Wu Haining
Director, Fixed Network Marketing
Huawei Technologies Co., Ltd.

In the mid-1990s, when the asymmetric digital subscriber line (ADSL) technology was tested on a small scale, no one could foresee that nearly 100 million people worldwide would now be using this technology. Also, no one could forecast the great impact of Internet technology on our lives. At present, when the scale of the broadband network is getting bigger, and its services are becoming richer and richer, the xDSL network architecture is brewing new improvements, enabling the broadband network to provide content and applications not imagined before. These improvements are based the Internet protocol (IP) direction of the broadband access network's development.

The xDSL technology was born in the beginning of the 1990s, when the asynchronous transfer mode (ATM) network was being constructed like a raging wildfire, and the Internet was emerging. As a prevailing and mature broadband network technology, ATM became the mainstream technology of the xDSL network at that time. In addition, the popularization and application of the xDSL technology improved the development of the ATM technology, and made ATM-to-the-desktop possible.

With the development of Internet applications, the Internet protocol (IP) and Ethernet technologies became prevailing in the data communication networks, while the ATM technology quit the stage gradually as a result of limited application. At beginning, the ATM connection-oriented network layer application declined and was replaced by the connectionless Internet protocol (IP). Then, the ATM connection-oriented link layer application declined and was replaced by connectionless Ethernet technology. Up to now, more than 90 percent of the network traffic uses the IP bearer mode, and more than 90 percent of the user terminals are connected through the Ethernet. Consequently, the IP and Ethernet technologies are playing an absolute and leading role in the broadband network. As a network layer technology, ATM has been replaced by IP in the xDSL network. However, considering the compatibility of the terminal and network, the ATM still exists in the xDSL and at the convergence layer.

With the decrease of the ATM application and increase of the IP/Ethernet application, the cost difference between an ATM port and an Ethernet port is becoming bigger and bigger. At present, the price of a synchronous transfer mode (STM)-4 (622 Mbps) interface in an ATM switch is 10 times to 20 times that of a gigabit Ethernet (1,000 Mbps) interface in a Layer-3 Ethernet switch. To construct an economical broadband network, the carriers worldwide have now begun to replace the ATM with the Ethernet for the main link-layer technology. The convergence layer of the xDSL network, the part between the DSL access multiplexer (DSLAM) and broadband-remote access server (B-RAS), is replaced first. The following is a typical application of the xDSL network, and explains how the Ethernet technology replaces the ATM in the convergence network:

Figure 1: Application of the Ethernet at the xDSL Convergence Layer

Ethernet, with its lower cost, has replaced the convergence network between the DSLAM and broadband-remote access server (B-RAS), and the network is simplified a lot. Comparing this with the traditional ATM-based xDSL network, we can get the difference between the two networks in construction costs according to different amounts of users.

Figure 2: Difference in Cost between ATM Convergence and Ethernet Convergence

As seen from the comparison in Figure 2, ATM convergence is quite different from Ethernet convergence in cost, and their differences are especially obvious in cases where the network scale is smaller and the networking is more complex.

With the increasing development of the xDSL technology, Ethernet technology begins to infiltrate into xDSL. For example, the asymmetric DSL (ADSL2+) and very-high-data-rate DSL (VDSL) all introduce IP/Ethernet packet-transmission capability. In addition, in the VDSL field, it is quite popular for the link layer to use Ethernet technology. The network architecture becomes simpler and more compact after the Ethernet is used in the xDSL. For this reason, it is estimated that the network construction cost of this architecture will be lower.

From the above analysis, we find that in a network with IP applications prevailing, using the IP/Ethernet instead of the ATM will simplify the network and reduce network-construction cost. This technology development guideline now gets more and more recognition from the carriers and equipment manufacturers worldwide.

The application of the IP/Ethernet to broadband access can not only save investment, but also improve the operational efficiency of the broadband network, bringing bigger benefits to carriers. The video-multicast application is a convincing example.

Many carriers earnestly long for the development of television services through the twisted pair, for they can not only deal in traditional telecommunication services, but also enter the profitable entertainment broadcast field. However, this is impossible for a carrier that adopts the ATM network architecture, because:

• The ATM technology has already stagnated for a long time, without any progress made in the aspect of a standard for this application
• The ATM connection-oriented technology cannot satisfy the applications (such as multicast) with high-volume traffic
• The ATM switch and ATM-based DSLAM do not take the mass-multicast applications into account during design. Their data-stream copy mechanisms are complicated and low in efficiency. Therefore, they cannot meet commercial application requirements.
• The ATM network bandwidth is expensive, and cannot support services (such as multicast) that require high bandwidth

The IP/Ethernet technology is very mature in supporting the video-multicast application, because of its natural technical advantage and continuous improvement during practice:

• The IP-based multicast protocol called Internet group management protocol (IGMP) has already been brought to perfection and been tested in practical applications..
• IGMP is simple and effective. It can support the application with high-volume traffic and improve the users' experiences.
• The Ethernet switch and IP/Ethernet-based DSLAM support the multicast application naturally. Their data-stream copy mechanisms are simple and effective. Therefore, they can meet the commercial application requirements on a large scale.
• The Ethernet network bandwidth is cheap, and suitable for the services (such as multicast) requiring high bandwidth.

Besides support for applications such as multicast, the IP/Ethernet-based broadband network has distinct advantages in supporting multiservice, combined applications such as triple play. In addition, the updated IP DSLAMs have the following features to meet the requirements of quality of service (QoS), multiservice bearer, and security:

This supports:

• The differentiated services (Diffserv) mode, and has the differentiated services code point (DSCP)/class of service (COS)/802.1P Mark and Remark capabilities
• Different traffic classification functions, including the traffic classifications aimed at ATM premises visit charge (PVC), Ethernet packet headers, and IP packet headers
• The multipriority scheduling capability, with each port supporting at least four priority queues
• Multiprotocol label switching (MPLS)-based traffic engineering
• The connection with the bandwidth manager, which can provide absolute bandwidth guarantee for some real-time services.

This contains the following benefits:

• Support for powerful multicast capability,
• A large number of multicast channels (such as more than 1,000 channels) and a strong multicast-processing capability (such as more than 400 messages per second).
• The fast leave function (such as less than 100 ms) which can improve the user experience.
• The multicast preview function and on-line users' statistics, which helps operators to optimize the program.
• Controllable multicast, which is convenient for user management

And it supports or has:

• Multiple access functions, such as fiber-to-the-x (FTTx) and worldwide interoperability for microwave access (WiMAX).
• The ATM-convergence function and multiservice over multiprotocol label switching (MPLS), such as pseudo wire emulation edge-to-edge (PWE3).

It supports:

• The PVC/MAC/IP binding function
• The access control list (ACL) function, and the filtering of illegal addresses
• The point-to-point protocol over Ethernet (PPPoE+), dynamic host configuration protocol (DHCP) Option 82 and QinQ functions, and locates the users' physical addresses

The single PVC supports multiple services, which facilitates the compatibility with the network terminals.

And it supports:

• The automatic adaptation of the port type (ADSL/ADSL2/ADSL2 + automatic adaptation).
• The auto-negotiation of the xDSL PVC.
• The ATM operations, administration, and maintenance (OAM) of the ADSL port.

It is the fifth year of the large-scale development of the broadband network. The broadband access network has finally entered the age of IP. This trend will bring to us networks with higher service income and lower operation and maintenance costs. With all these trends becoming reality, are you ready?

Educational content provided by Huawei Technologies, Co. Ltd.