5a1af1a7db
Also, move PDU Session Release diagram to new PDU Session Management section in rrc-dev.md. Changes: PDU Session Modify: - Update PDU session modification sequence diagram - Add DRB-To-Remove/To-Modify/To-Setup list handling - Document E1AP Bearer Context Modification flow - Add RRC reconfiguration trigger after E1AP response QoS Flows Handling: - Complete overview with 3GPP standards references (TS 23.501, 37.324, 38.463, 29.281, 38.331) - Comprehensive Mermaid sequence diagram covering control and data plane Technical details: - Multiple QoS flows per DRB supported - QFI to DRB mapping at RRC and SDAP layer - F1-U tunnels: 1 per DRB, no QFI marking - N3 tunnels: 1 per PDU session, with QFI marking Signed-off-by: Guido Casati <guido.casati@openairinterface.org>
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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 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:
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