In the loop

Next generation DSL has arrived in the local loop, helping to solve deployment issues associated with spectral interference and the cable binder group.

Carriers are battling to differentiate themselves through new methods of delivering voice and data, and copper-based high-speed access has long been touted as the answer. But in reality, today's high-speed solutions are not deployed easily and often incur significant additional costs.

For example, in the unbundled loop environment, moving dial-up solutions into the lucrative, growing DSL arena is daunting. Not only must carriers evaluate the types of DSL available, they also must overcome two distinct challenges: the existing telecommunications infrastructure and spectral interference between high-speed services. Currently, carriers are engaging in the costly, time-consuming process of re-engineering their infrastructures to accommodate these challenges. Even in a single-carrier environment, this cable plant engineering is required to deliver multiple services to customers.

The primary cause of today's deployment challenges relates to constant bit-rate (CBR) technology, which is the basis for current high-speed access solutions. CBR technology creates a steady transmission on a specific frequency and does not automatically adapt its frequency characteristics if interference and incompatibility occur during normal operation. Because of the rigid nature of CBR technology, spectral incompatibility occurs, causing services to be susceptible to interference that impedes communications.

However, there is an alternative to today's CBR technologies and re-engineering. Recent advances have enabled next generation DSL solutions to solve the deployment problem. Differing from traditional technologies, these new solutions rely on burst-mode transmissions, which are more resilient within existing telecom infrastructure and can manage spectral interference.

The importance of the binder group

Because of today's spectral compatibility uncertainties, service providers must follow deployment rules in order to deploy robust and well-maintained subscriber services. For example, with today's CBR technology, carriers must assess the condition of the local loop to discern if DSL can be deployed on the lines.

Once the local loop passes the first test, carriers must avoid interference between lines by ensuring that various DSL services do not reside in the same copper line bundles.

The existing copper infrastructure that carriers use originally was installed to handle only voice services. This infrastructure presents deployment barriers for advanced voice and data solutions, which were unheard of at the time that the copper was installed. The copper infrastructure, or local loop, uses unshielded twisted pair wires to service each phone. Cables are built by combining groupings of 25 pairs of twisted wires, referred to as binder groups. The name "binder group" refers to the coil of cotton ribbon that holds or binds the 25 unshielded twisted pairs together. Carriers often color-code these binders to assist in connecting the cables, which can contain 100 or more local loops (Figure 1).

The two sources of spectral compatibility problems within the binder group originate in the existing telecom infrastructure and in crosstalk. These issues, if not addressed, prohibit the deployment of DSL.

The current telecom infrastructure creates echoes from changes in line impedance. These changes occur whenever the gauge of the wire changes, or when bridged taps are present (Figures 2 and 3). Bridged taps are unterminated wires that hang off a local loop and often are created when carriers connect to a home or business by splitting off one of the UTPs present in a cable without disconnecting the unused portion of the loop. After many decades of using bridged taps, many undocumented connections have been made, leading to an unpredictable number of gauge changes and bridged taps adversely affecting current CBR-based DSL.

In addition, today's CBR technologies cause crosstalk, a phenomenon of transmitted energy leaking from one twisted pair into another twisted pair. The dominant crosstalk, near-end crosstalk, derives from transmitters from certain loops that are on the same end of the line as the receiver of another loop. Another source of crosstalk is interference from transmitters located at the opposite end of the line from the receiver; this is called far-end crosstalk (Figure 4).

Working toward the solution

The FCC is addressing the spectral compatibility issue by requesting a universal standard from the American National Standards Institute (ANSI). The task of drafting a standard was delegated to ANSI's T1E1.4 working group, an open forum in which manufacturers, technology developers and industry leaders contribute to industry standards. The group has created other industry standards such as the T1.413 asymmetrical DSL (ADSL) standard.

Based on the discussions in the working group, a draft standard has been developed that will qualify high-speed services as spectrally compatible. It does this in two ways:

- The first method relies on the service fitting into a defined class of transmission profiles and accommodating the engineering deployment rules attached to them when appropriate.

- The second method considers future developments that may not fit into defined classes. This approach - called "Method B" - enables a service to prove its spectral compatibility with other services through an analytical test of the technology's transmission characteristics.

Spectral compatibility is an important step in the right direction, but it is not the whole solution. Under the draft standard, interference still may occur between the defined classes of service. The standard's deployment rules are designed to address service interference, but for services to reach mass deployment, the number of constraints must be further reduced or even eliminated. Not only will services need to be compatible with one another within their service class, but services also must be spectrally agile to negotiate between each other without affecting one another in the spectrum of the binder group. This spectral agility creates versatile services that will not interfere with or be interfered with by other services.

To eliminate re-engineering gymnastics and provide a robust service delivering the quality and speed consumers and businesses demand, next generation DSL technology must be both spectrally compatible and agile. For DSL to evolve into a deployable solution, next generation high-speed solutions should combine the best features of proven technologies, including dial-up equipment and cable modems. Unlike CBR technologies, next generation DSL will employ burst-mode transmissions that allow it to "listen" to line characteristics and manage around potential interfering services, making it compatible with POTS, T-1, ISDN/ISDN DSL, high bit-rate DSL, symmetrical DSL, ADSL and G.Lite services.

This copper-based, next generation DSL solution should eliminate a truck roll because it is resilient to bridged taps and wire gauge changes. In addition, the ease of deployment should reduce costs by taking loop re-engineering out of the equation. The technology will evaluate activity in the binder group and adapt dynamically, ensuring maximum throughput with no interference. If this all sounds like a distant dream, it isn't - next generation DSL technologies already are being introduced and deployed. Elastic Networks' EtherLoop, for example, is a spectrally agile solution that seamlessly operates within existing plant infrastructures.

Combining spectral compatibility with spectral agility significantly affects the telecom industry - it eases and speeds up high-speed service deployment, which translates into increased value-added services with lowered costs to the customers. The bottom line is that spectrally compatible technologies create faster, more cost-effective and deployable solutions, meeting the growing demands of customers and businesses.