E1AP Procedures

[[_TOC_]] # Introduction The E1 interface is between the gNB-CU-CP (Central Unit - Control Plane) and gNB-CU-UP (Central Unit - User Plane) nodes. This interface is governed by the E1 Application Protocol (E1AP) outlined in the 3GPP release 16 specifications, specifically in the documents: * 3GPP TS 38.463 - E1 Application Protocol (E1AP) * 3GPP TS 38.460 - E1 general aspects and principles * 3GPP TS 38.461 - E1 interface: layer 1 * 3GPP TS 38.462 - E1 interface: signaling transport The E1AP protocol consists of the following sets of functions: * E1 Interface Management functions * E1 Bearer Context Management functions * TEID allocation function ## E1 Bearer Context Management function This function handles the establishment, modification, and release of E1 bearer contexts. * E1 Bearer Context Establishment: initiation of E1 bearer context is by gNB-CU-CP and acceptance or rejection is determined by gNB-CU-UP based on admission control criteria (e.g., resource availability). * E1 Bearer Context Modification: can be initiated by either gNB-CU-CP or gNB-CU-UP, with the receiving node having the authority to accept or reject the modification. * Release of Bearer Context: can be triggered either directly by gNB-CU-CP or following a request from gNB-CU-UP. * QoS-Flow to DRB Mapping Configuration: responsible for setting up and modifying the QoS-flow to DRB mapping configuration. gNB-CU-CP decides the flow-to-DRB mapping, generates SDAP and PDCP configurations, and provides them to gNB-CU-UP. # OAI implementation For the E1AP design in OAI, please refer to the [E1 Design](./E1-design.md) document. ## E1 re-establishment The purpose of this procedure is to follow-up the re-establishment of RRC connection over the E1 interface. For all activated DRBs a Bearer Context Modification from CU-CP to CU-UP is necessary, according to clause 9.2.2.4 of 3GPP TS 38.463. If any modification to the bearer context is required, the CU-CP informs the CU-UP with the relevant IEs (e.g. in case of PDCP re-establishment, PDCP Configuration IE clause 9.3.1.38), Current implementation in OAI: ```mermaid sequenceDiagram participant e as UE participant d as DU participant c as CUCP participant u as CUUP Note over e,c: RRCReestablishment Procedure e->>d: RRCReestablishmentRequest Note over d: initial_ul_rrc_message_transfer_f1ap d->>+c: Initial UL RRC Message Transfer (CCCH, new C-RNTI) Note over c: rrc_gNB_process_initial_ul_rrc_message Note over c: rrc_handle_RRCReestablishmentRequests c-->>+d: DL RRC Message Transfer note right of d: fallback to RRC establishment d-->>-e: RRCSetup e-->>d: RRCSetupComplete Note over c: rrc_gNB_generate_RRCReestablishment Note over c: cuup_notify_reestablishment Note over c: e1apCUCP_send_BEARER_CONTEXT_MODIFICATION_REQUEST c->>u: BEARER CONTEXT MODIFICATION REQUEST Note over u: e1apCUUP_handle_BEARER_CONTEXT_MODIFICATION_REQUEST Note over u: e1_bearer_context_modif Note over u: nr_pdcp_reestablishment c->>-d: DL RRC Message Transfer (old gNB DU F1AP UE ID) d->>e: RRCReestablishment Note over d: Fetch UE Context with old gNB DU F1AP UE ID and update C-RNTI e->>d: RRCReestablishmentComplete u->>c: BEARER CONTEXT MODIFICATION RESPONSE Note over c: e1apCUCP_handle_BEARER_CONTEXT_MODIFICATION_RESPONSE ``` ## PDU Session Release ```mermaid sequenceDiagram participant AMF participant CUCP participant CUUP participant DU participant UE AMF->>CUCP: NG PDU SESSION RESOURCE RELEASE COMMAND Note over CUCP: ngap_gNB_handle_pdusession_release_command CUCP->>CUCP: rrc_gNB_process_NGAP_PDUSESSION_RELEASE_COMMAND Note over CUCP: set status PDU_SESSION_STATUS_TORELEASE CUCP->>CUUP: E1 Bearer Context Modification Request Note over CUUP: release_gtpu_tunnel (GTP tunnel, PDCP, SDAP) CUUP->>CUCP: E1 Bearer Context Modification Response Note over CUCP: rrc_gNB_send_f1_drb_release_request CUCP->>DU: F1 UE Context Modification Request Note over DU: handle_ue_context_drbs_release (release in MAC/RLC) DU->>CUCP: F1 UE Context Modification Response Note over CUCP: rrc_CU_process_ue_context_modification_response Note over CUCP: replace existing CellGroupConfig Note over CUCP: rrc_gNB_generate_dedicatedRRCReconfiguration CUCP->>UE: RRCReconfiguration (DRB release list, NAS PDU) UE->>CUCP: RRCReconfigurationComplete Note over CUCP: handle_rrcReconfigurationComplete Note over CUCP: rrc_gNB_send_NGAP_PDUSESSION_RELEASE_RESPONSE CUCP->>AMF: NG PDU SESSION RESOURCE RELEASE RESPONSE Note over CUCP: rm_drbs_by_pdusession (from stored RRC list) Note over CUCP: rm_pduSession (from stored RRC list) ```