# O-RAN E2AP
###### tags: `ORAN`


## :notebook_with_decorative_cover: Table Of Contents
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- [x] General Explanation
- [x] Elementary Procedures
- [x] Near-RT RIC Functional Procedures
- [x] Global Procedures
- [x] References
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## :notebook_with_decorative_cover: Main Page

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### <center>General Explanation</center>
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<center>Fig.1. E2AP Connection Diagram [1] </center>
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<center>Fig.2. E2AP Layer Position [2] </center>
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E2 Application Protocol ~~or~~(E2AP) is a general protocol by which Near RT-RIC and disaggregated E2 Nodes communicate. E2 Nodes are O-DU, O-CU-CP, and O-CU-UP.
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### <center>Elementary Procedures</center>
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E2AP elementary procedure can be grouped into two class. **Class 1 means that RAN will response. Class 2 Means that RAN will not response**. Elementary procedures for E2AP could be seen in the figures below.
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**<i class="fa fa-book fa-fw"></i> Class 1 Elementary Procedures**
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<center>Fig.3. E2AP Class 1 Elementary Procedures [3]</center>
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**<i class="fa fa-book fa-fw"></i> Class 2 Elementary Procedures**
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<center>Fig.4. E2AP Class 2 Elementary Procedures [3]</center>
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**<i class="fa fa-book fa-fw"></i> RIC Services and E2AP Procedures Relationship**
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<center>Fig.5. Relationship Between RIC Services and E2AP Procedures [4]</center>
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### <i class="fa fa-book fa-fw"></i> Near-RT RIC Functional Procedures
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**<i class="fa fa-book fa-fw"></i> RIC Subscription Procedure**
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This is used to establish E2 subscriptions on E2 Node consisting of an event trigger and a sequence of actions, each with a corresponding subsequent action.
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**Successful Case**
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The Near-RT RIC initiates the procedure by sending the RIC SUBSCRIPTION REQUEST message containing a unique RIC Request ID to the target E2 Node. If the requested trigger and at least one required action are accepted by the target E2 Node, the target E2 Node shall reserve necessary resources and send the RIC SUBSCRIPTION RESPONSE message back to the Near-RT RIC.
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<center>Fig.6. Near-RT RIC Subscription Procedure Successful Operation [3]</center>
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<center>Fig.7. Subscription Procedure From xApp To E2 Node [5]</center>
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<center>Fig.8. RIC Subscription Request Message [3]</center>
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<center>Fig.9. RIC Subscription Response Message [3]</center>
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**Unsuccessful Case**
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If the target E2 Node does not admit at least one requested action, or detects an inconsistency in the sequence of actions or in the subsequent action definitions, or a failure occurs during the RIC Subscription procedure, the target E2 Node shall send the RIC SUBSCRIPTION FAILURE message to the Near-RT RIC
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> [name=jonathanrichard]+ Submgr will start the Subscription Delete Procedure to the E2 Termination in case of Subscription Failure. Here is the snippet of the code: 


<center>Fig.10. Near-RT RIC Subscription Procedure Unsuccessful Operation [3]</center>
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<center>Fig.11. RIC Subscription Request Message [3]</center>
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<center>Fig.12. RIC Subscription Failure Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> RIC Subscription Delete Procedure**
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This procedure is used to delete E2 subscriptions on E2 Node.
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**Successful Case**
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The Near-RT RIC initiates the procedure by sending the RIC SUBSCRIPTION DELETE REQUEST message to the target E2 Node. The target E2 Node shall release necessary resources and send the RIC SUBSCRIPTION DELETE RESPONSE message back to the Near-RT RIC.
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<center>Fig.13. Near-RT RIC Subscription Delete Procedure Successful Operation [3]</center>
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<center>Fig.14. RIC Subscription Delete Request Message [3]</center>
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<center>Fig.15. RIC Subscription Delete Response Message [3]</center>
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**Unsuccessful Case**
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If the target E2 Node does not admit at least one requested action, or detects an inconsistency in the sequence of actions or in the subsequent action definitions, or a failure occurs during the RIC Subscription procedure, the target E2 Node shall send the RIC SUBSCRIPTION FAILURE message to the Near-RT RIC
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<center>Fig.16. Near-RT RIC Subscription Delete Procedure Unsuccessful Operation [3]</center>
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<center>Fig.17. RIC Subscription Delete Request Message [3]</center>
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<center>Fig.18. RIC Subscription Delete Failure Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> RIC Indication Procedure**
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The purpose of the RIC Indication procedure is to transfer a message associated with a Report and/or Insert RIC Service to the Near-RT RIC corresponding to a previously successful RIC Subscription procedure and the corresponding detection of the Event Trigger.
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**Successful Case**
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An E2 Node initiates the procedure by sending RIC INDICATION message containing the associated RIC Request ID IE, RAN Function ID IE, RIC Action ID IE, optionally sequence number RIC Indication SN IE, RIC Indication Type IE, RIC Indication Header IE, RIC Indication Message IE and optionally a RIC Call Process ID IE to the Near-RT RIC.
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<center>Fig.19. Near-RT RIC Indication Procedure Successful Operation [3]</center>
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<center>Fig.20. RIC Indication Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> RIC Control Procedure**
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The purpose of the RIC Control procedure is to initiate or resume a specific functionality in the E2 Node.
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**Successful Case**
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The Near-RT RIC initiates the procedure by sending the RIC CONTROL REQUEST message containing the associated RIC Request ID IE, RAN Function ID IE, optionally RIC Call Process ID IE, RIC Control Header IE, RIC Control Message IE and optionally RIC Control Ack Request IE to the E2 Node. If the RIC CONTROL REQUEST message contains the RIC Control Ack Request IE set to “Ack” and the E2 Node has successfully processed the requested RIC Control procedure action, then the E2 Node shall respond with the RIC CONTROL ACKNOWLEDGE message.
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<center>Fig.21. Near-RT RIC Control Procedure Successful Operation [3]</center>
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<center>Fig.22. RIC Control Request Message [3]</center>
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<center>Fig.23. RIC Control Acknowledge Message [3]</center>
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**Unsuccessful Case**
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If the RIC CONTROL REQUEST message contains an optional RIC Call Process ID IE that is invalid or refers to an expired Call Process, then the E2 Node shall respond with a RIC CONTROL FAILURE message with an appropriate cause value.
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<center>Fig.24. Near-RT RIC Control Procedure Unsuccessful Operation [3]</center>
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<center>Fig.25. RIC Control Request Message [3]</center>
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<center>Fig.26. RIC Control Failure Message [3]</center>
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### <i class="fa fa-book fa-fw"></i> Global Procedures
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**<i class="fa fa-book fa-fw"></i> E2 Setup Procedures**
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The purpose of the E2 Setup procedure is to establish the signaling connection between E2 Node and Near-RT RIC. This procedure shall be initiated by the E2 Node.
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**Successful Case**
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An E2 Node initiates the procedure by sending the E2 SETUP REQUEST message including the appropriate data to a Near-RT RIC. The Near-RT RIC replies with the E2 SETUP RESPONSE message including the appropriate data.
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<center>Fig.27. E2 Setup Procedure Successful Operation [3]</center>
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<center>Fig.28. E2 Setup Request Message [3]</center>
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<center>Fig.29. E2 Setup Response Message [3]</center>
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**Unsuccessful Case**
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If the Near-RT RIC cannot accept the setup it shall respond with an E2 SETUP FAILURE message with an appropriate cause value.
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<center>Fig.30. E2 Setup Procedure Unsuccessful Operation [3]</center>
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<center>Fig.31. E2 Setup Request Message [3]</center>
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<center>Fig.32. E2 Setup Failure Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> Reset Procedures**
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The purpose of the Reset procedure is to align the resources in E2 Node and Near-RT RIC in the event of an abnormal failure. The procedure resets the E2 interface.
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**Successful Case**
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This procedure may be initiated by either Near-RT RIC or E2 Node.
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<center>Fig.33. Reset Procedure Successful Operation (Iniatiated By E2 Node) [3]</center>
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<center>Fig.34. Reset Procedure Successful Operation (Iniatiated By Near-RT RIC) [3]</center>
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<center>Fig.35. Reset Request Message [3]</center>
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<center>Fig.36. Reset Response Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> Error Indication**
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The Error Indication procedure is initiated by either the E2 Node or the Near-RT RIC to report detected errors in one incoming message, provided they cannot be reported by an appropriate failure message.
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**Successful Case**
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This procedure may be initiated by either Near-RT RIC or E2 Node.
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<center>Fig.37. Error Indication Successful Operation (Iniatiated By E2 Node) [3]</center>
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<center>Fig.38. Error Indication Successful Operation (Iniatiated By Near-RT RIC) [3]</center>
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<center>Fig.39. Error Indication Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> RIC Service Update**
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The purpose of the RIC Service Update procedure is to update application level configuration data needed for E2 Node and Near-RT RIC to interoperate correctly over the E2 interface.
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**Successful Case**
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An E2 Node initiates the procedure by sending a RIC SERVICE UPDATE message to the Near-RT RIC. Such message shall include an appropriate set of up-to-date Near-RT RIC service-related configuration data, including, but not limited to, the complete lists of added, modified and deleted supported Near-RT RIC Service functions that E2 Node has just taken into operational use along with a revision counter for each item in each list. After successful update of requested information, Near-RT RIC shall reply with the RIC SERVICE UPDATE ACKNOWLEDGE message to inform the initiating E2 Node that the requested update of application data was performed successfully
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<center>Fig.40. RIC Service Update Successful Operation [3]</center>
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<center>Fig.41. RIC Service Update Message [3]</center>
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<center>Fig.42. RIC Service Update Acknowledge Message [3]</center>
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**Unsuccessful Case**
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If the Near-RT RIC cannot accept the update it shall respond with a RIC SERVICE UPDATE FAILURE message and appropriate cause value.
If the RIC SERVICE UPDATE FAILURE message includes the Time To Wait IE, the E2 Node shall wait at least for the indicated time before reinitiating the RIC Service Update procedure towards the same Near-RT RIC. Both nodes shall continue to operate the E2 with their existing Near-RT RIC Service data.
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<center>Fig.43. RIC Service Update Unsuccessful Operation [3]</center>
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<center>Fig.44. RIC Service Update Message [3]</center>
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<center>Fig.45. RIC Service Update Acknowledge Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> E2 Node Configuration Update Procedure**
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The purpose of the E2 Node Configuration Update procedure is to update application level E2 Node configuration data needed for E2 Node and Near-RT RIC to interoperate correctly over the E2 interface.
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**Successful Case**
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An E2 Node initiates the procedure by sending a E2 NODE CONFIGURATION UPDATE message to the Near-RT RIC. After successful update of requested information, Near-RT RIC shall reply with the E2 NODE CONFIGURATION UPDATE ACKNOWLEDGE message to inform the initiating E2 Node that the requested update of application data was performed successfully.
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<center>Fig.46. E2 Node Configuration Update Successful Operation [3]</center>
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<center>Fig.47. E2 Node Configuration Update Message [3]</center>
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<center>Fig.48. E2 Node Configuration Update Acknowledge Message [3]</center>
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**Unsuccessful Case**
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If Near-RT RIC cannot accept the E2 NODE CONFIGURATION UPDATE message it shall respond with the E2 NODE CONFIGURATION UPDATE FAILURE message and appropriate cause value.
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<center>Fig.49. E2 Node Configuration Update Unsuccessful Operation [3]</center>
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<center>Fig.50. E2 Node Configuration Update Message [3]</center>
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<center>Fig.51. E2 Node Configuration Update Failure Message [3]</center>
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**<i class="fa fa-book fa-fw"></i> E2 Connection Update Procedure**
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The purpose of the E2 Connection Update procedure is to update the E2 interface connection between the E2 Node and Near-RT RIC.
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**Successful Case**
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The Near-RT RIC initiates the procedure by sending a E2 CONNECTION UPDATE message to the E2 Node. Such message shall include an appropriate set of up-to-date E2 interface connection data that the E2 Node shall take into account when modifying the E2 interface connection. After successful update of E2 interface connection(s), the E2 Node shall reply with the E2 CONNECTION UPDATE ACKNOWLEDGE message to inform the initiating Near-RT RIC that the requested E2 connection update was performed successfully.
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<center>Fig.52. E2 Connection Update Procedure Successful Operation [3]</center>
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<center>Fig.53. E2 Connection Update Message [3]</center>
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<center>Fig.54. E2 Connection Update Acknowledge Message [3]</center>
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**Unsuccessful Case**
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If the E2 Node cannot accept the update, it shall respond with a E2 CONNECTION UPDATE FAILURE message and appropriate cause value.
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<center>Fig.55. E2 Connection Update Procedure Unsuccessful Operation [3]</center>
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<center>Fig.56. E2 Connection Update Acknowledge Message [3]</center>
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<center>Fig.57. E2 Connection Update Failure Message [3]</center>
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### <center>References</center>
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[1]. [SD-RAN Techinar](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiTlt3Ki7_xAhXNDaYKHSm3AbIQFnoECAYQAw&url=https%3A%2F%2Fopennetworking.org%2Fwp-content%2Fuploads%2F2021%2F02%2F2021-02-09-SD-RAN-Techinar.pptx.pdf&usg=AOvVaw0TU6FaVPVuDXtl0nX7iOjS)
[2]. [Radisys ONF SD-RAN](https://www.google.com/url?sa=i&url=https%3A%2F%2Fopennetworking.org%2Fwp-content%2Fuploads%2F2020%2F09%2FPrakash-Siva-Final-Slide-Deck-1.pdf&psig=AOvVaw06IqlHkqcU_hWq8Az3Gght&ust=1625136520063000&source=images&cd=vfe&ved=2ahUKEwixlZi5l7_xAhWEDN4KHRaxB68Qr4kDegQIARBh)
[3]. [O-RAN.WG3.E2AP-v01.01](https://drive.google.com/file/d/1MvrXtRW4wMlB44VmXKQQPvJYwSv-hIYc/view?usp=sharing)
[4]. [O-RAN.WG3.E2GAP-v01.01](https://drive.google.com/file/d/126poL8qUq-8kcQtzzhpSvtg99CO6cEb3/view?usp=sharing)
[5]. [Subscription Manager Documentation](https://docs.o-ran-sc.org/projects/o-ran-sc-ric-plt-submgr/en/latest/user-guide.html)
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