FTTN: The 'other' fiber access option

Jan. 5, 2006
By Bryan Kennedy, ADC -- The FTTN option enables service providers to take advantage of existing copper infrastructure to deliver cost-effective, high-bandwidth solutions.

The FTTN option enables service providers to take advantage of existing copper infrastructure to deliver cost-effective, high-bandwidth solutions.


While fiber-to-the-premise (FTTP) continues gaining momentum in delivering triple-play services to customers, it may not be the optimal solution for every service provider. Some providers are opting to take fiber only as far as necessary to take advantage of existing copper infrastructure for a more cost-effective alternative.

Fiber-to-the-node (FTTN) solutions can be adapted to numerous scenarios and situations, including passive standalone architectures and active integrated architectures that embed the service delivery frame into an active cabinet. There is even a "combo" solution that provides FTTN today with a migration path to FTTP once the existing infrastructure is no longer suitable.

However, as with FTTP, there are challenges and considerations inherent in deploying a successful FTTN network. There is no such thing as a "one size fits all" solution when transforming the traditional switched network into a high-speed, high-capacity broadband network.

The challenges of FTTN

A major challenge facing FTTN deployment is the need to "resectionalize" the distribution area. With the introduction of VDSL and ADSL2/ADSL2+ technologies, loop length distances in the distribution area have been reduced from as much as 12,000 ft to between 3,000 ft and 5,000 ft.

This is also true of the 18,000-ft central office (CO)-fed distribution area, where copper outside plant (OSP) customers also must be resectionalized to shorten all loops to within 3,000 ft to 5,000 ft. In addition, the DSL access multiplexer (DSLAM) may also require upgrading or replacement to support the new services. But the main challenge is the fact that the CO will be limited to serving a 3,000- to 5,000-ft area.

The same holds true at the remote terminals, whether fed by copper or fiber. Each must be rearranged to support customers within a 3,000- to 5,000-ft area, where previously they may have served an area of around 12,000 ft.

Low-density areas--areas beyond 5,000 ft from the last cross box--also must be addressed. Traditionally, these are the areas that "do not qualify" for new services due to the cost involved with distributing service to sparsely populated areas. Still, service providers must acknowledge the need for the same services in these areas and consider how to construct the FTTN network accordingly.

Resectionalizing the distribution area, therefore, will be a major consideration as service providers reduce coverage areas from the traditional 12,000 ft to smaller loops of 3,000 ft to 5,000 ft. Customized solutions will be required, including the addition of new cabinets at the edge of the 5,000-ft boundaries.

Other challenges of FTTN include finding ways to accommodate the various existing OSP designs. Right-of-way issues for new cabinet placements must be considered, and existing cross boxes may not have spare binding posts to support a direct termination method for delivering new service. Providers must also consider that not all customers will choose to upgrade services.

Delivering the goods

New FTTN broadband technology also poses challenges to service delivery--the most significant being pair bonding. Pair bonding enables the electronic bonding of two output DSLAM ports inside the DSLAM to provide twice the bandwidth. However, successful pair bonding requires two continuous copper pairs to the customer premise, and it must be supported by the service delivery platform. Although there are methods to overcome this challenge, pair bonding remains a complicated and expensive endeavor for service providers.

Provisioning new ADSL2 and VDSL2 services requiring two copper pairs may not be possible with the existing infrastructure. Some homes may not have a second continuous pair available, forcing service providers to roll customers from their existing platform to a new platform that is closer to the subscriber--at additional expense.

Moreover, service providers must have clean copper pairs available; there cannot be any bridge taps on the line. Load coils and other means of stretching POT service also must be eliminated. Today's new services require a clean copper plant.

Leaping the FTTN hurdles

Since there are multiple approaches to deploying FTTN, multiple solutions are required. One approach is to use passive cabinets to upgrade DSLAMs at the remote terminal or where an existing cross box may lack capacity. This will also require small and medium integrated cabinets--active fiber-fed units--for resectionalizing the existing network infrastructure.

At the CO, the objective is to serve all customers within 3,000 ft to 5,000 ft. Copper-fed remote terminals with or without DSLAMs can be used to resectionalize the distribution area to serve customers within 5,000 ft. Existing fiber-fed remote terminals with or without DSLAMs also can service a 5,000-ft area. But new distribution areas will require new products, such as an all-in-one fiber-fed broadband cabinet with built-in copper distribution.

Architecture platforms that offer a migration path to FTTP or even offer both FTTP and VDSL/ADSL2+ out of the same cabinet also are available. Service providers want to ensure the architecture they choose will enable easy access points for testing the network. Easy service turn-up and troubleshooting operations equate to significant cost savings during the life of the network.

Another important consideration is the architecture's protection components. The protection elements should take up very little space inside the cabinet while providing over-voltage and over-current protection for the network element.

FTTN, FTTP or both?

As service providers make major decisions regarding the most cost effective, reliable method for delivering today's new voice, video, and data services to customers, both FTTP and FTTN architectures should be thoroughly investigated. Individual circumstances surrounding each deployment will dictate how far to push fiber toward the customer premise.

For Greenfield deployments, the cost parity between copper and fiber has made FTTP a viable choice. However, overbuilding existing networks with pure fiber may prove to be cost-prohibitive for many providers. In this case, FTTN becomes a very attractive alternative, particularly if it provides an easy migration to FTTP in the future. Whatever the choice, service providers should seek custom products that adapt to any deployment scenario while providing the most reliable, flexible, and cost-effective long-term solution.

Bryan Kennedy is an executive account manager at ADC (Minneapolis, MN). He may be reached via the company's Web site at www.adc.com.