Data Communications Networking Devices - 4th Ed.pdf

774 ___________________________________________________________ SPECIALIZED DEVICESSignal attenuationThe signal attenuation of optical ®bers are relatively independent of frequency. Incomparison, the signal attenuation of metallic cables increases with frequency. Thelack of signal loss at frequencies up to 1 GHz permits ®ber optic systems to beexpanded as equipment is moved to new locations. In comparison, conventionalmetallic cable systems may require the insertion of line drivers or other equipmentto regenerate signals at various locations along the cable.Electrical hazardOn ®ber optic systems light energy in place of electrical voltage or current is usedfor the transfer of information. The light energy alleviates the potential of a shockhazard or short circuit condition. The absence of a potential spark makes ®ber optictransmission particularly well suited for such potentially dangerous industrialenvironment uses as petrochemical operations as well as re®neries, chemical plants,and even grain elevators. A more practical bene®t of optical ®bers for most corporatenetworks is the complete electrical isolation they afford between the transmitterand receiver. This results in the elimination of a common ground which is arequirement of metallic conductors. In addition, since no electrical energy is transmittedover the ®ber, most building codes permit this type of cable to be installedwithout running the cable through a conduit. This can result in considerablesavings when compared to the cost of installing a conduit required for conventionalcables, whose cost can exceed $2500 for a 300-foot metal pipe.SecurityConcerning security, the absence of radiated signals makes the optical ®bertransmission TEMPEST acceptable. In comparison, metallic cables must often beshielded to obtain an acceptable TEMPEST level. Although ®bers can be tappedlike metallic cable, doing so would produce a light signal loss. Such a loss could beused to indicate to users a potential ®ber tap condition.Weight and sizeOptical ®bers are smaller and lighter than metallic cables of the same transmissioncapacity. As an example of the signi®cant differences that can occur, consider anoptical cable of 144 ®bers with a capacity to carry approximately 100 000 telephoneconversations. The cable would be approximately one inch in diameter and weighabout 6 ounces per foot. In comparison, the equivalent capacity copper coaxialcable would be about three inches in diameter and weigh about 10 pounds per foot.Thus, ®ber optic cables are normally easier to install than their equivalent metallicconductor cables.