PCI Express® (PCIe®) has been on a tear, doubling the data rate with each new generation in response to the torrid rise in data traffic and the needs of advanced workloads. But raising signaling rates gets harder and harder with each doubling. That’s why with PCIe 6.0, we have some of the most dramatic changes yet seen in the standard to enable the jump to 64 GT/s.
First and foremost among the changes is the shift to PAM4 (“Pulse Amplitude Modulation with four levels”) signaling. PAM4 combines two bits per clock cycle for four amplitude levels (00, 01, 10, 11) vs. PCIe 5.0, and earlier generations, which used NRZ modulation with one bit per clock cycle and two amplitude levels (0, 1).
There are always tradeoffs, however, and the transition to PAM4 signal encoding introduces a significantly higher Bit Error Rate (BER) vs. NRZ. This prompted the adoption of a Forward Error Correction (FEC) mechanism to mitigate the higher error rate inherent in PAM4. PCIe 6.0 adopts an FEC that is sufficiently lightweight to have minimal impact on latency.
But an FEC requires fixed-size packets, so PCIe 6.0 introduces FLIT mode, where packets are organized in Flow Control Units of fixed sizes, as opposed to variable sizes in past PCIe generations. FLIT mode also simplifies data management at the controller level and results in higher bandwidth efficiency, lower latency, and smaller controller footprint.
All else being equal, higher speeds mean higher power, so PCIe 6.0 introduces a new low-power state of operation called L0p Mode. L0p enables traffic to run on a reduced number of lanes to save power. L0p maintains at least one active lane at all times to ensure uninterrupted traffic flow. The link always trains at the highest possible width and can modulate down (and back up again) as needed by the traffic.
With over 20 years of leadership in PCIe solutions, Rambus can help you navigate all the new changes and implement high-performance PCIe 6.0 designs that deliver all the benefits of the new standard. You can get the technical details on our newly announced PCIe 6.0 Controller here.
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