TIA-568B Annex A details new fiber-connector requirements
Richard Akins Panduit Corp.
The upcoming TIA-568B standard will list a new set of performance requirements to ensure fiber-optic connectors provide the performance end users expect for premises applications.
The benefits of fiber-optic cabling include high-bandwidth, long-distance transmission capabilities and immunity to electromagnetic interference. With larger numbers of shorter links in the premises-cabling environment, reliability, another of fiber`s benefits, plays a critical role.
For decades, the global telecommunications industry has relied heavily upon fiber optics to provide reliable communications. The quality of service this transmission medium provides has allowed the delivery of world-class service in an environment where an hour of downtime can cost tens of millions of dollars.
One of the most critical components in any fiber-optic link is the connector. It is at this vulnerable point, more than any other, that mechanical or environmental factors can disrupt proper optical transmission and bring a network to its knees.
Even though the Telecommunications Industry Association (TIA) did not choose to standardize on any one of the new small-form-factor (SFF) connectors currently being marketed, the industry standards organization has been busy setting new performance standards for these and other connector styles, especially for premises-cabling applications. That sector has recognized the need for robust fiber-optic connectors. The local area network/building cabling environment typically subjects fiber connectors to higher levels of rematings and more potential mechanical damage than long-distance telecommunications systems. On the other hand, premises connectors do not face the extremes in environmental conditions that outdoor connectors must withstand.
To reflect this mixed bag of requirements, the TIA TR-41.8.1 Fiber Optic Task Group has developed a draft fiber-connector performance annex (Annex A) for the next revision of the Commercial Cabling Standard, TIA-568B. The draft Annex A, summarized in the table, lists the TIA-defined Fiber Optic Test Procedures (FOTPs) that connectors must pass. These FOTPs instruct connector vendors in the methods that must be followed when performing optical, mechanical, and environmental tests upon their connectors. By following these standardized test methods, the performance of various connector designs can be compared more easily and accurately. For each FOTP, Annex A also lists the required performance levels each connector must reach to be deemed acceptable in the specific application of commercial (premises) cabling.
Annex A is important because the draft TIA-568B3 standard (the fiber-optic component section of TIA-568B) allows any fiber connector to be used in the majority of premises-cabling applications as long as the connector passes the tests listed in the annex. It should be noted that to be recognized by TIA-568B, a connector must also be defined by a TIA Fiber Optic Connector Intermateability Standard (FOCIS)--for example, FOCIS-2, FOCIS-3, or FOCIS-6.
The history of Annex A
The current edition of TIA-568A does not include a fiber-connector test requirements section. Instead, a few tests are mentioned, along with required performance levels. For both singlemode and multimode fiber, insertion loss per mated connector set is to be less than 0.75 dB. Back-reflection (return loss) is specified at better than -20 dBm for multimode fiber and better than -26 dBm for singlemode fiber.
Mechanically, the connector is required to be able to withstand 0.5 lbs of force at a 0 angle (when terminated onto "buffered" fiber). When a plug is terminated onto a fully jacketed fiber cable (for example, as part of a patch cord), the cable-retention requirement is raised to 7.5 lbs.
Several years ago, TIA TR-41.8.1 asked the TIA FO6.3 committee to begin work on a "detail" specification for connectors to better define tests to cover the optical, mechanical, and environmental conditions fiber connectors may experience in use. This detail specification would cover the use of TIA FOCIS-2 BFOC/2.5-style fiber-optic connectors (often referred to as ST-style connectors) used within the premises-cabling environment.
According to the draft detail specification, "The intent...is to enable end users and manufacturers to specify fiber-optic connector set components and cable assemblies supporting the requirements of TIA/EIA-568A." The following components were to be specified within the detail specification (initially TIA-SP-3397, to become TIA-47500AB): kit, adapter, hybrid adapter, patch cord, pigtail cord, premises cable, and simplex and duplex plugs for singlemode and multimode fiber with various cable constructions and types.
By December 1997, the document had grown to 27 pages. Paragraph 1.3 of the document detailed a required 17-character product-designating sequence to be applied to all BFOC/2.5-style products. Generically, this product-designating sequence looked like "E-47500AB-PDMC2A17-B." Within the designator were characters that defined the specification number, the variant identification number (kit, adapter, etc.), the quality assessment level, the connector specification, the optical-fiber specification (for patch cords, etc.), and the fiber-optic communications cable specification (indoor/outdoor, etc.). A series of line drawings provided examples of various product configurations.
A separate section of the detail specification provided supplementary information such as how to clean products and color-code plugs and adapters. Section 2 of the draft specification discussed the physical intermateability requirements for the connectors. BFOC/2.5 products would follow the requirements of TIA FOCIS-2, while SC-style components (used in hybrid adapters and patch cords) were to be defined by TIA FOCIS-3. Section 3 discussed the requirements for qualification approval of various products. Toward the end of this section, Tables 4, 5, and 6 defined the test sequences each connector would go through, while Table 7 provided detailed information on each of the required test procedures (similar to the table).
In the November 1997 meeting of the Fiber Optic Task Group at TIA TR-41.8, it was decided to include the pertinent information from the detail specification directly into the TIA-568B. One reason was that the detail specification had grown quite large and had already been in progress for several years. Second, the detail specification covered only one connector style--thereby potentially requiring every other connector style (including the new breed of SFF connectors) to develop their own detail specifications. Third, the draft of TIA-568B provides a means by which the testing information could be kept in the same document as the application specifications, providing more easily accessed information to the end user and a more efficient manner by which to make updates.
Annex A does not make existing products obsolete. Some of the testing requirements have become more stringent (particularly cable retention), but for the most part, today`s leading-edge fiber connectors should be able to perform adequately. However, some of the older connector products will need to be more fully tested to prove themselves in compliance with TIA-568B. In addition, some of the new smaller connectors will have to provide the required test results from the start, since they may not have the benefit of years of "real-world" experience to back their reliability claims.
Providing the required performance
To answer the call for robust connectors in the premises-cabling market, vendors have spent hundreds of millions of dollars improving fiber-optic connector designs to reach the current state of the art, including the BFOC (TIA FOCIS-2), SC (TIA FOCIS-3), and the small-form-factor Fiber Jack (TIA FOCIS-6) connectors (see Fig. 1). These connectors share the same basic ferrule/split-sleeve fiber alignment mechanisms. As a group, they are well represented in a number of international standards, including the TIA-568A Commercial Cabling Standard.
Optical tests: There are essentially only two optical tests in Annex A: insertion loss (the amount of light lost through the connection) and backreflection (the amount of light that reflects back off the face of the fibers being joined within mated connectors). As the table shows, the annex covers the backreflection requirements of both multimode and singlemode fiber. Connectors used in premises-cabling applications are required to handle both fiber types due to the increasing use of singlemode fiber, even to the desktop. For transceiver and networking electronics manufacturers, the ability of any connector style to handle both singlemode and multimode fiber connectivity is crucial to their ability to cost-effectively provide products to end users over the long term.
The BFOC, SC, and Fiber Jack connectors use similar 2.5-mm ferrule and split-sleeve fiber-alignment mechanisms to provide excellent insertion loss and backreflection capabilities. In all three cases, the connectors individually protect each fiber within separate, spring-loaded ferrules. (Note that the BFOC is a simplex connector, the SC can be either simplex or duplex, and the Fiber Jack provides duplex connectivity in a half-size, RJ-45-style plug and jack.)
The individually spring-loaded feature ensures physical contact between mated fibers, eliminating end gaps and lowering connector insertion loss (see Fig. 2). The ferrules from mating connectors are aligned within a split sleeve, which limits the amount of angular misalignment between mated fibers, again lowering insertion loss (see Fig. 3). The use of a separate cylindrical ferrule for each mated fiber also reduces the loss generated by rotational offsets as shown in Figures 4a and 4b compared with ferrules that house multiple terminated fibers. Finally, the concentricity and circularity of the ferrules provide tolerances tight enough for both multimode and singlemode use.
The light reflecting off a connection point and back through the fiber can detrimentally affect optical transmitters, particularly in singlemode systems. One way to overcome backreflection is to introduce various end geometries to the ferrules. By using 2.5-mm ferrules, end surfaces can be made into a variety of geometries, including physical contact, ultra physical contact, and angled physical contact, to achieve the desired level of backreflection (down to -60 dB or lower, well in excess of the requirements in the draft Annex A).
Mechanical tests: A fiber-optic cabling system typically will include connectors terminated onto patch cords that will be handled by the end user on a consistent basis. The rest of the fiber connectors are typically protected within enclosures (see Table). For example, the cable retention for connectors used on patch cords is 15 lbs per cable sheath. Connectors within enclosed areas may be terminated directly onto buffered fibers (instead of jacketed fiber cables within added strength members such as aramid yarn). In this case, the cable-retention requirement is only 0.5 lb per buffered fiber.
However, as fiber makes its way into the horizontal cabling environment, many more fiber connectors are being placed within modular patch panels and outlets. In addition, many fiber-to-the-desktop cabling runs use duplex "zip-cord"-style fiber cables, not the multistrand fiber cables traditionally used in backbone cabling links. To provide the extra protection required and to take advantage of the extra strength of the duplex zip-cord cable construction, it makes sense that connectors allow for the termination of individually jacketed fibers in "behind the wall" connectors as well as in connectors used on patch cords. Connector plugs and jacks should allow for the option of terminating (typically) 3-mm jacketed fiber cables to provide a full 15 lbs of cable retention. The FOCIS-2, FOCIS-3, and FOCIS-6 connector implementations provide this capability.
Other mechanical tests include flexing and twisting the fibers. In both cases, the ability of a connector to accept both buffered and fully jacketed fibers is important to provide end users with the flexibility to add strength to the connection if desired. In the real world, the flex and twist tests are important to judge how the connectors will react to the forces applied to the fibers when the terminated cables are routed through enclosures, racks, and cable-management products.
The final mechanical test is for connector-coupling strength. This simple test determines the amount of force required to remove the plug from the jack or from the adapter, depending upon the connector style. As fiber moves to the desktop, vacuum cleaners, chair legs, and other moving objects can easily catch on a fiber patch cord. Coupling strength is critical to the survival of the product and the network connection.
The final mechanical test is for connector durability. The requirement is that the connector can be mated at least 500 times without significant increases in insertion loss. The 2.5-mm ferrules provide the mechanical stability that helps protect the fragile bare fibers through such a test. In addition, the ferrule end surface provides a good surface across which the end user can wipe the fiber ends clean, should they become dirty during the numerous rematings of the test or through common everyday use.
Environmental tests: The premises-cabling market typically does not see the extremes of temperature and humidity found in telecommunications applications. But Annex A does require testing under high- and low-temperature and high-humidity conditions.
In this regard, the primary benefit of the 2.5-mm ferrule is that the ferrule, typically with an adhesive or epoxy within it, encases the bare fiber lengthwise. This containment helps keep water and humidity away from the fiber itself, where it could physically degrade the fiber over time. This degradation may affect the longevity of the fiber when put under a bent or otherwise stressful condition. The 2.5-mm ferrule holds the fibers straight, reducing or eliminating one source of stress.
Data networks within the commercial-building environment are mission critical and will become more so in the future. The high-speed transmission capability provided by fiber-optic systems is only valuable to the end user if the fiber-cabling system continues to provide near-perfect quality of service. This kind of quality will require the use of proven connector technologies that perform to the terms of the new standards defined within the draft of Annex A of TIA-568B3. u
Rick Akins is fiber-optic product manager within the Network Systems Div. of Panduit Corp. (Orland Park, IL).