I/O Interfaces & Buses

Which is closest to the efficiency of the 128b/130b encoding adopted from PCI Express 3.0 onward?

  1. AAbout 80%
  2. BAbout 50%
  3. CAbout 98.5%
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Answer: C. About 98.5%

128b/130b encoding sends 128 data bits with a 2-bit header as 130 bits, so efficiency is 128/130 ≈ 98.5%, a big reduction in waste compared with 8b/10b's 80%. Transitions are ensured by scrambling the data (XOR with a fixed pseudo-random sequence) to mix 0s and 1s. This is why PCI Express 3.0 nearly doubled effective throughput while raising the signaling rate only from 5 to 8 GT/s.

  • A. 80% is the efficiency of 8b/10b.
  • B. The efficiency is far better than that.
  • C. Correct. 128 ÷ 130 ≈ 0.985.
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More I/O Interfaces & Buses questions

  1. Which describes a "lane" in PCI Express?
  2. What is closest to the effective one-direction bandwidth of a PCI Express 3.0 x16 link (8 GT/s per lane, 128b/130b encoding)?
  3. For PCI Express generations 3 through 6, how does the per-lane signaling rate change with each generation?
  4. What is a characteristic of PAM4 (four-level pulse amplitude modulation), adopted in PCI Express 6.0?
  5. Why does forward error correction (FEC) become more important with PAM4 than with NRZ?