTerminationUeCfg MO as its child.
The NrdcSnTermination and the NrdcSnTerminationUeCfg MOs are both
defined by the operator or created automatically. The
NrdcSnTerminationUeCfg.nrdcSnTermAllowed attribute is used to apply
different settings for each ARP applicable to the 5QI profile referencing the
parent NrdcSnTermination MO instance. The attribute is configured to control
whether a QoS flow with the 5QI profile is allowed to be Secondary Node
terminated or not.
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NR-NR Dual Connectivity
Note:
The NrdcSnTermination = SnTerminationProhibited MO instance is
automatically created with a child NrdcSnTerminationUeCfg = Base
MO instance. The NrdcSnTermination = SnTerminationProhibited
MO instance with a child NrdcSnTerminationUeCfg = Base MO instance
prohibits QoS flows for IMS signaling, voice, and latency-sensitive
services to be terminated by the Secondary Node. It is recommended
that this MO instance is used for such services, so that no manual
configuration is needed.
If the CUCP5qi.nrdcSnTerminationRef attribute is empty, any of the following
cases is applicable:
— If it exists, the automatically created NrdcSnTermination =
SnTerminationProhibited MO instance with a child
NrdcSnTerminationUeCfg = Base MO instance is used for the following
services:
•
IMS signaling
•
Voice services
•
Latency-sensitive services
— If it exists, the NrdcSnTermination MO instance with the
NrdcSnTermination.defaultNrdcSnTerm attribute set to true is used for
services other than IMS signaling, voice services, or latency-sensitive
services.
If no instance set as default of the NrdcSnTermination MO exists, the QoS
flow with the specific 5QI is not allowed to be Secondary Node terminated.
A PDU session is allowed to be Secondary Node terminated if all of its QoS flows
are allowed to be Secondary Node terminated. By controlling QoS flow
termination, a UE can use a combination of different types of PDU sessions. One
type of PDU session has only Master Node terminated QoS flows, while the other
has only Secondary Node terminated QoS flows. The same configuration,
whether Secondary Node termination is allowed or not, is recommended for all
QoS flows in the same PDU session.
RELATED INFORMATION
Bearer Type Change in NR-DC
PDU Session Resource Modify Procedure
Configure 5QI and ARP Levels for QoS Flows Allowed to Be Secondary Node
Terminated
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3.3
Bearer Type Change in NR-DC
A bearer type change is when one type of DRB is reconfigured into the other. This
occurs at different NR-DC procedures that trigger DRB reconfiguration. Bearer
type changes exist between the following types of DRBs supported in NR-DC:
— Master Node terminated MCG DRB
— Secondary Node terminated split DRB
The NR-DC-capable UE can be in one of the following states depending on the
DRB type it uses:
Idle Mode
The UE is in RRC_IDLE mode.
Connected Mode with NR-DC Not Configured
The UE is connected to the Master Node using one or
more Master Node terminated MCG DRBs only. Therefore,
user plane traffic is transmitted through the Master Node
only.
At this stage, if all the criteria for starting an A4
measurement are met, the UE is configured by the Master
Node to start A4 measurements to find a candidate
PSCell on a High-Band frequency.
At the reception of the A4 measurement report, the
Master Node initiates the Secondary Node Addition
procedure, if all the criteria for the procedure are met.
During the Secondary Node Addition procedure, one or
more Master Node terminated MCG DRBs that are
allowed to be split by configuration are reconfigured to
become Secondary Node terminated split DRBs.
Connected Mode with NR-DC Configured
The UE is connected in NR-DC both to the Master Node
and the Secondary Node. The UE can use the following
bearer types depending on the 5QI and ARP
configuration of the QoS flows the DRBs carry:
— One or more Secondary Node terminated split DRBs.
— A combination of one or more Master Node
terminated MCG DRBs and one or more Secondary
Node terminated split DRBs.
Different scenarios can trigger the Master Node initiated
or the Secondary Node initiated Secondary Node Release
procedure. The result is that the Secondary Node
resources for the UE are released. Secondary Node
terminated split DRBs are reconfigured into Master Node
terminated MCG DRBs. The UE remains connected to the
Master Node.
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NR-NR Dual Connectivity
The UE can be released to RRC_IDLE mode when
connected in NR-DC.
The different UE state changes and the NR-DC procedures that trigger bearer
type changes are shown in Figure 7.
A4 Measurement-based
Secondary Node Addition
Initial Context Setup
Connected Mode with
NR-DC Not Configured
Idle Mode
RRC Connection Release
Connected Mode with
NR-DC Configured
Master Node or Secondary Node
initiated Secondary Node Release
RRC Connection Release
L0003102B
Figure 7 UE State Changes at NR-DC Procedures
RELATED INFORMATION
NR-DC Deployment
PDU Session Resource Setup Procedure
PDU Session Resource Modify Procedure
Incoming Handover into a New PCell
Secondary Node Addition
Master Node Initiated Secondary Node Release
Secondary Node Initiated Secondary Node Release
3.4
User Plane Management
With the Secondary Node resources available to the UE, the bearer type can be
changed from Master Node terminated DRBs to Secondary Node terminated split
DRBs. For Secondary Node terminated split DRBs, the user plane provided to the
UE is set up in the following ways:
— Downlink traffic is transmitted through the MCG or SCG radio resources of
the Secondary Node terminated split DRB depending on the High-Band
downlink radio link quality. Downlink data can be aggregated over both the
MCG and SCG radio resources depending on the amount of data.
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— Uplink traffic is transmitted either through the MCG or SCG radio resources of
the Secondary Node terminated split DRB. The uplink data path through
which the uplink traffic is transmitted is based initially on configuration and
then the High-Band uplink radio link quality. Uplink user data can be
aggregated over both the MCG and SCG radio resources depending on the
following variables:
— The amount of user data.
— The UE capability to support splitting of uplink PDCP data on both the
MCG and SCG for the Secondary Node terminated split DRB.
— The configuration of the uplink data split threshold.
The uplink PDCP aggregation function is enabled for NR-DC UEs only if the
primary uplink data path is SCG.
The different user plane management functions used in NR-DC are provided by
the following features operable on the Secondary Node:
— The LTE-NR Dual Connectivity provides the means of configuring the
following functions also used in NR-DC:
— The initial primary path of uplink user data
— The switching of downlink MCG and SCG radio resources of the
Secondary Node terminated split DRB
— The switching of uplink MCG and SCG radio resources of the Secondary
Node terminated split DRB
— The LTE-NR Uplink Aggregation provides the means of configuring whether
uplink user data is aggregated over both the MCG and SCG radio resources in
NR-DC. The uplink data split threshold attribute
QciProfileEndcConfigExt.ulDataSplitThresholdMcg that is intended
for cases in which the primary path is MCG is not used for NR-DC UEs.
— The LTE-NR Downlink Aggregation provides the means of configuring the
following functions also used in NR-DC:
— The downlink aggregation of the MCG and SCG radio resources
— The flow control mechanism
RELATED INFORMATION
Bearer Type Change in NR-DC
Parameters for NR-NR Dual Connectivity
Secondary Node Initiated Secondary Node Modification
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NR-NR Dual Connectivity
LTE-NR Dual Connectivity
LTE-NR Uplink Aggregation
LTE-NR Downlink Aggregation
3.5
NR-DC Band Combination Selection
The band combination and feature set are selected among the allowed ones to
maximize SCG carrier aggregation and optimize Layer 1 and Layer 2
configuration. Band combinations contain the operating bands of the PCell
frequency and the SCG frequencies that support NR-DC. The support of the
different band combinations and feature sets is based on the NR-DC-specific
capabilities of the UE.
The Master Node requests the NR-DC-specific UE capabilities (if not received
from the AMF) from the UE at Initial Context Setup. A candidate PSCell is
considered if the UE is capable of a band combination supporting NR-DC that
contains the following:
— The band of the PCell frequency
— The band of the High-Band frequency of the candidate PSCell
Band combination and feature set selection is a distributed function between the
Master Node and the Secondary Node according to the following steps:
1. The Master Node assembles a list of allowed band combinations based on
the reported NR-DC-specific capabilities of the UE. This is done by the Master
Node checking the following criteria:
— If the band combination which contains the PCell frequency band
supports NR-DC.
— If the band combination and the feature set match the current PCell
configuration.
This check can result in one of the following outcomes:
— If the list is not empty and it contains the High-Band frequency of the
latest A4 measurement report in at least one band combination
supporting NR-DC for the UE, the Secondary Node Addition procedure
can be triggered.
— The list is not empty, but it does not contain the High-Band frequency of
the latest A4 measurement report in any band combinations supporting
NR-DC for the UE. In this case, the Master Node does not trigger the
Secondary Node Addition procedure at the reception of an A4
measurement report. However, subsequent A4 measurements can start.
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— If the list is empty, the Master Node does not trigger the Secondary Node
Addition procedure at the reception of an A4 measurement report. In
addition, no subsequent A4 measurement is started while the UE
remains in connected mode in the same PCell.
2. The Master Node signals the list of allowed band combinations to the
Secondary Node at the Secondary Node Addition procedure using the XnAP
S-NODE ADDITION REQUEST message.
3. At the reception of the XnAP S-NODE ADDITION REQUEST message, the
Secondary Node uses the band combination list to perform the following
actions:
— To finalize the band combination and feature set selection.
— To select a set of candidate High-Band SCells to be used in the SCG.
The Secondary Node selects the best possible band combination and feature
set based on whether that band combination supports NR-DC and is
supported by 3GPP. It selects, for example, the number of carrier
components, the MIMO layers and the type of modulation in a way that
maximizes the Secondary Node throughput.
4. The Secondary Node signals the finalized selection of the band combination
and feature set to the Master Node using the XnAP S-NODE ADDITION
REQUEST ACKNOWLEDGE message.
5. At the reception of the XnAP S-NODE ADDITION REQUEST ACKNOWLEDGE
message, the Master Node performs the following actions:
— The Master Node configures NR-DC for the UE based on the band
combination and the feature set selected by the Secondary Node.
— The Master Node removes SCells on the Master Node and all the SCell
measurements for the UE if carrier aggregation is applied on the Master
Node.
RELATED INFORMATION
Master Node Criteria for NR-DC Setup
PDU Session Resource Setup Procedure
PDU Session Resource Modify Procedure
Incoming Handover into a New PCell
Secondary Node Addition
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NR-NR Dual Connectivity
3GPP TS 38.101-3; NR; User Equipment (UE) radio transmission and reception;
Part 3: Range 1 and Range 2 Interworking operation with other radios
3.6
User Activity Supervision
For the UE configured with NR-DC, both the Master Node and the Secondary
Node monitor user activity. The Master Node does it on UE level, considering the
overall UE state of being active or inactive.
The PDCP layer in the Master Node monitors the activity of all the Master Node
terminated MCG DRBs for the UE. The DRB is inactive if it is not used for user
data transmission during the time period defined by the inactivity timer set on
the Master Node in the InactivityProfileUeCfg.tInactivityTimer attribute.
The Master Node also monitors if NAS RRC messages are sent or received during
the time period defined by the NAS inactivity timer. The timer is set in the Master
Node using the GNBCUCPFunction.nasInactivityTime attribute expires.
The PDCP layer in the Secondary Node monitors the activity of all Secondary
Node terminated split DRBs. These DRBs are inactive if they are not used for user
data transmission for the time value of the
InactivityProfileUeCfg.tInactivityTimerNrdcSn attribute. When the
Secondary Node detects that the activity state of the Secondary Node terminated
split DRBs change (from active to inactive or from inactive to active), it sends the
XnAP ACTIVITY NOTIFICATION message to the Master Node. This message is
used to notify the Master Node about the activity state of the Secondary Node
terminated split DRBs for the UE.
Master Node
Secondary Node
GNBCUCPFunction.nasInactivityTime
NAS RRC message
sent or received
Master Node
terminated MCG DRBs
active/inactive
InactivityProfileUeCfg.tInactivityTimer
Xn-C interface
User activity supervision in NR-DC
A
Inactivity timer
A
Secondary Node
terminated Split DRBs
active/inactive
XnAP ACTIVITY NOTIFICATION
L0003117B
Figure 8 User Activity Supervision in NR-DC
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The Master Node releases the UE to RRC_IDLE mode when all the following
criteria are fulfilled:
— The Master Node terminated MCG DRBs are inactive as indicated by the
expired inactivity timer.
— The Secondary Node terminated split DRBs are inactive as indicated by the
notification from the Secondary Node.
— No NAS RRC messages are sent or received during the configured time
period.
RELATED INFORMATION
UE Released to Idle Mode
3.7
Xn Link Break Handling
An Xn link break occurs when the connection between the Master Node and the
Secondary Node is lost over the Xn-C interface. The Xn link break can typically be
caused by any of the following cases:
— Transport failure occurs over the Xn interface.
— Either the Master Node or the Secondary Node restarts or is not operational.
— The local termination point of the Xn interface on the Master Node or on the
Secondary Node is locked. This occurs when the
TermPointToGNodeB.administrativeState attribute is set to LOCKED.
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NR-NR Dual Connectivity
UE
Master Node
Secondary Node
AMF
Xn link
break
1. Xn link break detected
1. Xn link break detected
UEs with Secondary Node terminated split DRBs using the Xn link
2. NG Reset procedure
NGAP NG RESET
3. RRC RRCConnectionRelease
NGAP NG RESET ACKNOWLEDGE
4. Local resource cleanup
4. Local resource cleanup
5. Xn link recovery
L0003114A
Figure 9 Sequence of Xn Link Break Handling at Lost Xn Connection
The Xn link break handling in NR-DC is done according to the following
sequence:
1. The Xn link break is detected both in the Master Node and the Secondary
Node independently.
When the Xn link break is caused by setting the
TermPointToGNodeB.administrativeState attribute to LOCKED, the SCTP
association shuts down for transport over the Xn interface before the
detection of the Xn link break. The TermPointToGNodeB.operationalState
attribute displays DISABLED. If the termination point is locked on the Master
Node, it is the Master Node that shuts down the SCTP association. If the
termination point is locked on the Secondary Node, it is the Secondary Node
that shuts down the SCTP association.
2. The NG Reset procedure is used between the Master Node and the AMF.
a. The Master Node sends the NGAP NG RESET message to the AMF. This
message contains the list of UEs using Secondary Node terminated split
DRBs served by the broken Xn link. The message indicates the release of
all the affected UEs.
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b. The AMF responds to the Master Node with the NGAP NG RESET
ACKNOWLEDGE message.
3. The Master Node initiates the release of all UEs served by the broken Xn link
to RRC_IDLE mode. This is triggered when the Master Node sends the RRC
RRCConnectionRelease message to the affected UEs.
4. All the MCG and the SCG radio resources of the Secondary Node terminated
split DRBs, as well as all other UE-specific resources, allocated to the
affected UEs are locally released. This occurs in both the Master Node and
the Secondary Node simultaneously.
5. The Master Node and the Secondary Node initiate new Xn link setup
attempts independently to restore connection over the Xn interface.
However, when the Xn link break is caused by locking the termination point,
no Xn link re-establishment is initiated either by the Master Node or the
Secondary Node.
The XnC Link to GNodeB Failure alarm is raised on both the Master Node and
the Secondary Node at Xn link break. However, when the Xn link break is caused
by locking the termination point, no alarm is raised either on the Master Node or
on the Secondary Node.
RELATED INFORMATION
Detection of an Xn Link Break in the Master Node
XnC Link to GNodeB Failure
3.8
Master Node Mobility Support in NR-DC
3.8.1
Intra-gNodeB Handover When NR-DC Is Configured
The following intra-gNodeB mobility cases are supported in NR-DC for the
Master Node:
— Intra-frequency intra-gNodeB handover
— Inter-frequency intra-gNodeB handover
The intra-gNodeB handover follows the procedure of the NR Mobility feature.
However, when used in NR-DC, the Master Node initiated Secondary Node
Release procedure is performed during handover.
After sending the RRC RRCReconfiguration to the UE, the Master Node initiates
the Secondary Node Release procedure. With it, all PDU sessions for the UE that
are Secondary Node terminated in the source cell are reconfigured to be Master
Node terminated in the target cell. PDU sessions that are already Master Node
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NR-NR Dual Connectivity
terminated are kept. If Xn-U downlink TEIDs are provided, packet forwarding is
performed for the Secondary Node terminated PDU sessions.
After the UE responds to the Master Node with the RRC
RRCReconfigurationComplete message, the Master Node uses the PDU Session
Resource Modify Indication procedure to notify the AMF of the new NG-U
downlink TEIDs provided during the Secondary Node Release procedure.
If all criteria for starting A4 measurements are met, the Master Node configures
the UE in the target cell to perform A4 measurements. The aim is to find a
candidate PSCell for the Secondary Node Addition procedure.
RELATED INFORMATION
Outgoing Intra-System Handover from the Source Cell
Incoming Handover into a New PCell
Master Node Initiated Secondary Node Release
Reception of an A4 Measurement Report
NR Mobility
3.8.2
Inter-gNodeB Handover When NR-DC Is Configured
The following inter-gNodeB mobility cases are supported in NR-DC for the
Master Node over the NG interface:
— Intra-frequency inter-gNodeB, NG-based handover
— Inter-frequency inter-gNodeB, NG-based handover
The inter-gNodeB, NG-based handover follows the procedure of the NR Mobility
feature. However, when used in NR-DC, the Master Node initiated Secondary
Node Release procedure is performed during the handover.
After sending the RRC RRCReconfiguration to the UE, the source Master Node
initiates the Secondary Node Release procedure. With it, all PDU sessions for the
UE, regardless if Secondary Node or Master Node terminated, are moved from
the source Master Node to the target gNodeB. On the target gNodeB, all the
moved PDU sessions are reconfigured to be Master Node terminated. If Xn-U
downlink TEIDs are provided, packet forwarding is performed for the Secondary
Node terminated PDU sessions from the Secondary Node to the target gNodeB.
Packet forwarding is also performed for Master Node terminated PDU session
from the source Master Node to the target gNodeB.
After the Secondary Node Release procedure, the source Master Node sends the
NGAP UPLINK RAN STATUS TRANSFER message to the AMF. The message
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contains the combined PDCP COUNT values from the source Master Node and the
Secondary Node for AM DRBs.
If all criteria for starting A4 measurements are met, the target gNodeB, acting as
the Master Node, configures the UE in the target cell to perform A4
measurements. The aim is to find a candidate PSCell for the Secondary Node
Addition procedure.
RELATED INFORMATION
Outgoing Intra-System Handover from the Source Cell
Incoming Handover into a New PCell
Master Node Initiated Secondary Node Release
Reception of an A4 Measurement Report
NR Mobility
3.8.3
IRAT Handover to LTE When NR-DC Is Configured
The IRAT handover to LTE follows the procedure of the NR Mobility feature.
However, when used in NR-DC, the Master Node initiates the UE context release
to the Secondary Node.
RELATED INFORMATION
Outgoing IRAT Handover from the Master Node
Master Node Initiated UE Context Release
NR Mobility
3.8.4
Incoming IRAT Handover from LTE into an NR-DC-Capable PCell
The handling of the incoming IRAT handover from LTE follows the procedure of
the NR Mobility feature. If all criteria for starting A4 measurements are met, the
target gNodeB, acting as the Master Node, configures the UE in the target cell to
perform A4 measurements. The aim is to find a candidate PSCell for the
Secondary Node Addition procedure.
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NR-NR Dual Connectivity
RELATED INFORMATION
Master Node Criteria for NR-DC Setup
Incoming Handover into a New PCell
NR Mobility
3.9
Scenarios for Master Node Triggered NR-DC Procedures
3.9.1
Reception of an A4 Measurement Report
The UE, as configured to perform inter-frequency A4 measurements, reports the
PCI of a candidate PSCell on a High-Band frequency to the Master Node.
When an A4 measurement report is received by the Master Node, the Secondary
Node Addition procedure starts if all the criteria for triggering the procedure are
met.
RELATED INFORMATION
Master Node Criteria for NR-DC Setup
Secondary Node Addition
3.9.2
UE Indicating SCG Radio Link Failure
The UE can indicate RLF by sending the SCGFailureInformation RRC message
to the Master Node in the following cases:
— It is not possible to perform random access to the PSCell during the
Secondary Node Addition procedure.
— The configured maximum number of allowed RLC retransmissions is reached
in uplink.
— The timer set in the Rrc.t310 attribute expires.
At the reception of the SCGFailureInformation RRC message, the Master Node
initiates the Secondary Node Release procedure.
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RELATED INFORMATION
Master Node Initiated Secondary Node Release
3.9.3
UE Released to Idle Mode
When the UE is released to RRC_IDLE mode by the Master Node, resources in the
Secondary Node are released. This is triggered in the Secondary Node at the
reception of the XnAP UE CONTEXT RELEASE message sent by the Master Node.
RELATED INFORMATION
Master Node Initiated UE Context Release
3.9.4
PDU Session Resource Setup Procedure
During the PDU Session Resource Setup procedure, resources over the Uu and
NG-C interfaces are assigned for one or more PDU sessions and their
corresponding QoS flows. The procedure aims to set up DRBs corresponding to
the QoS flows for a given UE.
Connected Mode with NR-DC Not Configured
Based on the configured 5QI and ARP values of each QoS flow in a PDU session,
the following cases occur:
— If the new PDU session is allowed to be Secondary Node terminated and all
the criteria for starting an A4 measurement are met, the Master Node
configures the UE to start A4 measurements if not already started.
— If the new PDU session is not allowed to be Secondary Node terminated by
configuration, the Master Node takes no action related to NR-DC at reception
of the NGAP PDU SESSION RESOURCE SETUP REQUEST message.
Connected Mode with NR-DC Configured
When the UE is configured with NR-DC, setting up additional Secondary Node
terminated PDU sessions is not supported. Such PDU sessions are set up as being
Master Node terminated, regardless if the 5QI and ARP configuration allows
Secondary Node termination. The PDU sessions remain to be Master Node
terminated until the next time NR-DC is configured.
RELATED INFORMATION
Master Node Criteria for NR-DC Setup
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NR-NR Dual Connectivity
Termination Point Control of QoS Flows in the Master Node
Reception of an A4 Measurement Report
3.9.5
PDU Session Resource Modify Procedure
During the PDU Session Resource Modify procedure, the AMF requests the
Master Node to modify the already existing PDU sessions for a given UE. The
procedure is used to set up or release a QoS flow within an established PDU
session. No action is taken when an existing QoS flow in a Secondary Node
terminated PDU session is modified.
A PDU session is modified to be allowed to be Secondary Node terminated by
releasing QoS flows that prevent Secondary Node termination by configuration.
A PDU session is modified not to be allowed to be Secondary Node terminated by
adding QoS flows that prevent Secondary Node termination by configuration.
Connected Mode with NR-DC Not Configured
Based on the configured 5QI and ARP values of each QoS flow in a PDU session,
the following cases occur:
— When a PDU session becomes allowed to be Secondary Node terminated by
releasing QoS flows and all the criteria for starting an A4 measurement are
met, the Master Node configures the UE to start A4 measurements, if not
already ongoing.
— When no PDU session is allowed to be Secondary Node terminated any
longer by adding QoS flows, the ongoing A4 measurement continues until
the measurement time is up. However, A4 measurements are not restarted
after that.
Connected Mode with NR-DC Configured
Based on the configured 5QI and ARP values of each QoS flow in a PDU session,
the following cases occur:
— When QoS flows are added to already established PDU sessions, the
following restrictions apply:
— If the established PDU session is Master Node terminated, newly-added
QoS flows are always Master Node terminated, regardless of 5QI and
ARP configuration.
— If the established PDU session is Secondary Node terminated, newlyadded QoS flows are always Secondary Node terminated, regardless of
5QI and ARP configuration. When a QoS flow preventing Secondary
Node termination is added to a Secondary Node terminated PDU
session, the PDU session remains Secondary Node terminated.
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— When the last QoS flow that prevents the PDU session from being Secondary
Node terminated is released, the PDU session remains Master Node
terminated and no action is taken. Such PDU sessions remain to be Master
Node terminated until the next time NR-DC is configured.
QoS Flow A1
QoS Flow B1
PDU Session A
PDU Session B
Master Node
Secondary Node
SDAP
SDAP
PDCP
PDCP
RLC
RLC
RLC
MAC and Layer 1
MAC and Layer 1
Set up QoS Flow
QoS Flow A1
Release QoS Flow
QoS Flow B1
PDU Session A
PDU Session B
Master Node
Secondary Node
SDAP
SDAP
PDCP
PDCP
RLC
QoS Flow B2
RLC
RLC
MAC and Layer 1
RLC
PDCP
RLC
MAC and Layer 1
L0003104B
Figure 10 Setting up and Releasing QoS Flow for Secondary Node Terminated
PDU Session
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NR-NR Dual Connectivity
RELATED INFORMATION
Master Node Criteria for NR-DC Setup
Termination Point Control of QoS Flows in the Master Node
Reception of an A4 Measurement Report
Master Node Initiated Secondary Node Modification
3.9.6
PDU Session Resource Release Procedure
During the PDU Session Resource Release procedure, resources of established
PDU sessions are released for a given UE.
Connected Mode with NR-DC Not Configured
If the released PDU session was the last that was allowed to be Secondary Node
terminated, the ongoing A4 measurement continues until the measurement time
is up. However, A4 measurements are not restarted after that.
Connected Mode with NR-DC Configured
When the UE is configured with NR-DC, releasing a subset of Secondary Node
terminated PDU sessions is not supported.
When a subset of Secondary Node terminated PDU sessions is released, the
following sequence applies:
1. Resources in the Secondary Node are released at the reception of the XnAP
UE CONTEXT RELEASE message sent by the Master Node.
2. The Master Node requests the UE context release by sending the NGAP UE
CONTEXT RELEASE REQUEST message to the AMF.
3. The UE is released to RRC_IDLE mode by the Master Node.
When all Secondary Node terminated PDU sessions are released, resources in the
Secondary Node are released at the reception of the XnAP UE CONTEXT RELEASE
message sent by the Master Node. The UE remains connected to the Master
Node.
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QoS Flow A1
QoS Flow B1
PDU Session A
PDU Session B
Master Node
Secondary Node
SDAP
SDAP
PDCP
PDCP
RLC
RLC
RLC
MAC and Layer 1
MAC and Layer 1
PDU Session Resource Release
QoS Flow A1
PDU Session A
Master Node
SDAP
PDCP
RLC
MAC and Layer 1
L0003116B
Figure 11 Release of the Last Secondary Node Terminated PDU Session
RELATED INFORMATION
UE Released to Idle Mode
Master Node Initiated UE Context Release
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NR-NR Dual Connectivity
3.9.7
Detection of an Xn Link Break in the Master Node
When an Xn link break is detected by the Master Node, all the UEs using
Secondary Node terminated split DRBs served by the broken Xn link are released
to RRC_IDLE mode by the Master Node. The outcome is the same if the Xn link
break is caused by locking the termination point of the Xn interface on the Master
Node.
RELATED INFORMATION
Xn Link Break Handling
UE Released to Idle Mode
3.9.8
Outgoing Intra-System Handover from the Source Cell
At all types of outgoing intra-system handover from the source cell on the Master
Node, the Master Node initiates the Secondary Node Release procedure as part
of the handover execution.
RELATED INFORMATION
Intra-gNodeB Handover When NR-DC Is Configured
Inter-gNodeB Handover When NR-DC Is Configured
Master Node Initiated Secondary Node Release
3.9.9
Incoming Handover into a New PCell
At incoming handover into a new PCell, the Master Node can configure the UE to
start A4 measurements to find a candidate PSCell on a High-Band frequency if
the NR-DC-specific UE capabilities are known in the new PCell.
The UE capabilities are not known in the new NR-DC-capable PCell in the
following cases when the UE moves to this PCell from a non-NR-DC-capable cell
the first time:
— After UE registration to the 5GC
— After incoming IRAT handover from LTE
Therefore, no A4 measurement starts. The A4 measurement starts when the UE
connects to the network the next time, or when a subsequent handover is
performed to another NR-DC-capable new PCell.
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RELATED INFORMATION
Master Node Criteria for NR-DC Setup
Intra-gNodeB Handover When NR-DC Is Configured
Inter-gNodeB Handover When NR-DC Is Configured
Incoming IRAT Handover from LTE into an NR-DC-Capable PCell
3.9.10
Outgoing IRAT Handover from the Master Node
At an outgoing IRAT handover, the Master Node initiates the UE context release
to the Secondary Node.
RELATED INFORMATION
IRAT Handover to LTE When NR-DC Is Configured
Master Node Initiated UE Context Release
3.10
Scenarios for Secondary Node Triggered NR-DC
Procedures
3.10.1
Secondary Node Detects Radio Link Failure
The Secondary Node initiates the Secondary Node Release procedure when it
detects that the configured maximum number of allowed RLC retransmissions is
reached in downlink.
RELATED INFORMATION
Secondary Node Initiated Secondary Node Release
3.10.2
NR Cell or Sector Carrier Is Locked on the Secondary Node
The Secondary Node initiates the Secondary Node Release procedure when
either of the following attributes of the PSCell and the associated sector carrier is
set to LOCKED:
— NRCellDU.administrativeState
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NR-NR Dual Connectivity
— NRSectorCarrier.administrativeState
RELATED INFORMATION
Secondary Node Initiated Secondary Node Release
3.10.3
Random Access Timer Expiry in Secondary Node
The Secondary Node initiates the Secondary Node Release procedure when it
detects that the time for performing a successful random access to the PSCell is
expired during the Secondary Node Addition procedure.
RELATED INFORMATION
Secondary Node Initiated Secondary Node Release
3.10.4
PDCP COUNT Threshold Is Reached
The Secondary Node initiates the Secondary Node Release procedure when the
uplink or the downlink PDCP COUNT threshold is reached for at least one
Secondary Node terminated split DRB. During the procedure, the PDCP COUNT
values are forwarded to the Master Node.
In this case, the purpose of the Secondary Node initiated Secondary Node
Release procedure is to prevent COUNT wrap-around.
RELATED INFORMATION
Secondary Node Initiated Secondary Node Release
3.10.5
High-Band Uplink Radio Link Quality Falls Below Configured Threshold in
the PSCell
If the High-Band uplink radio link quality falls below a configured threshold in
the PSCell, the Secondary Node initiates the Secondary Node Modification
procedure to change the uplink primary path from SCG to MCG.
Note:
The good quality of the MCG radio resources must be ensured by the UE
that performs mobility procedures on the Master Node.
The uplink PDCP aggregation is disabled when the primary uplink path is set to
MCG after switching from the SCG to the MCG radio resources of the Secondary
Node terminated split DRB.
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RELATED INFORMATION
Configuration Considerations for Switching of Uplink MCG and SCG Radio
Resources of the Secondary Node Terminated Split DRB
Secondary Node Initiated Secondary Node Modification
3.10.6
High-Band Uplink Radio Link Quality Rises Above Configured Threshold in
the PSCell
If the High-Band uplink radio link quality rises above a configured threshold in
the PSCell, the Secondary Node initiates the Secondary Node Modification
procedure to change the uplink primary path from MCG to SCG if all of the
following conditions apply:
— The primary uplink path for the transmission of PDCP PDUs is MCG.
— The transmission of PDCP PDUs in uplink is active.
— The GNBCUUPFunction.endcUlNrRetProhibTimer timer is not running.
If the primary uplink path is switched back to SCG, the uplink PDCP aggregation
is enabled.
RELATED INFORMATION
Configuration Considerations for Switching of Uplink MCG and SCG Radio
Resources of the Secondary Node Terminated Split DRB
Secondary Node Initiated Secondary Node Modification
3.11
NR-DC Procedures
3.11.1
Secondary Node Addition
The Secondary Node Addition procedure establishes a UE context in the
Secondary Node. This way, the Secondary Node can provide SCG and PDCP
resources to the NR-DC-capable UE.
At the Secondary Node Addition procedure, PDU sessions that are allowed to be
Secondary Node terminated by configuration are moved to the Secondary Node.
The following changes are applied:
— All DRBs in the PDU session are reconfigured from Master Node terminated
MCG DRBs to Secondary Node terminated split DRBs keeping the same DRB
ID.
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NR-NR Dual Connectivity
— New RLC and MAC entities are established for the Secondary Node
terminated split DRBs in the Secondary Node.
In the Master Node, the following options are available:
— The RRC Reconfiguration procedure is performed without an intra-cell
handover
The RLC entity in the Master Node is reestablished by releasing the old
RLC bearer and setting up a new RLC bearer for the reconfigured DRB.
The PDCP is always reestablished when this option is used.
This option is used when the number of the Master Node terminated
MCG DRBs plus twice the number of the DRBs that are reconfigured
from Master Node terminated to Secondary Node terminated is less than
or equal to eight.
— The RRC Reconfiguration procedure is performed with an intra-cell
handover
In this case, an intra-cell handover is performed in the Master Node to
reestablish the RLC entities of the reconfigured DRBs. The DRBs are
released and added again for the UE without PDCP re-establishment.
This option is only used when the number of the Master Node terminated
MCG DRBs plus twice the number of the DRBs that are reconfigured
from Master Node terminated to Secondary Node terminated is greater
than eight.
— The security key is changed from the master key to the secondary key.
— Downlink packet forwarding is performed over the Xn-U interface.
— Any ongoing A4 measurements stop.
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UE
Master Node
Secondary Node
AMF
1. RRC MeasurementReport
2. XnAP S-NODE ADDITION REQUEST
3. XnAP S-NODE ADDITION REQUEST ACKNOWLEDGE
4. XnAP XN-U ADDRESS INDICATION
5. XnAP SN STATUS TRANSFER
6. RRC RRCReconfiguration
7. PCell Random Access
8. RRC RRCReconfigurationComplete
9. XnAP S-NODE RECONFIGURATION COMPLETE
10. PSCell Random Access
11. NGAP PDU SESSION RESOURCE MODIFY INDICATION
12. NGAP PDU SESSION RESOURCE MODIFY CONFIRM
L0003144A
Figure 12 Sequence of Secondary Node Addition
1. The NR-DC-capable UE sends an A4 measurement report to the Master Node
in the RRC MeasurementReport message.
When the A4 measurement report is received, the Master Node evaluates if
all the criteria for the Secondary Node Addition procedure are met. If all the
criteria are met, the Master Node initiates the Secondary Node Addition
procedure to the Secondary Node that is the gNodeB that hosts the reported
candidate PSCell on a High-Band frequency.
2. The Master Node sends the XnAP S-NODE ADDITION REQUEST message to
the Secondary Node over the Xn-C interface. This message contains the
following information:
— The request to set up one or several PDU sessions terminated by the
Secondary Node. For existing PDU sessions, the Master Node forwards
information on the contained QoS flows and the NG-U uplink TEID.
— The secondary key to be used in the Secondary Node.
— The UE capabilities including the complete list of band combinations the
UE reported.
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NR-NR Dual Connectivity
— The PSCell candidate selected by the Master Node.
— The list of allowed NR-DC band combinations.
3. The Secondary Node responds to the Master Node with the XnAP S-NODE
ADDITION REQUEST ACKNOWLEDGE message. This message contains the
following information:
— The NG-U downlink TEID for each PDU session with QoS flows to be
Secondary Node terminated.
— The request to prepare MCG radio resources for the DRBs to be
reconfigured as split. This is done by the Secondary Node providing Xn-U
uplink TEIDs.
— TEIDs provided by the Secondary Node for downlink packet forwarding
for each DRB.
— The CG-Config message containing the selected band combination and
feature set. This list includes the PSCell and possibly SCells of the SCG.
At the reception of the message, the Master Node removes SCells on the
Master Node and all the SCell measurements for the UE, if carrier
aggregation is applied on the Master Node. If needed, measurement gaps
are configured for frequencies of the SCells to be removed.
4. The Master Node sends the XnAP XN-U ADDRESS INDICATION message to
the Secondary Node. The message contains the Xn-U downlink TEIDs for the
DRBs to be reconfigured as split. The DRBs to be reconfigured as split are
suspended in the Master Node.
5. The Master Node can send the XnAP SN STATUS TRANSFER message to the
Secondary Node for AM DRBs. The message is used to forward the PDCP
COUNT values.
At this stage, the DRBs to be reconfigured as split are suspended and
downlink packet forwarding starts.
6. The Master Node sends the RRC RRCReconfiguration message to the UE
over the Uu interface. The message includes the CG-Config message that
contains information about the SCG.
7. Optionally, random access is performed in the PCell. This depends on
whether an intra-cell handover is used or not.
8. The UE responds to the Master Node with the RRC
RRCReconfigurationComplete message. The UE can start sending uplink
user data in the MCG using the secondary key in the relocated PDCP.
9. The Master Node sends the XnAP S-NODE RECONFIGURATION COMPLETE
message to the Secondary Node over the Xn-C interface. The message
contains information on the successful reconfiguration of the UE. The DRBs
reconfigured as split resume in the Secondary Node. The Secondary Node
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can start sending downlink user data in the MCG using the secondary key in
the relocated PDCP.
10. The UE performs the random access procedure in the PSCell. After a
successful random access procedure, the Secondary Node can send and
receive user data in the SCG using the secondary key.
11. The Master Node sends the NGAP PDU SESSION RESOURCE MODIFY
INDICATION message to the AMF over the NG-C interface to provide the new
NG-U downlink TEID that is used by the UPF. After the path update, the UPF
starts sending downlink user data to the Secondary Node.
12. The AMF confirms the path update by responding to the Master Node with
the NGAP PDU SESSION RESOURCE MODIFY CONFIRM message.
The following messages in the sequence are initiated in parallel:
— XnAP SN STATUS TRANSFER
— RRC RRCReconfiguration
— NGAP PDU SESSION RESOURCE MODIFY INDICATION
RELATED INFORMATION
Master Node Criteria for NR-DC Setup
Bearer Type Change in NR-DC
NR-DC Band Combination Selection
Reception of an A4 Measurement Report
3.11.2
Master Node Initiated Secondary Node Modification
The Master Node initiated Secondary Node Modification procedure is used to set
up new QoS flows or release existing QoS flows for already established
Secondary Node terminated PDU sessions for a UE connected with NR-DC.
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NR-NR Dual Connectivity
UE
Master Node
Secondary Node
1. XnAP S-NODE MODIFICATION REQUEST
2. XnAP S-NODE MODIFICATION RESPONSE
3. XnAP XN-U ADDRESS INDICATION
4. RRC RRCReconfiguration
5. RRC RRCReconfigurationComplete
L0003147A
Figure 13 Sequence of Master Node Initiated Secondary Node Modification
1. The Master Node sends the XnAP S-NODE MODIFICATION REQUEST message
to the Secondary Node to request that the Secondary Node modifies an
already established Secondary Node terminated PDU session in either of the
following ways:
— If a new QoS flow is added to the PDU session, this QoS flow is set up as
Secondary Node terminated, regardless of 5QI and ARP configuration.
— If a QoS flow is released from the PDU session, the PDU session and all
its QoS flows remain Secondary Node terminated.
2. The Secondary Node replies to the Master Node with the XnAP S-NODE
MODIFICATION REQUEST ACKNOWLEDGE to indicate the modification of the
resources for PDU sessions terminated by the Secondary Node.
3. The Master Node sends the XnAP XN-U ADDRESS INDICATION message to
the Secondary Node if the PDU session resource modification includes
adding new QoS flows to already established Secondary Node terminated
PDU sessions.
4. The Master Node updates the UE with information on the modified PDU
sessions with the RRC RRCReconfiguration message.
5. The UE responds with the RRC RRCReconfigurationComplete message to
the Master Node.
RELATED INFORMATION
PDU Session Resource Modify Procedure
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3.11.3
Secondary Node Initiated Secondary Node Modification
The Secondary Node initiated Secondary Node Modification procedure is used to
transfer information about the new uplink primary path to the UE over the
Master Node if the High-Band uplink radio link quality drops below or rises above
a configurable threshold value. Afterwards, the UE is reconfigured to change the
uplink primary path of PDCP PDU transmission.
UE
Master Node
Secondary Node
1. XnAP S-NODE MODIFICATION REQUIRED
2. RRC RRCReconfiguration
3. RRC RRCReconfigurationComplete
4. XnAP S-NODE MODIFICATION CONFIRM
L0003309A
Figure 14 Sequence of Secondary Node Initiated Secondary Node Modification
1. The Secondary Node initiates the Secondary Node Modification procedure by
sending the XnAP S-NODE MODIFICATION REQUIRED message to the Master
Node over the Xn-C interface.
2. The Master Node sends the RRC RRCReconfiguration message to the UE
over the Uu interface.
3. The UE responds to the Master Node by sending the RRC
RRCReconfigurationComplete message.
4. The Master Node responds with the XnAP S-NODE MODIFICATION CONFIRM
message to the Secondary Node.
RELATED INFORMATION
High-Band Uplink Radio Link Quality Falls Below Configured Threshold in the
PSCell
High-Band Uplink Radio Link Quality Rises Above Configured Threshold in the
PSCell
User Plane Management
Parameters for NR-NR Dual Connectivity
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3.11.4
Master Node Initiated UE Context Release
Master Node
Secondary Node
XnAP UE CONTEXT RELEASE
L0003163A
Figure 15 Sequence of Master Node Initiated UE Context Release
The Master Node requests the Secondary Node to release all the SCG resources
for the UE by sending the XnAP UE CONTEXT RELEASE message over the Xn-C
interface to the Secondary Node.
RELATED INFORMATION
UE Released to Idle Mode
PDU Session Resource Release Procedure
Outgoing IRAT Handover from the Master Node
Master Node Initiated Secondary Node Release
Secondary Node Initiated Secondary Node Release
3.11.5
Master Node Initiated Secondary Node Release
The Master Node initiated Secondary Node Release procedure triggers the
release of the UE context at the Secondary Node.
The Master Node initiated Secondary Node Release procedure is triggered by one
of the following scenarios:
— The UE detects RLF in the SCG.
— Any type of intra-system handover is performed from the source cell on the
Master Node.
At the Master Node initiated Secondary Node Release procedure, Secondary
Node terminated PDU sessions are moved to the Master Node. The following
changes are applied:
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— All DRBs in the PDU session are reconfigured from Secondary Node
terminated split DRBs to Master Node terminated MCG DRBs keeping the
same DRB ID.
— RLC and MAC entities established at the Secondary Node Addition procedure
are released for the DRBs to be reconfigured from Secondary Node
terminated into Master Node terminated.
In the Master Node, the following options are available:
— The RRC Reconfiguration procedure is performed without an intra-cell
handover
The RLC entity in the Master Node is reestablished by releasing the old
RLC bearer and setting up a new RLC bearer for the reconfigured DRB.
The PDCP is always reestablished when this option is used.
This option is used when the number of the Master Node terminated
MCG DRBs plus twice the number of the DRBs that are reconfigured
from Secondary Node terminated to Master Node terminated is less than
or equal to eight.
— The RRC Reconfiguration procedure is performed with an intra-cell
handover
In this case, an intra-cell handover is performed in the Master Node to
reestablish the RLC entities of the reconfigured DRBs. The DRBs are
released and added again for the UE without PDCP reestablishment.
This option is only used in either of the following cases:
•
When the number of the Master Node terminated MCG DRBs plus
twice the number of the DRBs that are reconfigured from Secondary
Node terminated to Master Node terminated is greater than eight.
•
At least one UM DRB is reconfigured from Secondary Node
terminated into Master Node terminated.
— The security key is changed from the secondary key to the master key.
— Downlink packet forwarding is performed over the Xn-U interface.
— If all the criteria for starting an A4 measurement are met, the Master Node
configures the UE to start A4 measurements.
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UE
Master Node
Secondary Node
AMF
1. XnAP S-NODE RELEASE REQUEST
2. XnAP S-NODE RELEASE REQUEST ACKNOWLEDGE
3. XnAP XN-U ADDRESS INDICATION
4. XnAP SN STATUS TRANSFER
5. RRC RRCReconfiguration
6. PCell Random Access
7. RRC RRCReconfigurationComplete
8. NGAP PDU SESSION RESOURCE MODIFY INDICATION
9. NGAP PDU SESSION RESOURCE MODIFY CONFIRM
10. XnAP UE CONTEXT RELEASE
L0003145A
Figure 16 Sequence of Master Node Initiated Secondary Node Release
1. If one of the trigger conditions is detected, the Master Node initiates the
release of the Secondary Node. To initiate the procedure, the Master Node
sends the XnAP S-NODE RELEASE REQUEST message over the Xn-C interface.
With this message, the Master Node triggers the release of the SCG resources
for a specific UE.
2. The Secondary Node replies with the XnAP S-NODE RELEASE REQUEST
ACKNOWLEDGE message. The message contains the following information:
— The list of PDU sessions with Secondary Node terminated QoS flows to
be released.
— Information on the QoS flows that the Secondary Node terminated PDU
sessions contain.
— The request for downlink packet forwarding.
3. The Master Node sends the XnAP XN-U ADDRESS INDICATION message to
the Secondary Node with information on TEIDs for packet forwarding. When
the message is received, the downlink packet forwarding starts.
4. The Secondary Node can send the XnAP SN STATUS TRANSFER message to
the Master Node for AM DRBs. The message is used to forward the PDCP
COUNT values.
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5. The Master Node sends the RRC RRCReconfiguration message to the UE
over the Uu interface. The message indicates to the UE that the SCG
resources are released, and contains the A4 measurement configuration.
6. Optionally, random access is performed in the PCell. This depends on
whether an intra-cell handover is used or not.
7. The UE responds to the Master Node by sending the RRC
RRCReconfigurationComplete message. The Master Node can start
sending and receiving user data using the master key in the relocated PDCP.
After the UE reconfiguration is confirmed, the DRBs reconfigured as Master
Node terminated resume in the Master Node.
8. The Master Node sends the NGAP PDU SESSION RESOURCE MODIFY
INDICATION message to the AMF over the NG-C interface to provide the new
NG-U downlink TEID that is used by the UPF. After the path update, the UPF
starts sending downlink user data to the Master Node.
9. The AMF confirms the path update by responding to the Master Node with
the NGAP PDU SESSION RESOURCE MODIFY CONFIRM message.
10. The Master Node requests the Secondary Node to release all the SCG
resources for the UE by sending the XnAP UE CONTEXT RELEASE message
over the Xn-C interface to the Secondary Node.
The following messages in the sequence are initiated in parallel:
— XnAP SN STATUS TRANSFER
— RRC RRCReconfiguration
— NGAP PDU SESSION RESOURCE MODIFY INDICATION
— XnAP UE CONTEXT RELEASE
RELATED INFORMATION
Bearer Type Change in NR-DC
UE Indicating SCG Radio Link Failure
Outgoing Intra-System Handover from the Source Cell
3.11.6
Secondary Node Initiated Secondary Node Release
The Secondary Node initiated Secondary Node Release procedure triggers the
release of the UE context at the Secondary Node.
The Secondary Node initiated Secondary Node Release procedure is triggered by
one of the following scenarios:
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NR-NR Dual Connectivity
— The Secondary Node detects RLF.
— The PSCell or the underlying sector carrier is locked.
— The random access timer expires in the Secondary Node.
— The PDCP COUNT threshold is reached for a Secondary Node terminated split
DRB.
At the Secondary Node initiated Secondary Node Release procedure, Secondary
Node terminated PDU sessions are moved to the Master Node. The following
changes are applied:
— All DRBs in the PDU session are reconfigured from Secondary Node
terminated split DRBs to Master Node terminated MCG DRBs keeping the
same DRB ID.
— RLC and MAC entities established at the Secondary Node Addition procedure
are released for the DRBs to be reconfigured from Secondary Node
terminated into Master Node terminated.
In the Master Node, the following options are available:
— The RRC Reconfiguration procedure is performed without an intra-cell
handover
The RLC entity in the Master Node is reestablished by releasing the old
RLC bearer and setting up a new RLC bearer for the reconfigured DRB.
The PDCP is always reestablished when this option is used.
This option is used when the number of the Master Node terminated
MCG DRBs plus twice the number of the DRBs that are reconfigured
from Secondary Node terminated to Master Node terminated is less than
or equal to eight.
— The RRC Reconfiguration procedure is performed with an intra-cell
handover
In this case, an intra-cell handover is performed in the Master Node to
reestablish the RLC entities of the reconfigured DRBs. The DRBs are
released and added again for the UE without PDCP reestablishment.
This option is only used in either of the following cases:
•
When the number of the Master Node terminated MCG DRBs plus
twice the number of the DRBs to be reconfigured from Secondary
Node terminated to Master Node terminated is greater than eight.
•
At least one UM DRB is reconfigured from Secondary Node
terminated into Master Node terminated.
— The security key is changed from the secondary key to the master key.
— Downlink packet forwarding is performed over the Xn-U interface.
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— If all the criteria for starting an A4 measurement are met, the Master Node
configures the UE to start A4 measurements.
UE
Master Node
Secondary Node
AMF
1. XnAP S-NODE RELEASE REQUIRED
2. XnAP S-NODE RELEASE CONFIRM
3. XnAP SN STATUS TRANSFER
4. RRC RRCReconfiguration
5. PCell Random Access
6. RRC RRCReconfigurationComplete
7. NGAP PDU SESSION RESOURCE MODIFY INDICATION
8. NGAP PDU SESSION RESOURCE MODIFY CONFIRM
9. XnAP UE CONTEXT RELEASE
L0003146A
Figure 17 Sequence of Secondary Node Initiated Secondary Node Release
1. If one of the trigger conditions is detected, the Secondary Node initiates the
release of the Secondary Node. To initiate the procedure, the Secondary
Node sends the XnAP S-NODE RELEASE REQUIRED message to the Master
Node over the Xn-C interface. The message contains the following
information:
— The list of PDU sessions with Secondary Node terminated QoS flows to
be released.
— Information on the QoS flows that the Secondary Node terminated PDU
sessions contain.
— The request for downlink packet forwarding.
2. The Master Node replies with the XnAP S-NODE RELEASE CONFIRM message
to the Secondary Node with information on TEIDs for packet forwarding.
When the message is received, the downlink packet forwarding starts.
3. The Secondary Node can send the XnAP SN STATUS TRANSFER message to
the Master Node for AM DRBs. The message is used to forward the PDCP
COUNT values.
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NR-NR Dual Connectivity
4. The Master Node sends the RRC RRCReconfiguration message to the UE
over the Uu interface. The message indicates to the UE that the SCG
resources are released, and contains the A4 measurement configuration.
5. Optionally, random access is performed in the PCell. This depends on
whether an intra-cell handover is used or not.
6. The UE responds to the Master Node by sending the RRC
RRCReconfigurationComplete message. The Master Node can start
sending and receiving user data using the master key in the relocated PDCP.
After the UE reconfiguration is confirmed, the DRBs reconfigured as Master
Node terminated resume in the Master Node.
7. The Master Node sends the NGAP PDU SESSION RESOURCE MODIFY
INDICATION message to the AMF over the NG-C interface to provide the new
NG-U downlink TEID that is used by the UPF. After the path update, the UPF
starts sending downlink user data to the Master Node.
8. The AMF confirms the path update by responding to the Master Node with
the NGAP PDU SESSION RESOURCE MODIFY CONFIRM message.
9. The Master Node requests the Secondary Node to release all the SCG
resources for the UE by sending the XnAP UE CONTEXT RELEASE message
over the Xn-C interface to the Secondary Node.
The following messages in the sequence are initiated in parallel:
— XnAP SN STATUS TRANSFER
— RRC RRCReconfiguration
— NGAP PDU SESSION RESOURCE MODIFY INDICATION
— XnAP UE CONTEXT RELEASE
RELATED INFORMATION
Bearer Type Change in NR-DC
Secondary Node Detects Radio Link Failure
NR Cell or Sector Carrier Is Locked on the Secondary Node
Random Access Timer Expiry in Secondary Node
PDCP COUNT Threshold Is Reached
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4
Network Impact of NR-NR Dual Connectivity
The feature affects data throughput, retainability, and UE battery life. It
introduces and adds information to signaling messages over external
interfaces.
Capacity and Performance
Data throughput for the UE is expected to increase when Secondary Node
terminated split DRBs are used. This is because the SCG provides increased
bandwidth with High-Band frequencies. When using the Secondary Node
terminated split DRB, the following impact can be expected on Master Node and
Secondary Node traffic:
— Data traffic volume over the Secondary Node to the UE increases.
— Data traffic volume over the Master Node to the UE might decrease as most
of the traffic flows through the SCG.
— Total user throughput might increase.
— Signaling is expected to increase over the following interfaces:
— The NG-C interface between the Master Node and the 5GC
— The Uu interface between the Master Node and the UE
— The Xn-C interface between the Master Node and the Secondary Node
With the activation of the feature, impact on Retainability can be expected. This
is because at feature activation, both the Master Node with the feature activated
and also the neighbor gNodeBs without the feature start requesting NR-DCspecific UE capabilities. The increased number of RRC UECapabilityEnquiry
messages sent to UEs occurs for one of the following reasons:
— The AMF does not support storing or updating UE capabilities.
— The UE uses an RRC protocol version older than 15.6.0. Such UEs are not
able to indicate if NR-DC-specific capabilities were already requested by the
network or not.
This Retainability impact can affect the network in the following ways:
— An increase in the network load can be expected because of the frequent
requests for NR-DC-specific UE capabilities.
— The risk of dropping the connection increases for UEs that are at the cell edge
or in bad radio conditions.
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NR-NR Dual Connectivity
Interfaces
The feature affects the following external interfaces:
Xn-C
The feature introduces the following messages over the
Xn-C interface:
— ACTIVITY NOTIFICATION
— ERROR INDICATION (different message instances for
UE-associated and non-UE associated signaling)
— S-NODE ADDITION REQUEST
— S-NODE ADDITION REQUEST ACKNOWLEDGE
— S-NODE ADDITION REQUEST REJECT
— S-NODE MODIFICATION CONFIRM
— S-NODE MODIFICATION REFUSE
— S-NODE MODIFICATION REQUIRED
— S-NODE MODIFICATION REQUEST
— S-NODE MODIFICATION REQUEST ACKNOWLEDGE
— S-NODE MODIFICATION REQUEST REJECT
— S-NODE RECONFIGURATION COMPLETE
— S-NODE RELEASE REQUEST
— S-NODE RELEASE REQUEST ACKNOWLEDGE
— S-NODE RELEASE REJECT
— S-NODE RELEASE REQUIRED
— S-NODE RELEASE CONFIRM
— SN STATUS TRANSFER
— UE CONTEXT RELEASE
— XN-U ADDRESS INDICATION
NG-C
The feature introduces the following messages:
— PDU SESSION RESOURCE MODIFY INDICATION
— PDU SESSION RESOURCE MODIFY CONFIRM
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The feature uses the following messages at Xn link break:
— NG RESET
— NG RESET ACKNOWLEDGE
Uu
— The SCGFailureInformation RRC message is
introduced by the feature.
— The following RRC messages are modified containing
NR-DC information:
— RRCReconfiguration
— CG-Config
— CG-ConfigInfo
Other Network Elements
Higher UE battery consumption is expected when NR-DC-capable UEs perform
A4 measurements or use the Secondary Node resources.
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NR-NR Dual Connectivity
5
Parameters for NR-NR Dual Connectivity
The feature introduces new configurable parameters. It uses parameters of
the LTE-NR Dual Connectivity, the LTE-NR Downlink Aggregation, and the
LTE-NR Uplink Aggregation features for user plane management.
Table 2
Parameter
Type
Description
NrdcControl
Introduced
See MOM description.
NrdcMnCellProfile
Introduced
See MOM description.
NrdcMnCellProfileUeC
fg
Introduced
See MOM description.
NrdcSnTermination
Introduced
See MOM description.
NrdcSnTerminationUeC
fg
Introduced
See MOM description.
Table 3
56
MOs Introduced by NR-NR Dual Connectivity
Attributes for PCell Configuration on the Master Node
Parameter
Type
Description
NRCellCU.nrdcMnCellP
rofileRef
Introduced
See MOM description.
NrdcControl.nrdcCont
rolId
Introduced
See MOM description.
NrdcMnCellProfile.nr
dcMnCellProfileId
Introduced
See MOM description.
NrdcMnCellProfile.re
servedBy
Introduced
See MOM description.
NrdcMnCellProfileUeC
fg.nrdcMnCellProfile
UeCfgId
Introduced
See MOM description.
NrdcMnCellProfileUeC
fg.nrdcEnabled
Introduced
See MOM description.
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Table 4
Attributes on the Master Node to Configure PDU Session Termination
Parameter
Type
Description
CUCP5qi.nrdcSnTermin
ationRef
Introduced
See MOM description.
NrdcSnTermination.de
faultNrdcSnTerm
Introduced
See MOM description.
NrdcSnTermination.nr
dcSnTerminationId
Introduced
See MOM description.
NrdcSnTermination.re
servedBy
Introduced
See MOM description.
NrdcSnTerminationUeC
fg.nrdcSnTermAllowed
Introduced
See MOM description.
NrdcSnTerminationUeC
fg.nrdcSnTermination
UeCfgId
Introduced
See MOM description.
Table 5
Attributes to Configure Xn Interface Timers for NR-DC Procedures
Parameter
Type
Description
GNBCUCPFunction.tXnD
cOverall
Introduced
See MOM description.
GNBCUCPFunction.tXnD
cPrep
Introduced
See MOM description.
Table 6
Attributes to Configure A4 Measurements
Parameter
Type
Description
NrdcMnCellProfileUeC
fg.nrdcMeasRestartTi
me
Introduced
See MOM description.
NrdcMnCellProfileUeC
fg.nrdcMeasTime
Introduced
See MOM description.
NrdcMnCellProfileUeC
fg.rsrpPSCellCandida
te
Introduced
See MOM description.
The attribute contains
the ReportConfigA4Rsrp
struct with the following
configurable members:
— threshold
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NR-NR Dual Connectivity
Parameter
Type
Description
— hysteresis
— timeToTrigger
Table 7
Attributes to Configure Inactivity Timers
Parameter
Type
Description
InactivityProfileUeC
fg.tInactivityTimerN
rdcSn
Introduced
See MOM description.
GNBCUCPFunction.nasI
nactivityTime
Affecting
See MOM description.
InactivityProfileUeC
fg.tInactivityTimer
Affecting
See MOM description.
Table 8 Attributes on the Secondary Node to Configure Radio Link Failure
Conditions
The table contains the attributes used to configure Radio Link Failure (RLF)
conditions for the SCG resources available for the UE. For more information on
the configuration, see the relevant section of the LTE-NR Dual Connectivity
feature description.
Parameter
Type
Description
Rrc.t310
Affecting
See MOM description.
Rrc.t304
Affecting
See MOM description.
DataRadioBearer.dlMa
xRetxThreshold
Affecting
See MOM description.
DataRadioBearer.ulMa
xRetxThreshold
Affecting
See MOM description.
DataRadioBearer.tPol
lRetransmitDl
Affecting
See MOM description.
DataRadioBearer.tPol
lRetransmitUl
Affecting
See MOM description.
Table 9
Attributes of LTE-NR Dual Connectivity Used in NR-DC
The table contains the attributes used to configure the initial primary path of
uplink user data, and the uplink and downlink thresholds for MCG and SCG radio
resource switching of the LTE-NR Dual Connectivity feature used in NR-DC. For
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more information on the functions and configurations, see the relevant sections
of the LTE-NR Dual Connectivity feature description.
Parameter
Type
Description
GNBCUUPFunction.endc
UlNrRetProhibTimer
Affecting
See MOM description.
NRCellDU.endcUlLegSw
itchEnabled
Affecting
See MOM description.
NRCellDU.endcUlNrLow
QualThresh
Affecting
See MOM description.
NRCellDU.endcUlNrQua
lHyst
Affecting
See MOM description.
NRCellDU.endcDlNrLow
QualThresh
Affecting
See MOM description.
NRCellDU.endcDlNrQua
lHyst
Affecting
See MOM description.
QciProfileEndcConfig
Ext.initialUplinkCon
f
Affecting
See MOM description.
Table 10
Attributes of LTE-NR Uplink Aggregation Used in NR-DC
The table contains the attribute used to configure whether uplink user data is
aggregated over the MCG and SCG radio resources of the LTE-NR Uplink
Aggregation feature used in NR-DC. In addition, the uplink PDCP aggregation
function is applicable for NR-DC if the primary uplink data path is SCG. For more
information on the function and the configuration, see the relevant sections of
the LTE-NR Uplink Aggregation feature description.
Parameter
Type
Description
QciProfileEndcConfig
Ext.ulDataSplitThres
hold
Affecting
See MOM description.
Table 11
Attributes of LTE-NR Downlink Aggregation Used in NR-DC
The table contains the attributes used to configure the flow control mechanism
and the downlink aggregation of the MCG and SCG radio resources of the LTENR Downlink Aggregation feature used in NR-DC. For more information on the
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NR-NR Dual Connectivity
functions and configurations, see the relevant sections of the LTE-NR Downlink
Aggregation feature description.
Parameter
Type
Description
GNBCUUPFunction.endc
DlNrRetProhibTimer
Affecting
See MOM description.
GNBCUUPFunction.dcDl
AggExpiryTimer
Affecting
See MOM description.
GNBCUUPFunction.dlPd
cpSpsTargetTimeLTE
Affecting
See MOM description.
GNBCUUPFunction.dlPd
cpSpsTargetTimeNR
Affecting
See MOM description.
GNBCUUPFunction.dcDl
AggActTime
Affecting
See MOM description.
GNBCUUPFunction.dcDl
PdcpInitialMcgRate
Affecting
See MOM description.
GNBCUUPFunction.dcDl
PdcpInitialScgRate
Affecting
See MOM description.
CUUP5qi.dcDlPdcpAggr
PrioCg
Affecting
See MOM description.
CUUP5qi.dcDlPdcpAggr
TimeDiffCg
Affecting
See MOM description.
CUUP5qi.dcDlPdcpAggr
TimeDiffProhibit
Affecting
See MOM description.
CUUP5qi.dcDlPdcpAggr
TimeDiffThresh
Affecting
See MOM description.
Table 12
60
Attributes Used for AQM Configuration on the Secondary Node
Parameter
Type
Description
CUUP5qi.aqmMode
Affecting
See MOM description.
CUUP5qi.estimatedE2E
RTT
Affecting
See MOM description.
CUUP5qi.packetDelayB
udget
Affecting
See MOM description.
CUUP5qi.packetDelayB
udgetOffset
Affecting
See MOM description.
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RELATED INFORMATION
Master Node Criteria for NR-DC Setup
Termination Point Control of QoS Flows in the Master Node
User Plane Management
User Activity Supervision
TCP Optimization for the Secondary Node Terminated Split DRB in NR-DC
Configure 5QI and ARP Levels for QoS Flows Allowed to Be Secondary Node
Terminated
Secondary Node Initiated Secondary Node Modification
LTE-NR Dual Connectivity
LTE-NR Uplink Aggregation
LTE-NR Downlink Aggregation
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NR-NR Dual Connectivity
6
Performance of NR-NR Dual Connectivity
The feature introduces Performance Indicators to monitor Secondary Node
Addition and Release. New PM counters and PM events are introduced by
the feature, and existing ones can also be used to monitor the different
feature functions.
KPIs
This feature has no associated Key Performance Indicators.
Performance Indicators
The Performance Indicators for monitoring the Secondary Node Addition
procedure are shown in Table 13.
Table 13
Performance Indicators for Monitoring Secondary Node Addition
Performance Indicator
Formula
NR-DC Setup Success Rate Captured
on Master gNodeB (expressed in
percentage)
100 ×
NRCellCU.pmNrdcSetupUeMnSucc /
NRCellCU.pmNrdcSetupUeMnAtt
NR-DC Setup Success Rate Captured
on Secondary gNodeB (expressed in
percentage)
NRCellCU.pmNrdcSetupUeSnSucc /
NRCellCU.pmNrdcSetupUeSnAtt
100 ×
The Performance Indicators for monitoring the Secondary Node Release
procedures are shown in Table 14.
Table 14
Performance Indicators for Monitoring Secondary Node Release
Performance Indicator
Formula
NR-DC Relocation Success Rate
Captured on Master gNodeB
(expressed in percentage)
100 ×
NRCellCU.pmNrdcRelRelocUeMnSucc
/ NRCellCU.pmNrdcRelRelocUeMnAtt
NR-DC Release Abnormal Active
Rate Captured on Secondary gNodeB
(expressed in percentage)
62
100 × NrdcRellUeSnAbnormalAct /
NrdcRellUeTotal
NrdcRellUeSnAbnormalAct
NRCellCU.pmNrdcR
elUeSnAbnormalMn
InitAct +
NRCellCU.pmNrdcR
elUeSnAbnormalSn
InitAct
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Performance Indicator
Formula
NrdcRell NRCellCU.pmNrdcRelUeSn
UeTotal Normal +
NRCellCU.pmNrdcRelUeSn
AbnormalMnInit +
NRCellCU.pmNrdcRelUeSn
AbnormalSnInit
Counters
The following PM counters are used to monitor Accessibility for the Secondary
Node Addition procedure:
— NRCellCU.pmNrdcSetupUeMnAtt
— NRCellCU.pmNrdcSetupUeMnSucc
— NRCellCU.pmNrdcSetupUeSnAtt
— NRCellCU.pmNrdcSetupUeSnSucc
The following PM counters are used to monitor packet forwarding in NR-DC:
— GNBCUUPFunction.pmPdcpPktFwdRecDlQos
— GNBCUUPFunction.pmPdcpPktFwdRecDlDiscQos
— EP_XnU.pmPdcpPktFwdTransDlQos
— EP_XnU.pmPdcpPktFwdTransDlDiscQos
The following PM counters are used to monitor Retainability for the Secondary
Node Release procedures:
— NRCellCU.pmNrdcRelRelocUeMnAtt
— NRCellCU.pmNrdcRelRelocUeMnSucc
— NRCellCU.pmNrdcRelUeSnNormal
— NRCellCU.pmNrdcRelUeSnAbnormalMnInit
— NRCellCU.pmNrdcRelUeSnAbnormalMnInitAct
— NRCellCU.pmNrdcRelUeSnAbnormalSnInit
— NRCellCU.pmNrdcRelUeSnAbnormalSnInitAct
The following PM counters are used to monitor UEs connected with NR-DC:
— NRCellCU.pmRrcConnLevelMaxNrDc
— NRCellCU.pmRrcConnLevelSumNrDc
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NR-NR Dual Connectivity
— NRCellCU.pmRrcConnLevelMaxSa
— NRCellCU.pmRrcConnLevelSumSa
— NRCellCU.pmRrcConnLevelSamp
The following EBS counters are used to monitor UEs connected with NR-DC:
— pmEbsRrcConnLevelMaxSa, which can be used as a flexible counter.
— pmEbsRrcConnLevelSumSa, which can be used as a flexible counter.
— pmEbsRrcConnLevelSamp
The EBS counters are associated with the NRCellCU MO. For information on the
EBS counters, see Lists and Delta Lists.
The following EBS counters are used to monitor the intra-cell handover at
Secondary Node Addition and Release procedures in NR-DC:
— pmEbsNrdcRelRelocUeMnAttIcho
— pmEbsNrdcRelRelocUeMnSuccIcho
— pmEbsNrdcSetupUeMnAttIcho
— pmEbsNrdcSetupUeMnSuccIcho
The EBS counters are associated with the NRCellCU MO. For information on the
EBS counters, see Lists and Delta Lists.
The following EBS counters associated with the EUtranCellRelation and the
NRCellCU MOs can be used to monitor Master Node mobility with the
NrDcConfigured filter applied:
— pmEbsHoPrepAttOutEutran
— pmEbsHoPrepAttOutEutranPoorCov
— pmEbsHoPrepSuccOutEutran
— pmEbsHoPrepSuccOutEutranPoorCov
— pmEbsHoExeAttOutEutran
— pmEbsHoExeAttOutEutranPoorCov
— pmEbsHoExeSuccOutEutran
— pmEbsHoExeSuccOutEutranPoorCov
For information on the EBS counters, see Lists and Delta Lists.
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The following EBS counters associated with the NRCellCU and the
NRCellRelation MOs can be used to monitor Master Node mobility with the
NrDcConfigured filter applied:
— pmEbsHoPrepAttOutInterGnb
— pmEbsHoPrepAttOutInterGnbIntraF
— pmEbsHoPrepAttOutIntraGnb
— pmEbsHoPrepAttOutIntraGnbIntraF
— pmEbsHoPrepSuccOutInterGnb
— pmEbsHoPrepSuccOutInterGnbIntraF
— pmEbsHoPrepSuccOutIntraGnb
— pmEbsHoPrepSuccOutIntraGnbIntraF
— pmEbsHoExeAttOutInterGnb
— pmEbsHoExeAttOutInterGnbIntraF
— pmEbsHoExeAttOutIntraGnb
— pmEbsHoExeAttOutIntraGnbIntraF
— pmEbsHoExeSuccOutInterGnb
— pmEbsHoExeSuccOutInterGnbIntraF
— pmEbsHoExeSuccOutIntraGnb
— pmEbsHoExeSuccOutIntraGnbIntraF
For information on the EBS counters, see Lists and Delta Lists.
The following PM counters can be used to monitor the switching of downlink
MCG and SCG radio resources of the Secondary Node terminated split DRB:
— GNBCUUPFunction.pmPdcpRadioResSwitchDlMcg
— GNBCUUPFunction.pmPdcpRadioResSwitchDlScg
The GNBCUUPFunction.pmProhibitTimerRefuseUlLegSwitch PM counter can
be used to monitor how many times the uplink radio resource switching to SCG
radio resources is rejected because of the
GNBCUUPFunction.endcUlNrRetProhibTimer attribute setting.
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NR-NR Dual Connectivity
Events
The following PM events are used for monitoring the Secondary Node Addition
procedure and the messages used in the procedure:
— CuCpProcMnNrdcSetup
— CuCpProcSnNrdcSetup
— CuCpXnSNodeAdditionRequest
— CuCpXnSNodeAdditionRequestAcknowledge
— CuCpXnSNodeAdditionRequestReject
— CuCpXnXnUAddressIndication
— CuCpXnSNodeReconfigurationComplete
— CuCpXnSnStatusTransfer
The reception of A4 measurement reports can be monitored using the
CuCpMeasurementReportSaEval PM event.
The following PM events are used to monitor the Master Node Initiated
Secondary Node Modification procedure:
— CuCpXnSNodeModificationRequest
— CuCpXnSNodeModificationRequestAcknowledge
— CuCpXnSNodeModificationRequestReject
The following PM events are used to monitor the Secondary Node Initiated
Secondary Node Modification procedure:
— CuCpXnSNodeModificationConfirm
— CuCpXnSNodeModificationRefuse
— CuCpXnSNodeModificationRequired
The following PM events are used to monitor both the Master Node initiated
Secondary Node Modification and the Secondary Node initiated Secondary Node
Modification procedures:
— CuCpProcMnNrdcModification, which is monitored in the Master Node.
— CuCpProcSnNrdcModification, which is monitored in the Secondary Node.
The CuUpProcSnNrdcUlPrimaryCgSwitch PM event is used to monitor the NRDC switching of uplink MCG and SCG radio resources of the Secondary Node
terminated split DRB procedure.
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When the Master Node uses the PDU Session Resource Modify Indication
procedure, the following PM events can be monitored:
— CuCpNgPduSessionResourceModifyIndication
— CuCpNgPduSessionResourceModifyConfirm
The RLF detection can be monitored by using the
CuCpRrcScgFailureInformation PM event.
The CuCpPerCellRrcConnectedUe PM event is used for monitoring the
connected UEs in the following ways:
— The connected NR SA UEs, including those configured with NR-DC, in the
PCell
— The connected UEs configured with NR-DC in the PSCell
The following PM events are used for monitoring the Secondary Node Release
procedures and the messages used in the procedures:
— CuCpProcMnNrdcRelease
— CuCpProcSnNrdcRelease
— CuCpXnSNodeReleaseRequest
— CuCpXnSNodeReleaseRequestAcknowledge
— CuCpXnSNodeReleaseReject
— CuCpXnSNodeReleaseRequired
— CuCpXnSNodeReleaseConfirm
— CuCpXnXnUAddressIndication
— CuCpXnUeContextRelease
— CuCpXnSnStatusTransfer
The following PM events are updated with the NrDcConfigured filter to monitor
Master Node mobility and to include the nrdc_configured event parameter:
— CuCpProcNrHoPrepOut
— CuCpProcNrHoExeOut
The following PM events are used to monitor the Error Indication procedure
between the Master Node and the Secondary Node:
— CuCpXnErrorIndicationCommon
— CuCpXnErrorIndicationDedicated
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NR-NR Dual Connectivity
7
Activate NR-NR Dual Connectivity
To use the feature, it must be activated following the usual feature
activation procedure.
Prerequisites
— The license key is installed both in the Master Node and the Secondary Node.
— The conflicting features are deactivated on the Master Node and the
Secondary Node.
— CCTR is active for at least one week before this procedure to collect enough
troubleshooting data.
Steps
1. Set the attribute FeatureState.featureState to ACTIVATED in the
FeatureState=CXC4012582 MO instance on both the Master Node and the
Secondary Node.
When the feature is activated, to enable the use of NR-DC for UEs in the PCell on
the Master Node:
2. Set the NrdcMnCellProfileUeCfg.nrdcEnabled attribute to true.
After This Task
Keep CCTR active for at least one week after this procedure for continued
collection of troubleshooting data.
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8
Deactivate NR-NR Dual Connectivity
If the feature is no longer needed, it can be deactivated following the usual
feature deactivation procedure. It must also be deactivated before the
activation of any conflicting feature.
Prerequisites
— CCTR is active for at least one week before this procedure to collect enough
troubleshooting data.
Steps
1. Set the NrdcMnCellProfileUeCfg.nrdcEnabled attribute to false in the
PCell on the Master Node.
2. Set the attribute FeatureState.featureState to DEACTIVATED in the
FeatureState=CXC4012582 MO instance on both the Master Node and the
Secondary Node.
After This Task
Keep CCTR active for at least one week after this procedure for continued
collection of troubleshooting data.
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NR-NR Dual Connectivity
9
Engineering Guidelines for NR-NR Dual
Connectivity
9.1
Configure IP Connectivity in NR-DC for the Xn-U
Interface
In NR-DC, the user plane connection between the Master Node and the
Secondary Node is established over the Xn-U interface.
Prerequisites
Separate LocalIpEndpoint MO instances exist on both the Master Node and on
the Secondary Node. On the Master Node, the MO is under the
EndpointResource MO, which is a child to the GNBDUFunction MO. On the
Secondary Node, the MO is under the EndpointResource MO, which is a child to
the GNBCUUPFunction MO.
Steps
1. Configure the LocalIpEndpoint.addressRef attribute on the Master Node
to reference an instance of either the AddressIPv4 or AddressIPv6 MO.
The Master Node uses the configured IP address for local transport network
endpoints for the user plane connection.
2. Add the F1 value to the list in the LocalIpEndpoint.interfaceList
attribute on the Master Node.
The Xn-U interface uses the IP address referenced in the
LocalIpEndpoint.addressRef attribute on the Master Node to connect to
the Secondary Node.
3. Configure the LocalIpEndpoint.addressRef attribute on the Secondary
Node to reference an instance of either the AddressIPv4 or AddressIPv6
MO.
The Secondary Node uses the configured IP address for local transport
network endpoints for the user plane connection.
4. Add the XN value to the list in the LocalIpEndpoint.interfaceList
attribute on the Secondary Node.
The Xn-U interface uses the IP address referenced in the
LocalIpEndpoint.addressRef attribute on the Secondary Node to connect
to the Master Node.
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Results
The IP addresses to connect the nodes in NR-DC over the Xn-U interface are
defined.
After This Task
Both the Master Node and the Secondary Node need to be restarted, so that the
IP address configuration can take effect.
9.2
TCP Optimization for the Secondary Node Terminated
Split DRB in NR-DC
TCP optimization provides reduced queuing delay in downlink user plane traffic
to improve end user performance. AQM is used for TCP optimization of the
Secondary Node terminated split DRBs in NR-DC. AQM is performed in the
downlink PDCP buffer of the Secondary Node when configured using the
CUUP5qi.aqmMode attribute. The attribute can be configured to one of the
following values, depending on the traffic characteristics:
OFF
AQM is not used.
Packets are discarded if they become older than 1
second.
MODE1
AQM is active. Recommended AQM mode for DRBs with
non-GBR traffic.
A single packet is discarded when it becomes older than
the minimum age threshold. The minimum age of packets
is calculated as 2 × CUUP5qi.estimatedE2ERTT.
A prohibit timer, set as 4 × CUUP5qi.estimatedE2ERTT, is
present between discarding single packets.
All packets that are older than the maximum age
threshold are discarded. The maximum age of packets is
calculated as 10 × CUUP5qi.estimatedE2ERTT.
MODE2
AQM is active. Recommended AQM mode for DRBs with
GBR traffic.
Packets older than the sum of
CUUP5qi.packetDelayBudget and
CUUP5qi.packetDelayBudgetOffset are discarded.
Packets are discarded regardless of the value set in the CUUP5qi.aqmMode
attribute if the maximum number of packets is reached for each DRB.
For AQM used when the NR-DC-capable UE operates in NR SA mode, see NR
Standalone.
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Note:
9.3
The setting of the CUUP5qi.estimatedE2ERTT attribute must be aligned
with the configuration of AQM in DU5qi. The configured value for the
CUUP5qi.estimatedE2ERTT must not be higher than 50 ms, to avoid
unwanted AQM triggered packet discards.
Configure 5QI and ARP Levels for QoS Flows Allowed to
Be Secondary Node Terminated
By using different settings for each APR and 5QI, operators can determine which
QoS flows are allowed to be Secondary Node terminated.
Prerequisites
— An instance of the NrdcSnTermination MO exists under the
GNBCUCPFunction MO.
— The NrdcSnTermination.defaultNrdcSnTerm is set to false.
— An instance of NrdcSnTerminationUeCfg MO exists as the child of the
NrdcSnTermination MO.
— An instance of the CUCP5qi MO is configured as a 5QI profile representing
the configuration of a QoS flow.
Steps
1. Configure the CUCP5qi.nrdcSnTerminationRef attribute to contain a
reference to the instance of the NrdcSnTermination MO.
2. Configure a value to the NrdcSnTerminationUeCfg.nrdcSnTermAllowed
attribute that allows Secondary Node termination for an ARP or an ARP
combination.
Results
This specific QoS flow is allowed to be terminated by the Secondary Node. The
DRB carrying this QoS flow is configured as a Secondary Node terminated split
DRB.
RELATED INFORMATION
Termination Point Control of QoS Flows in the Master Node
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9.4
Configuration Considerations for Switching of Uplink
MCG and SCG Radio Resources of the Secondary Node
Terminated Split DRB
In deployments with a mix of EN-DC and NR-DC UEs, the value of the
NRCellDU.endcUlNrLowQualThresh attribute might have to be increased to
avoid RLF for NR-DC UEs. By adjusting the NRCellDU.endcUlNrLowQualThresh
attribute to be the most optimal for NR-DC, the uplink PDCP PDU transmission
throughput can be improved and maintained in variable High-Band radio
conditions.
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Appendix A: Feature Change History
This section lists changes that affected this feature and the impact it had on
the network.
Appendix A.a: 22.Q1: Switching of Uplink MCG and SCG Radio Resources of the Secondary
Node Terminated Split DRB in NR-DC
The enhancement enables the switching of the uplink primary path for
PDCP PDU transmission of the Secondary Node terminated split
DRBs based on the uplink radio quality in the SCG.
Feature Name:
NR-NR Dual Connectivity
Feature Identity:
FAJ 121 5380
Value Package Name:
Peak Rate Evolution
Value Package Identity:
FAJ 801 4005
Node Type:
Baseband Radio Node
Access Type:
NR
Summary and Benefits
By allowing the switching of the uplink primary path of Secondary Node
terminated split DRBs based on dynamic evaluation of the SCG uplink quality, the
enhancement provides the following benefits:
— It reduces the number of Secondary Node releases that are caused by SCG
RLF for UEs that are configured with NR-DC.
— It maintains uplink PDCP PDU transmission throughput in variable HighBand radio conditions.
Capacity and Performance
The switching of the uplink MCG and SCG radio resources of the Secondary Node
terminated split DRB improves Retainability for the Secondary Node in NR-DC.
Operation
The enhancement introduces the following attributes that are used to configure
Xn interface timers for NR-DC procedures:
— GNBCUCPFunction.tXnDcOverall
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Feature Change History
— GNBCUCPFunction.tXnDcPrep
The enhancement makes the following attributes applicable to configuring the
uplink MCG and SCG radio resource switching function of the LTE-NR Dual
Connectivity feature used in NR-DC:
— GNBCUUPFunction.endcUlNrRetProhibTimer
— NRCellDU.endcUlLegSwitchEnabled
— NRCellDU.endcUlNrLowQualThresh
— NRCellDU.endcUlNrQualHyst
The GNBCUUPFunction.pmProhibitTimerRefuseUlLegSwitch PM counter can
be used to monitor how many times the uplink radio resource switching to SCG
radio resources is rejected because of the
GNBCUUPFunction.endcUlNrRetProhibTimer attribute setting.
If the initial uplink primary path is SCG, the following PM counters in the
Secondary Node can decrease if switching of the uplink MCG and SCG radio
resources of the Secondary Node terminated split DRB for NR-DC is used:
— NRCellCU.pmNrdcRelUeSnAbnormalMnInit
— NRCellCU.pmNrdcRelUeSnAbnormalMnInitAct
— NRCellCU.pmNrdcRelUeSnAbnormalSnInit
— NRCellCU.pmNrdcRelUeSnAbnormalSnInitAct
— NRCellDU.pmMacTimeUlResUe
— NRCellDU.pmMacVolUl
— NRCellDU.pmMacVolUlResUe
— NRCellDU.pmMacVolUlResUeLate
— NRCellDU.pmMacVolUlResUeLastSlot
— NRCellDU.pmPuschSchedActivity
— NRCellDU.pmMacHarqUlAckQpsk
— NRCellDU.pmMacHarqUlDtxQpsk
— NRCellDU.pmMacHarqUlNackQpsk
— RpUserPlaneLink.pmPdcpPktLossUlQos
— RpUserPlaneLink.pmPdcpPktLossUlToDiscQos
— RpUserPlaneLink.pmPdcpPktLossUlToQos
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— RpUserPlaneLink.pmPdcpVolRecUlQos on the internal RpUserPlaneLink
— RpUserPlaneLink.pmPdcpPktRecUlQos on the internal RpUserPlaneLink
— RpUserPlaneLink.pmPdcpPktRecUlOooQos
If the initial uplink primary path is SCG, the following PM counters in the Master
Node can increase if switching of the uplink MCG and SCG radio resources of the
Secondary Node terminated split DRB for NR-DC is used:
— NRCellDU.pmMacTimeUlResUe
— NRCellDU.pmMacVolUl
— NRCellDU.pmMacVolUlResUe
— NRCellDU.pmMacVolUlResUeLate
— NRCellDU.pmMacVolUlResUeLastSlot
— NRCellDU.pmPuschSchedActivity
— NRCellDU.pmMacHarqUlAckQpsk
— NRCellDU.pmMacHarqUlDtxQpsk
— NRCellDU.pmMacHarqUlNackQpsk
If the initial uplink primary path is SCG, the following PM counters in the
Secondary Node can increase if switching of the uplink MCG and SCG radio
resources of the Secondary Node terminated split DRB for NR-DC is used:
— RpUserPlaneLink.pmPdcpVolRecUlQos on the external RpUserPlaneLink
— RpUserPlaneLink.pmPdcpPktRecUlQos on the external RpUserPlaneLink
The enhancement introduces the following PM events:
— CuCpXnSNodeModificationConfirm
— CuCpXnSNodeModificationRefuse
— CuCpXnSNodeModificationRequest
— CuCpXnSNodeModificationRequestAcknowledge
— CuCpXnSNodeModificationRequestReject
— CuCpXnSNodeModificationRequired
The following PM events are used to monitor both the Master Node initiated
Secondary Node Modification and the Secondary Node initiated Secondary Node
Modification procedures:
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Feature Change History
— CuCpProcMnNrdcModification, which is monitored in the Master Node.
— CuCpProcSnNrdcModification, which is monitored in the Secondary Node.
The enhancement deprecates the CuCpProcNrdcModification PM event.
The CuUpProcSnNrdcUlPrimaryCgSwitch PM event is used to monitor the NRDC switching of uplink MCG and SCG radio resources of the Secondary Node
terminated split DRB procedure.
Hardware
No special hardware requirements.
Interfaces
This change affects the following interfaces:
Xn-C
The enhancement introduces the following messages
over the Xn-C interface:
— S-NODE MODIFICATION CONFIRM
— S-NODE MODIFICATION REFUSE
— S-NODE MODIFICATION REQUIRED
Other Network Elements
It is recommended to use ENM version 22.1 or later to support the MOM changes
introduced by this feature enhancement. If the Release Independence Manager
application is used, the MOM changes are supported on earlier ENM versions
also.
RELATED INFORMATION
NR-NR Dual Connectivity
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