The 4 Parameters of Specifying Fiber

[IMAGE]376[/IMAGE]When deploying optical fiber, there are four important decisions that must be made. How long does the cable need to be? What type of fiber should be used? How many fibers should be in the cable? What type of cable jacket should be used?

Cable Length — Cable length is somewhat trivial since this parameter may easily be empirically determined based on measurements or estimates.

Fiber Type — There are two basic types of fiber; multimode and single-mode. Multimode fiber-optic modems are generally less expensive than their single-mode counterparts, but single-mode fiber is a few cents per foot per fiber less expensive than multimode. Consider multimode for applications with short optical path lengths and low data rates/bandwidth requirements. For example, multimode fiber may be used to pass one channel of analog video with serial camera control data using analog fiber modems cost effectively over distances of up to several kilometers. Gigabit Ethernet, on the other hand, could be limited to as little as 700 feet on multimode fiber. Single-mode fiber would be a better choice for such an application. Depending on the application and the optical path length, it may be useful to consider a hybrid fiber cable that allows for multimode and single-mode fibers in the same jacket.

While the following lists are by no means absolute, they offer a general guide.

Consider multimode fiber for the following:

  • One channel of analog video
  • One channel of analog video with serial camera control data
  • One channel of serial data
  • Contact closures
  • Four or eight channels of analog video on one fiber
  • One channel of Fast Ethernet
  • Optical path lengths of less than one kilometer
  • When using analog fiber optic modems

Consider single-mode fiber for the following:

  • More than eight channels of analog video on one fiber
  • Multiple channels of analog video with serial camera control data on one fiber
  • Gigabit Ethernet
  • Optical path lengths greater than one kilometer
  • When using digital fiber-optic modems

Fiber Count — Always pull as many fibers as you currently need while anticipating future requirements as best as possible. Better to have more fiber than not enough, but do not pull spares just for the sake of it. Individual fibers typically do not fail; cables fail, and usually catastrophically due to digging incidents and the like. Fibers are prebundled in multiples of six so it might be wise to round up to the next multiple of six. Availability might be better for such cables, as well.

Fiber Jacket — Although not as glamorous, the cable jacket deserves attention. The proper jacket will ensure long-term reliability, safety and compliance with local ordinances. It is the function of the jacket to protect the cable from things such as water, rodents, ultraviolet light, high and low temperatures, and petroleum and other chemicals. The jacket can also protect the cable from abrasion, crushing and impact. The proper cable jacket will allow the cable to be directly buried in the ground, hung aerially while being self-supporting, hung aerially while being lashed to a messenger cable, or installed in conduit. Finally, the cable jacket determines how safe a particular cable is for use in areas where people or animals will be. Cable jackets are available in low smoke and flame-resistant versions and, as with copper cables, available for use in plenum air spaces. As a side note, while an optical fiber, being a complete dielectric is not susceptible to interference from electromagnetic interference, proximity to high voltage lines may not be allowed for safety reasons.

John Nave is Technical Support Manager for Danbury, Conn.-based Communications Network (ComNet).


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