3GPP TS 38.401 V15.0.0 (2017-12)
`
`Technical Specification
`
`3rd Generation Partnership Project;
`Technical Specification Group Radio Access Network;
`NG-RAN;
`Architecture description
`(Release 15)
`
`
`
`
`
`
`The present document has been developed within the 3rd Generation Partnership Project (3GPP TM) and may be further elaborated for the purposes of 3GPP.
`The present document has not been subject to any approval process by the 3GPP Organizational Partners and shall not be implemented.
`This Specification is provided for future development work within 3GPP only. The Organizational Partners accept no liability for any use of this Specification.
`Specifications and reports for implementation of the 3GPP TM system should be obtained via the 3GPP Organizational Partners' Publications Offices.
`
`
`
`

`

`
`Release 15
`
`2
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`
`
`
`
`
`
`
`
`
`
`
`
`Keywords
`NG-RAN, Radio
`
`3GPP
`
`Postal address
`
`
`3GPP support office address
`650 Route des Lucioles - Sophia Antipolis
`Valbonne - FRANCE
`Tel.: +33 4 92 94 42 00 Fax: +33 4 93 65 47 16
`
`Internet
`http://www.3gpp.org
`
`Copyright Notification
`
`No part may be reproduced except as authorized by written permission.
`The copyright and the foregoing restriction extend to reproduction in all media.
`
`© 2017, 3GPP Organizational Partners (ARIB, ATIS, CCSA, ETSI, TSDSI, TTA, TTC).
`All rights reserved.
`
`UMTS™ is a Trade Mark of ETSI registered for the benefit of its members
`3GPP™ is a Trade Mark of ETSI registered for the benefit of its Members and of the 3GPP Organizational Partners
`LTE™ is a Trade Mark of ETSI registered for the benefit of its Members and of the 3GPP Organizational Partners
`GSM® and the GSM logo are registered and owned by the GSM Association
`
`3GPP
`
`

`

`
`Release 15
`
`3
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`Contents
`Foreword ...................................................................................................................................................... 4
`1
`Scope .................................................................................................................................................. 5
`2
`References .......................................................................................................................................... 5
`3
`Definitions and abbreviations .............................................................................................................. 6
`3.1
`Definitions ................................................................................................................................................... 6
`3.2
`Abbreviations............................................................................................................................................... 6
`4
`General principles ............................................................................................................................... 7
`5
`General architecture ............................................................................................................................ 7
`5.1
`General ........................................................................................................................................................ 7
`5.2
`User plane .................................................................................................................................................... 7
`5.3
`Control plane ............................................................................................................................................... 8
`6
`NG-RAN architecture ......................................................................................................................... 9
`6.1
`Overview ..................................................................................................................................................... 9
`6.2
`NG-RAN identifiers ................................................................................................................................... 10
`6.2.1
`Principle of handling Application Protocol Identities ............................................................................. 10
`6.2.2
`gNB-DU ID .......................................................................................................................................... 11
`6.3
`Transport addresses .................................................................................................................................... 11
`6.4
`UE associations in NG-RAN Node ............................................................................................................. 11
`7
`NG-RAN functions description ......................................................................................................... 12
`7.0
`General ...................................................................................................................................................... 12
`7.1
`Void .......................................................................................................................................................... 12
`8
`Overall procedures in gNB-CU/gNB-DU Architecture ...................................................................... 12
`8.1
`Void .......................................................................................................................................................... 12
`8.2
`Intra-gNB-CU Mobility .............................................................................................................................. 12
`8.2.1
`Intra-NR Mobility ................................................................................................................................. 12
`8.2.1.1
`Inter-gNB-DU Mobility ................................................................................................................... 12
`8.2.1.2
`Intra-gNB-DU inter-cell mobility..................................................................................................... 14
`8.2.2
`EN-DC Mobility ................................................................................................................................... 14
`8.2.2.1
`Inter-gNB-DU Mobility using MCG SRB ........................................................................................ 14
`8.2.2.2
`Inter-gNB-DU Mobility using SCG SRB ......................................................................................... 15
`8.3
`Mechanism of centralized retransmission of lost PDUs ............................................................................... 15
`8.3.1
`Centralized Retransmission in Intra gNB-CU Cases .............................................................................. 15
`8.4
`Multi-Connectivity operation...................................................................................................................... 17
`8.4.1
`Secondary Node Addition ..................................................................................................................... 17
`8.4.1.1
`EN-DC ............................................................................................................................................ 17
`8.4.2
`Secondary Node Release (MN/SN initiated) .......................................................................................... 17
`8.4.2.1
`EN-DC ............................................................................................................................................ 17
`8.5
`F1 Startup and cells activation .................................................................................................................... 19
`8.6
`Void .......................................................................................................................................................... 19
`9
`Synchronization ................................................................................................................................ 20
`9.1
`gNB Synchronization ................................................................................................................................. 20
`10 NG-RAN interfaces .......................................................................................................................... 20
`10.1
`NG interface .............................................................................................................................................. 20
`10.2
`Xn interface ............................................................................................................................................... 20
`10.3
`F1 interface ................................................................................................................................................ 21
`Annex A (informative):
`Deployment scenarios of gNB/en-gNB ....................................................... 22
`Annex B (informative):
`Change History .......................................................................................... 23
`
`
`3GPP
`
`

`

`
`Release 15
`
`4
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`Foreword
`This Technical Specification has been produced by the 3rd 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.
`
`3GPP
`
`

`

`
`Release 15
`
`5
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`Scope
`1
`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.
`
`References
`2
`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]
`
`[2]
`
`[3]
`
`[4]
`
`[5]
`
`[6]
`
`[7]
`
`[8]
`
`[9]
`
`[10]
`
`[11]
`
`[12]
`
`[13]
`
`[14]
`
`[15]
`
`[16]
`
`3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
`
`3GPP TS 38.300: "NR; Overall description; Stage-2".
`
`3GPP TS 23.501: "System Architecture for the 5G System".
`
`3GPP TS 38.473: "NG-RAN; F1 application protocol (F1AP)".
`
`3GPP TS 38.414: "NG-RAN; NG data transport".
`
`3GPP TS 38.424: "NG-RAN; Xn data transport".
`
`3GPP TS 38.474: "NG-RAN; F1 data transport".
`
`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".
`
`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".
`
`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)".
`
`ITU-T Recommendation G.8261/Y.1361 (2008-04): "Timing and Synchronization aspects in
`Packet networks".
`
`3GPP TS 37.340: "NR; Multi-connectivity; Overall description; Stage-2".
`
`3GPP TS 33.501: "Security Architecture and Procedures for 5G System".
`
`3GPP TS 38.410: "NG-RAN; NG general aspect and principles".
`
`3GPP TS 38.420: "NG-RAN; Xn general aspects and principles"
`
`3GPP TS 38.470: "NG-RAN; F1 general aspects and principles".
`
`3GPP
`
`

`

`
`Release 15
`
`6
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`3
`
`Definitions and abbreviations
`
`Definitions
`3.1
`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.
`NG-RAN node: as defined in TS 38.300 [2].
`
`Abbreviations
`3.2
`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
`AMF
`AP
`AS
`CM
`CMAS
`ETWS
`F1-U
`F1-C
`F1AP
`FDD
`GTP-U
`IP
`NAS
`O&M
`PWS
`QoS
`RNL
`RRC
`SAP
`SCTP
`SFN
`SM
`SMF
`TDD
`TDM
`TNL
`
`
`5G Core Network
`Access and Mobility Management Function
`Application Protocol
`Access Stratum
`Connection Management
`Commercial Mobile Alert Service
`Earthquake and Tsunami Warning System
`F1 User plane interface
`F1 Control plane interface
`F1 Application Protocol
`Frequency Division Duplex
`GPRS Tunnelling Protocol
`Internet Protocol
`Non-Access Stratum
`Operation and Maintenance
`Public Warning System
`Quality of Service
`Radio Network Layer
`Radio Resource Control
`Service Access Point
`Stream Control Transmission Protocol
`System Frame Number
`Session Management
`Session Management Function
`Time Division Duplex
`Time Division Multiplexing
`Transport Network Layer
`
`3GPP
`
`

`

`
`Release 15
`
`7
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`General principles
`4
`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
`
`General
`5.1
`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.
`
`User plane
`5.2
`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.
`
`3GPP
`
`

`

`
`Release 15
`
`8
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`Radio
`proto-
`cols
`(1)
`
`UE
`
`Non-Access Stratum
`
`Radio
`proto-
`cols
`(1)
`
`NG
`proto
`cols
`(2)
`
`NG
`proto
`cols
`(2)
`
`Access Stratum
`NG-RAN
`
` 5GC
`
`NG
`
`Radio
`(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
`
`Control plane
`5.3
`Figure 5.3-1 shows the control plane (signalling) protocol stacks on NG and Uu interfaces.
`
`CM,SM
`
` (3)
`
`Non-Access Stratum
`
`CM,SM
`
`(3)
`
`Radio
`proto-
`cols
`(1)
`
`UE
`
`Radio
`proto-
`cols
`(1)
`
`NG
`proto
`cols
`(2)
`
`NG
`proto
`cols
`(2)
`
`Access Stratum
`NG-RAN
`
` 5GC
`
`NG
`
`Radio
`(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.
`
`3GPP
`
`

`

`
`Release 15
`
`6
`
`6.1
`
`9
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`NG-RAN architecture
`
`Overview
`
`
`NG-RAN
`
`gNB
`
`5GC
`
`NG
`
`Xn-C
`
`gNB
`
`NG
`
`
`gNB-CU
`
`
`F1
`
`
`gNB-DU
`
`F1
`
`
`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: For resiliency, a gNB-DU may be connected to multiple gNB-CU 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 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 gNB is connected to all AMFs within an AMF Region. The AMF Region is 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.
`
`3GPP
`
`

`

`
`Release 15
`
`10
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`6.2
`
`NG-RAN identifiers
`
`Principle of handling Application Protocol Identities
`6.2.1
`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. 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 are shown below:
`RAN UE NGAP ID:
` ARAN 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 the AMF logical node.
`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
`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.
`
`3GPP
`
`

`

`
`Release 15
`
`11
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`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-DU ID
`6.2.2
`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 thevF1 Setup procedure. The gNB-DU ID is used only
`within F1AP procedures.
`
`Transport addresses
`6.3
`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].
`
`UE associations in NG-RAN Node
`6.4
`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
`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.
`UE-associated logical NG/Xn/F1 -connection:
`NGAP, XnAP and F1AP provide means to exchange control plane messages associated with the UE over the
`respectively NG-C, Xn-C or F1-C 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.
`
`3GPP
`
`

`

`
`Release 15
`
`12
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`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.
`
`NOTE: The UE-associated logical NG-connection may exist before the NG-RAN node UE context is setup in the
`NG-RAN node.
`
`The UE-associated logical Xn-connection may exist before the NG-RAN node UE context is setup in the
`target NG-RAN node.
`
`The UE-associated logical F1-connection may exist before the UE context is setup in the gNB-DU.
`
`7
`
`NG-RAN functions description
`
`General
`7.0
`For the list of functions refer to TS 38.300 [2].
`
`Void
`7.1
`Reserve for future use.
`
`8
`
`Overall procedures in gNB-CU/gNB-DU Architecture
`
`Void
`8.1
`Reserve for future use.
`
`8.2
`
`Intra-gNB-CU Mobility
`
`8.2.1
`
`Intra-NR Mobility
`
`Inter-gNB-DU Mobility
`8.2.1.1
`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.
`
`3GPP
`
`

`

`
`Release 15
`
`13
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`UE
`
`Source
`gNB-DU
`
`Target
`gNB-DU
`
`gNB-CU
`
`Downlink user data
`Uplink user data
`1. Measurement Report
`
`2. Uplink RRC Transfer
`(Measurement Report)
`
`3. UE Context Setup Request
`
`4. UE Context Setup Response
`5. UE Context Modification Request
`(RRCConnectionReconfiguration)
`6. RRCConnectionReconfiguration
`Downlink Data Delivery Status
`
`7. UE Context Modification Response
`
`Downlink user data
`
`8. Random Access Procedure
`
`9. RRCConnectionReconfigurationComplete
`
`Downlink user data
`Uplink user data
`
`10. Uplink RRC Transfer
`(RRCConnectionReconfigurationComplete)
`
`11. UE Context Release Command
`
`12. UE Context Release Complete
`
`
`
`Figure 8.2.1.1-1: Inter-gNB-DU Mobility for intra-NR
`
`1. The UE sends a Measurement Report message to the source gNB-DU.
`
`2. The source gNB-DU sends an Uplink RRC Transfer message to the gNB-CU to convey the received
`Measurement Report.
`
`3. The gNB-CU sends an UE Context Setup Request message to the target gNB-DU to create an UE context and
`setup one or more bearers.
`
`4. The target gNB-DU responds to the gNB-CU with an UE Context Setup Response message.
`
`5. The gNB-CU sends a UE Context Modification Request message to the source gNB-DU, which includes a
`generated RRCConnectionReconfiguration message and indicates to stop the data transmission for the UE. The
`source gNB-DU also sends a Downlink Data Delivery Status frame to inform the gNB-CU about the
`unsuccessfully transmitted downlink data to the UE. Downlink packets, which may include PDCP PDUs not
`successfully transmitted in the source gNB-DU, are sent from the gNB-CU to the target gNB-DU.
`
`6. The source gNB-DU forwards the received RRCConnectionReconfiguration to the UE.
`
`7. The source gNB-DU responds to the gNB-CU with the UE Context Modification Response message.
`
`8. A Random Access procedure is performed at the target gNB-DU.
`
`9. The UE responds to the target gNB-DU with an RRCConnectionReconfigurationComplete message.
`
`10. The target gNB-DU sends an Uplink RRC Transfer message to the gNB-CU to convey the received
`RRCConnectionReconfigurationComplete. Downlink packets are sent to the UE. Also, uplink packets are sent
`from the UE, which are forwarded to the gNB-CU through the target gNB-DU.
`
`11. The gNB-CU sends an UE Context Release Command message to the source gNB-DU.
`
`3GPP
`
`

`

`
`Release 15
`
`14
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`12. The source gNB-DU releases the UE context and responds the gNB-CU with an UE Context Release Complete
`message.
`
`Intra-gNB-DU inter-cell mobility
`8.2.1.2
`This procedure is used for the case that UE moves from one cell to another cell within the same gNB-DU during NR
`operation, and supported by UE Context Modification (gNB-CU initiated) procedure as specified in TS 38.473 [4].
`
`8.2.2
`
`EN-DC Mobility
`
`Inter-gNB-DU Mobility using MCG SRB
`8.2.2.1
`This procedure is used for the case the UE moves from one gNB-DU to another gNB-DU within the same gNB-CU
`when only MCG SRB is available during EN-DC operation. Figure 8.2.2.1-1 shows the inter-gNB-DU mobility
`procedure using MCG SRB in EN-DC.
`
`UE
`
`Source
`gNB-DU
`
`Target
`gNB-DU
`
`gNB-CU
`
`MeNB
`
`Downlink user data
`Uplink user data
`1. Measurement Report
`
`3. UE Context Setup Request
`
`4. UE Context Setup Response
`
`6. UE Context Modification Request
`
`Downlink Data Delivery Status
`
`7. UE Context Modification Response
`
`2. SgNB Modification Request
`
`5. SgNB Modification Request
`Acknowledge
`
`8. RRC Reconfiguration procedure is performed between the MeNB and the UE.
`
`Downlink user data
`
`9. SgNB Reconfiguration
`Complete
`
`10. Random Access Procedure
`Downlink user data
`Uplink user data
`
`11. UE Context Release Command
`
`12. UE Context Release Complete
`
`Figure 8.2.2.1-1: Inter-gNB-DU Mobility using MCG SRB in EN-DC
`
`
`
`1. The UE sends a Measurement Report message to the MeNB.
`
`2. The MeNB sends an SgNB Modification Request.
`
`3. The gNB-CU sends an UE Context Setup Request message to the target gNB-DU to create an UE context and
`setup one or more bearers.
`
`4. The target gNB-DU responds the gNB-CU with an UE Context Setup Response message.
`
`5. The gNB-CU responds the MeNB with an SgNB Modification Request Acknowledge message.
`
`6. The gNB-CU sends a UE Context Modification Request message to the source gNB-DU indicating to stop the
`data transmission to the UE. The source gNB-DU also sends a Downlink Data Delivery Status frame to inform
`the gNB-CU about the unsuccessfully transmitted downlink data to the UE.
`
`3GPP
`
`

`

`
`Release 15
`
`15
`
`3GPP TS 38.401 V15.0.0 (2017-12)
`
`7. The source gNB-DU responds the gNB-CU with an UE Context Modification Response message.
`
`8. The MeNB and the UE perform RRC Reconfiguration proced