New DSL Technologies and Related Broadband Operation

The broadband access market in China has kept a rapid growth since it was initiated. The Digital Subscriber Line (DSL) technology is the dominant broadband technology applied in the market. According to statistics, China had followed the United States to become the second largest DSL market with over 10 million subscribers by the beginning of 2004.

    However, many problems in DSL applications have emerged along with the fast growth of the market. High-speed access to the Internet is the most important DSL service available, which regards residential subscribers as its target users. But such an Internet access service cannot bring broadband operators great profits, since a large amount of investment in network construction is required and the service is related to numerous end users, which may bring heavy operation and maintenance workload, consuming much manpower and money. The revenue from the low charges fails to balance the large investment and operation costs. Therefore, DSL services can hardly help operators make profits at present.

    Gaining profits is important for the long-term development of DSL services. On one hand, low cost is necessary for users to enjoy DSL services at low prices; on the other hand, DSL operation can be improved through the use of new technologies, such as video multicast, online testing and user management.

1 Video Multicast
There are two kinds of Video services. One is the unidirectional broadcasting video service, such as traditional TV broadcasting services; another is bi-directional interactive video service such as Video on Demand(VoD). Although high-speed access to the Internet has become the dominant application of DSL technologies due to the popularization of the Internet, the initial design purpose of Asymmetric Digital Subscriber Line (ADSL) was actually to support the VoD service. DSL technologies do not suit for broadcast services because twisted-pair cables have far narrower bandwidth than coaxial cables, so multiple video traffics cannot be forwarded simultaneously. Each unicast stream is only sent to one client that requests it. Unicasting wastes bandwidth by sending multiple copies of the data, which makes great pressure to the backbone network. Even if video streams with high compression ratio are available for the Digital Subscriber Line Access Multiplexer (DSLAM) that can support 1 000 subscribers, the requested bandwidth at the network side will beyond 1 Gb/s. It is obviously unrealistic.

    However, with the multicasting technology, the same video stream only needs one time delivery to those clients who request it, which helps minimize the demand for bandwidth of the backbone network. On the downstream way, DSL only transfer user-requested video streams, TV channels or VoD programs. This makes it possible for operators to offer multicast video services.

    Typical Video Applications are as follows:
    (1) VoD: Stored non-real-time contents are sent as unicast streams to clients who request them. VoD is usually not interactive except for the control information.

    (2) Multicast Video: It can be regarded as an extension application of VoD. Program sources can be organized into channels and then sent via multicast.

    (3) Videoconferencing: A videoconference can be held between two parties as well as among multiple parties. The former can be regarded as a video call, and video data streams are transferred point-to-point.

    (4) Remote Learning: Multiple transmission schemes can be mixed and used in remote learning.
It is estimated that broadband video applications would certainly play an important role in future networks and change the network utilization to a certain extent.

    Video streams of multicast services are transferred in MPEG over IP or in MPEG over ATM. When in MPEG over IP, the transmission is implemented according to IP multicast protocols. While the MPEG over ATM scheme is chosen, point-to-multipoint ATM connections will support the transmission. Besides, a mixed mode can also implement the transmission. DSLAM is responsible for swapping requested channels and forwarding traffic.
There are two kinds of multicasting schemes: multicasting for special purposes and mixed multicasting. As for multicasting for special purposes, a special multicast access network is offered, and DSLAM provides special interfaces to connect the multicast network. The multicast services on DSLAM and Internet services are transported respectively. However, the mixed multicasting just takes advantage of the existing broadband access network. And the multicast services on DSLAM and Internet services are mixed to transport. Figure 1 is an example of multicast networking.

    It should be noted that, although all DSLAM equipment vendors declare that their products support multicasting, the fact is that a large amount of DSLAM equipment simply supports a technical protocol without taking the actual application of multicast services into consideration. Such equipment may work well in lab, but it cannot provide good-quality services on the real network. Operable multicasting asks for the following improvements in DSLAM.

    (1) DSLAM needs to support multicasting control protocols to implement controllable multicasting. The multicasting protocols include both public standards such as IGMP, IGMP PROXY and Snooping, as well as private protocols of vendors. So the interoperability between different equipment has to be carefully considered.

    (2) DSLAM’s core switching and the bandwidth and forward mode of DSLAM’s backboard bus are requested to sufficiently support multicast distribution. However, the original architecture of DSLAM is designed for repeated multiplexing of unicast, which is liable to bottleneck multicast traffics.

    (3) A complete user management function is necessary. Although the user management and authentication can be implemented via the Broadband Remote Access Server (BRAS), the closer multicast equipment is to users, the more beneficial it is to service response and bandwidth saving. Therefore the highest forward efficiency can be obtained by integrating multicast authentication with DSLAM. Moreover, the latency of channel selection can be reduced to minimum in this authentication mode.

    (4) Most multicast services are video services to meet demands of subscribers of high value, so QoS of multicast services must be guaranteed. For the satisfaction of video subscribers, DSLAM should be improved to support QoS of multicast services and to adopt special data queue and traffic flow controlling.

2 Online Testing
As DSL is used on a large scale, the correlative maintenance and line pre-selection become more important. Online DSL testing is a good solution, which helps offer parameters for primary subscriber line selection, analyze reasons for faults and troubleshoot them. Accordingly, the great demand for maintenance personnel can be avoided, and maintenance efficiency improved.

    Current DSL is still based on the traditional telephone line, belonging to narrow-band communication. The traditional telephone line was only used for voice signal transport early. The highest frequency of the voice signal is
3.4 kHz, so normal conversation wouldn’t be intensively interfered by noise, crosstalk, longitudinal unbalance and poor insulation. However, for DSL services, it is another picture. For example, ADSL services make use of the telephone line as their carrier, and the bandwidth of 4 kHz-1.1 MHz is divided into 256 subchannels to transfer data. Therefore, such services are extremely sensitive to quality of the line. Noise, crosstalk, longitudinal unbalance and poor insulation have great impacts on high-frequency band for data transmission.

    DSLAM and modem products are now matured and stable, so the capability and quality of the line to deliver high-frequency data signals are decisive factors in quality of DSL services.  Therefore, in order to guarantee the normal use of DSL service, the performance of the related subscriber line must be overall examined before deploying a DSL port. However, traditional testing and maintenance means by handset instrument cannot meet the demands of increasingly heavy line maintenance any more. How to guarantee the transport quality of  high-frequency data on telephone lines and what test means can be used for line examination and maintenance have currently become key problems in daily DSL maintenance.

    For operators, online testing can easily offer the following services: 
    (1) Pre-select the line and assess the service quality and service level.
    (2) Shoot troubles and quickly settle troubles.
    (3) Carry out long-term and periodical user line maintenance.
    (4) Collect, classify and store detailed user line data that can be used for launching other services.

    The online testing includes Double-End Line Testing (DELT) and Single-End Line Testing (SELT). ITU-T G.992.3 has described that DELT is an online test conducted at the central office end and CPE. It is mainly applied for line maintenance after a DSL service is launched. In addition, there are problems of interworking between the central office end and the user end, especially at the initial time. In fact, SELT has more significant applications, because it can help operators ensure the usability of a line before a subscription for DSL services is accepted. However, SELT also has its weaknesses, such as the unavailability of certain testing functions. DSLAM vendors should try their best to further improve the performance of SELT.

    The performance parameters of a user line can be measured through line testing. The parameters include the line length, bridge tap, coil, line damage, background noise, crosstalk and so on. In addition, line testing can implement measurement and assessment of DSL parameters, such as the maximum access rate, uplink/downlink rate, noise margin, transmission attenuation and crosstalk at remote/close ends. Therefore, a testing system often supports simulation of both the central office and remote ends, using testing equipment to simulate the real connection of DSL equipment with the opposite end and to test interworking between upper-layer services, including tests of IP Ping, FTP download and so on. This function gives a strong support to troubleshooting.

    The online DSL testing system can make centralized and automatic trouble detecting for all ADSL service subscribers in its coverage. If connected to the PSTN Maintenance System, it can also implement the centralized settlement, automatic assignment and unified management. The system consists of the ADSL network management system, DSLAM equipment, ADSL testing equipment and the PSTN maintenance system.

    A networking scheme of DSL testing is shown in Figure 2. The DSL equipment is connected to DSLAM through the test bus and the communication interface. The test bus implements line capture, while the communication interface functions as the data channel for control. The DSL network management system controls all equipment connected to DSLAM via the management network, working as a testing server. Meanwhile, its testing functions also include user port status inquiry. And its connection with the PSTN maintenance system can implement the centralized testing of DSL services.

3 User Management
With the rapid development of DSL technologies, DSL broadband access services have been accepted by common people. The DSL access adopts advanced line modulation technology, so a DSL modem is necessary at the user side. However, with growing applications and demands, functions of the modem become more specific and more complex. Consequently, modem configuration and management have become a work beyond users’ capability. In addition, operators don’t want their subscribers to change any modem configuration by themselves. They also monitor service users to prevent them from using services without subscription, and even to trace and locate illegal users or hackers. Based on the actual situation, operators have to pay much attention to solve problems such as how to launch services rapidly, change configuration remotely, locate faults accurately and find users’ exact information such as the location easily.

    User management in the current operation model and business model mainly involves service management and terminal equipment management at the user side.

    (1) Service Management
    Service management means to guarantee legal users to use their applied network resources, meanwhile to limit and refuse illegal users. User authentication management is usually implemented via BRAS, but there exist some problems in actual operation. Generally, BRAS can only authenticate and manage service accounts, failing to distinguish user’s position. Since different accounts have different limits and rights, this may cause illegal use of accounts with high-level rights. In the early days, DSLAM uplink in the ATM system can implement service binding in the Permanent Virtual Circuit (PVC). However, current IP DSLAM often provides Ethernet interfaces for uplink, and user information cannot reach authentication equipment. The Virtual Local Area Network (VLAN) with only 4 K ID won’t meet the demands for all subscribers in the Metropolitan Area Network (MAN).

    Regarding to this problem, the solution is to transfer user information from DSLAM to authentication equipment (BRAS) via the service data plane or the control plane. Transmission via the data plane means to change data frames by adding related user information, such as the physical port number, applied bandwidth and account status. On the other hand, transmission via the control plane refers to information exchange during the authentication process. During the authentication, management equipment inquires DSLAM for user information. In this way, upper equipment can judge whether the current status is identical with the actual service subscription, preventing "flying accounts". In addition, hostile users can be found by tracing their locations.

    (2) Customer Premises Equipment (CPE) Remote Management 
    Nowadays, the carriers are facing the pressure from users, because users are unable to manage DSL terminal devices by themselves. And with the openness of business modes, more and more vendors provide diversified terminals of various functions. The configuration and maintenance of the terminal device are becoming more complex, and users may also change the configuration at their will. Therefore, problems of access failure or improper configuration may occur. In this case, the user may resort to the carrier, and sometimes complain to authorities, which always leaves the carrier in an embarrassing situation. Therefore, centralized terminal management is quite necessary.
In a DSL network, the DSL terminal is equivalent to DSLAM equipment, so their configurations should be matched. That is to say, the remote management of DSL modem should start from DSLAM.The centralized management of terminal equipment is carried out via the network in the way that a special link can be established. Through the established channel, the management center can visit DSL terminals, while taking DSLAM as a proxy. As a result, the operator in the management center can remotely test, configure and monitor CPE terminals, which makes network management and maintenance easier and helps improve network operation efficiency.

    The available carrier-level equipment management models are mainly based on the Simple Network Management Protocol (SNMP), Web and Telnet. Each model has its special strength and weakness. One or more models can be used for management at the user side, but the SNMP model is the best centralized management. However, since the relevant Management Information Base (MIB) has no unified definition yet, a mixed way is often used in practice.

4 Conclusion
With the application of ADSL2+ and Very-high-data-rate Digital Subscriber Line (VDSL) technologies, DSL systems will support higher data rate, longer transmission distance and more powerful management. Moreover, new services and operation techniques will be supported better. Therefore, DSL will find much wider applications in the future.
 
Manuscript received: 2004-09-29