My 3GPP TS 38.401 Notes

3GPP TS 38.401 describes the overall architecture of the NG-RAN, including interfaces NG, Xn and F1 interfaces and their interaction with the radio interface.

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 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.

MBS Session context in a gNB-DU

The definition of an MBS Session context in a gNB-DU applicable for broadcast and multicast.

An MBS Session context in a gNB-DU

• is a block of information associated to an MBS Session, which may consist of one or several MRB Contexts;
• corresponds to either one or several F1-U tunnels.

MRB Context in a gNB-DU

An MRB Context is a block of information in a gNB-DU associated to one or several MRBs (MRB “instances”). The gNB-DU sets up resources for each MRB

• based on information provided within MBS Session Context related information as received by the gNB-DU (e.g. MRB QoS, MBS service area information, etc.), and,
• for multicast, based on the UE Contexts established for RRC_CONNECTED UEs within the gNB-DU containing joining information of the UE for the respective multicast session.
• for broadcast, the gNB-DU determines whether F1-U tunnels are setup per DU or per Area Session ID served by the DU.
• for multicast, the gNB-DU determines whether F1-U tunnels are setup per DU or per cell served by the DU or per Area Session ID served by the DU or for ptp restransmissions or for a ptp-only MRB leg.

For multicast, for each MRB, the gNB-DU provides the MRB specific Uu configuration to the gNB-CU to configure 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.

The UE-associated logical E1-connection uses the identities gNB-CU-CP UE E1AP ID and gNB-CU-UP UE E1AP ID.

When a node (AMF or gNB) receives a UE associated NGAP/XnAP/F1AP/E1AP message the node retrieves the associated UE based on the NGAP/XnAP/F1AP/E1AP ID.

UE-associated signalling

UE-associated signalling is an exchange of NGAP/XnAP/F1AP/E1AP messages associated with one UE over the UE-associated logical NG/Xn/F1/E1-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.

NOTE3: The general principle described in this clause also applies to ng-eNB and W1/E1 interface, if not explicitly specified otherwise.

MBS-associated logical F1/E1 -connection

F1AP and E1AP provide means to exchange control plane messages associated with an MBS session over the respective F1/E1 interface.

An MBS-associated logical connection is established during the first F1AP/E1AP message exchange between the F1/E1 peer nodes.

The connection is maintained as long as MBS associated F1AP/E1AP messages need to be exchanged over the F1/E1 interface.

The MBS-associated logical F1-connection uses the identities gNB-CU MBS F1AP ID and gNB-DU MBS F1AP ID.

The MBS-associated logical E1-connection uses the identities gNB-CU-CP MBS E1AP ID and gNB-CU-UP MBS E1AP ID.

When a node (DU or CU or CU-CP and CU-UP) receives an MBS associated F1AP/E1AP message the node retrieves the associated MBS session based on the F1AP/E1AP ID.

MBS-associated signalling

MBS-associated signalling is an exchange of F1AP/E1AP messages associated with one MBS session over the MBS-associated logical F1/E1-connection.

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