The capital costs to deploy FTTH have reached parity with other wire-line solutions, and the cost of ownership is actually lower (i.e., less maintenance issues), but understanding FTTH systems requires comprehension of entirely new products and a distinct vernacular. The developer who pushes through this montage of new vocabulary will discover that FTTH deployments reveal something familiar: a construction project. Deployment costs and decisions are influenced by the estimated subscription-adoption percentage rate, installation labor rate, lot sizes and layout of the community, cost of materials, and build-out schedule. And, like any other construction project, there are some basic pieces to the puzzle and some basic rules to follow.

A FTTH network can be broken down into three generic parts: a central equipment room, the optical distribution network (ODN) and the subscriber premise. The central equipment room may be alternately described as a "head-end," "central office" or just "equipment room," depending on whom you ask. Its function is to house all of the switching and apparatus necessary to receive voice, video and /or data services and redistribute those services to subscribers throughout the community.

In the case of a FTTH community consisting primarily of single-family homes, the subscriber premises piece of the network is going to be the small box that houses the electronics necessary for the homeowner to receive services. This box is commonly described as an "ONT" (optical network terminal) or "ONU" (optical network unit). It must be fully interoperable with the electronics in the central equipment room.

It is the third component of an FTTH system, the ODN, which highly distinguishes FTTH networks from other networks. The ODN consists of all the fiber cable and apparatus required to connect the central equipment room to the subscriber premise.

ODN Design Is Critical

The ODN is a critical portion of the FTTH network. The central office and subscriber side equipment could be upgraded multiple times over the next 20 to 30 years after a FTTH network is installed. However, the ODN must stand the test of time. It is both the most enduring and the most differentiating asset in a community network. Fortunately, it is also the "nuts and bolts" part of the network, and the area where a developer can apply good construction sensibilities without having years of telecommunications experience.

There are two basic families of FTTH networks and three basic elements of any ODN. A fiber-to-the-home network can typically be categorized as either point-to-point or passive optical network (PON). A point-to-point network is a relatively simple architecture where there is a direct connection between each subscriber and the electronics in the central equipment room or some intermediate equipment cabinet.

There are several varieties of passive optical network technology, but they are all distinguished by the deployment of optical splitters. Optical splitters are non-powered devices that split an optical signal to serve multiple locations. Typically, in PON, a single fiber will be split to serve up to 32 subscribers.

With either PON or point-to-point, there are usually three distinct elements to the ODN: feeder cabling, distribution infrastructure and the drop. The feeder cable is the connection between the central equipment room and an area of the community. The distribution infrastructure is the cabling and components that route nearest the homes. There is often a device that creates a clear demarcation between feeder cabling and distribution. Just as often, however, the line between those two elements can become blurry. In all instances, the drop consists of the cable and components used to connect the subscriber to the distribution infrastructure.

So what type of FTTH system should be deployed? Logically, the first question behind any ODN design is, "Are we deploying PON or point-to-point?" Many large carriers deploying FTTH have adopted PON because of its efficient use of fiber, native support of legacy telephone services, and easy accommodation of cable television services. Consequently, most of the independent telecommunications providers seeking business in new real estate developments have also adopted PON.

Point-to-point solutions usually only support traditional cable video if the number of fibers deployed to each residence is doubled. This still makes point-to-point FTTH a good alternative in multidwelling units and dense single-family residential applications where the material and labor costs for the extra fiber are a nominal part of the equation. After some consideration, many developers will decide that partnering with an entity that is already in the telecommunications business is a good idea.

Regardless of whether a developer opts to partner with someone or branch out on its own, the early history of FTTH in North America would indicate that PON is likely to be the chosen solution.

Once the technology has been selected, the ODN design may commence. The first purpose of any ODN is to be functional. Functionality encompasses myriad engineering rules and standards. Any decent vendor should be able to tell you if an ODN will meet the functionality standards. The more complicated issues in ODN design pertain to where, when and how the money is spent. Specifically, there is a balancing act between getting the maximum use from equipment in the head-end, controlling the cost of the feeder and distribution network and mitigating the cost to make the drop to the subscriber.

In most instances, the developer can arrive at the right answer by understanding the percentage of residents in a community who will subscribe, the labor rate for installation work in a given region, the lot sizes and layout of the community, the cost of materials, and the build-out schedule for the development. A solid engineering firm or reputable vendor will ask for that information before proposing a solution.

The design considerations for the ODN in a new community are often significantly different from the design considerations of a large carrier in a municipal build-out. For example, the developer often has a captive audience. Therefore, the calculation of subscription rate is more precise. The developer would typically prefer to postpone as much of the connection cost as possible until the moment when a home is built and occupied by a subscriber. Both of these variables can have a significant impact on ODN design and business case.

Guy Swindell is an applications engineering manager of FTTx Solutions for OFS (www.ofsoptics.com), and Calvin Martin is product marketing manager of FTTx Solutions for OFS, a manufacturer of fiber-optic cabling.