A new access architecture opens the door to mass-market DSL

In response to the demand for high-speed Internet connectivity, service providers have spent the last few years building and upgrading their network infrastructure. By leveraging the ubiquitous copper loops that deliver lifeline voice, DSL service has the potential to address nearly every home in the U.S. However, the real-world deployment of DSL has been slowed by the inherent restrictions of the current data overlay network. 

In the initial rush to deploy DSL, carriers have focused on central offices because they offer abundant space and power to accommodate additional data equipment, and the cost can be amortized over tens of thousands of subscriber lines. In this deployment model, DSL is carried as an overlay to the voice network. A subscriber transmits voice and high-speed data traffic simultaneously on a single telephone line, and a POTS splitter in the CO separates the two traffic types. Low-frequency POTS traffic is sent to an end-office voice switch while high-frequency DSL traffic is sent to a DSLAM, where it is aggregated and transported to a separate data network. 

Remote issues

Extending this overlay architecture to remote terminals is inefficient, cost-prohibitive and does not scale to meet anticipated subscriber demand. According to market research firm RHK, remote terminals already serve about a third of the subscriber lines deployed in today's network, and that number will jump to more than 40% by 2005. Thus, many subscribers, primarily those who are served from remote terminals, remain underserved for broadband DSL access, especially considering the attractive demographics present in many of the neighborhoods where these remotes have been placed within the past 20 years.

Unlike installations in the CO, remote terminal DSL deployments present some challenges. Right-of-way issues, aesthetics and the high costs of additional cabinets, pads and power supplies deter service providers from building cabinet farms at the edges of neighborhoods. Moreover, co-location issues can force carriers to implement non-standard wiring methods to access subscriber loops. As a result, service providers have been limited in their ability to quickly and profitably respond to broadband demand.

Integration is the key

Due to the impracticality of overlay DSL solutions, a fundamental redesign of the current access architecture is required. A new, integrated access architecture that focuses on voice and data convergence, efficiency and scalability is essential. Technology innovation is required at the loop interface, since it is this interface that dictates system power, density and cost. The successful integration of POTS and DSL must start at the silicon level to provide a single broadband termination point for these services. 

By using a silicon-based, integrated POTS+DSL architecture, the need for network-side POTS splitters is eliminated, and a true merging of POTS and DSL can be achieved. This merging of DSL modem and POTS line-termination functions creates a common data path that enables software provisionable for POTS+DSL to be provided on every line. This innovative approach minimizes cost, power and size, and enables profitable delivery of voice and data services to all customers.  

An integrated POTS+DSL chipset provides not only a compelling capital cost solution for a unified DLC and DSLAM architecture, but it also delivers significant operational savings. Specifically, having POTS+DSL on every line eliminates the need for truck rolls to the remote terminal site to turn DSL services up or down, or to change or add cards when a service mix changes--a recurring cost associated with today's so-called next-generation DLCs. Highly integrated POTS+DSL chipsets enable advanced broadband services to be software activated from the network operations center as service requests are received, and create an access architecture that efficiently scales to meet subscriber demand.

By addressing these operational issues, service providers can realize the lowest possible service-activation and lifecycle costs. Thus, splitterless technology at the network termination point is key to enabling DSL to achieve true mass-market deployment.

Not all splitterless technologies are created equal

Splitters add unnecessary costs, complicate loop testing and inhibit network evolution and convergence. Integrating splitterless POTS and DSL functionality onto the same chipset enables effective provisioning of advanced broadband services. However, not all splitterless technologies are the same. In fact, splitterless technologies vary widely in size, power consumption and heat dissipation.

Many purported splitterless service cards in NGDLC systems are not splitterless at all. Instead, an analog splitter has been moved from a separate shelf to the line card itself. In this implementation, discrete POTS and DSL chipsets are mounted side by side on the same board and a separate service card for each service is not required. Although this is a step in the right direction, such cards are typically very expensive, and therefore it is not economical to pre-deploy these cards in anticipation of service activation requests. The combo card approach, with separate chipsets and on-board splitters simply pushes the current overlay model down to the line cards, offering no operational benefits and only marginal capital savings.

A simple splitterless implementation is an analog front end that digitizes the POTS and DSL frequencies and passes the separate data streams back to separate POTS and DSL chipsets. This implementation replicates the functionality of an external splitter and offers minor footprint reduction but little benefit in power reduction and heat dissipation. Furthermore, integration by "half measures" makes it difficult to deal with transient currents resulting in dropped packets, unnecessary retrains, etc.

A complete integration of analog front end, voice coder/decoder and data digital signal processors yields the smallest footprint and minimizes power consumption and heat dissipation. This solves many of the problems associated with combo card designs, which use discrete components for voice and data. The result is a chipset that enables superior density, nominal power consumption, lower cost, increased reliability, excellent rate reach and interoperability.

 

Figure 1: Line termination evolution

 

Ubiquitous broadband 

A new class of broadband access system, the broadband loop carrier (BLC), is based on this highly integrated, splitterless chipset architecture. By using BLCs with POTS+DSL on every line, service providers can expand their DSL footprint without sacrificing POTS density, minimize both capital and operational expenditures, increase network reliability, and scale to meet ubiquitous broadband demand.

Through the addition of voice-over-packet software algorithms, this inherently converged voice and data architecture also enables a seamless, line-by-line packet migration capability. This capability provides a smooth migration path for the vast majority of subscribers--specifically those without specialized IADs--to take advantage of the service innovation possible with a packet-based softswitch architecture. Simultaneously, it allows service providers to drive operational efficiencies by enabling all subscribers to be moved onto a packet infrastructure. Service providers can realize these efficiencies and incremental revenue opportunities, while preserving lifeline commitments to all POTS subscribers. 

By implementing an integrated architecture within the access network, service providers can quickly respond to broadband demand and profitably deploy DSL to all subscribers--including those served from remote terminals--without reducing POTS densities. This integrated architecture also protects against technology obsolescence by supporting cost-effective deployment of new, converged voice and data services.

Jon Glass is Director of Product Management and Malcolm Loro is Director of Product Marketing for Catena Networks. 

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