1 .. This work is licensed under a Creative Commons Attribution 4.0 International License.
2 .. SPDX-License-Identifier: CC-BY-4.0
9 O-DU implements the functional blocks of L2 layer of a 5G NR protocol stack in SA(StandAlone) mode.
10 These layers primarily include NR MAC, NR Scheduler and NR RLC layers.
12 O-DU modules are developed as shown in the below diagram.
14 .. figure:: ODUArch.jpg
16 :alt: Figure 1 O-DU High Architecture Diagram
18 Figure 1 - O-DU High Architecture Diagram
20 O-DU High Thread Architecture
21 -------------------------------
23 As shown in Figure 1, there are multiple entities within O-DU High. Modules sharing a
24 given color belong to one thread. O-DU architecture can be defined at a thread
27 - Thread 1: O-DU thread
29 - Thread 2: DU APP inclusive of Config Handler, DU Manager, UE Manager, EGTP Handler and ASN.1 Codecs
31 - Thread 3: 5G NR RLC DL and MAC (inclusive of 5G NR SCH and Lower MAC)
33 - Thread 4: 5G NR RLC UL
35 - Thread 5: SCTP Handler
37 - Thread 6: Lower MAC Handler
41 --------------------------
45 This module configures and manages all the operations of O-DU.
46 It interfaces with external entities as follows:
48 - OAM: DU APP interacts with OAM on the O1 interface for configuration, alarms and performance management.
50 - O-CU: DU APP interacts with O-CU for RAN functionalities over the F1 interface which is built on SCTP. Control messages are exchanged on the F1-C interface and data messages on the F1-U interface.
52 - RIC: DU APP interacts with RIC on E2 interface over SCTP.
55 DU App submodules are as follows:
57 - Config Handler manages the configurations received on O1 interfaces and stores them within DU APP context.
59 - DU Manager handles all cell operations at the DU APP.
61 - UE Manager handles UE contexts at the DU APP.
63 - SCTP handler is responsible for establishing SCTP connections with O-CU, RIC on the F1AP and E2AP interfaces
66 - EGTP handler is responsible for establishing EGTP connection with O-CU for data message exchange on the F1-U
69 - ASN.1 Codecs contain ASN.1 encode/decode functions which are used for System information, F1AP and E2AP messages.
73 This module provides services for transferring the control and data messages
74 between MAC layer and O-CU (via DU App).
76 5G NR RLC UL and 5G NR RLC DL are the sub modules of this module that implement
77 uplink and downlink functionality respectively.
81 This module uses the services of the NR physical layer to send and receive data
82 on the various logical channels.
83 Functions of the 5G NR MAC module are as follows:
85 - 5G NR MAC is responsible for multiplexing and de-multiplexing of the data on various logical channels.
87 - 5G NR SCH schedules resources in UL and DL for cell and UE based procedures.
88 5G NR SCH is completely encapsulated within the 5G NR MAC i.e., all interactions of the 5G NR SCH is via the 5G NR MAC.
90 - Lower MAC interfaces between the MAC and the O-DU Low. It implements all the messages of FAPI
91 specification. It has a receiver thread to handle messages from L1.
94 O-DU Utility and Common Functions
95 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
96 These modules contain platform specific files and support O-DU High functionality and message exchanges.
100 ======================
103 This section describes the other modules that O-DU High interfaces with, as shown in below diagram.
105 .. figure:: O-DUHighInterfaces.jpg
107 :alt: O-DU High Interfaces
109 Figure 2 - O-DU High Interfaces
112 As shown in Figure 2, O-DU High interfaces with the following modules:
114 - O-CU: O-DU High communicates with O-CU on the F1AP interface. The control message exchanges are on F1-C while
115 data message exchanges are on F1-U interfaces. The below F1AP messages on F1-C are implemented, as per
116 3GPP 38.473-f60 v15.3:
118 - Interface Management
122 - gNB-DU Configuration Update
126 - UE Context Management
130 - RRC Message Transfer
132 - Initial UL RRC Message Transfer
134 - DL RRC Message Transfer
136 - UL RRC Message Transfer
138 - RRC Delivery Report
140 - Near RT RIC: O-DU High communicates with Near RT RIC on the E2 interface. The below E2AP messages are
141 implemented, as per ORAN WG3.E2AP v01.00:
147 - Near RT RIC Functional Procedures
153 - O-DU Low: O-DU High communicates with O-DU Low on the FAPI interface. The below FAPI messages are supported,
154 as per FAPI interface files shared by Intel:
156 - P5 messages - PHY mode control interface
158 - PARAM.request/PARAM.response
160 - CONFIG.request/CONFIG.response
168 - P7 messages - Main data path interface
188 - OAM: O-DU High communicates with OAM on the O1 interface.
192 O-DU High functionality
193 ========================
196 Cell Up and Broadcast Procedure
197 --------------------------------
199 This section describes the cell-up procedure within O-DU High.
201 .. figure:: CellUpAndBroadcast.png
203 :alt: Cell Up and Broadcast Procedure
205 Figure 3 - O-DU High Cell Up and Broadcast Procedure
208 As seen in the Figure 3,
209 - The DU APP module of O-DU High sends F1 Setup Request to O-CU. This message contains a list of cells that the O-DU High has been configured with.
211 - The O-CU responds with F1 Setup Response. This message contains a list of cells which must be activated.
213 - The O-DU High scans the list of cells received and sends corresponding cell configurations to 5G NR MAC.
215 - 5G NR MAC, in-turn configures the 5G NR SCH. It also configures the O-DU Low via the Lower MAC module.
217 - On receiving the cell config response, DU APP sends a gNB DU Config Update towards the O-CU. The O-CU responds with
218 gNB DU Config Update ACK towards the O-DU High.
220 - The DU APP now exchanges F1 Reset message with the O-CU to initialize the UE contexts.
222 - DU APP sends Cell Start Req towards 5G NR MAC. This message is translated by the Lower MAC into the FAPI message START.request towards the O-DU
225 - On receiving START.request, O-DU Low begins to send slot indications towards 5G NR MAC via the lower MAC.
226 The frequency of these slot indications is determined by the numerology(Mu) supported.
227 5G NR MAC forwards these slot indications to the 5G NR SCH and DU APP modules.
229 - When the first slot indication reaches the DU APP, cell is marked as up.
231 - The 5G NR SCH, keeps tracking the SSB and SIB1 ocassions on receiving regular slot indications.
232 On detecting the relevant ocassion, 5G NR SCH schedules SSB/SIB1 and forwards the DL Scheduling Information to 5G NR MAC.
234 - The 5G NR MAC mutiplexes the PDU and sends SSB/SIB1 packets towards the O-DU Low through the Lower MAC.
239 -----------------------
242 The O-DU High supports
244 - All physical channels - PBCH, PRACH, PDCCH, PDSCH, PUSCH, PUCCH
246 - All control logical channels - UL CCCH, DL CCCH, UL DCCH, DL DCCH
248 - All control transport channels - BCH, UL-SCH, DL-SCH, RACH
250 The above channels are used to achieve the below messages:
252 - Cell broadcast of System Information which includes SSB and SIB1.
258 - Random Access Response
264 - UE attach signalling flow
268 - Registraton Request
270 - Security Mode Command
272 - Security Mode Complete
276 - Registraton Complete
278 - Several NAS Message Exchanges
280 - RRC Reconfiguration
282 - RRC Reconfiguration Complete
284 Figure 4 below depicts the above call flow, inclusive of all interfaces:
286 .. figure:: UeAttach.png
288 :alt: O-DU High UE Attach Flow
290 Figure 4 - UE Attach Flow
294 OSC Testcases Supported
295 =========================
297 The O-DU High partially supports below use-cases: