The PCIe 4.0 interface is finally coming to market next month with the launch of AMD's Ryzen 3000 series processors, but the PCI-SIG, the standards body behind the PCIe specification, is already plowing forward with work on PCIe 6.0, which it announced will be completed in 2021. The announcement comes on the heels of the body's formal release of the PCIe 5.0 standard last month.
The new PCIe 6.0 interface double the Gigatransfer rate to 64 GT/s, which doubles effective bandwidth over PCIe 5.0 to 256 GB/s of throughput and retains the same maximum of 16 lanes.
|Row 0 - Cell 0||Bandwidth||Gigatransfer||Frequency||Encoding|
|PCIe 1.0||8 GB/s||2.5 GT/s||2.5 GHz||8b/10b|
|PCIe 2.0||16 GB/s||5 GT/s||5.0 GHz||8b/10b|
|PCIe 3.0||32 GB/s||8 GT/s||8.0 GHz||128b/130b|
|PCIe 4.0||64 GB/s||16 GT/s||16.0 GHz||128b/130b|
|PCIe 5.0||128 GB/s||32 GT/s||32.0 GHz||128b/130b|
|PCIe 6.0||256 GB/s||64 GT/s||32.0 GHz||PAM4|
The new PCIe 6.0 interface switches to PAM4 encoding from the NRZ 128b/130b encoding scheme used for PCIe 3.0, 4.0, and 5.0. The PAM4 encoding scheme is widely used in networking, so it is a known quantity that should be relatively easy to deploy. PAM4 uses forward error correction (FEC) to help boost transfer rates, and PCI-SIG also points to other unspecified mechanisms that will improve bandwidth efficiency.
As we've seen with all versions of the PCIe specification, PCIe 6.0 is backward compatible with the existing PCIe interfaces.
Channel reach, i.e., the "distance" the signal can travel, remains similar for PCIe 6.0 as it is with the PCIe 5.0 interface, which is critical to ensure broad industry uptake. The PCI-SIG doesn't provide a specific distance measurement for channel reach, instead it specifies the amount of tolerable signal loss, which remains at 36dB for PCIe 6.0.
The faster release of the PCIe 5.0 interface means PCI-SIG met its two-year goal of developing a newer revision of the spec, which is a welcome change after the seven-year wait for PCIe 4.0. The committee hopes to meet the same two-year cadence for the PCIe 6.0 interface and publish the specification in full in 2021, and if history holds true, we'll see devices arrive in 2022.
There are precious few PCIe 4.0 devices coming to market in the near term, like AMD's Navi graphics cards and a wave of SSDs based on Phison's new controller (exclusive PCIe 4.0 SSD testing here), but we aren't aware of any new PCIe 5.0 devices landing soon. Given the faster cadence of releases of the PCIe standard, it is possible that PCIe 4.0 will be a short-lived spec. That shouldn't impact those of us that want to move forward to PCIe 4.0 devices, though, as backwards compatibility means those devices will be useful for years to come.