By: Patrick Boshell
Today there are many considerations to be taken when it comes to fibre optic infrastructure and building. In this article we shall set forth some of the applications pertaining to the in-building wiring and its installation guidelines. For the purpose of this article we shall reference the National Electrical Code (NEC) and Siecor. These recommendations are guidelines for the installation of inbuild fibre optic infrastructure.
Cable System Management
Fibre optic cable system management is rapidly changing as fibre optic utilization increases. Fibre is no longer simply being used for trunks but for broadband services, metropolitan area networks, synchronous optical networks (SONET), feeder cables, and fibre to the curb or home in the near future. As fibre applications become more pervasive, the need to manage the fibre optic network in a consistent manner that allows for growth, changing topology, reconfiguration, and equipment upgrades will significantly increase.
Management of a fibre optic network requires that specific systems be correctly chosen and implemented, and that quality products and proven installation practices be used to ensure a reliable system. This management system consists of fibre optic cable, splices, connectors, cable assemblies, attenuators, patch panels, connector adapters, frames and raceway systems. Each component is critical to ensure a highly reliable, quality system.
Therefore, it is important that cable system management products meet or exceed the requirements, and that the vendor has a quality system in place for qualification, requalification, and process control to ensure they consistently meet these requirements.
Entrance Cable
The term entrance cable defines the cable that transitions from the building entrance point, typically the cable vault, to the termination point for fibre optic cable(s). The entrance cable may or may not be the same cable or cable type as used for the outside plant (OSP). The primary reasons for using an alternate fibre optic cable type are fire codes and safety.
The NEC provides the guidelines for all electrical installations in industrial, commercial, and residential buildings. The installation of optical fibre cable is subject to the NEC as defined in NEC Article 90-2.
This article provides a detailed description of the installations covered by the NEC guidelines. NEC Article 90-2 states that "installations of communication equipment under the exclusive control of communication utilities, located outdoors, or in building spaces used exclusively for such installations" are not covered by the NEC. Communication utility companies, independent and Regional Bell Operating Companies, and CATV companies, specify the regulations for the installation of fibre optic cable. In most instances, the communication utility company refers to the NEC. NEC Article 770 describes the guidelines for the installation of optical fibre cable inside a building. Siecor recommends that all optical fibre cables installed comply with the guidelines of the NEC Article 770 to assure the safety of persons and property.
NEC Article 770
NEC Article 770 states that optical fibre cables installed within a building be listed as resistant to smoke and flame propagation in accordance to its specific plenum, riser or general purpose building application. Siecor suggests that each installation employ dielectric, nonconductive-listed optical fibre cable in lieu of conductive-listed optical fibre cable. The use of conductive-listed cable that contains noncurrent-carrying conductive members are required to be grounded in accordance with NEC Article 250. Dielectric cables are not affected by this grounding requirement.
Plenum
A plenum is defined as a "compartment or chamber to which one or more air ducts are connected and which forms part of the air distribution system." The NEC advises that optical fibre cable installed in a plenum application comply with the Underwriters Laboratories UL-910 Standard, "Test for Flame Propagation and Smoke-Density Values for Electrical and Optical Fibre Cables Used in Spaces Transporting Environmental Air."
Cable tested in accordance to the UL-910 Standard must exhibit a maximum peak optical density < 0.5, a maximum average optical density < 0.15, and a maximum flame spread < five feet to qualify for plenum installations. Optical fibre cables that are installed in plenum applications are recommended to be marked as TYPE OFNP or OFCP and to be listed with Underwriters Laboratories to verify compliance to the UL-910 Standard. Utility companies do not typically utilize plenums as raceways and, therefore, do not employ plenum-rated optical fibre cables in most cases.
Riser
A riser is defined as a vertical cable run that travels from one floor to another floor within a building. The NEC advises that optical fibre cable installed in a riser application complies with the Underwriters Laboratories UL-1666 Standard, "Test for Flame Propagation of Electrical and Optical Fibre Cables Installed Vertically In Shafts." Cable tested in accordance to the UL-1666 Standard must exhibit a maximum flame spread < 12 feet to qualify for riser installations. Optical fibre cables that are installed in riser applications are recommended to be marked as TYPE OFNR or OFCR and to be listed with Underwriters Laboratories to verity compliance to the UL - 1666 Standard. CATV Headends and Telco Central Offices typically employ Type OFNR-listed optical fibre cable for installations between the cable vault and the fibre distribution frame (FDF).
The cable vault room is ordinarily located in the building's basement and the FDF is located on one of the upper floor levels. The use of Type OFNR-listed optical fibre cable serves to inhibit the spread of fire from one building floor to another.
General Purpose
General purpose (horizontal) is defined as the wiring area that extends along a building floor and those areas not addressed as plenum or riser environments. The NEC advises that optical fibre cable installed in a general purpose application comply with the Underwriters Laboratories UL-1581 standard, "Vertical-Tray Flame Test." Cable tested in accordance to the UL-1581 Standard must exhibit a maximum cable blister or char spread < eight feet to qualify for general purpose installations. Optical fibre cables that are installed in general purpose applications are encouraged to be marked as TYPE OFN or OFC and to be listed with Underwriters Laboratories to verify compliance to the UL-1581 Standard. Type OFN- or OFNR-listed optical fibre cables are usually employed for interconnect and cross-connect cables in and between FDFS.
NEC Exceptions
NEC Article 770 has defined exceptions that permit unlisted optical fibre cable to be installed in specific building applications. The primary exceptions stated in the NEC are:
Optical fibre cable shall not be required to be listed and marked when the cable enters the building from the outside and the cable length within the building does not exceed 50 feet and is terminated in a closure.
Optical fibre cable shall not be required to be listed and marked where the cable enters the building from the outside and is run in metal conduit or metal raceway.
These exceptions permit unlisted OSP cable to enter the cable vault room and to be transition spliced to a listed optical cable that runs to the FDF. Riser (Type OFNR)-listed optical fibre cable is normally spliced to the OSP cable at the cable vault. The use of the riser-rated cable inhibits the spread of flame from the vault to the FDF. If the unlisted OSP cable run inside the building is more than 50 feet from the building entrance, the cable must be closed in metallic tubing or an enclosed metal raceway to comply with NEC Article 770.
NEC Substitutions
The NEC permits optical fibre cables that pass more stringent UL-flame tests and are listed accordingly to be substituted for lower-rated designs (i.e., riser-listed cable may be used in a general purpose application).
Siecor recommends that Type OFNR-listed optical cable be deployed not only for riser applications, but also for interconnect and cross-connect cable. The use of OFNR-listed cable in general purpose applications allows for the efficient restructuring of the CO equipment layout, specifically from a single floor to multiple floor (riser) applications with the potential of reusing the existing cable.
Transition Splicing
Cables routed to a building location are generally unlisted OSP cables. As discussed previously, these cables should not be installed into buildings (unless protected by metallic tubing/raceway or terminated within 50 feet of building entry). Because of this requirement, numerous companies have standardized on a transition splice where unlisted OSP cables are spliced to listed cables. As Figure 3 depicts, transition splices can be executed in two separate locations. One location is the nearest appropriate manhole. However, the most commonly used location is the cable vault or an area in close proximity to the vault, often referred to as an alternate splicing area. Splice closures are commonly used at a manhole transition splice, while a number of options are available in the cable vault or alternate splicing areas. These options are discussed in the Product Specification section.
In either scenario, there is a transition from unlisted OSP cable to listed cable for the inside cable routing to the FDF.
At the transition splice location, there are various methods for completing the splices. These include fusion and mechanical splicing, either single or mass fibre.
Fusion splicing involves the alignment of two cleaved optical fibres and an electric arc to fuse the two fibres together. The result is a continuous single fibre. The single fibre method employs a fusion splicer, which maximizes each individual splice. A mass fusion splicer completes six to 12 fibres at the same time.
Mechanical splicing utilizes a mechanical alignment mechanism (V-grooves or alignment rods). The fibres are held together with a mechanical look or an epoxy/adhesive. In most mechanical splices, an index matching gel is used to reduce loss and reflection as the light crosses the gap between the fibres. Single-fibre mechanical splices typically require fibre cleaving only, whereas mass-fibre mechanical splices typically require the fibre end faces to be polished.
No matter which splicing method is utilized, splice tray(s) and a device to hold splice trays are needed to organize, secure, and protect the splices.
Stubbed Patch Panel
In some situations, it is often advantageous to install a stubbed patch panel to terminate the OSP fibre. The stubbed patch panel consists of a factory-manufactured cable assembly of specific (predetermined) length, unterminated on one end and terminated with connectors into a patch panel on the other end.
This reduces the time and project management needed to complete the installation of the fibre system, especially when used for transition splicing to OSP cables. The patch panel end is complete and requires only the installation of the patch panel into an equipment rack. The unterminated end is pulled to the desired location and then spliced to the OSP cable or terminated into another patch panel; the fibre highway is now ready. With the stubbed patch panel fibres directly connectorized at the factory and pre-installed into the patch panel for protection, the product is ready for immediate use. Installation is very simple - the patch panel end is installed into the equipment rack. Typically, the stubbed patch panel uses listed cable because of the NEC code and flame retardancy; however, unlisted OSP cable is also available.
Connector Specifications and Types
The term fibre optic connector describes the mechanical device used to provide an easily rematable, reusable junction between two fibres (Figure 4). Fibre optic connectors are used in a patch panel or at the connection to a fibre optic transmitter or receiver, such as at the fibre optic terminal (FOT) equipment. The proper selection and standardization of a fibre optic connector is critical and is based on a number of factors. The generic parameters are discussed here.
Performance
Attenuation and reflectance affect data transmission in a fibre optic system. Connector insertion loss (attenuation) and discrete reflectance (return loss) are measurements used to determine how much insertion loss and reflectance a connector junction adds to the system. These measurements are the critical performance parameters of connectors.
Insertion Loss (Attenuation)
Connector insertion loss measures how much power or light is attenuated through the connector junction. Measured in decibels (dB), typical insertion loss values for a single-mode junction are between 0.3 dB and 1.3 dB. The lower the insertion loss, the stronger the signal and the better the signal quality.
Reflectance (Return Loss)
Discrete reflectance measures the amount of reflection compared to the amount of light transmitted. Also measured in dB, current single-mode connector measurements range between -10 dB and -65 dB.
When light is reflected back to the equipment, the signal's shape and intensity are distorted. This distortion becomes critical as data transmission rates increase. The lower the reflectance, the lower the amount of light reflected (-65 dB is better than -10 dB). Therefore, the signal quality is better.
When mated fibres are in physical contact with each other, the amount of light reflected is decreased. Connectors that bring fibres into physical contact are referred to as PC connectors. However, if connectors with different reflectance measurements are connected, the best performance expected is that of the lower measurement.
Specifications
Bellcore's technical reference, "Generic Requirements for Optical Fibre Connectors and Connectorized Jumper Cables," defines the performance criteria and testing procedures for single-mode cable assemblies. The parameters observed throughout the variety of tests are insertion loss, reflectance and damage. The tests are designed to simulate real-life conditions cable assemblies might experience. The following is a list of the tests:
Fibre optic connectors can be installed on various cable and fibre coating types. Connectors may be installed on 250 pm coated fibre, on 900 pm buffered fibre without individual jackets and strength members, or on fibre that has individual jackets and strength members (i.e., 3 mm cable).
Features
Specific connector features affect performance and distinguish one connector type from another.
Ferrule
The part of the connector that holds the fibre is the ferrule. Some ferrules are cone-shaped (Biconic) while others are cylindrical. To mate properly, the ferrules should have the same shape and outside diameter. For cylindrical ferrules, 2.0 mm and 2.5 mm are the most common outside diameters, with a number of different connector types using the 2.5 mm ferrule (SC, ST Compatible, and FC).
Key
The key performs three functions in a connector design. First, the key ensures repeatable results by keeping the ferrule in the same position for each mating. The key also provides a stop to prevent ferrule rotation that could damage the connector's polish. Finally, on some designs the key is positioned to provide the lowest attenuation point, providing connectors with superior insertion loss performance.
Latching Mechanisms
The three main latching mechanisms that join the connector onto the adapter or equipment port are the threaded coupling, the push-twist coupling and the push-pull coupling. The different mechanisms affect ease-of-installation and packing density.
Types of Connectors
The most common types of single-mode connectors are SC, ST Compatible, and FC.
Field Connectorization
Field connectorization refers to the process of installing connectors at the installation site instead of having a manufacturer install them in a factory. Field connectorization is chosen when one or both ends of a cable need to be terminated on-site because installation conditions preclude the use of preconnectorized assemblies, or custom length jumpers are required.
In addition to insertion loss and reflectance, other parameters are important in choosing a field connector. These parameters include time-to-install, ease-of-installation, and reliability. All three parameters are affected by the installation method and the cable type.
Presently, three general installation methods exist for installing a connector on a fibre:
Method A requires epoxy curing time, while both Methods A and B require polishing time and skill. Method C simply requires the proper cleaving of the fibre.
Cable types include outside cable and indoor cable. The fibre coating in outside cable generally has a diameter of 250 pm. This fibre should be protected by the use of additional tubing. This provides protection to the fibre and helps support the weight of the connector. Fibre routed inside a housing is usually buffered out to 900 pm. Fibre routed in exposed areas should be buffered out to 3 mm.
Cross-Connect vs Interconnect
Cross-connect and interconnect are terms used to describe the method of passive connection between the entrance cable(s) and the equipment/ electronics (Figure 5). The cross-connect is per- formed by terminating both the entrance cable(s) and the jumpers to the equipment on the backside of the FDF shelf. A fibre optic cross-connect jumper is used to make a connection between the fibre terminations.
The interconnect is performed by terminating the entrance cable(s) to the backside of a fibre optic patch panel; however, the jumpers to the equipment make direct connection to the entrance cable fibres by connecting to the front side of the same panel. In the interconnect method, the cross-connect jumper and the second patch panel are not used; only one FDF panel with a jumper to the FOT equipment is used.
In the past, the interconnect method was used extensively for fibre optics; however, cross-connect has been the standard for copper distributing frames for years. As fibre deployment becomes more extensive, the benefits of the cross-connect method become more crucial. With this increase in fibre deployment, most companies are standardizing on cross-connect. One benefit of a cross-connect is increased flexibility and manageability by simplifying rearrangements, upgrades, and moves of fibre optic equipment. Also, cross-connection offers the economic advantages of accelerated system restorations and reduced coordination requirements between OSP and equipment jobs.
OSP Housing
The OSP housing is the patch panel in the building where the entrance cables are terminated. This patch panel is typically housed in a 23-inch rack, and when it contains strictly OSP housings, is commonly referred to as the OSP bay. The OSP housing/bay provides a demarcation point for incoming cables and OSP work while permitting maximum flexibility to reconfigure or reassign the optical circuits. OSP housings/bays can be employed in an interconnect or, more commonly, a cross-connect function.
Cross-Connect Jumpers
Cross-connect jumpers are typically one- or two-fibre connectorized cables that are used to connect the OSP housing/bay to the FOT housing/bay.
The most commonly used jumper type is the dual-fibre cable assembly, which reduces jumper routing or trough congestion.
FOT Housing
The FOT housing is the patch panel in the CATV Headend, the Telco CO, or remote site where the jumpers for the FOT equipment are terminated into a separate patch panel. The FOT housing is housed in a 23-inch rack and is commonly referred to as the FOT bay when it contains strictly FOT housings. The FOT housing/bay is only used in cross-connect applications.
The purpose of the FOT housing is to provide a means of cross-connecting fibres that are routed from the OSP housing and the end-equipment frames. The use of the FOT housings allows equipment testing, and the patching of fibres to be moved, reassigned, or even rerouted to other housings to achieve maximum system flexibility.
FOT Jumpers
FOT jumpers are typically 2, 4, 8, 12, 16, or higher fibre count connectorized cables that are used to connect the FOT housing/bay to the fibre optic electronics, i.e., optical multiplexer. These jumpers are sometimes referred to as mux jumpers and the fibre count is typically determined by the number of fibre ports in a specific equipment bay.
Optical Raceway
Optical raceways are the routing mechanisms used to support, secure, manage, identify, and protect optical jumpers/cables as they are installed between vaults, OSP housings, FOT housings, and end-equipment. Optical raceways include conduit, ladder racks, encased metal or plastic race-ways, and cable trays. Typically, the selection of a raceway is based on three primary applications: cable vault to OSP bay, FOT bay to FOT equipment, and between multiple lineups of FDFS, as in the FDF network.
Patrick Boshell is currently President of UTILiNK Co. Ltd. He is an expert both in electrical and fibre optic technologies. Special thanks to Siecor.
ET