Fiber Optic Networks

Basics of Fiber Optic Networks

• Light is used to transmit signals

• Signals travel along fiber channel, bouncing off walls (Fig. 25.8)
  

  Fibre Optic Transmission

  Bits are transmitted as pulses, at about 69% the speed of light (Fig 25.9).

  

  Fibre Optic Speed

• Advantage over copper: more bits per inch (bits are smaller, not faster).

• Electrical signals can be converted to optical signal at a few gigabits/sec. -> Potential: gigabit networks

Ultimate bandwidth of fibre optic transmission

• Three colors of light can be used to signal (at 850, 1300, and 1500 nm, about 200 nm wide).

  Each has about 25 terahertz of capacity.

• So aggregate capacity is about 50-75 terabits.

  In reality, only about 10-20 terabits is achievable.

SONET (Synchronous Optical Network)

• Important - because telephone companies will switch to it, making parts cheaper -> designed to replace copper.

• OC = Optical Carrier, STM = Synchronous Transport Module

   -----------------------------------------------------
   Data Rate       SONET (US)     CCITT (International)    
  (Mbits/sec)      Designation         Designation       
  -----------------------------------------------------
      51.84           OC-1                 -            
     155.52           OC-3               STM-1          
     622.08           OC-12              STM-4          
    1224.16           OC-24              STM-8          
    2488.32           OC-48              STM-16          
  -----------------------------------------------------
  

• In reality, telephone companies will only use STM-1, STM-4, and STM-16

WDM (Wavelength Division Multiplexing networks)

• Problem: current equipment cannot handle electrical signals for many gigabits per second

  Solution: use multiple transmitters/receivers with different wavelengths.

• Aggregate bandwidth is in the terabits, one node only gets gigabit

• Biggest problem: getting transmitters and receivers to agree on frequencies.