Dark (or they also say "blind" or "unlit") fibers are optical fibers that were laid, but are not yet used in a fiber-optic line.Since fiber optic cable carries information in the form of pulses of light, the term "dark fiber" refers to those through which no light pulse is transmitted.
There are thousands of kilometers of dark fiber! Typically, ISPs are laying fiber-optic cables “oversized” in the hope of traffic growth in the future. This is more profitable than subsequently expanding the already existing fiber-optic communication line. It is also assumed that excess capacity can be leased out - for cable television or telephone communications. That is, in fact, these are some promising projects.The fibers are not monitored or connected to the equipment in any way, but if necessary provide the infrastructure for quick commissioning.Of course, this mainly applies to local providers and telephone companies. On a large scale, this practice is unacceptable. Although, as a rule of thumb, in an optical cable for every six pairs of fibers involved, it is recommended to lay one redundant pair.
PS Honestly, we have not been able to find out where this indisputable "empirical" rule came from, and who gave the above recommendations for redundancy on backbone optical lines. If you know something on this issue - share information in the comments!
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