5 Functions of the W1 interface

37.4703GPPGeneral aspects and principlesRelease 17TSW1 interface

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.1 General

The following clauses describe the functions supported over W1-C and W1-U.

5.2 W1-C functions

5.2.1 Interface management function

The W1 setup function allows to exchange application level data needed for the ng-eNB-DU and ng-eNB-CU to interoperate correctly on the W1 interface. The W1 setup is initiated by the ng-eNB-DU.

The ng-eNB-CU Configuration Update and ng-eNB-DU Configuration Update functions allow to update application level configuration data needed between ng-eNB-CU and ng-eNB-DU to interoperate correctly over the W1 interface, and may activate or deactivate cells.

For NG-RAN, the W1 setup and ng-eNB-DU Configuration Update functions allow to inform the S-NSSAI(s) supported by the ng-eNB-DU.

The error indication function is used by the ng-eNB-DU or ng-eNB-CU to indicate to the ng-eNB-CU or ng-eNB-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 ng-eNB-DU and the ng-eNB-CU.

The W1 resource coordination function is used to transfer information about frequency resource sharing between ng-eNB-CU and ng-eNB-DU.

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

5.2.2 System Information management function

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

The ng-eNB-DU is responsible for the encoding of ng-eNB-MIB. In case broadcast of SIB1 and other SI messages is needed, the ng-eNB-DU is responsible for assembling SIB1, SIB2, SIB3, SIB8 and SIB16 and the ng-eNB-CU is responsible for assembling other SIBs.

5.2.3 Paging function

The ng-eNB-DU is responsible for transmitting the paging information.

The ng-eNB-CU provides paging information to enable the ng-eNB-DU to calculate the exact PO and PF. The ng-eNB-CU determines the PA. The ng-eNB-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.

5.2.4 UE context management function

The W1 UE context management function supports the establishment and modification of the necessary overall UE context.

The establishment of the W1 UE context is initiated by the ng-eNB-CU and accepted or rejected by the ng-eNB-DU based on admission control criteria (e.g., resource not available).

The modification of the W1 UE context can be initiated by either ng-eNB-CU or ng-eNB-DU. The receiving node can accept or reject the modification. The W1 UE context management function also supports the release of the context previously established in the ng-eNB-DU. The release of the context is triggered by the ng-eNB-CU either directly or following a request received from the ng-eNB-DU. The ng-eNB-CU requests the ng-eNB-DU to remove the UE Context when the UE enters RRC_IDLE or RRC_INACTIVE.

This function can be also used to manage DRBs and SRBs, i.e., establishing, modifying and releasing DRB and SRB resources. The establishment and modification of DRB resources are triggered by the ng-eNB-CU and accepted/rejected by the ng-eNB-DU based on resource reservation information and QoS information to be provided to the ng-eNB-DU. For each DRB to be setup or modified, the S-NSSAI may be provided by ng-eNB-CU to the ng-eNB-DU in the UE Context Setup procedure and the UE Context Modification procedure.

For NG-RAN, the mapping between QoS flows and radio bearers is performed by ng-eNB-CU and the granularity of bearer related management over W1 is radio bearer level, and the ng-eNB-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 ng-eNB-DU, and the ng-eNB-DU either accepts the request or rejects it with appropriate cause value.

With this function, ng-eNB-DU could also notify ng-eNB-CU whether the QoS for already established DRBs is not fulfilled any longer or it is fulfilled again.

The UE Inactivity Notification function is initiated by the ng-eNB-DU to indicate the UE activity event.

The Notify function is to enable the ng-eNB-DU to inform the ng-eNB-CU that the QoS of an already established GBR DRB cannot by fulfilled any longer or that it can be fulfilled again.

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

With this function, ng-eNB-CU requests the ng-eNB-DU to setup or change of the SpCell for the UE, and the ng-eNB-DU either accepts or rejects the request with appropriate cause value.

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

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

5.2.5 RRC message transfer function

This function allows to transfer RRC messages between ng-eNB-CU and ng-eNB-DU. RRC messages are transferred over W1-C. The ng-eNB-CU is responsible for the encoding of the dedicated RRC message with assistance information provided by ng-eNB-DU. This function also allows ng-eNB-DU to report to ng-eNB-CU if the downlink RRC message has been successfully delivered to UE or not.

5.2.6 Warning messages information transfer function

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

5.3 W1-U functions

5.3.1 Transfer of user data

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

5.3.2 Flow control function

This function allows to control the downlink user data flow to the ng-eNB-DU. The detailed protocol is specified in TS 38.425 [2]