Analysis of core technologies of Broadband Access Networks

Broadband access networks are worth learning. Here we mainly introduce the Network Positioning of broadband access servers and the system requirements of broadband access servers, including the extended functions of broadband access servers. With the increasing process of broadband data networks, Wide Area Network broadband access technology, represented by xDSL, HFCs, and even optical fiber LAN, has developed rapidly and has begun to be put into commercial use on a large scale. How to achieve unified network management more effectively and conveniently for the growing number of broadband access network architectures, providing flexible and diverse network application services is a real problem for network operators.

Network Positioning of Broadband Access Servers
It should be said that the launch of broadband access servers is largely due to the large-scale application of ADSL. We know that in the past, ADSL used to implement broadband access network services by creating a corresponding PVC connection for a service of each user, then, end these large numbers of PVC directly on the vro. As you can imagine, the problem is that the network structure is huge, the user connection is complex, and the actual maintenance is difficult. In addition, this access network structure is mainly based on the static IP access mode, which will cause low utilization of network IP addresses and difficult management. It is impossible to effectively implement centralized user authorization authentication and corresponding billing management. These problems will seriously restrict the large-scale commercialization of broadband access networks. In response to the technical characteristics and application requirements of broadband access networks and the role of narrowband dial-up access servers, broadband access servers have emerged, it becomes a network access device for the transition from a non-IP/IP Access Network to a backbone IP network, to achieve integration of network IP Access, the data access of the broadband IP/ATM network is mainly for xDSL/CableModem/Ethernet Access/wireless broadband data access, this solution brings together broadband users in terms of business, traffic, and management, and enables the user terminal to flexibly, autonomously, and conveniently select a service network through only one network connection, it adapts to the development trend of broadband access network applications and becomes an important network unit between the access layer and the backbone edge layer.

System Requirements for Broadband Access Servers
At present, the performance of the broadband network is obviously improved: in terms of access bandwidth, the dial-up access is being transferred to xDSL, hfc-or high-speed man access, and the access bandwidth has been expanded hundreds of thousands of times; in terms of processing performance, the powerful urban FR/ATM network or high-speed local area network gradually replaces a single PSTN network. In response, the performance and functions of the broadband access server must meet the following requirements:
1. Hardware design structure
Obviously, the broadband access server must have a high-speed and efficient packet forwarding feature to effectively solve the problems caused by high performance, high load, and high burst of broadband networks, broadband Access Servers with poor performance will inevitably become the "bottleneck" of the network ". At present, it has more than 2 GB of switching backplane capacity, and more than KPPS of independent packet forwarding performance has become the industry's basic requirements for broadband access servers. From the perspective of the hardware system structure, the Broadband Access Server has evolved from the inefficient centralized packet processing structure to the current distributed processing structure, and the front and back Plug board interface board and processing board have become the mainstream design ideas. Using this system structure, the processing module can directly process user traffic from the same slot interface module, in addition, the network traffic output on the same slot can be directly forwarded without going through the system backplane and the switching matrix module, thus effectively reducing the system load. On the other hand, in order to achieve packet forwarding between different slots, the system structure must provide the SwitchFabric module with high capacity, relatively independent, and redundant backup capabilities) and backplane bus of the corresponding capacity to ensure that the overall performance of the Broadband Access Server increases linearly with the increase of the interface module.

2. Interface Type and access method
Obviously, to support various broadband access types, the broadband access server must provide as many interface types as possible. Now, on the user side, the broadband access server can provide the DS3, OC3, and OC12 ATM optical interfaces to achieve pure ATM access or DSLAMDSL user concentrator) access, provides a 100/1000 M fast Ethernet interface to connect LAN users and HFCs users, and provides a high-density channelized or non-channelized E1/T1/DS3 frame relay interface to achieve access to frame relay users; on the network side, traffic aggregation and forwarding are generally achieved through the 100/1000 M fast Ethernet interface, OC3/OC12 ATM interface, or even OC12 POS interface, meet the bandwidth requirements of broadband services.
Unlike narrowband dial-up servers, the access process of Broadband Access Servers relies on the underlying data link layer, mainly the ATM layer and Ethernet layer) to encapsulate and reorganize data packets. Taking advantage of the technical features of the underlying layer, not only is the access networking flexible, but also can effectively bind the technical advantages of ATM and Ethernet to ensure service quality. Specifically, through RFC1490 and RFC1483 Layer 2 bridging technologies, RFC1577 Layer 3 IP routing technology can achieve static IP Access for broadband users; through PPPOverATM and PPPOverEthernet, users can access Dynamic IP addresses; the L2 VPN tunnel technology of L2TP meets the VPN access requirements of enterprise users and small ISP. From the current trend of application access, PPP access must be the mainstream application direction for broadband access. In PPP access technology, because PPPOE can be applied to multiple access networks, its applications are flexible, and it is easy to choose services. It also protects the existing investment of users, and its application prospects are the most promising.

3. Access quantity
Since the dial-up access server used TDM technology in the past, the system receives data from the PSTN network through each DS0 time slot. the maximum number of accesses to the system is the number of DS0 time slots that the system can terminate, that is, the maximum number of Modem that can be integrated by the system. To expand the number of system access, this time slot-based switching technology can only be achieved by expanding the system integration level to some extent, with considerable limitations. In a broadband network, the interface processing module directly encapsulates and restructured various protocol stacks, such as PPPOE or PPPOA calls. Because of the introduction of ASIC dedicated ic) technology, the system packet processing capability is significantly improved, and the length of access implementation is usually less than 5 seconds, including the radius Authentication time ); the reasonable cooperation of the processing modules of the system makes the system more stable and can well schedule and process the concurrent access of multiple users. At present, a medium-scale broadband access server should support more than 8000 concurrent PPP including PPPOA and PPPOE calls, and a large broadband access server can achieve million calls. The bandwidth access server has such processing capabilities that can fully meet the needs of large-scale broadband access applications.

Extended functions of Broadband Access Servers
The above section briefly introduces the system structure and access functions of the broadband access server. In fact, the broadband access server function is far from the same. In terms of specific network implementation, the bandwidth access server has changed the limits of a single, single, and single-function narrowband dial-up server, keeping up with the actual needs and development trends of broadband access, quickly expand functions. On the other hand, the Broadband Access Server is positioned at the edge of the backbone network, mainly to achieve business convergence and traffic convergence for various connected users, and should focus on its access processing capabilities, it is obviously unrealistic to expect the Broadband Access Server to undertake various network functions. Therefore, to meet the current needs and application trends of broadband access, you can expand the function of broadband access servers to achieve more efficient broadband access. The following describes the major extension functions of the broadband access server.

1. Business Selection
As the name suggests, this function is used to achieve the following functions: users can independently select various services provided by backend network operators through a connection to the Broadband Access Server. On the one hand, the specific implementation of various services has different technical focuses and different network performance requirements. In this way, an appropriate business model is allocated on the Convergence end of network services-the broadband access server, so that the system and its network resources can be bundled in a reasonable and orderly manner based on the characteristics of various services, implement various services with limited resources. On the other hand, from the perspective of the future development of network applications, the separation of the Network Content Service Provider ICP and the network access provider ISP is an inevitable trend. On the access aggregation side, the ISP must forward the selected business flow to the corresponding ICP. At the same time, the ISP network must strictly ensure the isolation of various businesses, that is, the isolation of ICP. Therefore, broadband access server is an inevitable application requirement for access users to achieve business selection. In specific implementation, each vendor has its own device, but basically adopts the following two modes: the terminal directly selects the service mode and selects the gateway ServiceSelectionGateway through the Background Service) mode.

In fact, the implementation kernels of these two selection modes are basically the same. The core of the Business selection is the implementation of the Broadband Access Server, and the difference is only in the form of user interfaces. However, considering the actual needs of operations, using the SSG service to select the gateway mode not only greatly improves the transparency of user access operations, but also reduces the configuration process of user terminals, it can also serve as a business portal to provide space for the next service expansion. For operators, this is indeed an ideal business operation mode and has become the future direction of business development.

2. QoS support
QoS has always been one of the focuses of packet switching networks, especially in IP networks. Today's broadband access networks, especially in backbone networks, primarily adopt point-to-point transmission networking, in exchange for QoS through high bandwidth. However, for a broadband access server, it undertakes the convergence of various services, and the uplink bandwidth is limited after all. It is obviously unrealistic to achieve QoS through high bandwidth. In this way, the implementation of QoS on the Access side is particularly important. As described above, the Broadband Access Server supports both ATM and FR Access. Obviously, the user's QoS problem can be well solved through the QoS implementation mechanism of ATM or FR itself. But do not forget that in addition to ATM and FR access, there are also various types of pure IP access in the broadband access server. For this type of access traffic, the service type of the IP header can be used to mark the ToS) field. The IP package is marked with the corresponding ToS on the business initiator side, and corresponding traffic ing or business ing is performed within the access server to distinguish various traffic levels and achieve network QoS.

3. VPN Virtual Private Network) Implementation
Virtual Private Network (VPC) is designed to ensure secure and efficient transmission of Private Network Data on the public network platform, so that the network has good scalability and scalability. The core of VPN technology is data packet encryption and network transmission. Now, IETF has developed some VPN Technical standards, such as L2 L2TP tunneling and L3 IPSec Encryption. The promulgation of these two technical standards laid a solid foundation for VPN applications. From the perspective of actual VPN applications, Most VPN services are implemented on the edge of the network, which is transparent to backbone network devices. As the role of network access and business convergence, the Broadband Access Server is often the initial initiator of a VPN application, which is crucial to the implementation of the VPN application. At present, in the VPN implementation of the second layer of the network, the Broadband Access Server provides L2TP tunnel encryption technology. Generally, it can be used as LACL2TP Access Concentrator or LNSL2TP network server) and flexible networking applications. The implementation of VPN on the third layer of the network is not popular because IPSec is a new protocol standard. Currently, only some broadband access servers have begun to support this function.

4. Port wholesale
Due to the need for business expansion, port leasing will also be the trend of broadband access network applications in the future. Especially for enterprise group users, they can quickly build their own networks through port leasing and bandwidth leasing on the Access side, saving a lot of investment in network construction. For such applications, you can use VLAN division or virtual router VirtualRouter in the broadband access server. In essence, these technologies divide sub-resources of the system. In each sub-system, the corresponding functions of the second and third layers of the network are independently completed to complete the port wholesale business. In fact, from the perspective of VPN, we can also think that port wholesale is another way to achieve VPN applications, and the application is flexible and convenient.

5. Support for Multicast
From the perspective of the entire network, the broadband access server must support multicast to distribute the end Of the multicast video stream at the network layer. The network host installs the corresponding multicast application to support the multicast protocol. By actively submitting a multicast application, select the desired multicast service, connect it to a local vro or multicast server that supports IGMP. From the perspective of technical implementation and the current support for multicast by actual devices, the Broadband Access Server forwards multicast traffic between network terminals and multicast servers or routers that support IGMP. Generally, it supports the Protocol standards of the First and Second editions of IGMP, but it only plays the role of IGMP Proxy) or IGMP spoofing Snooping to a large extent, the network end multicast package is transparently transmitted and distributed, and the end user does not feel different from the actual application. To further improve the flexibility of multicast applications for broadband access servers, some equipment vendors have begun to support multicast routing protocols such as PIM and DVMRP in their actual products.

6. manage IP traffic forwarding to implement firewall functions
The IP traffic forwarding management of the Broadband Access Server is to provide corresponding access capabilities to users based on the actual permissions of different users, to a certain extent to complete the IP firewall function, to achieve internal network security. To a large extent, IP traffic forwarding management is bound with the VPN and Service Selection of the Broadband Access Server, and works with the upper-layer backbone edge router, flexibly and effectively implement IP separation for various business types. In terms of technology implementation, this function can be achieved by filtering IPFilter through its own IP packet), flexible allocation of IP address segments and network-side NAT Network Address Translation for different businesses. At the same time, from the perspective of network security, broadband access servers should also provide protection against IP attacks and IP spoofing.

Today, a new round of data backbone networks and access networks, represented by optical communication, are developing rapidly, which puts forward higher requirements for broadband access servers in various aspects. The performance improvement of the Broadband Access Server focuses on the access processing capability, switching capacity, and interface bandwidth and density. According to the development plans of various manufacturers, the system performance requirements of the next generation of large Broadband Access Servers meet:
◆ The switching capacity should be at least 40 GB;
◆ The number of PPP calls supported at the same time reaches 20 K;
◆ The number of users can be configured to reach 100 K;
◆ The forwarding capability of an independent package exceeds 1 mpps.

At the same time, from the perspective of the development trend of IP addresses, the introduction of Multi-Protocol Label Switching MPLS can smoothly achieve network upgrades, making it easy to implement IP service quality assurance and VPN applications. These applications have incomparable advantages over other technologies. MPLS has become the industry's consensus on the development direction of the next generation of IP, and the support for broadband access servers is an inevitable choice.
In short, the Broadband Access Network Access Server is in the initial stage of large-scale application. With the development of the broadband access network, it has a very broad development prospect.

1. Detailed explanation of remote access technical difficulties
2. Detailed description of the development history of Broadband Access Networks
3. Discuss where broadband access network is going
4. Ethernet Access Technology Learning notes
5. Competition between ADSL Access Technology and other technologies