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If an 800 Gb/s optical transceiver consumes 16 W, what is the energy per bit?

  1. A50 pJ/bit
  2. B20 pJ/bit
  3. C2 pJ/bit
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Answer: B. 20 pJ/bit

Energy per bit is power consumption ÷ bit rate: 16 W ÷ (800×10⁹ b/s) = 2×10⁻¹¹ J/bit, or 20 pJ/bit (1 pJ = 10⁻¹² J). Remembering that W (= J/s) divided by b/s gives J/bit makes the calculation easier. pJ/bit makes it easy to compare the efficiency of links with different speeds and is often used when estimating the power of switches and optical components. 50 pJ/bit is the result of computing 800 ÷ 16, and 2 pJ/bit is off by one order of magnitude.

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