There is no doubt that Fiber-to-the-Home (FTTH) access architectures provide the most scalable and future-proof solutions for broadband service delivery. However, the economics associated with FTTH architectures are not justifiable for all service providers, especially for universal coverage. For most service providers to realize universal coverage for IPTV and enhanced broadband services, alternative access network technologies must be adopted.

Historically, Fiber-to-the-Node (FTTN) architectures based on single-pair ADSL2+ faced significant challenges in the successful deployment of competitive video solutions. The bandwidths just were not sufficient to keep up with market demands. With continued improvements in MPEG-4 encoders and the maturation of VDSL2, full-service video offerings with multiple HD streams and Video on Demand (VoD) content are now possible over FTTN access networks, making FTTN a more viable alternative to FTTH for multi-service access networks.

The Economics of FTTN and FTTH
The appeal of FTTN architectures is simple: it is all about the economics and time-to-market. If the debate was solely on technical merits, FTTH would win out over FTTN every time.

The question is fairly basic: do the bandwidth advantages, operational efficiencies, and service offerings enabled by FTTH access networks justify the additional deployment costs? While a small percentage of service providers are fortunate enough to make the business case work for FTTH as a universal access solution for 100 percent of their footprint, the numbers just don’t add up for most service providers.

In many cases, service providers have chosen to target FTTH for the areas where the economics do work, particularly new construction and high-density neighborhoods, while delaying a decision on FTTH in the lower-density areas of their network. In other cases, service providers have led with a FTTN strategy, banking on a faster time-to-market and lower cost per sub to capture customers today, while maintaining a transition path to FTTH. The decision is different for each service provider given that competitive threats, subscriber densities, and economic situations are different for each service provider.

The economic advantages of FTTN in terms of total cost of deployment are clear. The majority of CapEx spend associated with overbuild FTTH deployments is in the distribution and drop plant within the last 3,000 feet. A quick review of the FTTH deployment cost benchmarks shows that the incremental cost to connect a household with FTTH, once it has already been passed with fiber, accounts for nearly one-half of the total deployment costs associated with FTTH (total deployment costs in high-density FTTH applications easily exceeds $1,000 per subscriber). By utilizing the existing copper infrastructure within the last 3,000 feet to 6,000 feet, FTTN architectures are able to eliminate the majority of the incremental costs to connect a home, as well as the bulk of the costs associated with the fiber distribution network, allowing for deployment costs that are a fraction of FTTH costs per subscriber ($250-$400 per FTTN subscriber connected).

Historical Limitations of FTTN Architectures
Despite the economic advantages, FTTN architectures have been limited for many years in their ability to offer voice, video, and data services. Most importantly, single-pair ADSL2+ does not provide the bandwidth required for multi-HD IPTV services, a minimum requirement to keep up with the benchmarks already established by satellite and cable TV competitors. While ADSL2+ Bonding has provided some relief, mass market deployment has been limited due to the reliance on ATM in the access loop and the requirements for two pairs per subscriber.

VDSL has promised to resolve both of these issues by enabling 25+ Mbps services over a single pair and utilizing Ethernet framing in the access loop. However, early implementations, in advance of the VDSL2 standard, struggled with long loop performance, interoperability, and support for legacy xDSL services. These limitations minimized several of the key attributes required to make VDSL-based FTTN deployments a viable alternative to FTTH for the deployment of multimedia services. Given these deployment constraints, many service providers were compelled to bypass or limit FTTN as the preferred access architecture for the delivery of voice, video, and data services.

Advancements in FTTN
With all of the excitement surrounding in the $7.2 billion Broadband Stimulus Plan, FTTH architectures quickly moved to the forefront when discussing next-generation access networks and for good reason. With new sources of funding in place, the economic balance shifted toward FTTH solutions, particularly in the Tier 3 service provider market. In the shadow of the FTTH fanfare, technological advancements related to VDSL2 have quietly, but significantly, altered the viability of FTTN architectures.

In many ways, 2009 was a breakthrough year for VDSL2. With the introduction of second-generation VDSL2 chipsets, the promises of cross-vendor interoperability, long-loop performance, lower power, and legacy ADSL support were finally delivered. With this proven capability in place, many service providers launched large-scale deployments of standards-based VDSL2 FTTN services, while others moved forward with preparation for deployments in 2010.

The primary drawback to single-pair VDSL2 that remains is that it is still considered a short-loop technology, offering significant gains only within 5,000 feet of the subscriber. This year, there will be another major technical leap toward solving the reach problem with introduction of cost-effective and standards-based bonded VDSL2 CPE. Already well into the interoperability testing phases, VDSL2 bonding is finally positioned to extend the delivery of 25+ Mbps services to longer-loop subscribers. With single-pair VDSL2 already being mass deployed and VDSL2 bonding becoming available this year, service providers are finally able to implement a FTTN solution capable of delivering high-speed services to short-loop customers on a single pair and long-loop customers over 2 pairs. (See Figure 1.)

Figure 1. Last Mile Cost Comparison.

Traditional access platforms have been built around deploying ADSL2+ and POTS services out of CO and remote terminal cabinet locations. Even with the extended service range enabled by VDSL2 bonding, a remote node approach must be adopted if service providers expected to take advantage of the higher-speed services enabled by VDSL2.

Fortunately, small-form-factor access platforms are available today that are tailored around the deployment of high-speed VDSL2 services in low-density, remote node serving areas. These include sealed DSLAM solutions that eliminate the need for expensive enclosures and site construction. By implementing a remote node approach, service providers are able to take advantage of the economic advantages of VDSL2-based FTTN, while maintaining a progression path toward FTTH.

 About the Author
Robert Conger is a product manager with ADTRAN. He has more than 10 years of experience in marketing and engineering. For more information, please email robert.conger@adtran.com or visit www.adtran.com.

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