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Fiber's Future
 A Powerful Prospect FTTH
A Passive Optical Network System Strategy for the Final Mile
by Scott A. McCreary, Scott M. Chastain, David W. Kapella,
Tsuyoshi Imaizumi, Hiroki Ishikawa and Itaru Sakabe
Outside Plant Magazine December 2000
The majority of residential Internet users connect to it through modems that deliver up to 56 kilobits per second. People are now switching to cable modems and digital subscriber lines (DSL) to increase the download speed up to 200 megabits per second, and the need for higher bandwidth capacity is likely to continue. As a result, the increasing demand for high-speed Internet services and competition for customers are driving Regional Bell Operating Companies (RBOCs), Competitive Local Exchange Carriers (CLECs), and cable television providers to increase end user bandwidth.
Currently, the telephone industry has a reliable network to carry voice and data traffic, while the cable industry has a reliable network to carry video signals. Both, however, switch to copper or coaxial cable to go the final mile to the customer premises. The ultimate goal for most providers is a complete Fiber-To-The-Home (FTTH) architecture that eliminates the copper wire or coaxial cable from the signal path. Increased bandwidth requirements and recent cost reductions of optical transmission equipment are creating a competitive environment that is allowing fiber to be driven deeper into the outside plant network and making FTTH a reality for many customers.
A FTTH network architecture can offer unsurpassed bandwidth growth potential and network scalability. To be successful, a FTTH network should be capable of providing high quality broadband video, narrow band voice, and high-speed data simultaneously over a bi-directional Passive Optical Network (PON) with a minimum fiber count. A PON structure utilizes passive optical splitters in the outside plant instead of active electronics to distribute the transmission signals. Because the electronics are placed inside the home and are powered by the customer, power supplies in the outside plant cabinets and pedestals are not needed.
Cable Design Types
Generally, four types of cables are used in a FTTH network: trunk cables, distribution cables, drop cables, and indoor cables. Figure 1 shows an example of a typical FTTH network using the three outside plant cables. The first type of cable is a trunk or feeder cable and is usually the highest fiber count of the network. The trunk cable is typically a standard optical cable. There are no special requirements other than the ability to access fibers at mid-span; i.e., to intercept and splice selected fibers or ribbons with all other remaining fibers or ribbons passing through undisturbed.
The second type of cable in a FTTH network is a distribution cable. Distribution cables are usually shorter in length and lower fiber-count cables than trunk cables. Distribution cables can be either buried or aerial and must have the ability for mid-span accessibility of individual fibers.
Buried designs have been around for some time, but a newly developed aerial distribution cable, shown in Figure 2, offers several advantages when compared to conventional lashed or figure-eight cable designs. The cable uses a windowed web to connect the cable subunit to a metallic messenger. The windowed web, as opposed to a solid web construction, has gaps between the cable sub-unit and the messenger. This design improves field handling and reduces the possibility of injuries to the installer during installation and cable preparation. Instead of removing the continuous web of a standard figure-eight cable, the installer uses a standard utility knife to remove small two-inch portions of the web (see Figure 4).
Another unique aspect of the design is a completely dry construction. By eliminating all viscous water blocking gels, the cable is lighter and more craft friendly, thereby reducing installation costs. The dielectric cable sub-unit design utilizes ribbons to increase productivity when splicing to trunk cables, but maintains individual fiber access through standard peelable ribbon designs.
The third type of cable in the network is a drop cable. These cables are designed to run from the distribution cable directly to the home without the need to be accessed at mid-span. Drop cables are even shorter in length than distribution cables, and the fiber count is usually less than six. Like the distribution cables, drop cables can be either buried or aerial, as shown in Figure 3. The buried drop cable is a simple dielectric concentric design that has been optimized for FTTH applications.
The concentric design provides flexibility in all directions, and the cable conforms to UL flame requirements for riser locations.
Aerial applications have unique challenges that must be addressed for FTTH deployment. Self-supporting aerial cables are always under tension and are subject to complex mechanical and environmental conditions. Aerial cables are installed with an initial tension to meet application requirements such as sag and clearance. During service, factors such as wind, temperature, and even ice buildup will affect the cable. As a result, these cyclic changes in cable strain during the service life merit close design consideration. They can shorten the life of a cable.
The aerial drop cable design also uses a figure-eight, windowed-web technology to create cable core excess length, as shown in Figure 4. This excess length is the key element that allows the small cable to span 200 feet under National Electric Safety Code (NESC) heavy loading conditions with minimized fiber strain and fatigue. By minimizing the fiber strain, the aerial drop cable offers reliable long-term optical performance and limits the effects of dynamic fatigue on fiber reliability. Both the buried and aerial drop cables are flame-retardant and have all-dielectric cores so that additional grounding is not a factor, as it would be with cables that include metallic strength members. Also, both types of the cables can be factory terminated to create a plug-'n'-play system to further reduce preparation and installation times in the field.
The fourth type of cable in a FTTH network is an indoor cable that is typically used to run from the Network Interface Device (NID) on the side of the house to a network termination unit inside the customer's home. At that point, the optical signals will be converted to electrical signals. In instances where a NID terminal is not used, the drop cable can be run inside the home directly to the network termination unit. Like the drop cables, the indoor cable is all-dielectric and flame retardant.
Design Options for Enclosures
Innovative aerial enclosures have been developed specifically for the FTTH passive optical networks. Two enclosure options are discussed here. The first option, shown in Figure 5, provides environmental and mechanical protection to couplers and optical fiber splice points along the distribution cable. The closure is also a starting point for the drop cables to the home.
The aerial free-breathing closure, shown again in Figure 6 with two drop cables, is designed to be strand-mounted for mid-span taut sheath splices of a distribution cable and has entry ports for up to 16 drop cables on each end. The closure has been tested against Telcordia GR-771-CORE, Issue 1 requirements and meets all applicable specifications.
As an alternative to field splicing, drop cables can be pre-terminated at the factory. In this distance, Figure 7 shows the second enclosure option. In this option, a standard aerial closure is used on the distribution cable with one small drop cable routed to a pole-mounted coupler terminal housing. The pole-mounted coupler terminal housing accommodates passive couplers and acts as a starting point for the pre-terminated drop cables to the home.
This pole-mounted coupler terminal, shown in Figure 8, is very simple and small. The terminal houses one or two couplers inside a closed metal box and utilizes SC-type connectors. Since the drop cables are pre-terminated on both ends, cable slack is looped around the rear of the terminal for storage.
In instances where the other end of the pre-terminated drop cable cannot be routed directly inside the home, a small NID terminal has been engineered to act as a demarcation point on the side of each subscriber's home. The NID terminal utilizes SC-type connectors and allows for routing of both the drop cable and indoor cable breakouts.
When Passivity is Powerful
Together, these products create an effective passive optical network FTTH system that enables carriers to provide fiber the final subscriber mile. A PON, which currently holds the most potential for the future, utilizes passive optical splitters in the outside plant instead of active electronics to manage the transmission signals. The products discussed include two distribution and drop cable designs. The aerial distribution and drop cables are designed with a windowed-web technology to create cable sub-unit excess length. This excess length allows the cables to span long distances under NESC heavy loading conditions with minimal fiber strain.
Several new enclosure solutions engineered specifically to address unique FTTH requirements were also discussed. The enclosures developed for the FTTH system provide environmental and mechanical protection to couplers and optical fiber splice points along the distribution cable and are a starting point for the drop cables to the home.
Acknowledgement
The authors would like to express their appreciation to Yoshiro Yamane, Takeo Tsurumi, and Hiroshi Satani for their assistance.
Scott A. McCreary, Scott M. Chastain and David W. Kapella are with Sumitomo Electric Lightwave, Research Triangle Park, North Carolina.
Tsuyoshi Imaizumi, Hiroki Ishikawa and Itaru Sakabe are with Sumitomo Electric Industries, Yokohama, Japan.
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