ADSL Network Architecture

In 1989, bell proposed a passionate idea to transmit video, graphics, and other data at a speed of millions of bits/s over a common telephone line (twisted pair wires. The dream came true. An ADSL modem was born. Its downlink speed reaches Mbps and the uplink speed is 16 or 64 Kbps. This asymmetric rate is especially suitable for Vod ), at the same time, this asymmetric transmission technology can transmit farther distances than symmetric Transmission Technology. Unfortunately, the ADSL designers are not good at naming them. They give this technology a name (asypolicric Digital Subscriber Line) Short for ADSL, translated into Chinese is probably "asymmetric digital user line ". If the person named for ADSL had a little idea about the market at the beginning, he should be given a better and cooler name, such as megamodem or something, in this way, we can give him a better Chinese name, such as "Super modem" or something. But then again, isn't the modem name a mess? Today we are not too easy to call. We also gave modem a friendly Chinese nickname "cat ". Maybe tomorrow, ADSL will have a more friendly name.

Since 1989, ADSL has gone through a long journey. The uplink speed has also increased from 640 Mbps to 9 Mbps (at the cost of shortening the transmission distance) to kbps. More and more manufacturers are producing ADSL chips, and the size of ADSL devices is halved two years ago. Thousands of adslmodem instances have been installed all over the world, and a new network age is approaching. Along with the development of ADSL, its market goal has also undergone great changes. At the beginning, ADSL was designed and developed to meet the needs of people's VOD applications. people imagine that the next-generation network should be more adequate for telephone companies to provide video-on-demand services. Unexpectedly, however, the VOD market is not as optimistic as most people think, and VOD has not developed rapidly. Although the market for ADSL has failed to develop in the VOD market, it accidentally became a new favorite of the Internet and the company's network market. What people really need is a high-speed network access speed to meet the needs of internet access. For most Internet users, ADSL, an asymmetric transmission method, is particularly suitable for Internet access. At this time, people suddenly realized that what ADSL needed was not a TV but a computer. Of course, in this emerging market, ADSL also encountered a powerful Challenger-cable modem.

People need to use the internet or the company network for a large amount of data communication. ADSL provides people with a long-distance network access method. However, ADSL can only transmit one side of the bit line to the other side. The real need for communication between users and networks also requires access networks to achieve data connection, protocol conversion, channel reuse, and network management. For example, without an access network, ADSL is like a car without an engine.

The market demand of the Internet has made NSP focus on ADSL. At the same time, it also forces NSP (Network Service Provider) to increase network capacity and provide higher bandwidth for people. The rate adaptive capability of ADSL allows him to select the optimal transmission rate based on the line conditions. At the same time, the updated G. Lite standard makes it easier for people to install ADSL. The Internet and the company's network needs are also changing the access network architecture. On the one hand, for most networks, the dominant position of the IP protocol (a layer-3 Protocol) is unshakable. On the other hand, NSP cannot ignore the huge benefits brought by ATM, including support for real-time services such as video and voice, and QoS Services (QoS refers to the provision of different services according to users' needs. For example, it gives higher priority to the network communication of commercial users, of course, the cost will be relatively higher ). No one is sure whether or not the ATM will become the only network standard. However, today, the construction of an ATM-Based IP network is unquestionable.

Vro-Based Network Architecture

The original network architecture of the ADSL Access Network is a router-based network architecture, which is the same as that of most leased lines in China today. The Central Bureau (CO) uses a rack-mounted ADSL modem to connect to an IP router through an Ethernet Hub or a switch. In this way, the number of vro ports determines the number of ADSL users. If such a network structure changes a little today, dslam (DSL access multiplexing) replaces the ADSL rack modem and can directly connect the ADSL modem to the backbone network, the router may move outside the CO.

Here, protocol reuse is implemented through Ethernet between the PC and the router. A pc is connected to multiple points through the IP layer. In any case, such a network design concept still has merits: First, such a network architecture was designed in 1995, and there was only one such network device that people could think of at that time; such a network architecture does not require protocol conversion; such a network architecture does not need to introduce new protocols and is fully compatible with Internet and LAN protocols; finally, the most important thing is that he met the strong demand for high-speed Internet and LAN communication.

Of course, from today's standpoint, the shortcomings of this network are also obvious: He cannot meet the needs of network expansion, and ADSL targets ordinary Internet users; if a user needs to connect to multiple points (for example, the user needs to connect the ISP and the company network), then another router needs to implement this connection, it is impossible to provide a router for each service. The most important thing is that this network architecture cannot implement QoS services. In this way, NSP began to develop an ADSL Network Based on the second layer multiplexing of ATM.

ATM Access Network

Using the ATM technology, NSP can provide users with a more user-friendly ADSL Network. Through PVC (permanent virtual circuit), an ATM channel is established between users and information providers (such as ISP or corporate LAN gateway. PVC is a bit similar to ddnleased line, but ddnleased line is only a physical line. Compared with the preceding vro-based network, PVC serves as the connection channel between the user and the ISP or company network, and can transmit various information including IP addresses. The ATM access network model is designed to allow all data communication to go through the ATM Backbone Network. This network architecture provides multiple service connections and provides the best transmission mode for data and multimedia communication. In this network architecture, each Central Bureau (CO) is connected to each ADSL user through dslam, And the ADSL user is connected through the ADSL modem or ADSL pccard. The user's PC of the ADSL modem is generally connected to it through Ethernet. If it is a PC Card, the PC Card is required to be able to process the packets of the ATM. To meet the requirements of multiple user devices, you need to use a router behind the ADSL modem or a routing device with a built-in ADSL modem. In short, for this network architecture, PVC is used to directly connect users and service providers.

It was feasible to use the PVC connection method in the early market. However, with the development of the market, more and more users demanded the application of ADSL, And the PVC network architecture could not adapt to a large number of user access scale. We can imagine that each user needs to establish a PVC connection with two ISP or company networks, and the entire ATM network will be doubled. SVC can meet the needs of rapid network growth. Users can use SVC to connect to different networks as needed. However, due to the price and technical factors of SVC, It is not accepted yet. A new network architecture has emerged, that is, the establishment of PPP connections on the ATM network. That is to say, people can establish multiple PPP connections on a PVC link.

ATM-based PPP applications

In order to further adjust the ATM network architecture and adapt it to market development needs, most ADSL service providers adopt PPP applications based on ATM, the main reason is that the protocols and operation methods used in this network architecture can be seamlessly integrated with the existing network architecture of the ISP. For example, the ISP adopts the AAA (authentication, authorization, and accounting) Security Authentication mode, which is mainly used to authenticate access users. (Jason: The Authentication Mode of AAA is actually not mysterious. We usually need to enter the user name and password when using modem to access the Internet, at the ISP end, a server is required to authenticate our user name and password, and charges are made based on our Internet access time or data traffic. This Authentication mode is AAA). Based on the AAA Security Authentication mode, ISP can also provide other value-added services for users. All these applications (AAA) are implemented through a PPP connection between the user and the server. Today, we use a low-speed dial-up modem for PPP authentication connections. If ADSL can use the same connection method, the ISP can use the existing service platform to provide users with high-speed ADSL Services, including value-added services based on PPP in the future.

Nnsp gateway architecture

To transition to the next-generation ADSL access network, the NSP gateway architecture has become a choice for people. He is able to provide people with a short-term transitional solution and provide future support for SVC. The so-called NSP gateway refers to the concept of adding a LAC (Jason: LAC) between dslam and ISP (or other company networks, through the L2TP protocol, multiple PPP connections can be reused in a virtual channel, which can be an IP-based network or a PVC of an ATM and frame relay .) As mentioned above, the end-to-end ATM network architecture requires a large number of PVC connections, on which PPP connections from users to ISP or company networks are established, the LAC can reuse PPP connections to the same location into a PVC, which saves a lot of PVC connections and simplifies the network architecture, it is also suitable for expanding the network scale. For users, the specific operation is also very similar to the use of modem dial-up Internet access. Only the modem is used to dial the ISP's phone number, and the ISP's domain name must be called here. For example, if my user name is Jason and I need to connect to the 163 ADSL Network (its domain name is 163.com), then I will use the name of the Jason@163.com to dial, the NSP gateway will reuse my PPP connection application to the 163 PVC based on the domain name provided by me. At the same time, this network architecture can also provide applications of different service levels, provide different services for different user needs, establish different levels of PVC connections between dslam and an ISP, according to user needs, reuse the PPP connection to different PVCs. (Some users may need video conferencing and other applications with strong real-time performance, while some users only need simple applications such as email.) Of course, the corresponding charges vary depending on different levels, in the future, QoS will become one of the key issues that people will consider. "> In order to transition to the next-generation ADSL access network, the NSP gateway architecture has become a choice. He is able to provide people with a short-term transitional solution and provide future support for SVC. The so-called NSP gateway refers to the concept of adding a LAC (Jason: LAC) between dslam and ISP (or other company networks, through the L2TP protocol, multiple PPP connections can be reused in a virtual channel, which can be an IP-based network or a PVC of an ATM and frame relay .) As mentioned above, the end-to-end ATM network architecture requires a large number of PVC connections, on which PPP connections from users to ISP or company networks are established, the LAC can reuse PPP connections to the same location into a PVC, which saves a lot of PVC connections and simplifies the network architecture, it is also suitable for expanding the network scale. For users, the specific operation is also very similar to the use of modem dial-up Internet access. Only the modem is used to dial the ISP's phone number, and the ISP's domain name must be called here. For example, if my user name is Jason and I need to connect to the 163 ADSL Network (its domain name is 163.com), then I will use the name of the Jason@163.com to dial, the NSP gateway will reuse my PPP connection application to the 163 PVC based on the domain name provided by me. At the same time, this network architecture can also provide applications of different service levels, provide different services for different user needs, establish different levels of PVC connections between dslam and an ISP, according to user needs, reuse the PPP connection to different PVCs. (Some users may need video conferencing and other applications with strong real-time performance, while some users only need simple applications such as email.) Of course, the corresponding charges vary depending on different levels, in the future, QoS will become one of the key issues that people will consider.

Another similar network architecture is the use of BAS between ISP (or company network) and dslam (Broadband Access Server ). The previous NSP gateway only reused multiple PPP protocols into one PVC. Such a PPP connection is established between the user and the ISP (or company network. While the network model using BAS implements the terminal function of PPP. In short, Bas can process the user's PPP connection application, interpret the user name and domain name sent by the user, and send the user name and password to the corresponding ISP for authentication through the radius proxy through the IP network. This network architecture is very similar to the authentication method adopted by most ISPs today. It only places the Access Server away from the ISP, and such a bas can provide services for multiple ISPs. (The existing RAS in the ISP network is owned by the ISP.) The problem to be solved here is the ownership of BAS, in general, since he can provide services for multiple ISPs at the same time, he should be all for ilec or CLEC.