NG-RAN Overall Architecture and Interface Protocol Stack

NG-RAN Overall Architecture and Interface Protocol Stack

We have discussed various topics related to 5G so far in my previous articles related to 5G standards Evolution, 5G New Radio (5G NR) Key Technologies, 5G Network Migration Strategy etc. This article is different from previous articles, because I’m trying to explain more technical details based on 3GPP standards; basically for people who are interested to learn deep down the 5G architecture and so on.

Introduction

The main purpose of this article is to give the basic understanding about the overall architecture of next generation Radio Access Network for 5G technology and interface protocol stacks for main 5G network nodes interacting with the NG-RAN.

NG-RAN Overall Architecture

What is Next Generation Radio Access Network (NG-RAN)?

 

NG-RAN is a part of the 3GPP 5G system; responsible for providing Radio Access to 5G network. NG-RAN Node can be either a gNB or an NG-eNB according to the 3GPP TS 38.300 specifications.

gNB provides 5G NR access to the users; if technically explaining, provide NR control and user plane protocol termination toward user equipment side.

NG-eNB provides LTE Radio access to the users: similarly we can say as providing E-UTRA control and user plane protocol termination towards user equipment side.

What are the main interfaces related to NG-RAN?

I hope that you are familiar with the interface in EUTRAN/ LTE RAN /4G RAN. IN LTE, eNB connects to 4G core network through S1 interface and interconnection between eNBs happens through X1 interface. Similarly interface between gNB/ NG-eNB and 5G core network (5GC) is defined as NG interface and gNBs and NG-eNBs are connected each other through Xn interface.

What are the main 5GC nodes directly interacting with NG-RAN?

There are two main 5GC nodes defined in 3GPP interacting with NG-RAN; more specifically, 
Access and Mobility Management Function (AMF) is responsible for handling control plane traffic

User Plane Function (UPF) is responsible for handling user plane traffic. Therefore NG interface can be divided in to two parts NG-C to carry control plane traffic and NG-U to carry user plane traffic; connecting to AMF and UPF respectively.

NG-RAN architecture is illustrated in below figure according to the 3GPP TS 38.300.

Protocol Stacks of NG-RAN Interfaces

NG Interface

NG User Plane

 The NG user plane interface (NG-U) is defined between the NG-RAN node and the UPF. The transport network layer is built upon IP protocol stack and GTP-U, runs on UDP is used to carry the user plane PDUs between the NG-RAN nodes and the UPF. The user plane protocol stack of the NG interface is shown below.

NG Control Plane

The NG control plane interface (NG-C) is defined between the NG-RAN node and the AMF. The transport network layer is built upon IP protocol stack. Reliable transportation of signaling messages is ensured by the SCTP layer on top of IP; the SCTP layer provides guaranteed delivery of application layer messages and application layer is denoted as Next Generation Application Protocol (NG-AP). The control plane protocol stack of the NG interface is shown below.

The functions of NG-C as defined in 3GPP are as follow.

- NG interface management
- UE context management
- UE mobility management
- Transport of NAS messages
- Paging
- PDU Session Management

- Configuration Transfer

- Warning Message Transmission

Xn Interface

Xn User Plane

The Xn User plane (Xn-U) interface is defined between two NG-RAN nodes. The transport network layer is built upon IP protocol stack and GTP-U, runs on UDP is used to carry the user plane PDUs between two NG-RAN nodes. The user plane protocol stack of the Xn interface is illustrated below.

Xn Control Plane 

The Xn control plane interface (Xn-C) is defined between two NG-RAN nodes. The transport network layer is built upon IP protocol stack. Reliable transportation of signaling messages is ensured by the SCTP layer on top of IP; the SCTP layer provides guaranteed delivery of application layer messages and application layer is denoted as Xn Application Protocol (Xn-AP). The control plane protocol stack of the Xn interface is shown below.

The functions of Xn-C interface are defined as follows.

- Xn interface management
- UE mobility management, including context transfer and RAN paging
- Dual connectivity

Thanks for reading!!