Transmission Media -Cont

o    Uses step index fiber and a highly focused source of light that limits beams to a small Single Mode Fiber range of angles all close to the horizontal o    Single Mode fiber is manufactured with a much smaller Diameter than Multimode o    All  of  the  beams  arrive  at  the  destination  together  and  can  be  recombined  without distortion to the signal

™  Fiber Sizes o    Optical  Fibers  are  defined  by  the  ratio  of  the  diameter  of  their  Core  to  the diameter of their Cladding o    Both the diameters are expressed in Microns (Micrometers) o    A core is surrounded by cladding forming the Fiber. o    In most cases, fiber is covered by a Buffer layer that protects it from moisture. o    Finally the entire cable is encased in an outer jacket o    Both  core  and  cladding  can  be  made  of  either  glass  or  plastic  but  must  be  of different densities o    In  addition  the  inner  core  must  be  ultra  pure  and  completely  regular  in  size  and shape o    Chemical differences in material and even small variations in the size or shape of the core alter the angle of reflection and distort the signals o    Some applications can handle a certain amount of distortion and their cables can be made cheaply but others depend on complete uniformity o    The  outer  jacket  can  be  made  of  several  materials  including  Teflon,  Plastic, Fibrous Plastic, metal tubing Each of these materials have a purpose: -Plastic is lightweight and cheap but do not provide structural strength and can emit fumes when burnt -Metal tubing provides strength but is costly -Teflon is lightweight and can be used in open air but it is expensive and does not increase cable strength Light Sources for Optical Cable o    For  tx  to  occur  the  sending  device  must  have  a  light  source  and  the  receiving device with a photosensitive cell (Photodiode) o    Photodiode converts the light into current usable by the computer The light source can either be an LED or an ILD    LED: -Cheaper   but   provide   Unfocused   light  that   strikes   the   boundaries   of channel at uncontrollable angles -Limited to short distance use    LASER: -Can   be   focused   to   a   narrow   range   allowing   control   over   angle   of incidence ƒ     Fiber Optic Connectors o    Importance of Connectors o    If  connector  is  over  tight,  two  cores  can  be  compressed  and  angle  of reflection of the signal will be altered o    All  of  popular  connectors  are  Barrel  shaped  that  come  in  male  and female versions o    The  cable  has  a  male  connector  that  fixes  into  a  female  connector attached to the device to be connected ƒ     Advantages of Optical Fiber The major advantages of Fiber over twisted pair and coaxial cable are: ŠNoise Resistance: -Because fiber uses light rather than electricity, noise is not a factor -External light the only form of possible interference is blocked from the channel by the Outer jacket Less Signal Attenuation -Fiber  optic  Transmission  distance  is  significantly  greater  than  other media -A signal can run miles w/o regeneration Higher Bandwidth -Can support higher BWs and higher data rates -High rates are not utilized by absence of signal generation and reception technology Disadvantages of Optical Fiber COST -Expensive -No  impurities  or  imperfections  can  be  tolerated,  so  manufacturing  is costly -Laser light sources can be expensive INSTALLATION -Roughness & Cracking of core cannot be tolerated -All connections must be perfectly alligned ƒ     Disadvantages of Optical Fiber Fragility -Glass fiber is very fragile -Can  not  be  used  in  extreme  conditions  where  hardware  portability  is required