What is the difference between 5G bearer network?

Hello everyone, I am Xiaozaojun.

In today’s article, let’s talk about 5G bearer network.

What is a bearer network ? As the name suggests, a bearer network is a network that is specifically responsible for carrying data transmission .

In the past, we introduced more about the access network and the core network. If the core network is the human brain and the access network is the limbs, then the bearer network is the neural network connecting the brain and the limbs , responsible for transmitting information and instructions.

The bearer network, access network and core network work together to ultimately form a mobile communication network.

Although the importance of the bearer network is recognized by everyone, its presence is weak.

In most people's eyes, the bearer network is just a pipe. As long as it is not broken, you don't need to worry about it.

The communication network is a pipeline, and the bearer network is a "pipeline in the pipeline"

Many people also believe that the bearer network has low technical content and has no future as they are faced with optical fibers and network cables that can cause claustrophobia all day long.

In fact, these are all misunderstandings of the bearer network.

The bearer network may seem simple, but its internal structure is actually very complex. The scale of the entire technical system of the bearer network is not inferior to that of the access network and core network.

Especially in the 5G era, the development of the bearer network has reached a "crazy" level, introducing a lot of high-end black technologies that are dazzling and impressive.

Next, let me introduce it to you slowly.

From 1G to 4G, the bearer network has undergone tremendous changes from low bandwidth to high bandwidth and from small scale to large scale.

Today's bearer network is actually very powerful and complete, and the performance of bearer network equipment is also very strong.

The computer room is filled with fiber optic transmission equipment

Despite this, these existing equipment and technical solutions can only tremble in the face of 5G.

Entering the 5G era, communication network indicators have undergone significant changes, and some indicator standards have even increased by more than ten times. To meet the requirements, it is not possible to improve the wireless air interface alone. The entire end-to-end network architecture, including the bearer network, must undergo a self-revolution.

So what is the revolutionary goal of the bearer network? Mainly speaking, it includes the following aspects:

• Large bandwidth

Bandwidth! Bandwidth! Bandwidth!

There is no doubt that bandwidth is the most basic and important technical indicator of the 5G bearer network. The air interface rate has increased by dozens of times, and the bearer network must also be greatly improved accordingly. Especially at the current stage when 5G is just starting out, eMBB is the first service scenario to be realized, and the most concerned thing is bandwidth.

• Low latency, high reliability

Vertical industries such as Internet of Vehicles and industrial control have stringent requirements on network latency and reliability.

One of the most important requirements of 5G is low latency, which requires single-digit millisecond end-to-end latency. As part of the end-to-end network, the bearer network is not the focus of latency improvement, but it also has to share some of the indicator pressure.

In many 5G scenarios, the reliability requirement of "six nines (99.9999%)" is proposed. Therefore, the bearer network must also serve such requirements and have strong enough disaster recovery and fault recovery capabilities.

• High-precision synchronization capability

5G places high demands on the frequency and time synchronization capabilities of the bearer network.

What is synchronization used for?

Here are a few simple examples: 5G's carrier aggregation, multi-point coordination and ultra-short frames require very high time synchronization accuracy; 5G's basic services use the time division duplex (TDD) format, which requires precise time synchronization; and indoor positioning value-added services also require precise time synchronization.

• Easy to operate and maintain

The 5G bearer network will be extremely large, with a large number of devices and a complex network architecture. If the network cannot be flexible, intelligent, efficient and open, it will be a nightmare for operators and operation and maintenance staff.

• Low energy consumption

The network must be powerful enough and save as much power as possible. Saving power means saving money.

• Support slicing

We have introduced slicing before. It is a core capability of 5G networks. The bearer network must also support slicing.

The picture comes from the public account "Wireless Deep Sea"

The above aspects are the goals of the 5G bearer network's self-revolution. If any of the goals cannot be achieved, it will not be a qualified 5G bearer network.

Before introducing the 5G bearer network structure, let's first look at the changes in the access network .

Everyone is familiar with the 4G access network, which is composed of BBU (baseband processing unit), RRU (radio remote unit), and antenna feed system.

In 5G, the access network is reconstructed into three functional entities:

• CU (Centralized Unit )

• DU ( Distribute Unit )

• AAU (Active Antenna Unit )

BBU+RRU+antenna ▶▶▶ CU+DU+AAU

CU: The non-real-time part of the original BBU will be separated and redefined as CU, which is responsible for processing non-real-time protocols and services.

DU: The remaining functions of the BBU are redefined as DU, which is responsible for processing physical layer protocols and real-time services.

AAU: Part of the physical layer processing functions of the BBU are combined with the original RRU and passive antenna to form the AAU.

The reason for such a fine division is to better allocate resources, serve the diverse needs of services (such as reducing latency and energy consumption), and serve "network slicing". (For details, please see here: About 5G access network, just read this article! )

After the access network became AAU, DU, and CU, the bearer network also underwent tremendous changes.

I want to clarify a misunderstanding : Many people have always believed that the bearer network is just a connection between the access network and the core network, just like the picture at the beginning of this article:

In fact, it is not rigorous, and it is just for convenience. To be precise, the bearer network also includes the internal connection of the access network and the internal connection of the core network. Therefore, a more accurate way to draw the logical relationship should be like this:

This can truly reflect the mystery of "carrying"

The 5G bearer network is also responsible for connecting the network elements of the 5G access network, that is, the AAU, DU, and CU. Different connection locations have their own unique names, namely: fronthaul , midhaul , and backhaul .

Between AAU and DU, it is a prequel

Between DU and CU is the

Between CU and core network is backhaul

These three "transmissions" all belong to the bearer network

In real-life 5G networks, the locations of DU and CU are not strictly fixed. Operators can flexibly adjust them according to environmental needs.

I have previously introduced D-RAN and C-RAN . D-RAN is the Distributed RAN, and C-RAN is the Centralized RAN .

In the 4G era, the so-called distribution and concentration refers to the distribution or concentration of BBU. In the 5G era, it refers to the distribution or concentration of DU. This kind of concentration is also divided into "small concentration" and "large concentration".

5G access network will have multiple deployment modes

Once again, the purpose of using C-RAN for centralization is to achieve unified management and scheduling of resources, improve energy efficiency, and further realize virtualization (detailed introduction in the access network article).

Because of the diversity of deployment modes, the locations of fronthaul, midhaul, and backhaul are also different.

Different access network deployment methods = different bearer network locations

Telecom operators have different levels of computer rooms in different places. For example, the computer rooms in telecommunications buildings in large cities are often core computer rooms. The base station computer rooms in ordinary office buildings are site ( access ) computer rooms. In small cities or district-level telecommunications buildings, there are also computer rooms, which may be aggregation computer rooms.

Look a little dizzy?

I will draw a complete diagram of the load-bearing network structure to help you understand (although I think it may be even more confusing).

Bearer network structure diagram (example)

From a holistic perspective, in addition to the fronthaul, the bearer network is mainly composed of the metropolitan area network and the backbone network . The metropolitan area network is divided into the access layer, the aggregation layer and the core layer.

All data coming from the access network is eventually aggregated layer by layer and reaches the top-level backbone network.

What equipment and technologies are used for forward transmission? What about mid-transmission? What about back-transmission? Let’s continue reading.

Let’s start with the prequel.

Fronthaul is the part between AAU and DU. It includes many connection methods, such as:

• Fiber Direct Connection

• Passive WDM/WDM-PON

• Active equipment (OTN/SPN/TSN)

• microwave

Let us give a brief introduction.

The first one is the direct fiber connection method .

Each AAU and DU are directly connected to each other using optical fiber point-to-point networking, as shown below:

This is a typical "tycoon" method, simple and direct. However, this method takes up a lot of optical fiber resources and is more suitable for areas with relatively abundant optical fiber resources .

Moreover, this method is more suitable for the early stage of 5G construction. As 5G construction deepens, the number of base stations and carrier frequencies will increase sharply, and this method is definitely not affordable.

The second is the passive WDM method .

Install the color optical module on AAU and DU, complete the WDM function through passive devices, and use a pair or a single optical fiber to provide connections from multiple AAUs to DUs.

As shown below:

What is WDM?

WDM stands for Wavelength Division Multiplexing, which is a technology that combines two or more optical carrier signals of different wavelengths (carrying various information) at the sending end through a multiplexer and couples them into the same optical fiber of the optical line to transmit data.

What is a color light module?

The photoelectric converter in the optical multiplexing transmission link is also called WDM wavelength division optical module. Optical signals with different central wavelengths will not interfere with each other when transmitted in the same optical fiber, so the color optical module can combine optical signals of different wavelengths into one transmission, greatly reducing the link cost.

The opposite of colored light is gray light. Gray light is also called white light or black and white light. Its wavelength fluctuates within a certain range and there is no specific standard wavelength (center wavelength). Generally, the customer-side optical module will use a gray light module.

Although the passive WDM method saves optical fiber resources, it also has problems such as difficult operation and maintenance, difficult management, and difficult fault location.

The third method is active WDM/OTN .

Corresponding WDM/OTN equipment is configured in the AAU site and DU equipment room, and multiple fronthaul signals share optical fiber resources through WDM technology.

As shown below:

Compared with the passive WDM solution, this solution is more flexible in networking (supporting point-to-point and ring networks), and the consumption of optical fiber resources does not increase. In the long run, it is a very good way.

The fourth method is microwave.

This method is very simple, which is to transmit data via microwaves. It is very suitable for remote locations, clear line of sight, and where optical fiber cannot be put in place.

The advantages and disadvantages of the four methods are compared in the following table:

According to the current situation, in the early stage of 5G deployment, the fronthaul bearer is still mainly based on direct fiber drive, supplemented by passive WDM solutions.

Here I would like to introduce two concepts related to fronthaul, namely CPRI and eCPRI .

CPRI stands for Common Public Radio Interface. In the 4G era, this is the interface between BBU and RRU. It is a universal interface with multiple different versions, and different versions correspond to different network standards.

CPRI fiber between BBU and RRU

In the 5G era, the bandwidth between AAU and DU may reach hundreds of Gbps, and CPRI can no longer meet the requirements, so it has been upgraded to the eCPRI interface specification (enhanced CPRI), which significantly improves the interface bandwidth .

Speaking of bandwidth, we said earlier that 5G requires a very large bandwidth. How large is it?

The current 4G LTE network has a mainstream subcarrier bandwidth of 20MHz, and the peak throughput of a single base station is about 240Mbit/s. ( Yes, that’s right, the bandwidth of a base station is not as large as everyone thinks. )

The 5G network, especially the millimeter wave frequency band, has an air interface bandwidth of 100-400MHz or even higher. With the further support of air interface technologies such as Massive MIMO (enhanced multi-antenna), the bandwidth of a single base station will be dozens of times that of 4G.

5G base station bandwidth estimation reference

According to the calculation results, in the early stage of 5G construction, the operator's single base station bandwidth reference value will adopt the standard of 10GE or 25GE. ( In 4G, the standard of most sites is only 1GE. Even so, the waste of fronthaul bandwidth is still quite serious. )

Bandwidth standards for 5G fronthaul

The bandwidth of the access link nodes will be determined by the deployment method and type. The bandwidth of 5G hotspots will obviously be larger than that of general areas (more nodes and more high-frequency stations).

Next, let’s look at the mid-pass.

Due to bandwidth and cost reasons, direct fiber connection or passive WDM cannot be used for mid-haul transmission, and microwave is not realistic either.

The backhaul bearer solution in 5G mainly focuses on the transformation of existing technical frameworks such as PTN, OTN, and IPRAN.

( For these concepts, it is recommended to read this article first: Basic knowledge of transmission )

From a macro perspective, the essence of the 5G bearer network is to introduce a lot of black technologies through "additional installation and upgrading" on the basis of the existing technical framework of the 4G bearer network to achieve comprehensive enhancement of capabilities.

Taking the 5G mid-haul bearer network solutions of the three major domestic operators as an example, they basically strengthen and improve the existing solutions to achieve support for 5G.

First, let’s look at China Mobile, which has the strongest strength.

China Mobile believes that SPN is the most suitable solution for it and can meet all its needs.

SPN , which stands for Slicing Packet Network, is a technology system independently innovated by China Mobile.

China Mobile's 4G bearer network is based on PTN (Packet Transport Network). SPN is based on Ethernet transmission architecture, inherits the functional characteristics of PTN transmission solution, and enhances and innovates on this basis.

In the eyes of China Mobile, SPN is to " upgrade " an optical interface on Ethernet, which can make full use of the current very mature Ethernet ecosystem and achieve a relatively high cost-effectiveness.

Therefore, China Mobile is very optimistic about SPN and has made every effort to promote the establishment of SPN standards and vigorously support the development of SPN upstream and downstream industrial chains. Thanks to its efforts, SPN technology has indeed developed rapidly and the industrial chain has become increasingly complete.

China Telecom is promoting the M-OTN solution in the 5G bearer field. M-OTN is based on OTN and is an OTN technology optimized for mobile bearer ( Mobile-optimized OTN ).

The reason why China Telecom chose M-OTN is that China Telecom has a very complete and powerful OTN optical transmission network. As we all know, China Telecom's main business is fixed-line broadband, and it still has a strong foundation in optical transmission network infrastructure and sufficient bandwidth resources.

As an optical-based transmission network technology, OTN has the characteristics of large bandwidth and low latency, which can seamlessly connect to the 5G bearer requirements. Moreover, after years of development, OTN technology is stable and reliable, and is supported by mature systematic standards. For telecom, it can achieve smooth upgrades on the existing OTN network that has been deployed on a large scale, which is both cost-saving and efficient.

China Unicom is relatively short of money, so it is certain that it will make use of its own IPRAN .

IPRAN is the industry's mainstream mobile backhaul service bearer technology. It is widely used on the networks of domestic operators and has played an outstanding role in the 3G and 4G eras. Operators have also accumulated rich experience.

However, the existing IPRAN technology cannot meet the requirements of 5G, so China Unicom started to develop IPRAN2.0, which is enhanced IPRAN.

IPRAN2.0 has significantly improved port access capability and switching capacity. In addition, it has also made great improvements and innovations in tunnel technology, slice bearer technology, and intelligent maintenance technology.

China Unicom has been conducting functional verification and performance testing of the IPRAN 2.0 specification, and the overall situation looks good.

The above is the 5G backhaul network solution of the three major domestic operators.

There is a famous saying that " the economic base determines the superstructure . " In fact, this is somewhat similar to the current situation.

As the backbone of the communication network, the bearer network involves a large amount of capital investment. Operators will certainly fully consider factors such as resource reuse, construction costs, and industry maturity, and carefully choose the most suitable solution for themselves.

Faced with such a situation, it is actually very painful for companies upstream and downstream of the industrial chain.

It is easy for large equipment manufacturers, but it is difficult for small and medium-sized manufacturers to engage in research on multiple tracks at the same time. If the major solutions cannot develop in the direction of integration, the industry chain companies will be forced to choose to "stand in line". This will definitely restrict the expansion and sharing of the industry chain, and will also affect the reduction of the overall cost of bearer network construction.

Therefore, many experts have called for the solutions of major operators to be "integrated" as much as possible, preferably with different paths leading to the same goal. This will be a good thing for both the industry chain and operators, as well as for end users.

Okay, due to limited space, I’ll stop here for today.

The next issue will focus on the hard core part of the 5G bearer network.

I will focus on introducing the hierarchical structure of the 5G bearer network and the key technologies of each layer, including PAM4, FlexE and FlexO introduced to enhance bandwidth, SR segment routing introduced to enhance routing and forwarding efficiency, and SDN software-defined network introduced to support slicing, etc.

Stay tuned!

—— The End——

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