Digital Subscriber Line (DSL) Versus Cable

This chapter is from the book

Cabling Handbook, The, 2nd Edition

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This chapter is from the book

This chapter is from the book 

Cabling Handbook, The, 2nd Edition

Learn More Buy

Planning Implications for the Enterprise Network Manager

For the enterprise network manager, operator, and service provider, it is now an accepted, inescapable conclusion that xDSL technologies will one day be installed on a global basis in vast quantities. Despite other technology developments (not excluding recent announcements about the commercial viability of sending data down power lines), it is still the only viable technology capable of substantially increasing bandwidth on the local access loops without a substantial overhaul. These copper loops are ubiquitous over every home, and it is unlikely that any mass-scale upgrade to fiber will happen within the next decade.

The primary driver for xDSL is high-speed Internet service deployment to residential customers. So far, there have been two small commercial deployments and no less than 66 trials around the world, trying to prove that asynchronous DSL (ADSL) can provide a downstream connection of up to 8Mbps and upstream connection of up to 1Mbps over the existing telephony copper pair.

Both IP and ATM network architectures are currently on trial. ATM is considered the probable choice in the future. Initial trials have used standalone ADSL modems with discrete IP interfaces for reasons of time to market and availability; these are evolving to highly integrated digital subscriber loop access multiplexers (DSLAM) solutions.

The complete network typically consists of a core that is based around Synchronous Digital Hierarchy (SDH), wrapped with broadband ATM switches, ATM access switches, and DSLAMs. The DSLAMs provide the individual xDSL lines out to the customer premises and integrate with the existing POTS network connections. But even after having been in trial for more than 18 months, few operators seem close to commercial rollouts. What is taking them so long?

Some of the main barriers to large-scale adoption include the continuing standards battle and the lack of interoperability. The existence of two de facto standards—namely, DMT and CAP—provides network operators with a dilemma: deciding which to adopt before a leader is clearly identifiable in case the choice ends up as the Betamax of the standards. There are two standard camps, one apparently led by Israel's Amati, which has developed modems using discrete multitone technology (DMT) for the line coding. Motorola ²⁷ and Alcatel ²⁸ are among the major manufacturers that have developed DMT modems. The second camp has adopted a technology developed by the former AT&T Paradyne, which championed carrierless amplitude modulation/phase modulation (CAP) for the line coding. Westell, the enterprise that made the ADSL modems being used in Bell Atlantic's Virginia trial, uses CAP. With no clear stance being taken, a general "wait and see" attitude is developing.

Lack of interoperability is also becoming a matter of concern. The current xDSL units on the market, from a variety of vendors supporting different standards, do not interoperate. Interoperability between vendors' equipment is key to mass rollout and provisioning. The ultimate goal is for the end customers to be able to purchase their own ADSL termination units, much the same way as people buy off-the-shelf analog modems today. For this to happen, the ADSL termination unit technology needs to mature to the point that it is as simple as a modem to install and operate.

Operators are still deciding on suitable end-to-end architectures. Once these infrastructures have been agreed upon and the business and technology are in place, services can then be rolled out in volume. These new architectures need to address the integration of network, service, and enterprise management of these new broadband services. Without these seamless, end-to-end management systems in place, it is difficult to see how operators will be able to make any profit on broadband services.

Enterprise Network-Management Capabilities

The new broadband services being deployed by enterprise network managers, operators, and service providers are typically delivered over very complex network environments that include legacy equipment with primitive management capabilities and newer systems incorporating sophisticated telecommunications management network (TMN)–based element management. New network-management infrastructures are required to mold these disparate information sources into a cohesive view of the end-to-end services being delivered. This can be complicated by the large scale of such services—typically involving hundreds of thousands or millions of network elements. The major requirements can be summarized as follows:

• Cost-effective performance scalability—A system has the ability to manage small pilot networks and expand as the network grows without performance degradation.

• Multiple protocol support—Standards-based and proprietary management protocols must be supported in a transparent manner to the network applications.

• Powerful event management—As the network grows, the number of events grows exponentially. The ability to manage this volume of events in a way that helps the operator make sense of the status of the network is mandatory.

• Seamless application interworking—Operators will use a number of different applications to manage aspects of the network, but they require the ability to move easily between applications—to investigate problems or configure service for a particular customer, for example.

• User access management—The information within the management system must be protected, but it must also be feasible to partition the data and the access to it in a way that supports operational processes.

Enterprise Network-Management Architecture

Some of the key management challenges when installing xDSL networks include potential element volumes. xDSL network elements are extremely complex devices that require real-time management. As element volumes increase exponentially, operators are faced with the challenges of implementing end-to-end systems that can manage millions of highly complex access elements.

As an example, the 1994 BT video-on-demand (VOD) trial supported 2,000 ADSL lines, equating to 4,000 access elements. Although this was classified as a small marketing trial, it is still the largest single xDSL installation in the world. This network generated a tremendous amount of management traffic, primarily with SNMP and performance-monitoring information, all of which had to be processed in real time by the central management workstation.

Additionally, xDSL equipment, by virtue of its inherent built-in intelligence, generates a tremendous amount of management traffic. This information, be it alarms, performance-monitoring information, configuration data, diagnostic commands, or inventory data, has to be processed, stored, prioritized, formatted, and displayed by the central management console. A single element-management workstation cannot scale to support any major network growth.

Element and network complexity is also an issue that operators need to prepare for. xDSL equipment is becoming more complex with each generation. The latest generation uses rate-adaptive transmission, which causes a myriad of possible upstream and downstream speed permutations. Remote equipment now incorporates xDSL transmission hardware, IP routing equipment, and IP or ATM CPE interfaces into a single unit. In addition, it has to be seamlessly integrated with the existing POTS network leading to a wealth of configuration permutations, performance monitoring information, alarm and event reports, and diagnostic functions and inventory data being available to the network operator.

A typical end-to-end network capable of delivering high-speed Internet, on-demand services, and data and voice from multiple service providers is extremely complex. Many issues can be resolved only with the use of integrated management applications and systems. This often results in a number of incompatible element-management systems being installed to manage different parts of the network and then integrated together to provide an overall end-to-end service-management system. The eventual goal is to provide a zero-touch system to allow the end customers to control and manage a large part of their own service offering in real time.

xDSL equipment is normally installed over existing telephony circuits, but the devices at each end of the connection contain POTS splitters that allow both the telephony circuit and the xDSL circuit to use a single copper pair. This requires the integration of a new xDSL management system with existing POTS systems.

A range of issues also clouds the most efficient interface for the customer premises equipment. The two protocols in question are IP and ATM. Network operators are installing complex networking equipment into residential environments for the first time. This can lead to a multiplicity of new problems. These problems range from units being tampered with and cables being disconnected, to PCs being reconfigured. This could result in a huge increase in the number of service-related calls and queries directed at the network operator. The operator has to be able to diagnose, isolate, and analyze problems from the central management system. Only when this type of network, system management, and control is achieved will broadband services become commercially viable.

Viability

The density of the technology must increase dramatically to enable operators to fit all the required ADSL termination units into their existing exchange real estate. xDSL technologies are still costly, and this is holding back general acceptance due to the costs of equipment investment and the ongoing line-rental charge. Prices in the range of $370 to $420 would start to approach an acceptable level for the benefits of this new broadband technology.

In most European countries, however, high-capacity broadband ATM backbone networks have yet to be deployed. Without this backbone infrastructure, there is little benefit to be gained from introducing an upgraded access network. In the infrastructure context, the questions of how some major European operators will play off ADSL against their substantial ISDN services will be an interesting debate. As it now stands, current versions of ADSL cannot coexist with ISDN BRI lines. New versions of ADSL are being developed that will coexist with ISDN, but, as yet, they are available only in small trial volumes from less than a handful of vendors.

Telecommunication enterprises such as Deutsche Telekom ²⁹, which bet its future on ISDN, are now involved in desperately trying to increase ISDN usage before ADSL establishes itself. The situation it faces (in common with many other operators in Europe) is that too much emphasis and keenly priced ISDN will undermine ADSL; at the same time, not investing an adequate amount of resource on ADSL now might undermine its future as a broadband services provider.

So, what does the future hold for DSL and cable modems? Let's look at DSL first.