Andrew K. Straw / Siecor
Consultants and end users can lean on standards when specifying performance requirements.
Many structured cabling installations begin with the preparation of a detailed written specification for each system component. Such specifications may be necessary to ensure the correct product is used for the intended application. Specifications also ensure all products offered are equal in a competitive-bidding situation. Consultants and end users often find that they lack the breadth of knowledge or experience necessary to prepare a complete, detailed specification for optical fiber and fiber-optic cable. This fact is not surprising because it takes a significant effort to keep abreast of changing product offerings as well as the most recent product evaluation and measurement technology. However, consultants and end users can use industry standards to specify the necessary products without having to delve into excruciating detail.
Specifications for optical fiber and cable can be, and often are, long and arduous; they address a myriad of technical considerations. These considerations include mechanical, environmental, and dimensional characteristics, not to mention the finished product's compatibility with building codes and transmission equipment. Consultants and end users frequently prepare lengthy specifications, attempting to cover all pertinent details. This effort requires a thorough knowledge of cable design, optical science, and the details of scores of measurement methods. Panels of industry experts have developed national and international standards for fiber and cable; these standards can be the basis for the desired brief, accurate, detailed specifications.
Breaking down the variables
Optical fiber can be categorized into two general types: singlemode and multimode. Fiber-optic cables generally can be categorized into three types: outdoor, indoor, and indoor/outdoor. All these variables lead to a number of choices and decisions necessary to specify the right product for a particular job, which makes the job difficult enough without having to worry about details such as environmental and mechanical performance, cable construction, and fiber dimensions.
Specifications for fiber and fiber-optic cable can be divided into four categories: optical, mechanical, environmental, and dimensional. Optical specifications of particular concern are attenuation and bandwidth, which are currently specified at two operating wavelengths for each fiber type. Performance is specified at 850 and 1,300 nm for multimode fiber and 1,310 and 1,550 nm for singlemode. Mechanical specifications include such characteristics as tensile strength and resistance to crush, impact, and twist. Environmental considerations include temperature ranges for operation and storage; sensitivity to moisture and sunlight; and for outside-plant (OSP) cables, protection from lightning or rodent attack. Fiber specifications contain detailed measurements and tolerances for core and cladding diameter, the degree of roundness of both the core and the cladding, and their respective concentricity.
A definitive specification for an optical-fiber cable could contain details on all of these aspects and more, and would be quite lengthy. In addition to specifying each parameter or characteristic, it is often necessary to call out a particular test method, without which the parameter is meaningless. Fortunately, knowledge of the applicable standards and how to reference those standards can make optical-fiber specification a brief and easy task.
There are two types of multimode fiber in popular commercial use today. Physically, they each have a common cladding diameter of 125 microns but differ in core diameter. One has a core diameter of 50 microns and the other, 62.5 microns; these fibers are commonly called 50/125- and 62.5/125-micron fiber. Both fiber types are addressed in several standards, in varying degrees of depth.
The ANSI/TIA/EIA-492AAAA and 492AAAB standards, developed by the Telecommunications Industry Association/
Electronic Industries Alliance (TIA/EIA-Arlington, VA), provide all the pertinent requirements for 62.5/125- and 50/125-micron fiber that would need to be in a specification. They include references to approved test methods and options but do not spell out in detail attenuation and bandwidth, for which there are several options. Therefore, a simple reference to one of these standards will provide all necessary details for fiber, leaving only the optical-performance level-attenuation and bandwidth-to be added.
Also, a number of standards cover the optical-performance aspects of the specification. A reference to one of these standards will complete the specification. The most publicized fiber standard today is the IEEE-802.3z standard for Gigabit Ethernet, published by the Institute of Electrical and Electronics Engineers (IEEE-New York City).
There is, however, one area in which standards fall short of fully encompassing multimode-fiber characteristics: laser-transceiver performance. Historically, with a few notable exceptions, multimode systems have been based on light-emitting diodes. The advent of Gigabit Ethernet brought laser transmitters onto the multimode scene, and the performance of multimode fibers with laser transmitters is not yet adequately covered by available test-and-measurement standards.
Fiber manufacturers have responded by developing alternate measurements and product guarantees to fill the void. The resulting "laser-optimized fibers," as they are being called, show high-level performance in laser-based systems. They are currently offered as a performance option not covered by standards and must be addressed separately in a user specification.
Singlemode-fiber technology has become more complex than ever in recent years. The introduction of the erbium-doped fiber amplifier has extended long-haul system reach, and technologies for dense wavelength-division multiplexing have led to the introduction of new fiber types for singlemode applications. Singlemode has been, and remains, the mainstay of long-haul and metropolitan applications in the telephony and cable-TV industries. With increasing demands for bandwidth on data networks and local area networks (LANs), however, singlemode fiber is becoming increasingly popular in these applications. Many installations include multimode fiber for today's systems and singlemode fiber for future expansion. Therefore, it is important to understand the basic types of singlemode fiber and when to use one versus another.
Standard singlemode fiber was introduced into the commercial telephony market in 1983. While dimensional tolerances and optical performance have been somewhat improved over the years, the design has remained basically the same. In the 1990s, LAN standards began to include options for using standard singlemode fiber. To distinguish "plain-old singlemode" from some of the newer specialized fibers, the first-generation fiber is referred to as dispersion-unshifted singlemode fiber. Just as for multimode, there is a national standard that enumerates all requirements for dispersion-unshifted singlemode fiber. TIA/EIA-492CAAA includes all details necessary to specify this fiber type as well as several optical-performance options.
A key point about singlemode fiber is that standard practice is to specify dispersion rather than bandwidth. The simpler nature of light propagation in singlemode fiber makes dispersion an easier and more meaningful characterization method. Dispersion may be specified in terms of a limit or in terms of a slope and zero-dispersion wavelength, which are then used to calculate performance at a specific operating wavelength. For LAN applications today, a fiber meeting the TIA requirements is sufficient, and an end-user specification need not address dispersion any further.
While the fiber specification covers compatibility with and performance of the optical system, the fiber must be physically encased in a cable to survive a service environment. Therefore, it is important to specify the type of cable and that cable's characteristics, in addition to those of the fiber.
In the United States, indoor cables must be tested to meet one of four classifications for flame resistance-a primary design consideration. Outdoor cables, on the other hand, are optimized for more extreme temperatures and for resistance to the effects of water, sunlight, moisture exposure, and wind and ice loading. Indoor/outdoor cables must have the characteristics of both and are typified by more specialized materials and additional components. While they generally are the most costly of the three cable types, their use can frequently eliminate a splice point in the system, resulting in overall benefit.
Just like optical-fiber specifications, cable specifications have several aspects: optical, mechanical, environmental, and dimensional. Dimensional characteristics, particularly the cable diameter, are addressed in specifications only infrequently. However, they can be important when cables must be installed in pre existing conduits or innerducts.
The optical characteristics pertinent to optical fiber typically are relevant to fiber-optic cable, as well, with the exception of attenuation. A specification should clearly state that the maximum allowable attenuation value is for finished cable, since that is the only meaningful attenuation value for the end user. The bulk of consideration in cable specification should go to mechanical and environmental properties.
Indoor optical cables are designed for flexibility, tensile strength, ease of handling, and flame retardancy. They are usually not waterblocked or sunlight-resistant. In the United States, indoor cables must also meet one of four listing categories of the National Electrical Code (NEC): OFN/OFC or OFNG/OFCG for general purpose, OFNR/OFCR for riser applications, and OFNP/OFCP for plenum areas.
Tight-buffered cables, which include individual fibers, each directly coated with a protective plastic layer, are the most popular type of indoor cable today. Their tight-buffered design makes them easy to route in cabinets and outlets and provides compatibility with field-installable connectors. When installers terminate tight-buffered cables inside cabinets, those installers do not need fanout or other preparatory kits.
Outdoor cables are designed to withstand the rigors of outdoor installation for a lifetime of 20 to 40 years. They must have a wide operating-temperature range, be resistant to sunlight and moisture, and have sufficient tensile strength for long pull distances. The cable structure must isolate the glass fibers from the mechanical stresses that can be induced throughout the cable's service life. This isolation is best accomplished with a loose-tube design. Field-connecting loose-tube cable requires either spliced or preterminated connector pigtails or the use of a fanout or other preparatory kit to protect the individual fibers.
Waterblocking is an important attribute of outdoor cables because it prevents the accumulation of water in the cable core, which can freeze and stress the glass fiber. Historically, grease or gel in the cable core has accomplished waterblocking capability. More recently, dry tapes and yarns impregnated with absorbent polymer compounds have been used successfully.
Cables intended for direct burial underground should, in most cases, incorporate a steel tape armor for protection against gnawing rodents. Metallic armor has proven to be an effective rodent protection in controlled tests.
For aerial installations, outdoor fiber-optic cables require a messenger wire for lashing. However, some cables have an integrated messenger wire in a figure-eight cross section. Also available today are some all-dielectric self-supporting aerial cables, which contain no metallic elements but have sufficient tensile strength to support their own weight and a reasonable ice and wind load.
The most popular loose-tube cable includes buffer tubes, each of which contains one to 12 color-coded fibers. Some other cables include ribbons of fiber, rather than loose fibers in buffer tubes. Up to 24 fibers, joined together side-by-side, form a fiber ribbon. Stacks of these ribbons are in buffer tubes, creating a cable with a large number of fibers in a small cross-sectional area. Ribbon cables are popular in telephony and other long-haul applications and useful in crowded ducts where dense packing offers an advantage. Mass-fusion-splicing technology allows installers to splice 12 fibers in a ribbon simultaneously, offering labor savings in high-fiber-count applications. Ribbon cables are not common in premises applications but may become more popular as fiber becomes more prevalent in premises networks.
Indoor/outdoor cables incorporate the characteristics necessary for both applications; they are waterblocked for moisture protection and are sunlight-resistant. Indoor/outdoor cables also meet one or more of the code requirements for flame-spread resistance and smoke generation.
They can be useful in eliminating a splice point for a building-to-building run in a campus environment. They resemble outdoor cables more closely than indoor cables in design and appearance and usually require a fanout kit for field termination.
A detailed cable specification considers all environmental and regulatory factors that concern the installation environment, including temperature, mechanical loading, moisture, sunlight, flammability, rodents, and chemicals. Fully specifying each cable also requires a test or measurement method and criteria. Once again, standards are available to simplify this task.
Two sister documents published by the Insulated Cable Engineers Association (ICEA-South Yarmouth, MA) are useful as detailed product specifications. The ANSI/ICEA S-87-640 Standard for Outside Plant Communications Cable and the ANSI/ICEA S-83-596 Standard for Fiber Optic Premises Distribution Cable cover outside- and inside-plant cables, respectively. A third document, which will be designated S-83-696 and is currently in the works, will address 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 the optical fiber. ANSI/ICEA S-83-596 includes a summary of the flammability listing requirements from the NEC. Referencing these documents as appropriate for indoor or outdoor cable ensures you have included a full battery of environmental and mechanical testing and failure criteria in your specification.
In the United States, the federal government is another source for a detailed product specification. The Department of Agriculture's Rural Utilities Service (RUS) has published a document entitled Specification for Filled Fiber Optic Cables, which provides detailed product specifications for singlemode and multimode fiber, as well as OSP loose-tube cable. The RUS specification even outlines requirements for production and type testing as well as data reporting and manufacturers' recordkeeping. The RUS conducts technical reviews of cable manufacturers' products and programs and includes compliant companies' products on an "accepted list." By specifying that an OSP cable must be RUS-listed, an end user can take advantage of the RUS's efforts.
Writing your specification
By addressing only a few characteristics, then referencing the appropriate standards, you can fully specify a cable for a given application-indoor, outdoor, or indoor/outdoor-and a given fiber type. Areas you should address include cable application, cable type, any special requirements, fire-performance listing, fiber type, optical performance, and number of fibers.
With few exceptions, cable manufacturers conduct testing and specification development on the honor system. If a manufacturer claims the product has been tested and meets a certain specification, the buyer can accept the statement at face value or insist on a factory visit to verify that the manufacturer in fact performs the tests. Of course, the expense involved is most often beyond the means and desire of all but the largest end users. Certification to ISO 9001-a model for quality assurance in design, development, production, installation, and servicing-provides some assurance that the cable manufacturer has demonstrated to an independent third party that the product is developed to meet defined requirements and customer specifications are subject to credible technical review.
RUS acceptance, though only pertinent to OSP cable, is another form of third-party verification of a manufacturer's claims. Because the RUS specification and list of accepted suppliers is in the public domain, it costs nothing to access and use. Another third-party verification, which is always required, is an NEC-based listing for flame-retardant cables. These NEC-based listings are performed by organizations such as Underwriters Laboratories (Northbrook, IL) or Intertek Testing Services (Cortland, NY) and printed on the cable jacket.
Andrew K. Straw, PE, is manager of applications engineering at Siecor (Hickory, NC). He has also served as quality manager and engineering manager at the company's manufacturing plants in North Carolina. Straw presented a condensed version of this article at the BICSI Winter Conference in Orlando, FL, last January. The paper was entitled "Understanding and Specifying Optical Fiber Cables."
More on singlemode fiber
Dispersion-shifted singlemode fiber was introduced in the 1980s as an innovation that combined low dispersion with low attenuation at the 1,550-nm operating wavelength. A departure from the 1,310-nm window, it offered the potential for long-haul transmission over longer distances. In dispersion-shifted singlemode fiber, an advanced index-of-refraction profile causes the various types of positive and negative dispersion to cancel each other out. The result is virtually zero net dispersion at 1,550 nm. That's beneficial for single-channel transmission, but it has drawbacks that were discovered later.
In the 1990s, researchers developed diffraction gratings that allowed multiple transmission channels at separate wavelengths, spaced only a few nanometers apart. Systems today having 40 channels in the wavelength band from 1,530 to 1,565 nm are in widespread use, and the future promises availability of 65 or more channels in the region from 1,570 to 1,620 nm. This technology is referred to as dense wavelength-division multiplexing (DWDM).
As pioneers gained experience with DWDM, they learned that a small amount of dispersion is useful in mitigating interference among the closely multiplexed signals. Another generation of fiber, with a reduced but still-existent dispersion at 1,550 nm, soon was introduced to facilitate DWDM. This fiber type is called nonzero dispersion-shifted fiber (NZ-DSF). While very popular in the long-haul industry, it has no real benefits for local-area-network applications. The phenomena that NZ-DSF was developed to defeat are compounded with distance, while in short-distance applications of less than 20 km, those phenomena are insignificant. Therefore, there is no need for anything other than a dispersion-unshifted singlemode fiber in most premises and campus applications, even when potential upgrades to DWDM are considered.