TS 38.470
NG-RAN — F1 interface –
General Aspects and Principles

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V16.5.0 (PDF) 2021/06 19 p.

V15.8.0 2021/06 15 p.

Rapporteur:: Mr. Olofsson, Henrik Huawei Technologies Co. Ltd.

Content for TS 38.470 Word version: 16.5.0

1 Scope Word‑p. 6

The present document is an introduction to the 3GPP TS 38.47x series of technical specifications that define the F1 interface. The F1 interface provides means for interconnecting a gNB-CU and a gNB-DU of a gNB within an NG-RAN, or for interconnecting a gNB-CU and a gNB-DU of an en-gNB within an E-UTRAN.

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]

TR 21.905: "Vocabulary for 3GPP Specifications".

[2]

TS 38.401: "NG-RAN; Architecture Description".

[3]

TS 38.471: "NG-RAN; F1 layer 1".

[4]

TS 38.472: "NG-RAN; F1 signalling transport".

[5]

TS 38.473: "NG-RAN; F1 Application Protocol (F1AP)".

[6]

TS 38.474: "NG-RAN; F1 data transport".

[7]

TS 38.425: "NG-RAN; Xn interface user plane protocol".

[8]

TS 38.300: "NR; Overall description; Stage-2".

[9]

TS 37.340: "NR; Multi-connectivity; Overall description; Stage-2".

[10]

TS 38.321: "NR; Medium Access Control (MAC) protocol specification".

[11]

TS 38.331: "NR; Radio Resource Control (RRC); Protocol specification".

3 Definitions and abbreviations

3.1 Definitions

For the purposes of the present document, the terms and definitions given in TR 21.905 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.

en-gNB:

as defined in TS 37.340

gNB-CU:

as defined in TS 38.401

gNB-DU:

as defined in TS 38.401

gNB:

as defined in TS 38.300

3.3 Abbreviations Word‑p. 7

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905.

Backhaul

DRB

Data Radio Bearers

F1-U

F1 User plane interface

F1-C

F1 Control plane interface

F1AP

F1 Application Protocol

GTP-U

GPRS Tunnelling Protocol

IAB

Integrated Access and Backhauling

Internet Protocol

NR-MIB

NR-Master Information Block

O&M

Operation and Maintenance

Paging Area

Paging Frame

Paging Occasion

QoS

Quality of Service

RIM

Remote Interference Management

RLC

Radio Link Control

RRC

Radio Resource Control

SCTP

Stream Control Transmission Protocol

SRB

Signalling Radio Bearers

SIB1

System Information Block 1

SIB10

System Information Block 10

SIB12

System Information Block 12

SIB13

System Information Block 13

SIB14

System Information Block 14

Sidelink

TNL

Transport Network Layer

V2X

Vehicle-to-Everything

4 General aspects

This clause captures the F1 interface principles and characteristics.

4.1 F1 interface general principles

The general principles for the specification of the F1 interface are as follows:

the F1 interface is open;
the F1 interface supports the exchange of signalling information between the endpoints, in addition the interface supports data transmission to the respective endpoints;
from a logical standpoint, the F1 is a point-to-point interface between the endpoints.

NOTE:
A point-to-point logical interface should be feasible even in the absence of a physical direct connection between the endpoints.
the F1 interface supports control plane and user plane separation;
the F1 interface separates Radio Network Layer and Transport Network Layer;
the F1 interface enables exchange of UE associated information and non-UE associated information;
the F1 interface is designed in a future proof way to fulfil different new requirements, support new services and new functions;
one gNB-CU and a set of gNB-DUs are visible to other logical nodes as a gNB or an en-gNB where the gNB terminates the Xn and the NG interfaces, and the en-gNB terminates the X2 and the S1-U interfaces;
the gNB-CU may be separated in control plane (CP) and user plane (UP).

4.2 F1 interface specification objectives Word‑p. 8

The F1 interface specifications facilitate the following:

inter-connection of a gNB-CU and a gNB-DU supplied by different manufacturers.

4.3 F1 interface capabilities

The F1 interface supports:

procedures to establish, maintain and release radio bearers for the NG-RAN part of PDU sessions and for E-UTRAN Radio Access Bearers;
procedures to establish, maintain and release BH RLC channels;
the separation of each UE on the protocol level for user specific signalling management;
the separation of each IAB-MT on the protocol level for IAB-MT specific signalling management;
the transfer of RRC signalling messages between the UE and the gNB-CU.

4.4 Void

5 Functions of the F1 interface

5.1 General

The following clauses describe the functions supported over F1-C and F1-U.

5.2 F1-C functions

5.2.1 F1 interface management function

The error indication function is used by the gNB-DU or gNB-CU to indicate to the gNB-CU or gNB-DU that an error has occurred.

The reset function is used to initialize the peer entity after node setup and after a failure event occurred. This procedure can be used by both the gNB-DU and the gNB-CU.

The F1 setup function allows to exchange application level data needed for the gNB-DU and gNB-CU to interoperate correctly on the F1 interface, and exchange the intended TDD DL-UL configuration originating from the gNB-DU or destined to the gNB-DU. The F1 setup is initiated by the gNB-DU.

The gNB-CU Configuration Update and gNB-DU Configuration Update functions allow to update application level configuration data needed between gNB-CU and gNB-DU to interoperate correctly over the F1 interface, and may activate or deactivate cells. For cross-link interference mitigation, the gNB-CU may coordinate the exchange of intended TDD DL-UL configuration by merging, forwarding and selective forwarding of intended TDD DL-UL configuration(s) between its gNB-DUs, or between its gNB-DUs and other gNBs, gNB-CUs. With the gNB-CU Configuration Update function, energy saving with cell activation/deactivation can be supported as defined in TS 38.300.

The F1 setup and gNB-DU Configuration Update functions allow to inform the S-NSSAI(s), CAG ID(s) and NID(s) supported by the gNB-DU.

The F1 setup and gNB-CU Configuration Update functions allow to inform the NID(s) available at the gNB-CU.

The F1 resource coordination function is used to transfer information about frequency resource sharing between gNB-CU and gNB-DU. In case of split gNB architecture, the gNB-CU may consolidate the outgoing messages from multiple gNB-DUs and distribute the incoming messages to the involved gNB-DUs, to perform resource coordination.

The gNB-DU status indication function allows the gNB-DU to indicate overload status to gNB-CU.

The network access rate reduction function is used to indicate to the gNB-DU that the rate at which UEs are accessing the network need to be reduced.

The F1 removal function is used to remove the interface instance and all related resources between the gNB-DU and the gNB-CU in a controlled manner.

5.2.2 System Information management function Word‑p. 9

Scheduling of system broadcast information is carried out in the gNB-DU. The gNB-DU is responsible for transmitting the system information according to the scheduling parameters available.

The gNB-DU is responsible for the encoding of the NR-MIB message. In case broadcast of SIB1 and other SIBs is needed, the gNB-DU is responsible for the encoding of the SIB1 message, SIB10, SIB12, SIB13 and SIB14 and the gNB-CU is responsible for the encoding of other SIBs. The gNB-DU may re-encode SIB9. The gNB-DU is responsible for the generation of the SystemInformation message.

The gNB-CU is responsible for receiving the positioning assistance information from LMF, e.g the positioning related SIBs. The gNB-CU notifies gNB-DU about the SIBs, and the gNB-DU signals them directly.

To support Msg3 based on-demand SI as described in TS 38.331, the gNB-CU can confirm the received SI request from the UE by including the UE identity, and command the gNB-DU to broadcast the requested other SIs.

5.2.3 F1 UE context management function

The F1 UE context management function supports the establishment and modification of the necessary overall UE context.

The establishment of the F1 UE context is initiated by the gNB-CU and accepted or rejected by the gNB-DU based on admission control criteria (e.g., resource not available).

The modification of the F1 UE context can be initiated by either gNB-CU or gNB-DU. The receiving node can accept or reject the modification. The F1 UE context management function also supports the release of the context previously established in the gNB-DU. The release of the context is triggered by the gNB-CU either directly or following a request received from the gNB-DU. The gNB-CU request the gNB-DU to release the UE Context when the UE enters RRC_IDLE or RRC_INACTIVE.

This function can be also used to manage DRBs, SRBs and SL DRBs, i.e., establishing, modifying and releasing DRB, SRB and SL DRB resources. The establishment and modification of DRB, or SL DRB resources are triggered by the gNB-CU and accepted/rejected by the gNB-DU based on resource reservation information and QoS information to be provided to the gNB-DU. For each DRB to be setup or modified, the S-NSSAI may be provided by gNB-CU to the gNB-DU in the UE Context Setup procedure and the UE Context Modification procedure. In case of NG-RAN sharing, the gNB-CU includes the serving PLMN ID (for SNPNs the serving SNPN ID) in the UE Context Setup procedure.

For Uu, the mapping between QoS flows and radio bearers is performed by gNB-CU and the granularity of bearer related management over F1 is radio bearer level. For NG-RAN, the gNB-CU decides an aggregated DRB QoS profile for each radio bearer based on received QoS flow profile, and provides both aggregated DRB QoS profile and QoS flow profile to the gNB-DU, and the gNB-DU either accepts the request or rejects it with appropriate cause value. With this function, gNB-DU could also notify gNB-CU whether the QoS for already established DRBs is not fulfilled any longer or it is fulfilled again. The function can be also used to inform the gNB-DU the alternative QoS Parameters Sets when available for a QoS flow. To support packet duplication for intra-gNB-DU CA as described in TS 38.300, one data radio bearer should be configured with at least two GTP-U tunnels between gNB-CU and a gNB-DU.

For SL, the mapping between QoS flows and radio bearers is performed by gNB-CU and the granularity of bearer related management over F1 is radio bearer level. For NG-RAN, the gNB-CU decides an aggregated SL DRB QoS profile for each radio bearer based on received QoS flow profile, and provides both aggregated SL DRB QoS profile and QoS flow profile to the gNB-DU, and the gNB-DU either accepts the request or rejects it with appropriate cause value.

With this function, gNB-CU requests the gNB-DU to setup or change of the SpCell (as defined in TS 38.321) for the UE, and the gNB-DU either accepts or rejects the request with appropriate cause value. This function also enables the gNB-DU to inform the gNB-CU of which cell the UE has successfully accessed during conditional mobility.

With this function, the gNB-CU requests the setup of the SCell(s) at the gNB-DU side, and the gNB-DU accepts all, some or none of the SCell(s) and replies to the gNB-CU. The gNB-CU requests the removal of the SCell(s) for the UE.

With this function, the gNB-CU indicates the UL UE AMBR limit to the gNB-DU, and the gNB-DU enforces the indicated limit.

With this function, the gNB-DU indicates that a bearer, or a UE is inactive or active. The gNB-CU consolidates all the serving gNB-DUs for the UE and takes further action.

With this function, the gNB-CU indicates the gNB-DU that the UE context concerns mobility enhancement operation, and the gNB-DU takes corresponding actions.

In addition, for IAB-nodes and IAB-donors:

The F1 UE context management function is used for managing BH RLC channels, i.e. establishing, modifying and releasing BH RLC channel resources. The establishment of BH RLC channels is triggered by the IAB-donor-CU. The establishment and modification is accepted/rejected by the IAB-node's parent, based on e.g. resource reservation information and QoS information provided to the IAB-node's parent.
The DRB QoS profile framework is reused for BH RLC channels carrying DRBs. Prioritization of traffic on the F1-C interface is based on traffic type (e.g. UE-associated F1AP signalling, non-UE-associated F1AP signalling) and is enforced in the IAB-donor-DU and in IAB-nodes, considering that the traffic on the F1-C interface has higher priority than other traffic; in-sequence delivery over the signaling connection is always ensured.
The IAB-donor-CU associates each BH RLC channel carrying control plane traffic with one of the signaled control plane traffic type values.

5.2.4 RRC message transfer function Word‑p. 10

This function allows to transfer RRC messages between gNB-CU and gNB-DU. RRC messages are transferred over F1-C. The gNB-CU is responsible for the encoding of the dedicated RRC message with assistance information provided by gNB-DU. This function also allows gNB-DU to report to gNB- CU if the downlink RRC message has been successfully delivered to UE or not.

For IAB-nodes, this function allows to transfer RRC messages for setting up and configuring the IAB-MT side of the BH RLC channel. These RRC messages are carried on F1-C between the IAB-donor-CU and the parent IAB-DU i.e. the gNB-DU side of the BH RLC channel.

5.2.5 Paging function

The gNB-DU is responsible for transmitting the paging information according to the scheduling parameters provided.

The gNB-CU provides paging information to enable the gNB-DU to calculate the exact PO and PF. The gNB-CU determines the PA. The gNB-DU consolidates all the paging records for a particular PO, PF and PA, and encodes the final RRC message and broadcasts the paging message on the respective PO, PF in the PA.

5.2.6 Warning messages information transfer function Word‑p. 11

This function allows to cooperate with the warning message transmission procedures over NG interface. The gNB-CU is responsible for encoding the warning related SI message and sending it together with other warning related information for the gNB-DU to broadcast over the radio interface.

5.2.7 Remote Interference Management (RIM) message transfer function |R16|

This function enables the transfer of Remote Interference Management (RIM) backhaul messages between the gNB-CU and the gNB-DU. RIM messages are transferred over F1-C. The gNB-CU acts as a coordinator on behalf of its affiliated gNB-DUs, by merging the RIM information received from its gNB-DUs and forwarding the merged information to the target gNBs or gNB-CUs, transparently via the core network. Similarly, a gNB-CU distributes an incoming RIM backhaul message to all its concerned gNB-DUs.

5.2.8 Trace function |R16|

The Trace function provides means to control trace sessions for a UE over F1 interface.

5.2.9 Load management function |R16|

The load management function allows an gNB-CU to request the reporting of load measurements to gNB-DU and is used by gNB-DU to report the result of measurements admitted by gNB-DU.

5.2.10 Self-optimisation support function |R16|

This function allows the gNB-CU to provide information to the gNB-DU in order to support self-optimization functionality.

5.2.11 Positioning function |R16|

This function allows to transfer location management messages between gNB-CU and gNB-DU. With this function, gNB-CU request TRP information from gNB-DU, and gNB-DU response to gNB-CU with the TRP information. With this function, gNB-CU request positioning measurements from gNB-DU, and gNB-DU response to gNB-CU with the positioning measurements.

The function allows to transfer the positioning assistance data from gNB-CU to gNB-DU. The gNB-DU is responsible for broadcasting the positioning assistance data according to the scheduling parameters available.

5.2.12 IAB support function |R16|

5.3 F1-U functions Word‑p. 12

5.3.1 Transfer of user data

This function allows to transfer of user data between gNB-CU and gNB-DU.

5.3.2 Flow control function

This function allows to control the downlink user data flow to the gNB-DU. The detailed protocol is specified in TS 38.425.

5.4 TEIDs allocation

The gNB-DU is responsible for the allocation of the F1-U DL GTP TEID for each data radio bearer.

6 Procedures of the F1 interface

6.1 Control plane procedures

6.1.1 Interface Management procedures

The F1 Interface management procedures are listed below:

Reset procedure
Error Indication procedure
F1 Setup procedure
gNB-DU Configuration Update procedure
gNB-CU Configuration Update procedure
gNB-DU Resource Coordination procedure
gNB-DU Status Indication procedure
F1 Removal procedure
Network Access Rate Reduction procedure

6.1.2 Context Management procedures

The F1 Context management procedures are listed below:

UE Context setup procedure
UE Context Release Request (gNB-DU initiated) procedure
UE Context Release (gNB-CU initiated) procedure
UE Context Modification (gNB-CU initiated) procedure
UE Context Modification Required (gNB-DU initiated) procedure
UE Inactivity Notification procedure
Notify procedure
Access Success procedure

6.1.3 RRC Message Transfer procedures Word‑p. 13

The F1 RRC message transfer procedures are listed below:

Initial UL RRC Message Transfer procedure
UL RRC Message Transfer procedure
DL RRC Message Transfer procedure
RRC Delivery Report procedure

6.1.3A Warning Message Transmission procedures

The F1 Warning message transmission procedures are listed below:

Write-Replace Warning procedure
PWS Cancel procedure
PWS Restart Indication procedure
PWS Failure Indication procedure

6.1.4 System Information procedures

The F1 System information procedures are listed below:

System Information Delivery procedure

6.1.5 Paging procedures

The F1 Paging procedures are listed below:

Paging procedure

6.1.6 Void

6.1.7 Radio information transfer procedures |R16|

The F1 Radio information transfer procedures are listed below:

DU-CU Radio Information Transfer procedure
- CU-DU Radio Information Transfer procedure

6.1.8 UE Tracing procedures |R16|

The following procedures are used to trace the UE:

Trace Start procedure
Deactivate Trace procedure
Cell Traffic Trace procedure

6.1.9 Load management procedures |R16|

The load management procedures are listed as below:

Resource Status Reporting Initiation procedure
Resource Status Reporting procedure

6.1.10 Self-optimisation support procedure |R16| Word‑p. 14

The self-optimisation support procedure is used to transfer failure and mobility related information from the gNB-CU to the gNB-DU to enable self-optimisation

Access and Mobility Indication procedure

6.1.11 Positioning procedures |R16|

The F1 Positioning procedures are listed below:

Positioning Assistance Information Control procedure;
Positioning Assistance Information Feedback procedure;
Positioning Measurement procedure;
Positioning Measurement Report procedure;
Positioning Measurement Abort procedure;
Positioning Measurement Failure Indication procedure;
Positioning Measurement Update procedure;
TRP Information Exchange procedure;
Positioning Information Update;
Positioning Information Exchange procedure;
Positioning Activation procedure;
Positioning Deactivation procedure;
E-CID Measurement Initiation;
E-CID Measurement Failure Indication procedure;
E-CID Measurement Report procedure;
E-CID Measurement Termination procedure.

6.1.12 IAB procedures |R16|

6.2 User plane procedures

7 F1 interface protocol structure Word‑p. 15

7.1 F1 Control Plane Protocol (F1-C)

Figure 7.1-1 shows the protocol structure for F1-C. The TNL is based on IP transport, comprising the SCTP on top of IP. The application layer signalling protocol is referred to as F1AP (F1 Application Protocol).