5G NR: Architecture

Overall Architecture of NR RAN (Radio Access Network) would not look much different from LTE RAN  Architecture. However getting into details, you would start seeing some differences as well. You see the different name of each node and interface. MME/S-GW in LTE is replaced by AMF/UPF in NR and  X2/S1 in LTE are replaced by Xn/NG-C/U in NR. 

One of the most outstanding one would be that the gNB internal structure is split into two parts called CU (Central Unit) and DU (Distributed Unit) as shown below and these two entities are connected by a new an interface called F1 (For the details of F1 Interface, refer to 38.473).

Central Unit (CU): 
It is a logical node that includes the gNB functions like Transfer of user data, Mobility control, Radio access network sharing, Positioning, Session Management, etc. except those functions allocated exclusively to the DU. CU controls the operation of DUs over front-haul (Fs) interface. A central unit (CU) may also be known as BBU/REC/RCC/C-RAN/V-RAN

Distributed Unit (DU): This logical node includes a subset of the gNB functions, depending on the functional split option. Its operation is controlled by the CU. Distributed Unit (DU) also known with other names like RRH/RRU/RE/RU.

This page on 5G NR network interfaces describes various 5G interfaces used in 5G architecture. It includes Xn interface, NG interface, E1 interface, F1 interface and F2 interface used in 5G NR (New Radio) network architecture. It covers functions and locations of these 5G NR interfaces used between 5G RAN and 5GC.

5G NR overall architecture is shown in the following figure-2. This is as defined in the 3GPP TS 38.300 specification. The 5G NR network composed of NG RAN (Next Generation Radio Access Network) and 5GC (5G Core Network). As shown, NG-RAN composed of gNBs (i.e. 5G Base stations) and ng-eNBs (i.e. LTE base stations).

• Xn interface exists between these base stations viz. between gNB-gNB, between (gNB)-(ng-eNB) and between (ng-eNB)-(ng-eNB). Xn is the network interface between NG-RAN nodes. Xn-U stands for Xn User Plane interface and Xn-C stands for Xn Control Plane interface.
• NG interface exists between 5GC and these base stations (i.e. gNB & ng-eNB).

5G NR Xn Interface

• Location: Xn interface lies between NG-RAN Nodes viz. gNB & ng-eNB.
• Control Plane Functions are as follows:
-interface management and error handling (e.g. setup, reset, removal, configuration update)
-connected mode mobility management (handover procedures, sequence number status transfer, UE context retrieval)
-support of RAN paging
-dual connectivity functions (secondary node addition, reconfiguration, modification, release, etc.)
• User Plane Functions are as follows:
-Data Forwarding
-Flow Control
• References: TS 38.420 to TS 38.424

5G NR NG Interface

• Location: Between 5G RAN and 5G Core Network. Both control plane and user plane lies between them and hence there are two interfaces under NG interface viz. NG-C and NG-U. It is similar to LTE interfaces viz. S1-C and S1-U respectively.
• Functions/Objectives:
-NG interface supports the exchange of signalling information between NG-RAN and 5GC.
-It defines inter connection of NG-RAN nodes with AMFs supplied by different manufacturers.
-It specifies the separation of NG interface Radio Network functionality and Transport Network functionality to facilitate introduction of future technology.
• Capabilities:
-procedures to establish, maintain and release NG-RAN part of PDU sessions
-procedures to perform intra-RAT handover and inter-RAT handover
-the separation of each UE on the protocol level for user specific signalling management
-the transfer of NAS signalling messages between UE and AMF
-mechanisms for resource reservation for packet data streams
• References: From TS 38.410 to TS 38.414

5G NR E1 Interface

• Location: From logical perspective, E1 interface is point-to-point interface between a gNB-CU-CP and a gNB-CU-UP as shown in fig-2.
• Functions:
-E1 interface supports the exchange of signalling information between the endpoints.
-It separates Radio Network Layer and Transport Network Layer.
-It enables exchange of UE associated information and non-UE associated information.
• References: From TS 38.460 to TS 38.463

5G NR F1 Interface

• Location: Between gNB-CU and gNB-DU. It is also separated into F1-C and F1-U based on control plane and user plane functionalities.
• Functions:
-F1 interface defines inter-connection of a gNB-CU and a gNB-DU supplied by different manufacturers.
-It supports control plane and user plane separation.
-It separates Radio Network Layer and Transport Network Layer.
-F1 interface enables exchange of UE associated information and non-UE associated information.
• References: From TS 38.470 to TS 38.475

5G NR F2 Interface

This interface lies between lower and upper parts of the 5G NR physical layer. It is also separated into F2-C and F2-U based on control plane and user plane functionalities.

Following are the interfaces and nodes as shown in the figure-1 and figure-2.
• NG-C: control plane interface between NG-RAN and 5GC..

• NG-U: user plane interface between NG-RAN and 5GC.

• gNB: node providing NR user plane and control plane protocol terminations towards the UE, and connected via the NG interface to the 5GC. The 5G NR (New Radio) gNB is connected to AMF (Access and Mobility Management Function) and UPF (User Plane Function) in 5GC (5G Core Network). The protocol layers are mapped into three units viz. RRH (Remote Radio Head), DU (Distributed Unit) and CU (Central Unit) as shown in the figure-2.

• ng-eNB: node providing E-UTRA user plane and control plane protocol terminations towards the UE and connected via the NG interface to the 5GC.

While the NGAP is for control plane message exchange, the GTP-U protocol is used for user data delivery between RAN and User Plane Function (UPF).

Access and Mobility Management Function (AMF) AMF including the termination of RAN CP interface (N2) and of the NAS interface (N1

• NAS Signalling Termination

•NAS Signalling Security

• AS Security Control

•Inter CN Node Signalling For Mobility Between 3GPP Access Networks

•Idle Mode UE Reachability (Including Control And Execution Of Paging Retransmission)

•Registration Area Management

•Support Of Intra-system And Inter-system Mobility

•Access Authentication

•Access Authorization Including Check Of Roaming Rights

•Mobility Management Control (Subscription And Policies)

•Support Of Network Slicing

•SMF Selection

Session Management Function (SMF) The SMF is connected to the UPF via the N4 interface

UE IP address allocation and management

Selection and control of UP function

Configures traffic steering at UPF to route traffic to proper destination

Control part of policy enforcement and QoS.

Downlink Data Notification

User Plane Function (UPF), UPF is connected to gNB via N3 interface

1.Anchor point for intra‐and inter‐RAT mobility.

2.External PDU session point of interconnection

3.Packet routing and forwarding

4.UP QoS handling

5.Packet inspection and policy rule enforcement

6.Lawful interception

7.Traffic accounting, and reporting.

Authentication Server Function (AUSF) (AUSF) supports authentication for 3GPP access and un-trusted non-3GPP access (e.g. toWLAN).

Providing authentication

Authorization functionalities

Policy Control Function (PCF)

Ensures policy and charging control for service data flows

Provides authorized Quality of Service [QoS] for a SDF)

(PDU) Session related policy control and event reporting.

Unified Data Management (UDM)

Supporting The Authentication Credential Repository

Processing Function

Storing The Long‐term Security Credentials And Subscription Information

Access Authorization, Storage,

Management Of Privacy-protected Subscription Identifiers

Application Function (AF),

Representing any additional CP function which might be required by specific network slices, and which is potentially provided by third parties

Application Function (AF) interacts with the 3GPP Core Network in order to provide additional network services

Radio Access Network

•Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection

•Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling)

•IP header compression, encryption and integrity protection of data

•Selection of an AMF at UE attachment when no routing to an AMF can be determined from the information provided by the UE

•Routing of User Plane data towards UPF(s)

• Routing of Control Plane information towards AMF

•Connection setup and release

•Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection

•Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling)

•- IP header compression, encryption and integrity protection of data

•- Selection of an AMF at UE attachment when no routing to an AMF can be determined from the information

•provided by the UE

•Routing of User Plane data towards UPF(s)

•Routing of Control Plane information towards AMF

Connection setup and release