Bridgingthe DSL divide

With the rise of telecommuting, file sharing and Internet gaming applications, residential bandwidth demands have reached new heights.  As consumers clamor for more bandwidth, service providers gain the opportunity to cash in by delivering high-value broadband services to the home.  

While cable operators have captured an early lead with cable modem deployments, local exchange carriers (LECs) are currently planning a new approach to digital subscriber line (DSL) that is turning heads in the broadband world.  By harnessing the power of passive optical networking (PON) as a fiber based feeder for remote DSL installations, service providers can bridge the DSL divide, expanding their broadband service portfolios, making DSL available to more customers, and paving the way for a future evolution to fiber to the home (FTTH).

Burning the Broadband Bridge

According to the Gartner Group, approximately 40% of all potential DSL subscribers are beyond 18,000 feet of the nearest carrier’s central office (CO).  This means that almost half of the bandwidth-hungry households in North America alone are ineligible for DSL service, and with most CO DSL builds complete, that figure doesn’t look to be improving.  

These crippling distance limitations are burning the bridge for DSL’s success, forcing frustrated consumers to turn to competing broadband alternatives.  With cable modem users outnumbering DSL subscribers by a factor of two ⁽¹⁾, it is clear that LECs must take action to avoid losing the broadband battle.  But what’s an LEC to do when almost half of its potential subscriber base is too far away to serve?  There are two choices: 

• bring customers closer or

• push fiber deeper.

ROI return on investment How much "return," usually profit or cost saving, results. --from TechTarget.com Read more

If You Can’t Raise the Bridge, Lower the River

Of course, bringing customers closer to the CO is not an option.  However, driving the fiber network closer to residences provides an excellent opportunity to extend the addressable market for broadband services while maximizing the return on investment (ROI) on the 160 million copper access lines that exist in the United States today ⁽²⁾.  By shortening the distance between DSL endpoints and aggregation facilities, carriers can offer high-speed DSL services to 100% of the customers in their serving areas while enhancing their service mix to include value-added voice and IP data services.  But while benefits abound, building a remote DSL network is not as easy as dropping a new piece of hardware into the central office.  Designing and deploying such an access network is a challenge, but one that brings with it tremendous opportunity.

There are a number of factors to consider when planning a remote DSL deployment, including the ability to provision new subscribers and services rapidly, the flexibility to accommodate changing needs, and the versatility to manage the entire access network.  As with all access builds, price/performance is king.  Therefore, remote DSL deployments must be cost effective and allow service providers to deploy capital opportunistically--only when infrastructure investments are supported by revenue projections.  To stand the test of time and maximize ROI, this new network must also scale to support future bandwidth needs, additional users and enhanced service offerings. 

From Scientific American The Broadest Broadband A Less Expensive Way to Bring Fiber to the Home by Paul W. Shumate, Jr.

Bridging the Gap

While some carriers have evaluated T-1 lines and Sonet links to connect remote DSL facilities, neither technology meets the current and future requirements of the access network.  T-1 lines are relatively inexpensive to deploy, but offer no flexibility and/or scalability as bandwidth needs increase.  Sonet offers increased bandwidth, but is far too expensive to fan out to multiple serving areas and requires a significant capital expenditure before customer one can be served.  To bring economical, scalable broadband connections to remote DSL facilities opportunistically, passive optical networks (PONs) are the best bet for bridging the gap.

DSLAM Digital subscriber line access multiplexer Delivers exceptionally high-speed data transmission over existing copper telephone lines. A DSLAM separates the voice-frequency signals from the high-speed data traffic and controls and routes digital subscriber line (xDSL) traffic between the subscriber's end-user equipment (router, modem, or network interface card [NIC]) and the network service provider's network. --from the IEC Web ProForum on DSLAMs by Nokia Read the tutorial

By connecting remote terminal DSLAMs or next-generation digital loop carriers (NGDLCs) with PONs, carriers can take advantage of a flexible, economical network architecture that is ideal for serving today’s residential needs and grows to accommodate tomorrow’s increasing bandwidth requirements, take rates, and service offerings.  Using PONs, service providers can provision fractional wavelength services between 1 and 100 Mb/s to remote DSL networks, increasing bandwidth with the click of a mouse as a serving area’s needs increase.  Furthermore, service providers can overlay wavelengths on a PON without upgrading the outside plant infrastructure between the CO and the remote DSL facility, enabling a highly scaleable access network that stands the test of time.

Because many of today’s 1.8 million remote DSL installations ⁽³⁾ are fed with T-1 lines, migrating to a PON infrastructure can free up many of these connections for redeployment.  By doing so, carriers can capture additional business customers with traditional voice and data services using the existing copper plant to maximize ROI on their networks while preparing for next-generation optical access service offerings. As an added benefit, eliminating T-1 lines from the copper binder group reduces spectral interference, which reeks havoc on DSL lines by reducing the reach and bandwidth available for residential customers being served by the CO.

PONs also allow service providers to harness their existing fiber and copper plants to make the most of the investments they have already made, while enabling opportunistic fiber “fan outs” to support new customers and new service rates.  As opposed to Sonet systems, which require costly add/drop multiplexers at each junction point, a PON infrastructure takes advantage of inexpensive passive couplers to fan existing fiber out to new serving areas through a point-to-multipoint fiber architecture.  Therefore, a service provider’s sales and marketing divisions can target new clusters of customers while their engineering organization brings fiber to the new serving area; only installing electronic components when a significant revenue opportunity is uncovered.

Bringing Home the Fiber In a PON fed Remote DSL architecture, service providers are easily evolving their networks to provide fiber-based broadband services to the home.  By tapping into a passive coupler near the Remote DSL facilities, service providers are pulling fiber directly to early adopters that require more bandwidth than DSL can provide.   Once again, this opportunistic approach to network evolution only requires investments in fiber facilities and electronic equipment when service revenues support them.  Furthermore, as additional residences require high-bandwidth fiber-based services, this architecture supports the evolution to ubiquitous Fiber to the Home services.

By adopting this opportunistic model for extending the reach of DSL, carriers can broaden their addressable market not only by serving new residential areas, but also by breaking into the lucrative business market.  For example, carriers can leverage the same PON network to serve remote terminal DSLAMs, NGDLCs, and small and medium businesses (SMBs), enabling significant operational savings throughout the network planning, deployment, provisioning and maintenance processes.  

Through this unified access network approach, carriers can also expect increased ROI on their access infrastructures through a range of high-value service offerings including virtual private networks (VPNs), transparent local area networks (TLANS), and virtual local area networks (VLANs).  Furthermore, a PON-fed remote DSL deployment is the ideal stepping-stone to fiber to the home deployments (See Sidebar: Bringing Home the Fiber).

Conclusion

With North American remote DSL installations expected to serve over 8 million users by 2004 (4), it is imperative that service providers examine feeder technologies that maximize return on their existing network investments while providing a future-proof bridge to tomorrow’s fiber-based access services.  PON technology has emerged as the most efficient means of leveraging both fiber and copper assets to extend the reach of DSL services and increase the addressable residential broadband market.  

Furthermore, with its ability to leverage a single outside plant infrastructure to serve both residential and business customers, PON has the potential to unify the access network to streamline operations and boost network efficiency-–thereby extending the optical network closer to the door of every business and residence in a carrier’s territory.
Tony Zona is Founder, President and CEO of Quantum Bridge Communications, Andover, MA. He can be reached at tzona@quantumbridge.com.

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