20.0 Fiber Optic Cable

     20.1 Multimode (MM) Fiber
     Step index or graded index fiber. In North America the most common 
     size is 62.5/125; in Europe, 50/125 is often used.  These numbers 
     represent the diameter of the core (62.5) and diameter of the 
     cladding (125) in microns.  Multimode fiber is typically used in 
     applications such as local area networks, at distances less than 2 km.

     20.2 Single Mode (SM) Fiber
     Single mode fiber has a very small core.  Typical values are
     5-10 microns.  Single mode fiber has a much higher capacity and
     allows longer distances than multimode fiber.  Typically used
     for wide area networks such as telephone company switch to switch
     connections and cable TV (CATV).

     20.3 Loose Buffer
     The fiber is contained in a plastic tube for protection.
     To give better waterproofing protection to the fiber, the space 
     between the tubes is sometimes gel-filled. Typical applications 
     are outside installations. One drawback of loose buffer construction 
     is a larger bending radius. Gel-filled cable requires the installer 
     to spend time cleaning and drying the individual cables, and 
     cleaning up the site afterwards.

     20.4 Tight Buffer
     Buffer layers of plastic and yarn material are applied over the fiber.
     Results in a smaller cable diameter with a smaller bending radius.
     Typical applications are patch cords and local area network connections.
     At least one mfr. produces this type of cable for inside/outside use.

     20.5 Ribbon Cable
     Typically 12 coated fibers are bonded together to form a
     ribbon.  There are higher density ribbons (x100) which have
     the advantage of being mass-terminated into array connectors.
     A disadvantage is that they are often harder, and require special 
     tools to terminate and splice.

     20.6 Fiber Connectors
     There are a lot of different types of connectors, but the ones 
     commonly found in LAN/MAN/WAN installations are:

     FSD - Fixed Shroud Device, such as the FDDI MIC dual-fiber connector.
     SC  - A push-pull connector. The international standard.
    The SC connectors are recommended in SP-2840A.  The SC
    connector has the advantage (over ST) of being duplexed
    into a single connector clip with both transmit/receive fibers.
     SMA - Threaded connector, not much used anymore because of losses
    that change with each disconnection and reconnection.
     ST  - Keyed, bayonet-style connector, very commonly used.


     20.7 Fiber Optic Test Equipment
     Continuity tester: used to identify a fiber, and detect a break. 
     One type resembles a f/o connector attached to a flashlight.
   
     Fault locator:  used to determine exact location of a break. 
     Works by shining a very bright visible light into the strand. 
     At the break, this light is visible through the cable jacket.
         
     Tone Generator and Tracer: used to identify a cable midspan or 
     to locate a strand at its far end. Similar in purpose to the
     tone testers used on copper cable. The tone generator imposes 
     a steady or warbling audio tone on light passing down the cable. 
     The tracer detects and recovers the tone from light lost through 
     the cable jacket as a result of bending the cable slightly.
        
     Optical Source and Power Meter: used to measure the end-to-end 
     loss through a f/o strand, or system of cable, connectors and 
     patch cables. Measurements are more accurate than an OTDR.
      
     Optical Time Domain Reflectometer (OTDR): used to measure the length 
     of a cable, and detect any flaws in it. Can also be used to measure 
     end-to-end loss, although less accurately than a power meter.
     
     Fiber Talk set: allows using a pair of f/o strands as a telephone line.

     Fiber Optic Testing, standards: see EIA-455-171 (FOTP-171), EIA 526-14.