What does a 5G network look like? A simple article to understand

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Whether it is 2G, 3G, 4G or 5G, the mobile communication network is mainly composed of two subsystems: the radio access network (RAN) and the mobile core network.

RAN is responsible for managing the wireless part to efficiently utilize spectrum resources and meet users' service quality requirements. The key component of RAN is called base station. The base station in the 2G era is called BTS, 3G is called NodeB, 4G is called eNB, and 5G is called gNB.

The mobile core network builds a bridge between the RAN and the Internet. Its main functions include:

• Provides Internet connectivity for data and voice services

• Ensure QoS requirements for connections

• Manage user mobility to ensure uninterrupted service

• Billing

Core Network

Before understanding the core network, let's first understand two concepts: control plane and user plane.

When you pick up your mobile phone to surf the Internet, the base station will establish a wireless channel connection for your mobile phone. This process is mainly divided into two parts: control plane connection and user plane connection. The control plane carries signaling or control messages. The user plane, also called the data plane or forwarding plane, carries data traffic.

First, the base station establishes a "control plane" connection between the mobile phone and the control plane of the core network and forwards signaling messages between the two. These messages include mobile phone authentication, registration, mobility management, etc.

The base station then establishes a “user plane” connection between the mobile phone and the user plane of the core network to forward user data traffic.

The 5G network architecture is divided into NSA and SA. In the NSA architecture, the 5G base station is still connected to the 4G core network; in the SA architecture, the 5G base station is independently connected to the 5G core network. Therefore, let's first look at the 4G core network architecture.

The 4G core network is called EPC, which is mainly composed of MME, HSS, PCRF, SGW and PGW. Among them, MME, HSS and PCRF are control planes, and SGW and PGW contain user planes.

• MME (Mobility Management Entity): responsible for tracking and managing the mobility of terminals in the RAN.
• HSS (Home Subscriber Server): A database that stores information related to user subscriptions.
• PCRF (Policy and Charging Rules): responsible for policy control decisions and traffic charging.
• SGW (Service Gateway): Mainly responsible for forwarding IP data packets between RAN.
• PGW (Packet Gateway): Essentially an IP router that connects the core network to the external Internet.

The 4G core network can be flexibly deployed according to the geographical location. For example, MME and PGW can be deployed in central cities, and SGW can be deployed in small cities.

However, the 5G core network is more flexible. The 5G core network is called NG-Core, and its design has undergone unprecedented changes.

First, the control plane and user plane of the 5G core network are completely separated, which allows the user plane UPF to be flexibly sunk and distributedly deployed together with edge computing (MEC) at the edge closer to the user side, thereby reducing network latency and enabling low-latency 5G applications.

UPF is deployed with edge computing

Secondly, and more importantly, the 5G core network adopts a service-based architecture (SBA) design, which makes the 5G core network more elastic, efficient and flexible through cloud-native, stateless VNF (virtualized network function) and shared data layer to meet the diverse 5G use cases including eMBB, mMTC, and URLLC. It can also perform network slicing through microservices, thereby providing customized slicing services for various industries.

Simply put, SBA = Network Function Service + Service-Based Interface. NFV decouples virtualized, software-based network functions (NFs) from traditional hardware-software integrated dedicated telecommunications equipment, replacing traditional physical network elements, and now we need to further decompose NFs into multiple "network function services". NFs can be composed of multiple modular "network function services" through "service-based interfaces".

The difference between 4G core network and 5G core network

On the control surface:

• AMF (Access and Mobility Management Function) is responsible for the mobility and access management of the terminal, including access, connection, mobility management, authentication, location services, etc. It roughly corresponds to the mobility management part of MME in the original 4G EPC.
• SMF (Session Management Function) is responsible for session management functions, including IP address allocation, QoS control, forwarding routing control, etc. It roughly corresponds to some functions of MME in the original 4G EPC + the control plane part of S/PGW.
• PCF (Policy Control Function), responsible for policy control, roughly corresponds to PCRF in EPC.
• UDM (Unified Data Management) manages user data, including user identification, user contract data, authentication data, etc., which roughly corresponds to some functions of EPC's HSS.
• AUSF (Authentication Server Function) is an authentication server that works with UDM to be responsible for user authentication.
• NEF (Network Exposure Function) is similar to SCEF (Service Capability Exposure Function) in 4G core network. It opens business capabilities to third-party service providers through API interface to create new business value. However, unlike SCEF, NEF is connected to all NFs through SBI in bus mode, and the API functions opened by the two are also different.
• NRF (Network Function Library) is responsible for network function service registration and status monitoring, etc., to achieve automatic management, selection and scalability of network function services, and allow each network function to discover the services provided by other network functions.
• NSSF (Network Slice Selection Function) manages network slice related information, such as being responsible for selecting network slices for terminals.

On the user side:

• UPF (User Plane Function) is responsible for forwarding traffic between the wireless access network and the Internet, reporting traffic usage, implementing QoS policies, etc. It corresponds to the user plane of S/PGW in 4G EPC.

From 4G core network to 5G core network

Wireless Access Network

To understand wireless access networks, let's first look at the inner workings of a base station.

As shown above:

• RRC (Radio Resource Control) runs on the control plane of the wireless access network. As the radio resource control layer, RRC is responsible for connection management, access control, status management, system information broadcasting and other functions, but is not responsible for processing user data packets on the user plane.
• PDCP (Packet Data Convergence Protocol) is responsible for compressing and decompressing IP headers, encryption and integrity protection, etc. It is worth mentioning that in the NSA networking mode, the PDCP layer is responsible for data diversion and aggregation between 4G base stations and 5G base stations.
• RLC (Radio Link Control) is responsible for data segmentation/reassembly, ARQ error correction, duplicate packet detection, etc.
• MAC (Media Access Control) is responsible for real-time resource scheduling decisions, multiplexing/demultiplexing, buffering and other functions.
• PHY (physical layer), responsible for coding, modulation, FEC, etc.

According to the above protocol process, the 5G base station is divided into three parts: CU, DU and RU.

• CU, the central unit, handles high-level protocols such as RRC and PDCP and is responsible for non-real-time configuration and control decisions.

CU can be deployed in a cloud-based manner, and can even be integrated with the core network UPF and edge computing.

• DU, distributed unit, handles layer 2 functions with high real-time requirements and some physical layer functions.
• RU, antenna unit, refers to part of the physical layer and RF, antenna part.

As mentioned above, since the MAC layer is responsible for real-time scheduling of wireless resources, the DU needs to be deployed close to the RU (within 1ms). A typical deployment method is to deploy the DU and RU at the same site; for scenarios such as campuses, factories, and shopping malls, one DU can serve multiple distributed RUs.

In this way, the backhaul of the wireless access network is divided into three parts: fronthaul, midhaul and backhaul. The fronthaul is between RU and DU, the midhaul is between DU and CU, and the backhaul is from CU to the core network.