What is an access network?

Nowadays, whether it is the fixed network (broadband) used at home or the mobile network (mobile phone) that can be used at any time at home or outdoors, they have become an indispensable part of daily life, just like water, electricity and gas.

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Watching TV series, playing games, online shopping, remote online classes, video conferencing, etc., while enjoying the various conveniences brought by the Internet, have you ever thought about how we can get these services anytime and anywhere?

All of this starts with the access network that is closest to us and most accessible.

What is an access network?

To put it simply, the access network is the network composed of all communication devices from user terminals (such as mobile phones, computers, tablets, Internet TVs, etc.) to the operator's metropolitan area network.

The transmission distance of the access network is generally from a few hundred meters to several kilometers, so it is often figuratively referred to as the "last mile."

Our mobile phones, computers and other terminals can access the operator's metropolitan backbone network through this "last mile" service, and then access the Internet.

Classification of access networks

There are many ways to classify access networks. Currently, the most widely used method is to divide them into wired access networks and wireless access networks based on the access method.

Wired access networks are mainly divided into three categories according to the cables used:

• Copper cable access uses xDSL (x Digital Subscriber Line) technology, which is the same technology used to dial up to the Internet using telephone lines in the past.
• Fiber-coaxial hybrid access is a flexible technology that uses a mixture of optical fiber and coaxial cable. This technology is used in home cable TV.
• Fiber optic access, using all-fiber access PON (Passive Optical Network) technology, is the mainstream technology for wired access networks. FTTH (Fiber to the Home) allows everyone to enjoy the convenience brought by ultra-high Internet speeds.

Wireless access networks are mainly divided into two categories based on the mobility of access terminals:

• Fixed wireless access serves users at fixed locations or users moving within a small range. The main technologies include Bluetooth, Wi-Fi, WiMAX, etc.
• Mobile wireless access serves a large number of users who use mobile terminals (such as mobile phones), and the main technologies are cellular mobile technologies such as 4G and 5G.

Let’s take the fiber-optic access PON network technology and wireless access 5G mobile technology, which are closest to our lives, as examples to understand what wired access networks and wireless access networks are.

PON Network Technology

Since the advent of the Internet, people have never stopped pursuing higher network speeds.

From early dial-up Internet access via telephone lines to today's FTTH fiber-to-the-home, technology is advancing and the Internet is constantly speeding up.

FTTH, also known as fiber to the home, means installing optical fiber directly into the user's home, providing the user with sufficient bandwidth to meet various network needs such as voice, high-definition video, large-scale games, etc.

The convenience of FTTH benefits from the development of PON network technology, so what does the PON network look like?

The PON network is connected behind the fiber-optic broadband of every household. Let us trace the source of the optical fiber and take a look at the devices that the PON network consists of.

The PON network connects to the networks of various service providers (such as the Internet, IPTV, telephone/video services, etc.) through the metropolitan area network, and connects users' home telephones, IPTV televisions, computers and other terminals downward to provide network services.

Based on the distance from the user, the PON network is mainly composed of three parts.

• ONU (Optical Network Unit): The device is closest to the user. It is usually installed and deployed directly in the user's home. The common types are SFU (Single Family Unit) and HGU (Home Gateway Unit). One end of the ONU is connected to the optical splitter through optical fiber, and the other end is connected to the terminal device at home through wired or wireless means.

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SFU (which can be understood as an optical modem) generally needs to be used in conjunction with HGW (Home Gateway, which can be understood as a home router). HGU is more powerful and integrates the functions of an optical modem and a router.

• ODN (Optical Distribution Network): provides "transmission channels" for ONU and OLT through optical fibers and splitters. The splitter can divide one optical signal into multiple channels in a certain ratio.
• OLT (Optical Line Terminal): Farthest from the user, OLT is the core device of PON. It is generally installed and deployed in the operator's computer room to aggregate user data and send it to the metropolitan area network for transmission.

The above are all the components of PON network. In actual application, it is PON network technology that plays a decisive role in network speed. In the process of continuous evolution over the years, technologies such as EPON, GPON, 10G-EPON and XG(S)-PON have emerged.

For the relationship and differences between these technologies, students who are interested can click on the extended reading at the end of the article to learn more.

5G Mobile Access Technology

In the mobile communication network, the wireless access network is the closest link to us, connecting users and the business core network. The base stations (iron towers) that can be seen everywhere are the symbol of the wireless access network.

When we make a call or surf the Internet with a mobile phone, the base station will receive the mobile phone's signal, and the signal will be transmitted to the core network through the bearer network. The core network will then process the signal and pass it on to the call destination city or the network application on the Internet.

The core of the wireless access network is the base station. The traditional base station is composed of BBU (Base Band Unit, mainly responsible for baseband signal modulation), RRU (Remote Radio Unit, mainly responsible for radio frequency processing) and antenna (responsible for transmitting or receiving electromagnetic waves). Although the structure of the wireless access network has been continuously upgraded in the 2G ~ 4G era, the functional allocation of these three modules has basically not changed.

In the 4G era, wireless access networks provided users with unprecedented user experience (such as short videos, distance education, etc.). However, with the emergence of 5G business demands such as driverless cars, the Internet of Things, and ultra-high-definition video, wireless access networks are required to have higher spectrum, larger bandwidth, and lower latency.

In the 5G era, wireless access networks require a complete transformation from architecture to technology. Compared with the 4G era, 5G wireless access networks have undergone two disruptive upgrades:

(1) Baseband unit (BBU) is restructured into CU and DU

In order to cope with multiple application scenarios under 5G network slicing, the BBU function is reconstructed into CU and DU. CU (Centralized Unit) is a central unit responsible for processing high-level protocols and non-real-time services, and can control and coordinate multiple cells within the access network; DU (Distributed Unit) is a distributed access point responsible for processing physical layer protocols and real-time services. At the same time, the functions of RRU and antenna are merged in AAU.

This functional splitting approach reduces the burden on network transmission on the one hand, and makes network deployment more flexible on the other.

(2) Moving towards virtualization

In the 2G~4G era, one BBU can complete communication processing within the range of multiple antennas, but 5G uses higher frequency bands for communication. More base stations are needed to cover an area of ​​the same size. It is not practical to simply increase the number of CUs and DUs to solve the problem.

5G wireless access networks solve this problem with the help of virtualization technology. Virtualization technology enables the ability to run multiple different operating systems on the same physical server. For access networks, CUs are originally dedicated hardware devices that are relatively expensive. After virtualization, running software with CU functions on a virtual machine can realize the functions of the original CU.

5G wireless access networks can aggregate a large number of underlying resources through virtualization technology. These resources will be dynamically allocated in real time based on actual conditions such as business needs and user distribution.

In the future, not only will the wireless access network evolve towards virtualization, but the entire 5G mobile network will virtualize multiple virtual networks to achieve resource sharing and isolation, making end-to-end network slicing a reality and providing operators with lower-cost solutions.

Future Development Trends of Access Networks

In summary, both wired access and wireless access are constantly developing as users demand greater bandwidth.

In fact, in the future, the network will provide services to users based on business rather than access methods. Users will not care whether it is wireless access or wired access. The core network that provides services will gradually become cloud-based and unified. Business integration will accelerate network integration. The existing multiple independent access networks such as fixed access, mobile bearer, government and enterprise access will become the focus of network integration.

As the last mile of user access, the optical access network (PON network) also plays an important role in the fixed-mobile convergence in the 5G era. Based on a unified ODN network, WDM-PON, 10GPON, Combo PON and other technologies can be flexibly selected according to different application scenarios (wired and wireless), which can not only provide fixed broadband access, but also realize the fronthaul service of 5G base stations. The unified optical access network can be built and managed in a unified manner, which greatly saves network investment costs and improves network utilization.

In the 5G era, based on the ubiquitous ODN optical fiber resources and combined with the evolving PON technology and SDN&NFV technology, integrated access to wired and wireless services can be achieved. While sharing resources, service deployment and operation and maintenance can be simplified, which will bring better network services to users.