ETSI TS 138 401 V15.7.0 (2020-01)

ETSI TS 138 401 V15.7.0 (2020-01)






TECHNICAL SPECIFICATION
5G;
NG-RAN;
Architecture description
(3GPP TS 38.401 version 15.7.0 Release 15)

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3GPP TS 38.401 version 15.7.0 Release 15 1 ETSI TS 138 401 V15.7.0 (2020-01)



Reference
RTS/TSGR-0338401vf70
Keywords
5G
ETSI
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3GPP TS 38.401 version 15.7.0 Release 15 2 ETSI TS 138 401 V15.7.0 (2020-01)
Intellectual Property Rights
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Legal Notice
This Technical Specification (TS) has been produced by ETSI 3rd Generation Partnership Project (3GPP).
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ETSI

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3GPP TS 38.401 version 15.7.0 Release 15 3 ETSI TS 138 401 V15.7.0 (2020-01)
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) . 20
8.3 Mechanism of centralized retransmission of lost PDUs . 20
8.3.1 Centralized Retransmission in Intra gNB-CU Cases . 20
8.4 Multi-Connectivity operation . 22
8.4.1 Secondary Node Addition . 22
8.4.1.1 EN-DC . 22
8.4.2 Secondary Node Release (MN/SN initiated) . 22
8.4.2.1 EN-DC . 22
8.5 F1 Startup and cells activation . 24
8.6 RRC state transition. 25
8.6.1 RRC connected to RRC inactive. 25
8.6.2 RRC inactive to other states . 26
8.7 RRC connection reestablishment . 27
8.8 Multiple TNLAs for F1-C . 29
8.9 Overall procedures involving E1 and F1 . 30
8.9.1 UE Initial Access . 30
8.9.2 Bearer context setup over F1-U . 31
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3GPP TS 38.401 version 15.7.0 Release 15 4 ETSI TS 138 401 V15.7.0 (2020-01)
8.9.3 Bearer context release over F1-U . 32
8.9.3.1 gNB-CU-CP initiated bearer context release . 32
8.9.3.2 gNB-CU-UP initiated bearer context release . 33
8.9.4 Inter-gNB handover involving gNB-CU-UP change . 34
8.9.5 Change of gNB-CU-UP . 35
8.9.6 RRC State transition . 36
8.9.6.1 RRC Connected to RRC Inactive . 36
8.9.6.2 RRC Inactive to other states . 37
8.10 Multiple TNLAs for E1 . 39
8.11 Support of Network Sharing with multiple cell-ID broadcast . 40
8.11.1 General . 40
8.11.2 Initial Registration – separate PLMN signalling . 40
8.11.3 RRC Connection Reestablishment – separate PLMN signalling . 41
8.11.4 Support of shared signalling transport . 42
9 Synchronization . 42
9.1 gNB Synchronization . 42
10 NG-RAN interfaces . 43
10.1 NG interface . 43
10.2 Xn interface . 43
10.3 F1 interface . 43
10.4 E1 interface . 43
10.5 Antenna interface - general principles . 43
11 Overall procedures in NG-RAN Architecure . 43
11.1 Multiple TNLAs for Xn-C . 43
Annex A (informative): Deployment scenarios of gNB/en-gNB . 45
Annex B: NG-RAN Architecture for Radio Access Network Sharing with
multiple cell ID broadcast (informative) . 46
Annex C (informative): Change History . 47
History . 48

ETSI

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3GPP TS 38.401 version 15.7.0 Release 15 5 ETSI TS 138 401 V15.7.0 (2020-01)
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.
ETSI

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3GPP TS 38.401 version 15.7.0 Release 15 6 ETSI TS 138 401 V15.7.0 (2020-01)
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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3GPP TS 38.401 version 15.7.0 Release 15 7 ETSI TS 138 401 V15.7.0 (2020-01)
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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3GPP TS 38.401 version 15.7.0 Release 15 8 ETSI TS 138 401 V15.7.0 (2020-01)
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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3GPP TS 38.401 version 15.7.0 Release 15 9 ETSI TS 138 401 V15.7.0 (2020-01)
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.
ETSI

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3GPP TS 38.401 version 15.7.0 Release 15 10 ETSI TS 138 401 V15.7.0 (2020-01)
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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3GPP TS 38.401 version 15.7.0 Release 15 11 ETSI TS 138 401 V15.7.0 (2020-01)
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
supp
...