Structured cabling specifications ensure you purchase and install the right product for every job.
Most significant installations of structured cabling begin with written specifications for each system component. Specifications ensure that you purchase and install the right product for every job. Specifications also guarantee all products offered are equivalent in a competitive bid situation.
In most cases, installing contractors follow specifications written by electrical consulting engineers. But many data-cabling installations don't follow the usual architect — engineer — general contractor — electrical contractor hierarchy. In these cases, the installer must write the specifications and make the purchasing decisions for optical cabling. This is especially true for jobs that follow the design/build pattern. In a request for proposal (RFP) process, the installing contractor must present the installation's design and specifications to the owner and then complete the project.
Therefore, it is important for installing contractors to know how to write effective specifications. However, many installers lack the knowledge and/or experience to properly prepare a detailed specification for optical fiber and optical-fiber cable.
Let's review the popular types of fiber and cable in use today and look at how you can use industry standards to specify the products necessary to confidently complete your next installation.
Optical fiber falls into one of two categories: single mode and multimode. Finished cables can be categorized as outdoor, indoor, or indoor/outdoor. These possibilities present a number of choices and decisions for electrical contractors when specifying the right product for a particular job or application.
Two popular sizes of multimode fiber exist today for use in commercial applications: 50 micron and 62.5 micron. Each has a common cladding diameter (125 microns), but different core diameters (50 microns and 62.5 microns). Several standards address both sizes to varying degrees of depth. ANSI/TIA/EIA-492-AAAA and ANSI/TIA/EIA-492-AAAB are two popular sources of information. Either provides all of the necessary details for complete fiber.
However, you must add the optical performance level (attenuation and bandwidth). There are a number of standards you can reference to cover the optical performance aspects of the specification. The most well-known standard today is IEEE 802.3z for Gigabit Ethernet. The Table, above, compares several specifications of international standards.
There is, however, one area in which the standards fall short of fully encompassing all the characteristics of a multimode fiber. Fiber manufacturers have responded by developing alternate measurements and product guarantees to fill the void. The resulting laser-optimized fibers show superior performance in laser-based systems. They are currently offered as a performance option not covered by standards, and they must be addressed separately in a user specification. Lasers are proving to be better light sources than LEDs, which are traditionally used with multimode fiber. As their prices fall and their performance improves, lasers are gaining wider acceptance.
Recent advances have increased single-mode fiber versatility. The introduction of the erbium doped fiber amplifier has extended long-haul (cross-country, not local links) system reach, and technologies for dense wavelength division multiplexing have led to the introduction of new fiber types for single-mode applications. Nearly all long distance telephone, Internet, and cable-TV links use single-mode fiber.
With demand increasing for bandwidth on data networks and LANs, single-mode fiber is becoming steadily more popular in new applications. Many installations include multimode fiber for current systems and single-mode fiber in the event of future expansion — sometimes in the same cable. In these cases, the multimode fibers are terminated for immediate use, and the single-mode fibers are coiled, taped, and left for future use.
Standard single-mode fiber was introduced to the commercial telephony market in 1983. While dimensional tolerances and optical performance have improved over the years, the design of single-mode fiber has, for the most part, remained the same. In the 1990s, LAN standards began to include options for standard single-mode fiber. To distinguish it from some of the newer specialized fibers, older versions of single-mode are referred to as dispersion-unshifted single-mode fiber. Just as for multimode, a national standard enumerates all the requirements for this type of fiber. ANSI/TIA/EIA-492-CAAA includes all of the details necessary to specify this type of fiber, as well as several additional optical performance options.
While the fiber specification covers the compatibility with (and performance of) the optical system, the fiber must be physically encased in a cable to survive its environment. It is important to specify the type of cable and its characteristics in addition to those of the fiber.
Commercial optical cables can be categorized as one of three types: outdoor, indoor, or indoor/outdoor. In the United States, indoor cables must meet one of four classifications for flame resistance. This is a primary design consideration. Outdoor cables, on the other hand, must be able to withstand extreme temperatures and resist the effects of water, sunlight, moisture, and wind and ice loading.
Indoor/outdoor cables must possess the characteristics of both cable types. However, it is the specialized materials and additional components that go into these cables that distinguish them from others. Though they are generally more expensive, indoor/outdoor cables can frequently eliminate the need for a splice point in the system.
Indoor optical cables are designed for flexibility, tensile strength, ease of handling, and flame retardancy. In the United States, indoor cables must also meet one of the listing categories in Art. 770 of the NEC.
The most popular type of indoor cable is tight-buffered, in which the individual fibers are directly coated with a protective layer of plastic. This makes the cable easy to route in cabinets and outlets, and it provides compatibility with field-installable connectors. When they are terminated in cabinets, tight-buffered cables may be directly connectorized and do not need a fan-out or end preparation kit. The Sidebar, at the top of page 76, is an example of an indoor-cable specification.
Outdoor cables are designed to withstand the rigors of outdoor installation for a lifetime of 20 to 40 years. Outdoor cables must have a wide operating temperature range, be resistant to sunlight and moisture, and have sufficient tensile strength for long pull distances. Use loose tube design in the cable structure to isolate the glass fibers from the mechanical stresses that can be induced throughout the service life. To field-connectorize a loose tube cable, splice on preterminated connector pigtails, or apply an end preparation kit to protect the individual fibers. The Sidebar, at the bottom of the page, is an example of an outdoor plant cable specification.
Waterblocking is important in outdoor cables for preventing the accumulation of water in the cable core, where the water can freeze and transfer stress to the glass fibers. Historically, waterblocking has been accomplished with grease or gel in the cable core. Recently, dry tapes and yarns impregnated with super absorbent polymer compounds have begun to replace grease and gel. In most cases, cables intended for direct burial underground should incorporate a steel tape armor to prevent rodents from chewing through to the core. For aerial installation, outdoor optical-fiber cables must be lashed to a messenger wire. In addition, integrated messenger wire cables with a figure eight cross section are available, as are all dielectric self-supporting aerial cables.
Indoor/outdoor cables incorporate the characteristics necessary for both applications. They are waterblocked, sunlight-resistant, and meet one or more of the Code requirements for resistance to flame spread and smoke generation. An indoor/outdoor cable can be useful in eliminating a splice point for a building-to-building run in a campus environment. In design and appearance, indoor/outdoor cables more closely resemble outdoor cables. A kit is usually necessary for field termination.
A detailed cable specification would consider all of the environmental and regulatory factors that pertain to the installation environment. These include temperature, mechanical loading, moisture, sunlight, flammability, rodents, and chemicals. To fully specify each cable, a test or measurement method and criteria are also necessary. Fortunately, you can use standards to simplify this task.
Two sister documents published by the Insulated Cable Engineers Association (ICEA) are very useful as detailed product specifications. ANSI/ICEA S-87-640, “Standard for Outside Plant Communications Cable,” and ANSI/ICEA S-83-596, “Standard for Fiber Optic Premises Distribution Cable,” cover outside and inside plant cables, respectively. A third document, to be designated S-83-696, is under preparation for indoor/outdoor cables. In addition to detailed references for test method, loading, and failure criteria for finished cable, these documents also include similar details for optical-fiber cable. ANSI/ICEA S-83-596 also includes a summary of the flammability listing requirements from the NEC.
By referring to these documents, one can ensure inclusion of a full battery of environmental- and mechanical-type testing and failure criteria.
With single-mode and multimode fiber, as well as indoor, outdoor, and indoor/outdoor cable choices, installers have a multitude of cabling permutations to choose from. Purchasing the right combination can be a daunting task. However, knowing how to write effective specifications can reduce the confusion and make installing structured cabling a manageable project.
Outside Plant Cable Specification Example
This sample specification is written for both 62.5 micron/125 micron and single-mode fiber.
Cable: Loose tube, all-dielectric
Fiber count and type: 12 dispersion-unshifted single-mode and 24 multimode 62.5 micron/125 micron
Requirements: The outside plant cable shall be listed and accepted by the USDA RUS as compliant with 7 CFR 1755.900, “Specification for Filled Fiber Optic Cable.”
Multimode fibers shall comply with ANSI/EIA/TIA-492-AAAA, except as otherwise noted herein. Single-mode fibers shall comply with ANSI/EIA/TIA-492-CAAA, except as otherwise noted herein.
Indoor Cable Specification Example
Cable: Tight buffered
Fiber count and type: 12 multimode 62.5 micron/125 micron
Requirements: The tight-buffered cable shall be listed as Type OFNR in accordance with the NEC. The cable shall meet the requirements of ANSI/ICEA S-83-596, “Premises Distribution Cables.”
Optical fibers shall comply with ANSI/EIA/TIA-492-AAAA, except as otherwise noted herein. Optical performance shall be compliant with IEEE 802.3z. It shall be guaranteed for Gigabit Ethernet transmission up to 300 m at 850 nm. A data sheet containing attenuation and bandwidth measurements shall be packaged with each reel of finished cable greater than 1000 m in length.
The cable and fiber manufacturers shall be registered to ISO 9001.