I/O Interfaces & Buses
What is a characteristic of PAM4 (four-level pulse amplitude modulation), adopted in PCI Express 6.0?
- AIt sends 1 bit per symbol using two voltage levels for 0 and 1
- BIt sends 2 bits per symbol with four voltage levels, doubling the bit rate at the same symbol rate
- CIt works only over optical fiber, not copper
Show answer and explanation
Answer: B. It sends 2 bits per symbol with four voltage levels, doubling the bit rate at the same symbol rate
Conventional NRZ uses two voltage levels to send 1 bit per symbol. PAM4 uses four levels to represent 00, 01, 10 and 11, sending 2 bits per symbol. Channels attenuate signals more at higher frequencies, so doubling the bit rate without raising the symbol rate is the advantage. PCI Express 6.0 reached 64 GT/s with PAM4 at the same symbol rate as Gen 5.
- A. That describes NRZ (two-level signaling).
- B. Correct. It raises speed without raising frequency.
- C. PCI Express uses it over copper.