Loose tube or tight-buffered? Installation costs and reliability determined by the capabilities of different cabling types.

Stranded loose-tube cable has historically been deployed for campus backbone applications in accordance with industry standards, such as ICEA-640, "Standard For Optical Fiber Outside Plant Communications Cable." Work has recently been completed on ICEA-696, "Optical Fiber Indoor/Outdoor Cable Standard," which provides cable design and performance guidance that includes a tight-buffered cable option.

The Insulated Cable Engineers Association (ICEA) is responsible for providing cable standards, test procedures and guidelines for the telecommunications industry. This is the first ICEA and industry standard that provides guidance on tight-buffered cable intended for deployment in the outside plant as well as the inside plant. To date, tight-buffered cables have been designed for deployment in the more benign inside plant environment.

Historically, stranded loose-tube cable has been the cable design of choice for outside plant applications. This cable establishes a strain-free environment for the optical fiber that diminishes the influences of mechanical and environmental effects. The strain-free environment established in the loose-tube cable design compensates for movement in the cable structure without inducing mechanical forces on the fiber. This characteristic enhances the operating temperature range and the mechanical robustness of the loose-tube design.

Tight-buffered cables typically do not isolate the fibers from external forces to the same extent; therefore, temperature-related expansion and contraction effects, as well as mechanical forces, applied to any component of a tight-buffered cable can be translated to the optical fiber. As a result, tight-buffered cables are typically more sensitive to temperature extremes and mechanical disturbances than are loose-tube cables.

DIFFERENT TEST CRITERIA

The ICEA-696 document covers optical-fiber communications cables intended for use in indoor/outdoor optical-fiber applications and is not intended to be a carte blanche approval of tight-buffered cable in the outside plant. The document contains mechanical and environmental test requirements that address various optical-fiber cable designs that include tight-buffered cable.

The document includes essentially the same test requirements as the ICEA-640 document, but with less-demanding test criteria, since the deployed lengths are shorter. Of particular interest are the operating temperature range, water-blocking and jacket ultraviolet light-resistance test requirements.

ICEA-696-compliant tight-buffered cables are specified to operate across a -40[degrees]C to + 70[degrees]C temperature range. When tested in accordance with ICEA-696 and the test procedure FOTP-3, "Procedure to Measure Temperature Cycling Effects on Optical Fibers, Optical Cable, and Other Passive Fiber-Optic Components," the change in attenuation at extreme operational temperatures (-40[degrees] C and +70[degrees] C) cannot exceed 0.40 dB/km at 1,550 nm for single-mode fiber or 0.60 dB/kin at 1,300 nm for multimode fiber.

By comparison, the maximum allowable change in attenuation at extreme temperatures for ICEA-640-compliant cables cannot exceed 0.15 dB/kin at 1,550 nm for single-mode fiber or 0.30 dB/km for multimode fiber. Stable cabled fiber-attenuation performance across the operating temperature range is critical to ensure reliable transmission performance of high-data-rate systems, such as 10-Gigabit Ethernet.

Indoor/outdoor, flame-retardant, tight-buffered cables incorporate gel-flee, dry water-blocking technology to ensure long-term water-blocking performance. When tested in accordance with the industry standard test, FOTP-82, "Fluid Penetration Test for Fluid-Blocked Fiber Optic Cable," the tight-buffered cable must withstand water ingress and migration in accordance with industry standards for outside plant cable.

WATER-BLOCKING PERFORMANCE

Conforming water-blocked performance eliminates reliability concerns normally found in typical industry tight-buffered cables deployed in the outside plant. Outside plant cable should exhibit superior water-blocking performance to mitigate potential ice crush, material-compatibility and water-migration issues.

Outside plant application spaces include aerial deployment. To meet the demands of that environment, tight-buffered cable should be compatible with exposure to ultraviolet (UV) light. The ICEA-696 standard requires that the cable jacket withstand a light-exposure test in accordance with ASTM G 155, Cycle 1, except that the exposure is 720 hours minimum.

Proper selection of UV-resistant jacket materials is critical to conform to this requirement. The simple use of a black jacket is not adequate to provide the necessary UV resistance. Unless UV-resistant jacket materials are used, jacket cracking and brittleness can develop, which will compromise system performance or lead to total cable failure.

End-users have requested a low-fiber-count (no more than 24 fibers), flame-retardant, tight-buffered indoor/outdoor cable to use in short-distance campus backbone applications. The flame-retardant feature eliminates transition splicing at the building entrance, and the tight-buffered fiber offers direct connection without breakout kits. These features offer system installation cost savings for low-fiber-count indoor/outdoor applications.

An economic analysis of a typical indoor/outdoor campus backbone installation-with 24 fibers composed of 12 multimode and 12 single-mode fibers-indicates that the plenum indoor/outdoor tight-buffered cable has the lowest-cost system solution. The plenum tight-buffered cable offers installation labor savings based on the elimination of connectors, splicing and patch panels at the entrance facility.

Since the cable is plenum rated, it is deployed directly from the equipment room in one facility to the equipment room in the other facility, which minimizes the installation time compared to other installation scenarios. In addition, the cable eliminates or minimizes termination materials, such as buffer tube breakout kits.

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Doug Coleman is manager of technology and standards for Coming Cable Systems, Hickory, N.C.