Nvidia’s new graphics cards are the first standard GPUs for gaming PCs to support PCI Express 5.0 instead of PCIe 4.0, but they are a bit late to the party as PCIe 5.0 support has been expected from the 2022’s generation of GPUs already. Development of these interfaces is well ahead of adoption, and it looks like the specification for the n+2 generation that awaits us sometime in the future will be ready this year.
PCI Express is being developed by the PCI-SIG consortium and it has now announced that PCI Express 7.0 will be ready later this year. The completion here refers finalizing the specification of the standard in terms of making the finished version available to companies developing chips and other hardware. The actual arrival of the hardware to the market on the other hand may take quite a bit longer. And as we’ve seen with PCIe 5.0, it can vary quite a bit across different devices – motherboards and CPUs offered PCIe 5.0 support in 2021 to 2022, SSDs came a year and a bit later, while gaming GPUs are only rolling in support now. That’s with a standard that had the specification published in 2019.
PCI-SIG has now released the PCIe 7.0 specification in version 0.7 to its members (the previous version 0.5 came last April). As we approach version 1.0, the technology should be largely complete and the details of the specification are being fine-tuned, so device development can probably already be underway. Exactly when in this year version 1.0 will be ready and released is not yet known.
16 GB/s per lane
PCI Express doubles the bandwidth of a single interface lane in each generation (or so it has been until now), so PCI Express 7.0 will again be 4× faster in terms of transfer capacity in gigabytes per second than PCIe 5.0 (2× faster than PCIe 6.0). And eight times faster than the PCI Express 4.0 that is the currently most widely used generation. The speed is listed as 128 GT/s (gigatransfers per second). In practice, this will mean 16 GB/s per lane in one direction. The sum of the bandwidth available in both directions would be 32 GB/s. However, for most use cases, such as SSDs, it is the bandwidth in one direction that is of interest, as that is what determines the maximum sequential read and write speeds.
An SSD in a four-lane M.2 slot could therefore achieve read speeds of up to around 60 GB/s with PCI Express 7.0 ×4 (some of the theoretical 64 GB/s will be consumed by overhead). But again, it will probably take a few years for boards and SSDs using this interface to appear on the market. We estimate this could again take 3–4 years, so we could see this available in 2027 or 2028.
PCI Express 7.0 will still be based on an electrical principle instead of using optical (light-based) signals, which has long been explored as a possible alternative, so it will still be running standard copper traces in a PCB. The technology adopts the PAM4 pulse-amplitude modulation type, which is already used in PCI Express 6.0 and extracts double data bandwidth from a certain frequency of communication over the signal paths. One “cycle” transmits two bits because the PAM4 signalling distinguishes four signal levels instead of two.
Since PCIe 7.0 will use the same PAM4 as PCIe 6.0, the communication frequency itself will have to increase. Therefore, PCI-SIG’s development is said to focus on the parameters of the signal paths and the physical length of the connection that the signal will be able to travel reliably before an active element (repeater or switch) has to be inserted. The interface is supposed to maintain low latencies and also have better power efficiency than previous versions. However, this does not necessarily mean that device and interface power consumption will not increase. In fact, power efficiency is stated as the ratio of transfer speed to power consumption. The fact that the speed goes up by a factor of two means that the efficiency will always go up as well as long as the power consumption increase can be kept below a factor of two.
Due to the continued use of electrical communication, PCIe 7.0 will still be compatible with previous versions. Therefore, new devices can be plugged into older computers or, conversely, older cards can be plugged into a PCI Express 7.0 slot. The device and the host controller will auto-negotiate the highest speed that both sides can support during initialization and this will be used. PCIe 7.0 will therefore maintain continuity of hardware compatibility in PCs and devices based on the platform.
Source: PCI-SIG
English translation and edit by Jozef Dudáš
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