The structure of ADSL-based Internet access system: Li Hua Published: 2001/01/08
Thesis:
Based on ADSL-based access network architecture involves a wide variety of factors, this paper proposes a feasible Internet access architecture based on ready-to-see communication protocols (ATM, PPP, L2TP, etc.). This article describes the current discussion of more ADSL-based Internet access architectures. Key words: access network; end-to-end model; asynchronous transmission mode; point-to-point protocol; Layer 2 Tunnel Agreement
text:
ADSL-based Internet Access Architecture Introduction ADSL technology is a new type of access technology that provides broadband services to family and small companies. It can support a variety of high-speed broadband applications, such as high-speed Internet access, remote office, virtual private network, etc. The traditional dial-up access data transfer technology does not support these applications, or the efficiency is low. So current business is increasingly tending to ADSL technology. This article describes the current discussion of more ADSL-based Internet access architectures. 1 end-to-end ADSL interactive network architecture This architecture is based on existing standards, ie PPP over ATMs, and selection of these standards because they meet: • Access to Internet, Enterprise.com, local information providers, etc. Entity communication · Easy to implement transplant from existing ISP internal structures · Can connect multiple services · Multi-protocol support · Security · Support multicast · Support multiple service types · Guaranteed QoS (1) ATM end-to-end The proposed ADSL end-to-end mutual service model is based on an end-to-end ATM network between the client network and the service provider network. The ATM endpoint includes all devices of the client and termination of the service provider network of the end to the end ATM network. With ATM over ADSL, users can access to broadband Internet. It provides a seamless connection from remote users to ATM networks (including other ATM main networks, enterprise internal networks) or internet seamless connections. In addition, ATM can provide direct connection to the Internet / Enterprise Internal Network Server. This greatly improves Internet services in performance, resource sharing and redundancy. As a Layer 2 protocol on the ADSL access, ATM provides some obvious benefits, such as the transparency of protocols; support for multiple QoS classes and ensures the ability of QoS levels; ATM bandwidth adjustability; The development of XDSL members. (2) Once the PPP Over ATM establishes an ATM layer connection between the user and the service provider network, the session can be implemented with PPP in the link level and network level. Using the features of PPP can provide the necessary operational function on ATM: identification (PAP, CHAP, tagged system); Layer 3 automatic configuration (example, automatic configuration of domain name, allocation of the target network) Multiple parallel connections (ie, multiple PPP sessions); transparency to the third layer (example, PPP can support IP, IPX); encryption; compression; accounting and RADIUS service providers . (3) PPP Over ATM's airborne packages and VC multiplexing this interoperable architecture specifies that PPP is loaded to ATM using VC multiplexed AAL5 protocol. This means that each VC carrying the PPP session can only have a PPP session, ie, no other protocol parallel to PPP. Because the PPP itself supports protocol multiplexing, the IP and other PPP layers, this also means that the PPP Over AAL5 mapping uses an empty package. The PPP Over ATM-based end-to-end service architecture discussed above meets the network service requirements mentioned earlier. 2 Ethernet terminals Many ATM-based ADSL remote modems have an ATM25 network interface, but many of this modem will also have an Ethernet interface. This provides an opportunity to extend the advantages inherent in PPP to users who use Ethernet to connect their PCs to ADSL networks. There are two ways to support Ethernet users of ADSL-NT: one method is to load PPP to Ethernet (eg, local L2TP tunnel method). The other is to turn PPP (and ATM) at an ADSL-NT end. Both methods will be discussed below.
(1) Local L2TP Tunnel Since PPP is designed for serial connection, it is necessary for some new ideas and new technologies to work in Ethernet. One technology that extends PPP to Ethernet is a local L2TP tunnel. ADSL-NT maps PPP packets from the L2TP tunnel (between each PC on ADSL-NT and LAN) to an ATM VCC, and vice versa. People can use ADSL-NT as a network access server: accepted internal "call" through the local tunnel on Ethernet, and then transfer them to an ATM connection carrying a PPP end session. The local L2TP tunnel containing the PPP packet is loaded onto the IP packet and placed on the Ethernet frame. ADSL-NT restores PPP packets, insert them into ATM cells. In the downstream direction, ADSL-NT receives ATM cells, restructuring cells from the network through DSLAM, restores into PPP packets, and then forwards packets to the final destination via Ethernet. In order to support PPP sessions between remote users and ISP routers, ADSL-NT must be able to map ATM VC to the local L2TP tunnel through the LAN. The mappings and protocols used are shown in Figure 1. Service Telco ATU-R ClientPC Provider Figure 1 L2TP-based PPP Over Ethernet protocol architecture PPP-over-L2TP tunnel operation: In order to start a PPP session, PC establishes a L2TP tunnel to ADSL-NT (ie ATU-R) . Local communication is performed on a local IP subnet with a private IP address assigned to PC through DHCP. When ADSL-NT gets a L2TP session request from the PC, it is dial into a specific ATM address to initialize an SVC of the ATM network. The L2TP request includes a LAN address or name, and E.164 (the modem can convert E.164 addresses into NSAP format, and use it to start to the network's ATM call creation). If it is a PVC service, E.164 can be mapped to the PVC. Once the ATM connection is established, the L2TP tunnel request is known that the PPP session can be performed on this connection circuit. When the tunnel or SVC is released, the Modem will release another part. (2) The PPP agent can be implemented by the PPP agent for PPP Over ATM through Ethernet. Here, the host of ADSL-NT on the LAN appears as a simple ADSL router, but the ISP server or remote access server appears as a PPP host. This ADSL-NT can be a separate hardware device or a PC-based server, the latter has both an ATM / ADSL NIC, and an Ethernet network card, which connects ADSL access network and customer LAN. There are two possible ways to support the PPP proxy. First, each host on an ATM VCC uses a different global IP address; the other is a global IP address to share several hosts on the same VCC. 3 ADSL access network architecture using end-to-end ATM connection is mainly discussed by the Architecture of the ADSL Access System. The core network architecture of two non-ended-end ATM connections that support ADSL access services: L2TP Access Concentration (LAA) and PPP Terminals are set (PTA). They are similar in network topology, but data is different from regional broadband networks. (1) L2TP Access Concentration L2TP is an agreement to extend session on either network to a remote network server called L2TP Network Server (LNS). It enables users to call to his destination ISP or company dial server, but to an L2TP Access Concentrator (LAC) and then connect the PPP session to the LNS by the LAC.
The basic principle of L2TP is to put PPP into a tunnel transfer on the regional broadband. The function of the L2TP Access Concentrator (LAC) is implemented in the area of the regional broadband network to DSLAM. The function of the L2TP Network Server (LNS) is implemented in the router of ISP or company to the regional broadband network. Each user has an ATM PVC: Starting from ATU-R, cross-connect in DSLAM, terminating at LAC. PPP over ATM is used in the ADSL access network. LAC and LNS can be connected to any network that supports L2TP protocols. The PPP link is enclosed in the tunnel, passing through the area broadband network, transmitted from the LAC to the NSP end. The protocol stack of LAA is like Figure 2. Figure 2 The protocol stack DSLAM and ATU-R of the LAA are part of the ADSL access network. Assume that the PPP over ATM over ADSL is used in this part of the network. DSLAM is connected to a LAC. With each ATU-C a PVC, DSLAM is connected to the LAC, which simplifies the service activity process. What is the NSP of the NSP from the LAC decision user needs to be used and sent the PPP session through the tunnel. LAC is connected to the regional broadband network, which has a network topology of NSP requirements. NSP utilizes a router with LNS function, removes the L2TP package, and terminate the user's PPP connection. The router also provides the conversion of the techniques used from regional broadband technology to NSP. This provides the NSP connection with the mode of access to the Dial-up modem access. PPP provides Link Control Protocol (LCP) and Network Control Protocol (NCP) to determine the options for link layers and network layers. It can be implemented on the end-to-end line between the PC and the destination router. (2) The PPP terminal is concentrated in this structure, and the PPP session is not transmitted on the road to the NSP, but is terminated to the broadband access server (BAS). The IP packet is extracted in the BAS side, forwarded to an appropriate NSP via an IP-based area broadband network. This architecture is not required to use an ATM structure to connect to NSP. Any web technology that can encapsulate IP packets can be used between NSP and ADSL access networks. The user initiates a session by establishing a PPP connection between CPE and BAS. The BAS terminates the PPP session to forward the user's IP service stream to the corresponding NSP. NSP does not need to understand or support PPP functions. Its protocol stack is as shown in Figure 3. Figure 3 PTA protocol stack DSLAM and ATU-R are part of the ADSL access network. Assume that this part of the network uses PPP over ATM over ADSL. DSLAM is connected to BAS through an ATM. Pre-allocated to each user ATM PVC to BAS. Like the LAA model, this assignment is purely driven by the user process of access provider, regardless of NSP. Similarly, when a user changes NSP, the access provider has no effect. The features provided by the BAS include: IP address configuration, user identification, recognition, and billing by the PPP protocol group. BAS receives the user registration and password, and confirms the user's identity with NSP through a query language of the NSP's RADIUS server. IP addresses and other user configuration information are also available from NSP. On the other hand, BAS provides an IP routing interface for NSP. The BAS is configured to support any physical layer transmission of IP packets to NSP. When it establishes a PPP session with the user, it maps a user identifier to the NSP port, which can be a session identifier or a user port identifier. This unique map transmits the user's IP packet to the destination NSP. One thing is important, in the upstream direction, the BAS functional icon session drive mechanism router. Indeed, the BAS transfer does not rely on the target IP address to determine NSP, but is completely dependent on the session-session combination between users and NSPs. In addition, NSP utilizes an IP router corresponding to receive and forwarding these packets.
