What is E2 Interface in Open RAN ?

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Introduction

The E2 interface is an open interface between two end points, i.e., the near-RT RIC and the so-called E2 nodes, i.e., DUs, CUs in 5G and O-RAN compliant eNBs in 4G.  E2 allows the RIC to control procedures and functionalities of the E2 nodes.

E2 nodes interface also enables the collection of metrics from the RAN to the near-RT RIC, either periodically or after pre-defined trigger events. Both control and data collection procedures can connect to one or more cells, slices, QoS classes, or specific UEs.

To support the above operations, the O-RAN  uses a variety of unique identifiers, these identifiers based on 3GPP specifications for the gNB, slice, and QoS class. Regarding specific UEs, the O-RAN introduced a common user identifier (i.e., the UE-ID) across its specifications. This provides a consistent and uniform user identity across the system without exposing sensitive information related to the user.

E2 Interface Characteristics

  • E2 interface is an open interface between near-RT RIC and  DUs, CUs in 5G
  • It runs on top of the SCTP protocol over IP
  • E2 interface supports two protocols i.e. E2 Application Protocol (E2AP) and E2 Service Model (E2SM)
  • E2AP messages can embed different E2 Service Models, which implement functionalities related to RAN metrics and RAN Control
  • E2 Service Model includes  E2 Report, E2 Insert, E2 Control and E2 Policy
  • E2 service model can be used for three applications Key Performance Matrix (KPM), Network Interfaces (NIs) and RAN Control (RC)

E2 Interface Protocols

The E2 being a logical interface organized and composed of in following two protocols:

  • E2 Application Protocol (E2AP)
  • E2 Service Model (E2SM)

The E2AP  is a basic procedural protocol that coordinates how the near-RT RIC and the E2 nodes communicate with each other, and provides a basic set of services. E2AP messages can embed different E2SMs, which implement specific functionalities (i.e., the reporting of RAN metrics or the control of RAN parameters).

The E2 interface runs on top of the SCTP protocol. Each E2 node exposes a number of RAN functions, i.e., the services or capabilities it supports. For example, DUs from different vendors may expose different control knobs depending on which parameters and functionalities can be tuned, as well as their capability in collecting and reporting different performance metrics.

By using publish-subscribe mechanics, E2 nodes can publish their data and the xApps on the near-RT RIC can subscribe to one or more of these RAN functions through the E2 interface. This makes it possible to clearly separate the capabilities of each node and to define how the xApps interact with the RAN.

At the lowest level, the E2AP handles interface management procedure like E2 setup, E2 reset, E2 Indication and E2 reporting of errors and near-RT RIC service updates i.e. the exchange of the list of the RAN functions supported by the E2 node.

E2AP Setup Procedure

The E2 setup procedure is shown in below figure. At first, the SCTP connection is established between the near RT RIC and the E2 node which is aware of the IP address and port of the E2 termination of the near-RT RIC.

Then, the E2 node transmits an E2 setup request, in which it lists the RAN functions and configuration it supports, together with the identifiers for the node. The near-RT RIC processes this information and replies with an E2 setup response. After the connection is established, the E2AP provides four services which can be combined in different ways to implement an E2 Service Model.

E2 Service Model (E2SM) Overview

After the E2 connection is established, the E2AP provides following four services which can be combined in different ways to implement an E2 Service Model – E2SM. These procedures are then combined to create a service model. The service model message is inserted as payload in one of the E2AP messages, as shown in Figure below. The actual content is encoded using ASN.1 notation.

  • E2 Report:  The reporting procedure involves E2 RIC Indication messages that contain data and telemetry from an E2 node. The E2 report service is activated upon subscription from an xApp to a function offered by the E2 node. During the subscription negotiation, the xApp in the near-RT RIC can specify trigger events or the periodicity with which the E2 node should send report messages. Based on this periodicity, a timer is set in the E2 node and a report is sent whenever the timer expires. The RIC Indication message is of type report.
  • E2 Insert:  Similarly, the insert procedure involves messages sent from an E2 node to an xApp in the near-RT RIC to notify the xApp about a specific event in the E2 node (e.g., a UE signaling the possibility to perform a handover). It is activated upon subscription from an xApp and involves a RIC Indication message (of type insert). In this case, the trigger is associated to a radio resource management procedure which is suspended when the insert message is sent. A wait timer is also started, and, if the RIC does not reply before the timer expires, the procedure in the E2 node can be resumed or definitely halted.
  • E2 Control: The control procedure can be autonomously initiated by the RIC, or it can be the consequence of
    the reception of an insert message at the near-RT RIC. This procedure uses two messages, a RIC Control Request from the RIC to the E2 node, and a RIC Control Acknowledge in the opposite direction. The control procedures can influence parameters exposed by the RAN functions of the E2 node.
  • E2 Policy: This procedure involves a subscription message which specifies an event trigger and a policy that the E2 node should autonomously follow to perform radio resource management.

E2 Service Models

At the time of this writing, the O-RAN Alliance WG3 has standardized three service models:

  • E2SM Key Performance Matrix (KPM)
  • E2SM Network Interfaces (NI)
  • E2SM RAN Control (RC)

E2SM KPM reports performance metrics from the RAN, using E2 report messages. The procedures associated to the KPM service model are shown in Figure. During the E2 setup procedures, the E2 node advertises the metrics it can expose. An xApp in the near-RT RIC can then send a subscription message specifying which KPMs are of interest, and whether the reporting is periodic or trigger-based. Finally, the E2 node uses E2 Indication messages of type report to stream the selected KPMs. Different KPM messages are generated from different E2 nodes, i.e., the specifications defines performance metrics containers with different fields to be populated for DU, CU-UP, and CU-CP. Recent KPM specifications has introduced cell-specific and user-specific performance containers, which are based on the metrics defined in 3GPP technical specifications for LTE and 5G NR.

The E2SM NI  is used to take the messages received by the E2 node on specific network interfaces and forward them to the near-RT RIC domain via E2 report messages over the E2 interface. The E2 node advertises which interfaces it supports during the subscription procedure, and they include X2 (which connects LTE eNBs), Xn (which connects different NR gNBs), and F1, which connects DUs and CUs).

E2SM RAN Control (RC), introduced in July 2021, implements control functionalities through E2 control procedures. One of the goals of the xApps is to optimize the radio resource management in the E2 nodes. this is achieved through the recently-standardized RAN Control (RC) service model

References:

  • O-RAN near-real-time RAN intelligent controller E2 service model (E2SM), RAN function network interface (NI) 1.0, ORANWG3.E2SM-NI-v01.00.00 Technical Specification,
  • O-RAN near-real-time RAN intelligent controller E2 service model, ran control 1.0, ORAN-WG3.E2SM-RC-v01.00 Technical Specification
  • O-RAN Working Group 2, “O-RAN Non-RT RIC Architecture 1.0,” O-RAN.WG2.Non-RT-RIC-ARCH-TS-v01.00 Technical Specification
  • 3GPP TS 28.552 – 5G performance measurements,” Technical Specification
  • 3GPP TS 32.425 Performance measurements Evolved Universal Terrestrial Radio Access Network (E-UTRAN)

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