I/O Interfaces & Buses

On a link using 8b/10b encoding with a signaling rate of 5 Gb/s, what is the effective data rate?

  1. A4 Gb/s
  2. B5 Gb/s
  3. C6.25 Gb/s
Show answer and explanation

Answer: A. 4 Gb/s

With 8b/10b encoding, 8 of every 10 bits sent are data, so the effective rate is 8/10 of the signaling rate: 5 Gb/s × 0.8 = 4 Gb/s. This is the one-direction speed of a single PCI Express 2.0 lane, or 500 MB/s in bytes. When comparing interfaces, the signaling rate (such as GT/s) and the coding efficiency must be considered separately.

  • A. Correct. 5 × 8/10 = 4.
  • B. The extra coding bits must be subtracted.
  • C. That multiplies by 10/8, the wrong way round.
Practice this topic Back to topic

More I/O Interfaces & Buses questions

  1. Which is closest to the efficiency of the 128b/130b encoding adopted from PCI Express 3.0 onward?
  2. Which describes a "lane" in PCI Express?
  3. 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)?
  4. For PCI Express generations 3 through 6, how does the per-lane signaling rate change with each generation?
  5. What is a characteristic of PAM4 (four-level pulse amplitude modulation), adopted in PCI Express 6.0?