Fiber to the "X" (FTTX)

Traditionally, telco lines consist of a single pair of wires extending all the way from the central office to the subscriber. This architecture has served us well for over 100 years and is now capable of providing impressive bandwidth via DSL (digital subscriber line) and coax for triple play (voice, video and data) requirements. But bandwidth over copper wire comes at a cost. The operational expense of ensuring those copper wires are un-impaired by legacy analog components, splices, and even exposure to rain and wind damage is a daunting and expensive task. In addition, telco service providers need to deploy powered cabinets near the neighborhoods served by copper technologies to ensure the proper signal levels are maintained. The operational expense of these powered CATV or DSLAM (Digital Subscriber Loop Access Module) components is considerable when taking in to account the number of potential subscribers and the many pairs of wires serving a neighborhood. When new repairs are needed, the troubleshooting of a subscriber connection may have to occur at multiple points along the service delivery path. In addition, the long term prospect of pursuing copper-based technologies that must be flexible and keep pace with end user bandwidth demands is a risky consideration for service providers. This model is particularly risky given how the market is subjected to increased competition and shrinking budgets. So fiber based service delivery has been introduced to alleviate these operational inefficiencies. There are many ways to deploy fiber and collectively all these technologies are referred to as Fiber to the "X" or FTTX. Let us now explore each of the specific PON service strategies.

FTTH

By far, the most prevalent application of PON technology is Fiber to the Home or FTTH. The approach is straight forward: serve a fiber directly to the subscriber's home and install the ONT either on the inside or the outside of the premises. Although this represents one of the most costly PON deployments for service providers to undertake, the long term value is much greater in terms of take rate, perceived value by the homeowner, and the ability for the service provider to stay flexible in their provision of service. In an FTTH architecture a fiber distribution hub or FDH is installed in a strategic location within the neighborhood. The placement of this hub is selected based upon a number of factors mostly having to do with maximizing the geographical reach of the homes along the fiber path. Placing a hub too far on the peripheral of a neighborhood will result in unnecessarily long distribution fibers and present a more difficult repair if the fibers are cut or damaged by weather, heavy equipment, or even vehicular accidents. The FDH is of course passive and is available in many sizes and installation types. The FDH will be sized according to the number of potential subscribers being served by it. Typical FDH fiber capacities are 144, 288, or 432 fibers. While there are larger FDH's available (e.g., 576 or 768), the smaller hubs are typically deployed to keep the fiber counts and distances from the hub manageable. The FDH can be pole-mounted, pedestal mounted, or in some cases, installed underground as in a fiber vault or man-hole. The FDH takes in 12 to 24 feeder fibers and terminates them on the input of a 1x32 fiber splitter. A 1x32 split is typical in North American and European markets however some Asian PON systems may utilize as high as a 1x64 split for each PON port. The splitter then subtends to the distribution fibers that serve the individual homes. In most cases, the distribution fibers will be multiple loose tube or ribbon fibers enclosed in a common outside plant (OSP) sheath. Whether they are aerial or below-ground, they will eventually terminate in a fiber distribution terminal or FDT. The FDT, like the FDH is available in a variety of sizes and form factors but its sole purpose is to break out the multiple fiber cables into a single subscriber interface. At this point the fiber cable is reduced to perhaps 6, 12, or 24 individual fibers. These fiber interfaces are usually capped with a weather tight seal until an order for service is received.

The installer will then connect a ruggedized fiber service drop from the FDT to the subscriber's home. Here again, this last fiber run could be an aerial or underground fiber cable. At the subscriber's home the fiber cable terminates on the ONT which may be installed on the exterior of the home just as the copper lines would have been installed at the customer demarcation point. Alternatively, the ONT may be wall mounted in a basement or garage. The power for the ONT is provided by the home owner which does lend some complexity to the whole concept of the telephone availability for life safety reasons. If the power goes out in a home, there must be a means to ensure that dial tone is available for access to 911 emergency services. The answer for this issue is the integration of a small uninterruptible power supply or UPS. This arrangement powers the ONT during a power outage and ensures that dial tone is available for up to 4 hours or more. From the ONT, there are multiple options in connecting to a broadband home router (BHR). Often, an unshielded twisted pair (UTP) will be installed between the ONT and the BHR. This is typically a CAT 5e or CAT 6 cable and is installed in the conventional telco service provider model just as a phone line would be installed. Alternatively, the BHR may connect to the ONT via a Coaxial cable which uses the Multimedia over Coax Alliance (MOCA) protocol to convert the subscribed signals into Ethernet and vice versa. If all of this sounds complicated, there are many who would agree with you. The service provider can spend several hours installing this infrastructure and consequently minimum two year contracts to cover the cost of installation or even hefty installation charges are not uncommon for FTTH installations. Ultimately however, the FTTH service can ensure a very robust architecture for providing future proof and flexible service options. When compared to coaxial cable and DSL technologies, FTTH provides the most promising value despite the high cost of initial turn-up.