Fibre optic cable designs for the outside plant fall into four categories: 1) loose tube, 2) central tube, 3) ribbon, and 4) slotted core. The impetus behind using ribbon fibre was to increase fibre density, achieve higher count cables, and take advantage of mass fusion splicers, which splices multiple fibres in one operation. Ten years ago the design focus was on high count, but over the last eight or nine years cable manufacturers also have focused on reducing the size of fibre optic cables. The trend in higher counts and decreased diameters can be seen in slide 10.
Ribbon fibre generally consists of up to 12-fibres side by side. Versions with 24-fibres also are available. The ribbons are encased in an acrylate material. For high-count cables the 12-fibre ribbons are stacked one on top of the other to form a larger matrix that can be cabled in a stranded loose tube design or in a central tube cable. Another advantage of ribbon fibre is the option of using a mass fusion splicer that is capable of splicing 12- or more fibres at once, which leads to cost savings in splice time during cable deployment.
One variation of ribbon cable is the slotted core design that was invented by NTT and is most commonly used in Japan and other Asian countries. There also have been deployments of slotted core in Italy, Sweden and the Middle East. Slotted core designs use either 4-fibre or 8-fibres which are stacked and contained in an extruded channel. These cable designs are used throughout the network.
There are variations in design especially for aerial deployment that can be classified as loose tube such as Figure-8, all-dielectric self-supporting (ADSS) and optical fibre ground wire (OPGW.) Figure-8 has a steel messenger wire linked to the cable, which forms a figure-8 cross section.
ADSS and OPGW are used on power utilities' rights-of-way. To withstand the strain between poles or towers ADSS cable as strength elements. ADSS is usually installed in areas with a high incidence of lightning strikes; in the US this is in Florida and in the Pacific Northwest.
OPGW is stranded aluminium-clad steel with empty stainless steel tubes containing optical fibres and gel filling. The cable serves two functions: 1) It acts as a ground to conduct short circuit currents resulting from faults in the electrical system and to protect the line from lightning, and 2) It provides internal communications capability for supervisory control and data acquisition, and additional revenue generating option through third-party leases or other business models for serving telecom markets.
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