ETSI TS 138 401 V15.6.0 (2019-07)

5G; NG-RAN; Architecture description (3GPP TS 38.401 version 15.6.0 Release 15)

RTS/TSGR-0338401vf60

General Information

• Status: Published
• Publication Date: 22-Jul-2019
• Technical Committee: 3GPP RAN 3 - lub specification, lur specification, lu specification and UTRAN & O&M
• Current Stage: 12 - Completion
• Completion Date: 23-Jul-2019

Standards Content (Sample)

TECHNICAL SPECIFICATION
5G;
NG-RAN;
Architecture description
(3GPP TS 38.401 version 15.6.0 Release 15)

3GPP TS 38.401 version 15.6.0 Release 15 1 ETSI TS 138 401 V15.6.0 (2019-07)

Reference
RTS/TSGR-0338401vf60
Keywords
5G
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3GPP TS 38.401 version 15.6.0 Release 15 2 ETSI TS 138 401 V15.6.0 (2019-07)
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ETSI
3GPP TS 38.401 version 15.6.0 Release 15 3 ETSI TS 138 401 V15.6.0 (2019-07)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 5
1 Scope . 6
2 References . 6
3 Definitions and abbreviations . 7
3.1 Definitions . 7
3.2 Abbreviations . 7
4 General principles . 8
5 General architecture . 8
5.1 General . 8
5.2 User plane . 8
5.3 Control plane . 9
6 NG-RAN architecture. 10
6.1 Overview . 10
6.1.1 Overall Architecture of NG-RAN . 10
6.1.2 Overall architecture for separation of gNB-CU-CP and gNB-CU-UP . 11
6.2 NG-RAN identifiers . 12
6.2.1 Principle of handling Application Protocol Identities . 12
6.2.2 gNB-DU ID . 13
6.3 Transport addresses . 13
6.4 UE associations in NG-RAN Node . 13
7 NG-RAN functions description . . 14
7.0 General . 14
7.1 NG-RAN sharing . 14
8 Overall procedures in gNB-CU/gNB-DU Architecture . 15
8.1 UE Initial Access . 15
8.2 Intra-gNB-CU Mobility . 16
8.2.1 Intra-NR Mobility . 16
8.2.1.1 Inter-gNB-DU Mobility . 16
8.2.1.2 Intra-gNB-DU handover . 18
8.2.2 EN-DC Mobility . 18
8.2.2.1 Inter-gNB-DU Mobility using MCG SRB . 18
8.2.2.2 Inter-gNB-DU Mobility using SCG SRB (SRB3) . 19
8.3 Mechanism of centralized retransmission of lost PDUs . 19
8.3.1 Centralized Retransmission in Intra gNB-CU Cases . 19
8.4 Multi-Connectivity operation . 21
8.4.1 Secondary Node Addition . 21
8.4.1.1 EN-DC . 21
8.4.2 Secondary Node Release (MN/SN initiated) . 21
8.4.2.1 EN-DC . 21
8.5 F1 Startup and cells activation . 23
8.6 RRC state transition. 24
8.6.1 RRC connected to RRC inactive. 24
8.6.2 RRC inactive to other states . 25
8.7 RRC connection reestablishment . 26
8.8 Multiple TNLAs for F1-C . 28
8.9 Overall procedures involving E1 and F1 . 29
8.9.1 UE Initial Access . 29
8.9.2 Bearer context setup over F1-U . 30
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8.9.3 Bearer context release over F1-U . 31
8.9.3.1 gNB-CU-CP initiated bearer context release . 31
8.9.3.2 gNB-CU-UP initiated bearer context release . 32
8.9.4 Inter-gNB handover involving gNB-CU-UP change . 33
8.9.5 Change of gNB-CU-UP . 34
8.9.6 RRC State transition . 35
8.9.6.1 RRC Connected to RRC Inactive . 35
8.9.6.2 RRC Inactive to other states . 36
8.10 Multiple TNLAs for E1 . 38
8.11 Support of Network Sharing with multiple cell-ID broadcast . 39
8.11.1 General . 39
8.11.2 Initial Registration – separate PLMN signalling . 39
8.11.3 RRC Connection Reestablishment – separate PLMN signalling . 40
8.11.4 Support of shared signalling transport . 40
9 Synchronization . 41
9.1 gNB Synchronization . 41
10 NG-RAN interfaces . 41
10.1 NG interface . 41
10.2 Xn interface . 42
10.3 F1 interface . 42
10.4 E1 interface . 42
10.5 Antenna interface - general principles . 42
11 Overall procedures in NG-RAN Architecure . 42
11.1 Multiple TNLAs for Xn-C . 42
Annex A (informative): Deployment scenarios of gNB/en-gNB . 44
Annex B: NG-RAN Architecture for Radio Access Network Sharing with
multiple cell ID broadcast (informative) . 45
Annex C (informative): Change History . 46
History . 47

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3GPP TS 38.401 version 15.6.0 Release 15 5 ETSI TS 138 401 V15.6.0 (2019-07)
Foreword
rd
This Technical Specification has been produced by the 3 Generation Partnership Project (3GPP).
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the document.
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3GPP TS 38.401 version 15.6.0 Release 15 6 ETSI TS 138 401 V15.6.0 (2019-07)
1 Scope
The present document describes the overall architecture of the NG-RAN, including interfaces NG, Xn and F1 interfaces
and their interaction with the radio interface.
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
[2] 3GPP TS 38.300: "NR; Overall description; Stage-2".
[3] 3GPP TS 23.501: "System Architecture for the 5G System".
[4] 3GPP TS 38.473: "NG-RAN; F1 application protocol (F1AP)".
[5] 3GPP TS 38.414: "NG-RAN; NG data transport".
[6] 3GPP TS 38.424: "NG-RAN; Xn data transport".
[7] 3GPP TS 38.474: "NG-RAN; F1 data transport".
[8] ITU-T Recommendation G.823 (2000-03): "The control of jitter and wander within digital
networks which are based on the 2048 kbit/s hierarchy".
[9] ITU-T Recommendation G.824 (2000-03): "The control of jitter and wander within digital
networks which are based on the 1544 kbit/s hierarchy".
[10] ITU-T Recommendation G.825 (2001-08): "The control of jitter and wander within digital
networks which are based on the synchronous digital hierarchy (SDH)".
[11] ITU-T Recommendation G.8261/Y.1361 (2008-04): "Timing and Synchronization aspects in
Packet networks".
[12] 3GPP TS 37.340: "NR; Multi-connectivity; Overall description; Stage-2".
[13] 3GPP TS 33.501: "Security Architecture and Procedures for 5G System".
[14] 3GPP TS 38.410: "NG-RAN; NG general aspect and principles".
[15] 3GPP TS 38.420: "NG-RAN; Xn general aspects and principles"
[16] 3GPP TS 38.470: "NG-RAN; F1 general aspects and principles".
[17] 3GPP TS 38.460: "NG-RAN; E1 general aspects and principles".
[18] 3GPP TS 33.210: "3G security; Network Domain Security (NDS); IP Network Layer Security".
[19] 3GPP TS 36.300: "Evolved Universal Terrestrial Radio Access (E-UTRA), Evolved Universal
Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2".
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3 Definitions and abbreviations
3.1 Definitions
For the purposes of the present document, the terms and definitions given in TR 21.905 [1] and the following apply.
A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1].
en-gNB: as defined in TS 37.340 [12].
gNB: as defined in TS 38.300 [2].
gNB Central Unit (gNB-CU): a logical node hosting RRC, SDAP and PDCP protocols of the gNB or RRC and PDCP
protocols of the en-gNB that controls the operation of one or more gNB-DUs. The gNB-CU terminates the F1 interface
connected with the gNB-DU.
gNB Distributed Unit (gNB-DU): a logical node hosting RLC, MAC and PHY layers of the gNB or en-gNB, and its
operation is partly controlled by gNB-CU. One gNB-DU supports one or multiple cells. One cell is supported by only
one gNB-DU. The gNB-DU terminates the F1 interface connected with the gNB-CU.
gNB-CU-Control Plane (gNB-CU-CP): a logical node hosting the RRC and the control plane part of the PDCP
protocol of the gNB-CU for an en-gNB or a gNB. The gNB-CU-CP terminates the E1 interface connected with the
gNB-CU-UP and the F1-C interface connected with the gNB-DU.
gNB-CU-User Plane (gNB-CU-UP): a logical node hosting the user plane part of the PDCP protocol of the gNB-CU
for an en-gNB, and the user plane part of the PDCP protocol and the SDAP protocol of the gNB-CU for a gNB. The
gNB-CU-UP terminates the E1 interface connected with the gNB-CU-CP and the F1-U interface connected with the
gNB-DU.
NG-RAN node: as defined in TS 38.300 [2].
PDU Session Resource: This term is used for specification of NG, Xn, and E1 interfaces. It denotes NG-RAN interface
and radio resources provided to support a PDU Session.
3.2 Abbreviations
For the purposes of the present document, the terms and definitions given in TR 21.905 [1] and the following apply.
A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1].
5GC 5G Core Network
AMF Access and Mobility Management Function
AP Application Protocol
AS Access Stratum
CM Connection Management
CMAS Commercial Mobile Alert Service
ETWS Earthquake and Tsunami Warning System
F1-U F1 User plane interface
F1-C F1 Control plane interface
F1AP F1 Application Protocol
FDD Frequency Division Duplex
GTP-U GPRS Tunnelling Protocol
IP Internet Protocol
NAS Non-Access Stratum
O&M Operation and Maintenance
PWS Public Warning System
QoS Quality of Service
RET Remote Electrical Tilting
RNL Radio Network Layer
RRC Radio Resource Control
SAP Service Access Point
SCTP Stream Control Transmission Protocol
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SFN System Frame Number
SM Session Management
SMF Session Management Function
TDD Time Division Duplex
TDM Time Division Multiplexing
TMA Tower Mounted Amplifier
TNL Transport Network Layer
4 General principles
The general principles guiding the definition of NG-RAN architecture as well as the NG-RAN interfaces are the
following:
- Logical separation of signalling and data transport networks.
- NG-RAN and 5GC functions are fully separated from transport functions. Addressing scheme used in NG-RAN
and 5GC shall not be tied to the addressing schemes of transport functions. The fact that some NG-RAN or 5GC
functions reside in the same equipment as some transport functions does not make the transport functions part of
the NG-RAN or the 5GC.
- Mobility for an RRC connection is fully controlled by the NG-RAN.
- The NG-RAN interfaces are defined along the following principles:
- The functional division across the interfaces have as few options as possible.
- Interfaces are based on a logical model of the entity controlled through this interface.
- One physical network element can implement multiple logical nodes.
5 General architecture
5.1 General
The protocols over Uu and NG interfaces are divided into two structures:
- User plane protocols
These are the protocols implementing the actual PDU Session service, i.e. carrying user data through the access
stratum.
- Control plane protocols
These are the protocols for controlling the PDU Sessions and the connection between the UE and the network
from different aspects (including requesting the service, controlling different transmission resources, handover
etc.). Also a mechanism for transparent transfer of NAS messages is included.
5.2 User plane
The PDU Session Resource service is offered from SAP to SAP by the Access Stratum. Figure 5.2-1 shows the
protocols on the Uu and the NG interfaces that linked together provide this PDU Session Resource service.
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Non-Access Stratum
Radio
Radio
NG NG
proto-
proto proto
proto-
cols
cols cols
cols
(1)
(1)
(2) (2)
Access Stratum
5GC
UE
NG-RAN
Radio
NG
(Uu)
NOTE 1: The radio interface protocols are defined in 3GPP TS 38.2xx and TS 38.3xx.
NOTE 2: The NG interface protocols are defined in 3GPP TS 38.41x.

Figure 5.2-1: NG and Uu user plane
5.3 Control plane
Figure 5.3-1 shows the control plane (signalling) protocol stacks on NG and Uu interfaces.
(3)
CM,SM CM,SM (3)
Non-Access Stratum
NG
NG
Radio
Radio
proto-
proto proto
proto-
cols cols cols
cols
(1)
(1) (2) (2)
Access Stratum
5GC
UE
NG-RAN
Radio
NG
(Uu)
NOTE 1: The radio interface protocols are defined in 3GPP TS 38.2xx and TS 38.3xx.
NOTE 2: The protocol is defined in 3GPP TS 38.41x. (Description of NG interface).
NOTE 3: CM, SM: This exemplifies a set of NAS control protocols between UE and 5GC. The evolution of the
protocol architecture for these protocols is outside the scope of the present document.

Figure 5.3-1: NG and Uu control plane
NOTE: Both the Radio protocols and the NG protocols contain a mechanism to transparently transfer NAS
messages.
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6 NG-RAN architecture
6.1 Overview
6.1.1 Overall Architecture of NG-RAN

5GC
NG NG
NG-RAN
Xn-C
gNB
gNB
gNB-CU
F1 F1
gNB-DU gNB-DU
Figure 6.1-1: Overall architecture
The NG-RAN consists of a set of gNBs connected to the 5GC through the NG interface.
An gNB can support FDD mode, TDD mode or dual mode operation.
gNBs can be interconnected through the Xn interface.
A gNB may consist of a gNB-CU and one or more gNB-DU(s). A gNB-CU and a gNB-DU is connected via F1
interface.
One gNB-DU is connected to only one gNB-CU.
NOTE: In case of network sharing with multiple cell ID broadcast, each Cell Identity associated with a subset of
PLMNs corresponds to a gNB-DU and the gNB-CU it is connected to, i.e. the corresponding gNB-DUs
share the same physical layer cell resources.
NOTE: For resiliency, a gNB-DU may be connected to multiple gNB-CUs by appropriate implementation.
NG, Xn and F1 are logical interfaces.
For NG-RAN, the NG and Xn-C interfaces for a gNB consisting of a gNB-CU and gNB-DUs, terminate in the gNB-
CU. For EN-DC, the S1-U and X2-C interfaces for a gNB consisting of a gNB-CU and gNB-DUs, terminate in the
gNB-CU. The gNB-CU and connected gNB-DUs are only visible to other gNBs and the 5GC as a gNB. A possible
deployment scenario is described in Annex A.
The node hosting user plane part of NR PDCP (e.g. gNB-CU, gNB-CU-UP, and for EN-DC, MeNB or SgNB
depending on the bearer split) shall perform user inactivity monitoring and further informs its inactivity or (re)activation
to the node having C-plane connection towards the core network (e.g. over E1, X2). The node hosting NR RLC (e.g.
gNB-DU) may perform user inactivity monitoring and further inform its inactivity or (re)activation to the node hosting
control plane, e.g. gNB-CU or gNB-CU-CP.
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UL PDCP configuration (i.e. how the UE uses the UL at the assisting node) is indicated via X2-C (for EN-DC), Xn-C
(for NG-RAN) and F1-C. Radio Link Outage/Resume for DL and/or UL is indicated via X2-U (for EN-DC), Xn-U (for
NG-RAN) and F1-U.
The NG-RAN is layered into a Radio Network Layer (RNL) and a Transport Network Layer (TNL).
The NG-RAN architecture, i.e. the NG-RAN logical nodes and interfaces between them, is defined as part of the RNL.
For each NG-RAN interface (NG, Xn, F1) the related TNL protocol and the functionality are specified. The TNL
provides services for user plane transport, signalling transport.
In NG-Flex configuration, each NG-RAN node is connected to all AMFs of AMF Sets within an AMF Region
supporting at least one slice also supported by the NG-RAN node. The AMF Set and the AMF Region are defined in
3GPP TS 23.501 [3].
If security protection for control plane and user plane data on TNL of NG-RAN interfaces has to be supported, NDS/IP
3GPP TS 33.501 [13] shall be applied.
6.1.2 Overall architecture for separation of gNB-CU-CP and gNB-CU-UP
The overall architecture for separation of gNB-CU-CP and gNB-CU-UP is depicted in Figure 6.1.2-1.
gNB-CU-CP
gNB-CU-UP
E1
F1-C F1-U
gNB-DU gNB-DU
gNB
Figure 6.1.2-1. Overall architecture for separation of gNB-CU-CP and gNB-CU-UP
- A gNB may consist of a gNB-CU-CP, multiple gNB-CU-UPs and multiple gNB-DUs;
- The gNB-CU-CP is connected to the gNB-DU through the F1-C interface;
- The gNB-CU-UP is connected to the gNB-DU through the F1-U interface;
- The gNB-CU-UP is connected to the gNB-CU-CP through the E1 interface;
- One gNB-DU is connected to only one gNB-CU-CP;
- One gNB-CU-UP is connected to only one gNB-CU-CP;
NOTE 1: For resiliency, a gNB-DU and/or a gNB-CU-UP may be connected to multiple gNB-CU-CPs by
appropriate implementation.
- One gNB-DU can be connected to multiple gNB-CU-UPs under the control of the same gNB-CU-CP;
- One gNB-CU-UP can be connected to multiple DUs under the control of the same gNB-CU-CP;
NOTE 2: The connectivity between a gNB-CU-UP and a gNB-DU is established by the gNB-CU-CP using
Bearer Context Management functions.
NOTE 3: The gNB-CU-CP selects the appropriate gNB-CU-UP(s) for the requested services for the UE. In case
of multiple CU-UPs they belong to same security domain as defined in TS 33.210 [18].
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NOTE 4: Data forwarding between gNB-CU-UPs during intra-gNB-CU-CP handover within a gNB may be
supported by Xn-U.
6.2 NG-RAN identifiers
6.2.1 Principle of handling Application Protocol Identities
An Application Protocol Identity (AP ID) is allocated when a new UE-associated logical connection is created in either
an NG-RAN node or an AMF. An AP ID shall uniquely identify a logical connection associated to a UE over the NG
interface or Xn interface within a node (NG-RAN node or AMF) or over the F1 interface or over the E1 interface. Upon
receipt of a message that has a new AP ID from the sending node, the receiving node shall store the AP ID of the
sending node for the duration of the logical connection. The receiving node shall assign the AP ID to be used to identify
the logical connection associated to the UE and include it as well as the previously received new AP ID from the
sending node, in the first returned message to the sending node. In all subsequent messages to and from sending node,
both AP IDs of sending node and receiving node shall be included.
The definitions of AP IDs as used on NG interface or Xn interface or F1 interface or E1 interface are shown below:
RAN UE NGAP ID:
A RAN UE NGAP ID shall be allocated so as to uniquely identify the UE over the NG interface within an gNB.
When an AMF receives an RAN UE NGAP ID it shall store it for the duration of the UE-associated logical NG-
connection for this UE. Once known to an AMF this is included in all UE associated NGAP signalling.
The RAN UE NGAP ID shall be unique within the logical NG-RAN node.
AMF UE NGAP ID:
An AMF UE NGAP ID shall be allocated so as to uniquely identify the UE over the NG interface within the
AMF. When a NG-RAN node receives an AMF UE NGAP ID it shall store it for the duration of the UE-
associated logical NG-connection for this UE. Once known to a NG-RAN node this ID is included in all UE
associated NGAP signalling.
The AMF UE NGAP ID shall be unique within an AMF Set as specified in 3GPP TS 23.501 [3].
Old NG-RAN node UE XnAP ID:
An Old NG-RAN node UE XnAP ID shall be allocated so as to uniquely identify the UE over the Xn interface
within a source NG-RAN node. When a target NG-RAN node receives an Old NG-RAN node UE XnAP ID it
shall store it for the duration of the UE-associated logical Xn-connection for this UE. Once known to a target
NG-RAN node this ID is included in all UE associated XnAP signalling. The Old NG-RAN node UE XnAP ID
shall be unique within the logical NG-RAN node.
New NG-RAN node UE XnAP ID:
A New NG-RAN node UE XnAP ID shall be allocated so as to uniquely identify the UE over the Xn interface
within a target NG-RAN node. When a source NG-RAN node receives a New NG-RAN node UE XnAP ID it
shall store it for the duration of the UE-associated logical Xn-connection for this UE. Once known to a source
NG-RAN node this ID is included in all UE associated XnAP signalling. The New NG-RAN node UE XnAP ID
shall be unique within the logical NG-RAN node.
M-NG-RAN node UE XnAP ID:
An M-NG-RAN node UE XnAP ID shall be allocated so as to uniquely identify the UE over the Xn interface
within an M-NG-RAN node for dual connectivity. When an S-NG-RAN node receives an M-NG-RAN node UE
XnAP ID it shall store it for the duration of the UE-associated logical Xn-connection for this UE. Once known to
an S-NG-RAN node this ID is included in all UE associated XnAP signalling. The M-NG-RAN node UE XnAP
ID shall be unique within the logical NG-RAN node.
S-NG-RAN node UE XnAP ID:
A S-NG-RAN node UE XnAP ID shall be allocated so as to uniquely identify the UE over the Xn interface
within an S-NG-RAN node for dual connectivity. When an M-NG-RAN node receives a S-NG-RAN node UE
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XnAP ID it shall store it for the duration of the UE-associated logical Xn-connection for this UE. Once known to
an M-NG-RAN node this ID is included in all UE associated XnAP signalling. The S-NG-RAN node UE XnAP
ID shall be unique within the logical NG-RAN node.
gNB-CU UE F1AP ID:
A gNB-CU UE F1AP ID shall be allocated so as to uniquely identify the UE over the F1 interface within a gNB-
CU. When a gNB-DU receives a gNB-CU UE F1AP ID it shall store it for the duration of the UE-associated
logical F1-connection for this UE. The gNB-CU UE F1AP ID shall be unique within the gNB-CU logical node.
gNB-DU UE F1AP ID:
A gNB-DU UE F1AP ID shall be allocated so as to uniquely identify the UE over the F1 interface within a gNB-
DU. When a gNB-CU receives a gNB-DU UE F1AP ID it shall store it for the duration of the UE-associated
logical F1-connection for this UE. The gNB-DU UE F1AP ID shall be unique within the gNB-DU logical node.
gNB-CU-CP UE E1AP ID:
A gNB-CU-CP UE E1AP ID shall be allocated so as to uniquely identify the UE over the E1 interface within a
gNB-CU-CP. When a gNB-CU-UP receives a gNB-CU-CP UE E1AP ID it shall store it for the duration of the
UE-associated logical E1-connection for this UE. The gNB-CU-CP UE E1AP ID shall be unique within the
gNB-CU-CP logical node.
gNB-CU-UP UE E1AP ID:
A gNB-CU-UP UE E1AP ID shall be allocated so as to uniquely identify the UE over the E1 interface within a
gNB-CU-UP. When a gNB-CU-CP receives a gNB-CU-UP UE E1AP ID it shall store it for the duration of the
UE-associated logical E1-connection for this UE. The gNB-CU-UP UE E1AP ID shall be unique within the
gNB-CU-UP logical node.
6.2.2 gNB-DU ID
The gNB-DU ID is configured at the gNB-DU and used to uniquely identify the gNB-DU at least within a gNB-CU.
The gNB-DU provides its gNB-DU ID to the gNB-CU during the F1 Setup procedure. The gNB-DU ID is used only
within F1AP procedures.
6.3 Transport addresses
The transport layer address parameter is transported in the radio network application signalling procedures that result in
establishment of transport bearer connections.
The transport layer address parameter shall not be interpreted in the radio network application protocols and reveal the
addressing format used in the transport layer.
The formats of the transport layer addresses are further described in 3GPP TS 38.414 [5], 3GPP TS 38.424 [6] and
3GPP TS 38.474 [7].
6.4 UE associations in NG-RAN Node
There are several types of UE associations needed in the NG-RAN node: the "NG-RAN node UE context" used to store
all information needed for a UE and the associations between the UE and the logical NG and Xn connections used for
NG/XnAP UE associated messages. An "NG-RAN node UE context" exists for a UE in CM_CONNECTED.
Definitions:
NG-RAN node UE context:
An NG-RAN node UE context is a block of information in an NG-RAN node associated to one UE. The block of
information contains the necessary information required to maintain the NG-RAN services towards the active UE. An
NG-RAN node UE context is established when the transition to RRC CONNECTED for a UE is completed or in the
target NG-RAN node after completion of handover resource allocation during handover preparation, in which case at
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least UE state information, security information, UE capability information and the identities of the UE-associated
logical NG-connection shall be included in the NG-RAN node UE context.
For Dual Connectivity an NG-RAN node UE context is also established in the S-NG-RAN node after completion of S-
NG-RAN node Addition Preparation procedure.
If radio bearers are requested to be setup during a UE Context setup or modification procedure, the UE capabilities are
signalled to the receiving node as part of the UE context setup or modification procedures.
Bearer context:
A bearer context is a block of information in a gNB-CU-UP node associated to one UE that is used for the sake of
communication over the E1 interface. It may include the information about data radio bearers, PDU sessions and QoS-
flows associated to the UE. The block of information contains the necessary information required to maintain user-plane
services toward the UE.
UE-associated logical NG/Xn/F1/E1 -connection:
NGAP, XnAP, F1AP and E1AP provide means to exchange control plane messages associated with the UE over the
respectively NG-C, Xn-C, F1-C or E1 interface.
A UE-associated logical connection is established during the first NGAP/XnAP/F1AP message exchange between the
NG/Xn/F1 peer nodes.
The connection is maintained as long as UE associated NG/XnAP/F1AP messages need to be exchanged over the
NG/Xn/F1 interface.
The UE-associated logical NG-connection uses the identities AMF UE NGAP ID and RAN UE NGAP ID.
The UE-associated logical Xn-connection uses the identities Old NG-RAN node UE XnAP ID and New NG-RAN node
UE XnAP ID, or M-NG-RAN node UE XnAP ID and S-NG-RAN node UE XnAP ID.
The UE-associated logical F1-connection uses the identities gNB-CU UE F1AP ID and gNB-DU UE F1AP ID.
When a node (AMF or gNB) receives a UE associated NGAP/XnAP/F1AP message the node retrieves the associated
UE based on the NGAP/XnAP/F1AP ID.
UE-associated signalling:
UE-associated signalling is an exchange of NGAP/XnAP/F1AP messages associated with one UE over the UE-
associated logical NG/Xn/F1-connection.
NOTE1: The UE-associated logical NG-connection may exist before the NG-RAN node UE context is setup in the
NG-RAN node.
NOTE2: The UE-associated logical F1-connection may exist before the UE context is setup in the gNB-DU.
7 NG-RAN functions description
7.0 General
For the list of functions refer to TS 38.300 [2].
7.1 NG-RAN sharing
NG-RAN supports radio access network sharing as specified in TS 23.501 [3] and TS 38.300 [2] and TS 36.300 [19].
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8 Overall procedures in gNB-CU/gNB-DU Architecture
8.1 UE Initial Access
The signalling flow for UE Initial access is shown in Figure 8.1-1.

gNB-CU AMF
UE gNB-DU
1.RRCSetupRequest
2.INITIAL UL RRC MESSAGE
TRANSFER
3. DL RRC MESSAGE TRANSFER
4.RRCSetup
5.RRCSetupComplete
6.UL RRC MESSAGE TRANSFER
7.INITIAL UE MESSAGE
8.INITIAL CONNTEXT SETUP
REQUEST
9.UE CONTEXT SETUP REQUEST
10.SecurityModeCommand
11.UE CONTEXT SETUP RESPONSE
12.SecurityModeComplete
13.UL RRC MESSAGE TRANSFER
14.DL RRC MESSAGE TRANSFER
15.RRCReconfiguration
16.RRCReconfigurationComplete
17.UL RRC MESSAGE TRANSFER
18.INITIAL CONTEXT SETUP
RESPONSE
Figure 8.1-1: UE Initial Access procedure
1. The UE sends an RRCSetupRequest message to the gNB-DU.
2. The gNB-DU includes the RRC message and, if the UE is admitted, the corresponding low layer configuration
for the UE in the INITIAL UL RRC MESSAGE TRANSFER message and transfers to the gNB-CU. The
INITIAL UL RRC MESSAGE TRANSFER message includes the C-RNTI allocated by the gNB-DU.
3. The gNB-CU allocates a gNB-CU UE F1AP ID for the UE and generates a RRCSetup message towards UE.
The RRC message is encapsulated in -the DL RRC MESSAGE TRANSFER message.
4. The gNB-DU sends the RRCSetup message to the UE.
5. The UE sends the RRC CONNECTION SETUP COMPLETE message to the gNB-DU.
6. The gNB-DU encapsulates the RRC message in the UL RRC MESSAGE TRANSFER message and sends it to
the gNB-CU.
7. The gNB-CU sends the INITIAL UE MESSAGE message to the AMF.
8. The AMF sends the INITIAL CONTEXT SETUP REQUEST message to the gNB-CU.
9. The gNB-CU sends the UE CONTEXT SETUP REQUEST message to establish the UE context in the gNB-DU.
In this message, it may also encapsulate the SecurityModeCommand message.
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10. The gNB-DU sends the SecurityModeCommand message to the UE.
11. The gNB-DU sends the UE CONTEXT SETUP RESPONSE message to the gNB-CU.
12. The UE responds with the SecurityModeComplete message
13. The gNB-DU encapsulates the RRC message in the UL RRC MESSAGE TRANSFER message and sends it to
the gNB-CU.
14. The gNB-CU generates the RRCReconfiguration message and encapsulates it in the DL RRC MESSAGE
TRANSFER message
15. The gNB-DU sends RRCReconfiguration message to the UE.
16. The UE sends RRCReconfigurationComplete message to the gNB-DU.
17. The gNB-DU encapsulates the RRC message in the UL RRC MESSAGE TRANSFER message and send it to
the gNB-CU.
18. The gNB-CU sends the INITIAL CONTEXT SETUP RESPONSE message to the AMF.
8.2 Intra-gNB-CU Mobility
8.2.1 Intra-NR Mobility
8.2.1.1 Inter-gNB-DU Mobility
This procedure is used for the case when the UE moves from one gNB-DU to another gNB-DU within the same gNB-
CU during NR operation. Figure 8.2.1.1-1 shows the inter-gNB-DU mobility procedure for intra-NR.
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Source Target
UE gNB-CU
gNB-DU gNB-DU
Downlink user data
Uplink user data
1. MeasurementReport
2. UL R
...