Transceiver Signal Strength Issues

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• fiber does incur some signal strength loss
  • due to microscopic imperfections in the structure of glass fiber and smoothness of the edge of the core
    • leads to some small fraction of light within the core being scattered or absorbed
• attenuation can be tested using an optical source and optical power meter (fiber light meter)
• optical link budget is the amount of loss suffered by all components along a fiber transmission path
  • aka loss budget
  • calculated using:
    • attenuation
      • loss over the length of the cable
      • based on fiber type and wavelength used
      • single mode has loss of up to 0.4 dB/km
      • multimode has loss of 0.8 dB/km to 3 dB/km
    • connectors
      • each connector in a path incurs a loss
        • around 0.75 dB
    • splices
      • splices in the cable account for a loss of 1 dB for mechanical and 0.3 dB for fusion
• estimated loss budget is calculated when planning a link
  • tested at deployment to derive actual value
  • can reveal installation fault or unexpected source of signal loss

Info

• FOA has a loss budget calculator here

• loss budget must be less than the power budget
• power budget is calculated from the transceiver transmit power and receiver sensitivity
  • both measured in dB per milliwatt (dBm)
  • E.g., Tx = -8 dBm, Rx = -15 dBm, then power budget = 7 dBm
  • if loss budget is 5 dB, margin between the power budget and loss budget will be 2
    • margin is a safety factor to account for suboptimal installation conditions, aging, repair of accidental damage, and performance under different thermal conditions
  • if margin is low, the link is less likely achieve the expected bandwidth
    • can improve performance with better or fewer splices
    • can use amplifier to boost signal
  • outdoor plans are designed with a margin of at least 5 dBm
  • datacenter may have lower margin bc conditions are less variable

dBm

• dBm measures signal strength against a reference value, where 0 dBm is 1 milliwatt
• negative dBm is typical of Ethernet transceivers, which output less than 1 mw