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Internet access Architecture Based on ADSL
Introduction ADSL is a new access technology that provides broadband services to families and small companies using the original copper twisted pair telephone user line. It supports a variety of high-speed broadband applications, such as high-speed Internet access, remote office, virtual private network, and so on. Traditional dial-up access data transmission technology either cannot support these applications or is inefficient. Therefore, the current business is more and more inclined to ADSL technology.
This article introduces a widely discussed Internet access Architecture Based on ADSL. 1. End-to-End ADSL Interactive Network Architecture This architecture is based on existing standards, namely PPP over ATM. These standards are chosen because they meet:
· Access to Internet, enterprise network, and local information providers and communication with peer-to-peer entities
· Easy migration from the existing internal ISP Structure
· Multiple services can be connected at the same time
· Multi-protocol support
· Security
· Support for Multicast
· Supports multiple service types
· QoS Assurance (1) atm end-to-end
The proposed end-to-end interactive service model for ADSL is based on an end-to-end ATM network between the client network and the service provider network. The ATM endpoint includes all devices on the client and the service provider network that terminates the end-to-end ATM network.
Using ATM over ADSL, users can access the broadband Internet. It provides seamless connections from remote users to any terminal (including other ATM Backbone Networks, enterprise intranets) or the Internet. In addition, ATM can provide direct connections to Internet/enterprise intranet servers. This greatly improves Internet services in terms of performance, resource sharing, and redundancy.
As the layer-3 protocol on the ADSL access network, ATM provides some obvious advantages, such as protocol transparency, support for multiple QoS types and ensure the QoS level; bandwidth tonality of the ATM; it facilitates the development of different xDSL members. (2) PPP over ATM
Once an ATM layer connection is established between the client and the service provider network, you can use PPP to establish and release sessions at the link level and network level.
The features of PPP can be used to provide the necessary operation functions on the ATM: Identity Authentication (PAP, chap, tagged system); automatic configuration of layer-3 addresses (for example, automatic configuration of the domain name, IP Address Allocation in the target network); multiple parallel connections (that is, multiple PPP sessions); transparency to Layer 3 (for example, currently, PPP supports both IP and IPX), encryption, compression, billing, and interaction with RADIUS service providers. (3) Empty encapsulation and VC multiplexing of PPP over ATM
This interoperability architecture requires PPP to load to the ATM using the AAL5 protocol based on VC multiplexing. This means that each VC carrying a PPP session can only have one PPP session, that is, other protocols parallel to PPP are not reused. Since PPP itself supports protocol multiplexing, IP address and other Protocols Above the PPP layer, this also means that PPP over AAL5 ing uses empty encapsulation.
The above-mentioned end-to-end service architecture based on PPP over ATM meets the network service requirements described above. 2 Ethernet Terminal Many atm-based ADSL Remote modems have an ATM 25 network interface, but many of them also have an Ethernet interface. This provides an opportunity to extend the inherent advantages of PPP to users who use Ethernet to connect their PCs to the ADSL Network. There are two ways to support Ethernet users at the ADSL-NT end: one is to load PPP over Ethernet (for example, the local L2TP tunneling method ). The other is to close the PPP (and ATM) on the ADSL-NT end. The two methods are discussed below. (1) local L2TP Tunnel
Since PPP is designed for serial connections, it requires new ideas and technologies to work on Ethernet. A technology that extends PPP to Ethernet is a local L2TP tunnel.
The ADSL-NT maps PPP groups from an L2TP tunnel (between each PC on the ADSL-NT and LAN) to an atm vcc, and vice versa. People can think of ADSL-NT as a network access server: accept internal "calls" through a local tunnel over Ethernet, and then forward them to an ATM connection carrying PPP end-to-end sessions.
The local L2TP tunnel containing the PPP group is loaded to the IP Group and then placed on the Ethernet frame. ADSL-NT restores PPP groups and inserts them into ATM cells in segments. In the downstream flow direction, the ADSL-NT receives ATM cells from the network through dslam, reassembles the cells, restores them to PPP groups, and then forwards the packets to the final destination over Ethernet.
To support PPP sessions between remote users and ISP routers, The ADSL-NT must be able to map the atm vc over a LAN to a local L2TP tunnel. The ing and Protocol 1 used.
Service telco ATU-R (clientpc)
Provider Figure 1 protocol architecture of PPP over Ethernet based on L2TP PPP-over-L2TP tunnel operations:
To start a PPP Session, the PC establishes an L2TP tunnel to the ADSL-NT (that is, the ATU-R. Use the private IP address allocated to the PC through DHCP to perform local communication on the local IP subnet.
When the ADSL-NT gets an L2TP Session Request from the PC, it dials to a specific ATM address to initialize an SVC of the ATM network. An L2TP request includes a LAN address or name and e.164 (the modem can convert the e.164 address to NSAP format and use it to start creating an ATM call to the network ). For the PVC service, e.164 can be mapped to the PVC.
Once the ATM connection is established, the L2TP tunnel request is known, and the PPP session can be performed on this connection circuit. When the tunnel or SVC is released, the modem releases another part. (2) PPP proxy
You can also use a PPP proxy to support PPP over ATM over Ethernet. Here, the ADSL-NT represents a simple ADSL Router for hosts on the LAN, but a PPP host for the ISP Server or remote access server. This ADSL-NT can be an independent hardware device or a PC-based server, which has both an ATM/ADSL Nic and an Ethernet NIC, respectively connecting the ADSL Access Network and the customer's LAN.
There are two possible methods to support PPP proxy. One is to use a different global IP address for each host on an atm vcc; second, several hosts on the same VCC share a Global IP address. 3 architecture of non-end-to-end ATM connections in the ADSL Access System We have discussed the architecture of the ADSL Access Network Using end-to-end ATM connections. The following describes two core network architectures that support non-end-to-end ATM connections for ADSL Access Services: L2TP access aggregation (LAA) and PPP terminal aggregation (PTA ). They are similar in terms of network topology, but data is transmitted through regional broadband networks in different ways. (1) L2TP access set
L2TP is a protocol that extends sessions on any network to remote network servers called L2TP network servers (LNs. It enables a user to connect a PPP session to the LNS instead of directly dialing to the target ISP or corporate dialing server, but to an L2TP Access Concentrator (LAC.
The basic principle of L2TP is to put PPP in a tunnel for transmission over a regional broadband network. The L2TP Access Concentrator (LAC) function is implemented by connecting a regional broadband network to a dslam device. The L2TP network server (LNs) function is implemented in the ISP or company's router connecting to the regional broadband network. Each user has an atm pvc: Starting from the ATU-R, crossover in dslam, terminated at Lac. PPP over ATM is used in the ADSL Access Network. The LAC and LNS can be connected to any network that supports the L2TP protocol. The PPP link is enclosed in the tunnel and transmitted from the LAC to The lNS of the NSP through the Regional Broadband Network.
Protocol Stack 2 of LAA.
Figure 2 LAA protocol stack Dslam and ATU-R are both part of the ADSL Access Network. Assume that PPP over ATM over ADSL is used in this part of the network. Dslam is connected to a lac. By connecting dslam to the lac in the form of a PVC per ATU-C, This simplifies the service activity process. The LAC determines where the user needs to connect to and sends the PPP session to the NSP through a tunnel. The LAC is connected to the Regional Broadband Network, which has the network topology structure required by NSP. NSP uses a router with lNS function to terminate the user's PPP connection in addition to the L2TP encapsulation. The router also provides a conversion from the Regional Broadband Network Technology to the NSP technology. This is similar to the method provided by the dial-up modem for access to NSP.
PPP provides the Link Control Protocol (LCP) and Network Control Protocol (NCP) to determine the link layer and network layer options. It can be implemented on an end-to-end line between the PC and the destination router. (2) PPP terminal centralization
In this structure, the PPP session is not transmitted through a tunnel on the entire route to NSP, but ended on the Broadband Access Server (BAS ). Extracts IP groups from the bas end and forwards them to an appropriate NSP through an IP-based regional broadband network. This architecture does not require that an ATM structure be used to connect to the NSP. Any network technology that can encapsulate IP groups can be used between NSP and ADSL access networks.
A user initiates a session by establishing a PPP connection between the CPE and Bas. Bas terminates the PPP session and forwards the user's IP business flow to the corresponding NSP. NSP does not need to understand or support the PPP function. Its Protocol Stack 3.
Figure 3 PTA protocol stack Dslam and ATU-R are both part of the ADSL Access Network. Assume that this part of the network uses PPP over ATM over ADSL. Dslam connects to BAS through an ATM network. Pre-allocate an atm pvc to each user. Like the LAA model, this allocation is purely driven by the access provider's user process and has nothing to do with NSP. Similarly, when a user changes the NSP, the access provider is not affected.
The functions provided by Bas include IP Address Configuration, user identity authentication, approval, and billing through PPP protocol groups. Bas receives the user registration and password, uses a query language to the nsp radius server, and uses NSP to confirm the user identity. IP addresses and other user configuration information are also obtained from NSP.
On the other hand, Bas provides an IP routing interface for NSP. BAS is configured to support any physical layer transmission from IP groups to NSP. After a PPP session is established with the user, it maps a User Identifier to the NSP port. This User Identifier can be a session identifier or user port identifier. This unique ing transfers the user's IP group to the destination NSP. It is very important that in the upstream direction, Bas functions like session-driven routers. Indeed, the bas forwarder does not rely on the target IP address to determine the NSP, but relies entirely on the session-session combination between the user and the NSP. In addition, NSP uses an IP router to receive and forward these groups.
When a user wants to establish a connection to NSP, the user establishes a PPP session to BAS. Users and Bas exchange PPP groups on the PPP link. Bas and NSP exchange IP groups on regional broadband networks using specific network technologies specified by NSP. When the user wants to end the connection to NSP, he will interrupt the PPP session to BAS. Bas deletes the user-network service provider ing in its routing table and returns the IP address to NSP.
Similar to today's dial-up remote network, the PTA protocol stack uses PPP to encapsulate IP groups. The ADSL access network adopts PPP over ATM over ADSL. Each user pre-allocates an atm pvc to BAS. From bas, IP groups are extracted from PPP frames and sent to NSP through different network technologies. For an ATM, an atm vc is allocated between bas and NSP. 4 Conclusion The architecture of the access network based on ADSL involves many factors. The scheme proposed in this paper is a feasible scheme based on the existing communication protocols and makes full use of the functions and advantages of ATM, PPP, and L2TP.
References
[1] T. Kwok, ATM: The New paradugn for Internet, intranets and residential broadband services and applications, Upper Saddle River: Prentice Hall, 1997.
[2] T. Kwok, "residential broadband Internet services and applications requirements," IEEE communications magazine, June 1997.
[3] J. hernanen, "Multiprotocol encapsulation over ATM Adaptation Layer 5," IETF request for comments: 1483, July 1993.
[4] K. hamzeh, Et Al., "layer two Tunneling Protocol" L2TP "", Internet Draft,/draft-ietf-pppext-l2tp-04.txt, June 25/1997.
The Internet access ubuntures Based on ADSL
Lihua
(Department of optical information technology, Nanjing University
Of Posts and Telecommunications, Nanjing 210003, China) Abstract The access ubuntures Based on ADSL involves restricted factors. In this paper, we propose a kind of Internet access architecture based on the present protocols, such as ATM, L2TP and so on. Key words access network end-to-end model Asynchronous Transfer Mode (ATM) Point-to-Point Protocol (PPP) Layer 2 Tunneling Protocol (L2TP) Author member name: karinli |
