What exactly is BRAS?

Students who are good at English may be a little confused about BRAS because the word "BRAS" means "bra" in English.

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As a communication science public account, we certainly won’t study underwear. The BRAS mentioned in today’s article is another BRAS, whose full name is Broadband Remote Access Server, which is also known as Broadband Access Server.

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As the name suggests, BRAS is related to our broadband Internet access services. In fact, it is very important to everyone. Without it, our home broadband will not work properly and we will not be able to surf the Internet.

What is BRAS

Before introducing BRAS in detail, let us first lay some foundation.

In previous articles, I have introduced the transmission network to you many times.

The transmission network is the foundation of our entire communication network, responsible for connecting home users, government and enterprise users, and data centers around the world. It is also the backbone of the Internet.

Our entire transmission network consists of the backbone network and the metropolitan area network.

The backbone network is divided into the national backbone network (the first backbone) and the provincial backbone network (the second backbone).

As the name suggests, a metropolitan area network is a communication network within a single city (abbreviated as MAN, Metropolitan Area Network).

The metropolitan area network is also further subdivided into three layers: core layer, aggregation layer, and access layer.

We now all use fiber optic Internet access, and every household has an optical modem.

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I have introduced to you before that the scientific name of "optical modem" is ONT, which belongs to the PON (Passive Optical Network) system. The PON system belongs to the access layer of the metropolitan area network. (For more information about PON, see here: link)

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To put it simply, PON transforms one optical fiber into N optical fibers, thus realizing "optical connection" for thousands of households.

You may think of routers. Routers also have the function of distribution and aggregation, turning one network cable into N network cables.

So, is PON a "fiber-optic version" of a router?

To be precise, no.

PON is the underlying system, it only manages light and is only responsible for delivering light to your home. In other words, it only connects the water pipe to your home, but there is no water in the pipe.

If you want to have water, you must first open an account with the water company.

So, we need BRAS.

If you have tinkered with wireless routers or optical modems, you must be familiar with this interface:

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Yes, PPPOE dial-up interface.

After entering the username and password, click Dial, and if successful, you can access the Internet. If you do not dial, just the network port lights up, you still cannot access the Internet.

This PPPOE dial-up is equivalent to you telling the operator your broadband account information, and then asking the operator to turn on the water pipe and deliver water to your door.

This is a typical authentication process. One of the main functions of BRAS is authentication.

To put it bluntly, without BRAS, operators cannot identify users, determine user permissions, or charge users.

After all this talk, where is BRAS?

As shown in the figure below, BRAS is generally deployed at the core layer of the metropolitan area network.

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The currently popular layered naming method also calls the layer where the BRAS is located the service control layer.

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CR: Core Router

SR: Service Router

SW：Switch

As can be seen from the network diagram, BRAS is a bridge between the access network and the backbone network. It is a gateway that firmly controls the user's data in and out of the backbone network. In other words, it is like a toll station on a highway, managing and charging users.

Above it is the core router CR of the backbone network, which is the entrance and exit of the entire metropolitan area network traffic.

If a user wants to access the Internet, he must first ensure that the optical path of the PON is OK. Then, a PPP session is established between the optical modem (which can also be a wireless router) and the BRAS.

PPP, Point to Point Protocol, is a data link layer protocol. After establishing a PPP session, users can access the Internet (access the backbone network).

The PPPOE we mentioned earlier is PPP over Ethernet (there was also PPPOA, which is PPP over ATM in the past).

The process of establishing a PPP session is shown in the figure below. These are all protocol flows, so I will not introduce them in detail. You can check the relevant information.

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BRAS completes authentication and billing functions by cooperating with the authentication system and the billing system.

It is worth mentioning that in order to complete the authentication, there is another important network element, which is the RADIUS server (Remote Authentication Dial In User Service). In the above figure, after the BRAS receives the username and password from the terminal side, it will look for RADIUS authentication.

In addition to authentication, authorization and billing, BRAS can also be used for QoS, security management, multicast and VPN.

As the control center of the broadband network, it is actually the core network of the broadband network. It is combined with the operator's operation support platform and can also carry out many imaginative value-added services, making it a veritable "big housekeeper".

Development and Evolution of BRAS

BRAS was born in the ADSL era. At that time, the number of broadband users increased sharply, and BRAS effectively simplified the network architecture and realized centralized management functions, laying the foundation for the explosion of broadband services.

Later, it not only supported xDSL, but also supported Cable Modem, Ethernet access (LAN), wireless broadband data access (WLAN), FTTx (that is, fiber optic into buildings and homes) and other network types, supporting data access of broadband IP networks and ATM networks, and became the favorite of operators and government and enterprise customers.

Around 2017, BRAS began a new evolution.

I mentioned earlier that BRAS is actually like the core network in the mobile network. More than 10 years ago, the core network had an obvious evolution, which is the separation of control and forwarding. In other words, the equipment is decoupled, and the control plane and data forwarding plane are separated, each taking its own path. (Reference: From 2G to 5G, core network, what have you experienced?)

This trend also happens to BRAS.

Traditional BRAS, as a gateway, is responsible for both user management and data flow forwarding. The burden is heavy and it is difficult to improve performance.

Moreover, there are a large number of BRAS devices. As the network scale continues to expand and new services are launched, the maintenance workload of BRAS is increasing. The speed of deploying new services is also very slow, which affects the long-term development of the network.

Therefore, BRAS began to decouple, extracting and centralizing the user management functions on multiple BRAS devices to form the control plane (CP). On the BRAS device, the control plane of the router and the forwarding plane of the BRAS were retained to form the forwarding plane (User Plane, UP).

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Like the mobile core network, in addition to centralizing the control plane, virtualization (cloudification) is also introduced to form vBRAS. The benefits are the same as those of the cloud core network, which can flexibly expand and shrink capacity, simplify operation and maintenance, unify standard interfaces, and improve equipment performance.

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In addition to using vBRAS, the forwarding plane also becomes flexible.

For high-traffic services, high-performance hardware and distributed deployment can be used to meet forwarding performance requirements. For large-session and low-traffic services, x86 cloud devices can be used for centralized deployment to save costs.

The emergence of vBRAS reflects the trend of cloudification of metropolitan area networks. Its underlying evolution logic is the same as that of 4G/5G mobile communication networks.

It is worth mentioning that in addition to the morphological changes, the positioning of BRAS has also changed.

As the performance of equipment improves, the BRAS and SR (service router) that we saw earlier are gradually integrated into MSE (Multi-Service Edge) or BNG (Broadband Network Gateway). This is also a form of integration.

Well, that's all about BRAS. BRAS is extremely important for fiber broadband services. If there is a problem with it, we will not be able to access the Internet and enjoy a smooth network experience.