Cross the road to alternate in-ground cables

Feb. 1, 2004
Alternative cable design for campus-backbone applications a hearty option.

Alternative cable design for campus-backbone applications a hearty option.

Available duct space for optical-fiber cable in the campus backbones of many LANs is approaching or is at its maximum capacity. Concerns about business disruption and damage to existing infrastructure, combined with the high cost associated with directional boring and open trenching, have made the installation of new ductwork unappealing.

These issues, however, can be overcome by using a cable type designed for alternate in-ground applications—road cable.

Robust design

Road cable is a robust, single-tube cable designed for installation in private-network campus backbone environments. The cable is optimized for installation in roads, parking lots, and sidewalks, providing significant cost and time savings in cable installation. The cable is fully compliant to the TIA/EIA-568B Commercial Building Telecommunications Cabling Standard, which requires outside-plant cable to conform to the ICEA-640 Standard for Optical Fiber Outside Plant Telecommunications Cable.

The design of this cable, intended for alternate in-ground applications, offers high fiber density while maintaining a small diameter. The cable is made of a copper central tube containing optical fibers color-coded in accordance with TIA/EIA-598 Optical Fiber Cable Color Coding Standard, and segregated in groups by color-coded binders.

The copper central tube is filled with a waterblocking gel to prevent the ingress of water and is covered with a polyethylene sheath. The yellow sheath represents singlemode fiber cable, while orange represents multimode. A 60-fiber road cable has a diameter of just 7.0 mm, and the 144-fiber design is 9.5 mm in diameter.

This cable type is rugged and craft-friendly, and can be accessed with commercially available tools. You'll need a hook blade or suitable knife to remove the outer jacket, while a common tubing cutter can be used to score the copper tube so youcan access the optical fibers. Connectorization is achieved with a single-tube furcation kit.

Groove-cut installation

Installation involves placing the cable in a groove cut into a surface, as opposed to the typical open trench. During installation, the cable is first secured with special foam hold-down spacers pressed into the groove on top of the cable. A rubber hold-down filler rod is then pressed in on top of the foam spacer(s).

The black rubber-like filler material provides a hold-down force for properly securing the cable in the groove. It is rated for temperatures in excess of 240° C (400° F) and acts as a thermal barrier for hot bitumen (a common road-repair material).

The groove can be cut to suitable widths and depths; however, to prevent any structural damage to the road, it is recommended that groove depth not exceed two-thirds of the overall road thickness.

Both the cable and its installation methods are advantageous for several reasons, specifically, that the cable can be installed quickly (up to 1 kilometer per day) using commonly available equipment and materials. In addition, installation costs a fraction of such traditional methods as directional boring and trenching.

Road cable is also ideal for traditional direct-buried applications at any depth, as the cable is extremely crush-, rodent-, and corrosion-resistant. This characteristic makes transitioning from road or parking lot simple, and eliminates the requirement for a transition splice or the installation of ducts.

If ducts are preferred, the cable can be pulled like any other cable and is specified for tensile loads of up to 2,700 N (600 pounds). Drop cables can be spliced to the road cable using standard splice closures and modified splice closure end caps designed to strain-relieve the cable. The closures can be installed in manholes, handholes, or pedestals.

Performance attributes

The road cable offers crush performance far beyond that of conventional cables. Road cable is specified to handle crush forces up to 440 N/cm (251 pounds/inch) and has been tested to more than 800 N/cm (457 pounds/inch) without an increase in optical attenuation. Crush resistance, provided by the hoop strength of the copper central tube, enables installation with minimal disruption to city or business traffic flow.

The cable's outstanding crush resistance also helps ensure its long-term reliability in the harshest of climates. Despite the use of the best sealing materials, water may eventually migrate into the cable grooves, as with any groove cut in the road surface. During winter months, water can freeze and exert a considerable crush force on the cable. But road cable has been successfully tested in accordance with FOTP-98, Method B, Fiber Optic Cable External Freezing Test, and does not exhibit degraded performance even when completely surrounded by ice in an enclosed environment.

Another challenge in this environment is that road surfaces easily can exceed 140° F in the summer. In other words, a groove cut into a solid surface may constrict the expansion of the cable due to temperature changes. If a traditional cable were installed in this type of groove, one of two things would likely occur with elevated temperature:

  • A conventional loose tube or central-tube cable design would expand, and could pop out of the groove onto the road surface. Bitumen and other road-sealant materials typically soften with high temperatures, making it easier for the cable to push out.
  • A light buffer-tube-type cable design would expand, but probably would not have sufficient force to exit the groove. But this expansion contained by the groove likely would cause unacceptable attenuation levels due to macrobending of the optical fiber.

Road cable has a low coefficient of thermal expansion to overcome this challenge. Specifically, the coefficient of thermal expansion for copper is significantly less than that of the polyethylene sheath and offsets thermal expansion at elevated temperatures.

LANs have traditionally relied on the standard loose-tube optical-fiber cable design as the campus backbone link between the main cross-connect and the intermediate cross-connect. But alternative optical-cable designs like road cable can enhance installation and termination options such that increased productivity and reduced cost can be realized without compromising system reliability.

Doug Coleman is manager of technology and standards for private networks with Corning Cable Systems (

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