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Your Ultimate Guide to Understanding PCIe 5.0

By ·Categories: Tech Explained·Published On: May 24th, 2022·6.9 min read·

What is PCIe Gen 5.0?

PCIe 5.0 is the next evolution of the widely used, high-speed interface known as the Peripheral Component Interconnect Express, or PCIe. It is the common motherboard interface for peripheral connections, or endpoints, such as graphics cards, SSDs, Wi-Fi and Ethernet hardware.

PCIe History

PCIe gen 1.0 was originally introduced in 2003 by the Peripheral Component Interconnect Special Interest Group (PCI-SIG). The specs for PCIe generations 2.0, 3.0, 4.0, and 5.0 were released in 2007, 2010, 2017, and 2019 respectively. (Check out our Ultimate Guide to PCIe Gen 4.0.)

Once the PCI-SIG releases the spec, it takes time for CPUs and peripherals to deliver on those specifications. The specification for PCIe 5.0 was released in 2019, but CPUs that support PCIe 5.0 weren’t launched until early 2022. Furthermore, the peripherals that support PCIe 5.0 won’t be released until later in 2022. 

What about PCIe Gen 6.0?

Just as PCIe 5.0 is becoming commercially available, the PCI-SIG has already defined the next PCIe generation. The specification for PCIe 6.0 was just released in January of 2022. We can expect it to take months, or even years for commercially available products to support PCIe 6.0. 

“PCI-SIG is pleased to announce the release of the PCIe 6.0 specification less than three years after the PCIe 5.0 specification. PCIe 6.0 technology is the cost-effective and scalable interconnect solution that will continue to impact data-intensive markets like datacenter, artificial intelligence, machine learning, HPC, automotive, IoT, and military/aerospace, while also protecting industry investments by maintaining backwards compatibility with all previous generations of PCIe technology.”

Al Yanes, PCI-SIG chairperson and president

PCIe Generational Differences

On the surface, newer PCIe slots look the same as the previous generations. However, each generation of PCIe is twice as fast as its predecessor. This speed at which data or information travels from one location to another is measured using the metric of Gigatransfers per second, or GT/s. This is the rate of bits (0’s and 1’s) transferred per second from the host to the end device or endpoint. Keep in mind, the provided GT/s of the specification is the theoretical max speed. The real-world speeds may be slower. 

The data transfer rate of PCIe 3.0 is 8 Gigatransfers per second or 8 GT/s. PCIe 4.0 transfers data at 16 GT/s, and PCIe 5.0 at 32 GT/s. The data transfer rate of PCIe 6.0 will be 64 GT/s, according to PCI SIG. More speed is desired because it allows computers to process more data and enables them to use the latest graphics cards (GPUs) and solid state drives (SSDs) to their full potential. 

PCIe Forward and Backward Compatibility

As mentioned in the quote above from Al Yanes, PCIe generations feature both backward and forward compatibility. That means that you can connect a PCIe 3.0 SSD to a PCIe 4.0 slot and you can also connect a PCIe 4.0 SSD into a PCIe 3.0 slot. But speed will be limited to the lowest generation. So your PCIe 4.0 SSD plugged into a PCIe 3.0 slot will transfer data at Gen 3.0 speeds. 

PCIe Lanes Explained

PCIe lanes are the physical link between the PCIe-supported device and the processor/chipset. You can think of a single PCIe lane like a highway where data travels in both directions simultaneously. As one would expect, the bandwidth will increase linearly with the number of PCIe lanes. 

The number of lanes available include 1 lane (x1), 4 lanes (x4), 8 lanes (x8)  and 16 lanes (X16). That means that up to 16 of these bidirectional PCIe lanes can be allotted to a single device, enabling it to achieve a high-bandwidth, low-latency transfer of data.

Real World Example of PCIe 4.0 and PCIe 5.0 on Sixteen Lanes

Let’s look at a theoretical example of a GPU operating at PCIe 4.0 and PCIe 5.0.

  • Your GPU uses 16 PCIe lanes. 
    • Your peak theoretical bit rate via PCIe 4.0 would be: 
      • 16 Lanes x 16 Gigatransfers per lane = 256 GT/s
    • Your peak theoretical bit rate via PCIe 5.0 would be:
      • 16 Lanes x 32 Gigatransfers per lane = 512 GT/s

Data transfer rate is measured in Gigabits (Gb) per second. On the other hand, bandwidth is measured in Gigabytes (GB) per second: 8 Gigabits = 1 Gigabyte. See reference table below.

PCIe Reference table comparing PCIe generations data tranfer rate and total bandwidth from Gen 1 to Gen 6

Another Advantage of More Bandwidth

The additional bandwidth of PCIe 5.0 means that devices may be able to achieve the same throughput while using fewer lanes, thus freeing up the number of lanes available. For example, a graphics card that used to require x16 bandwidth to run at full speed may now run at the same speed with x8, making an additional 8 lanes available. Using fewer lanes is important because CPUs provide a limited number of lanes, which need to be distributed among devices.

The Importance of PCIe Encoding

The above quoted data transfer speed – GT/s – is a measure of raw speed. That is, how many bits can be transferred in a second. The data rate has to take into consideration the overhead for encoding the signal. Understanding the encoding technique is necessary to determine the actual amount of data that can be transferred. The encoding impact on the data rate translates directly to an impact on bandwidth.

PCIe Gen 3.0, PCIe Gen 4.0, and PCIe Gen 5.0 use a 128b/130b encoding scheme. Older generations such as PCIe Gen 2.0 use 8b/10b encoding. This encoding technique transforms 128-bit data into 130-bit line code. This allows for reasonable clock recovery (which is the process of extracting timing information from a data stream) and ensures alignment of the data stream. The two extra bits contain a preamble which aids in the clock recovery of the serial data stream to allow the receiver to decode the transmitted signals. This means, the maximum theoretical bandwidth in Gbps (Gigabit per second) of each PCIe Gen 5.0 looks like this:

32GT/s * (128b/130b) = 31.51Gbps

Knowing the encoding allows us to calculate the overhead required to transmit PCIe data streams. 

([130b-128b]/130b)*100 = 1.54%

PCIe Gen 6.0 Encoding

The new PCIe 6.0 interface uses Flit (flow control unit) encoding which, according to PCI-SIG, supports PAM4 modulation and works in conjunction with the FEC and CRC to enable double the bandwidth gain. That is a lot of acronyms – to put it simply – PCIe 6.0 offers improved bandwidth efficiency. In fact, PCI-SIG considers there to be no overhead in the data encoding itself.   

To learn more about the math behind the PCIe process, TECHGAGE wrote a helpful article. The engineering behind PCIe gets pretty complicated and involves a lot of science. Check out this article to learn the details behind the development of PCIe 5.0.

How does PCIe 5.0 affect my choice of SSD, NVMe, and GPU?

As of blog publication, there are no endpoints available that are compatible with PCIe 5.0. If you are buying chipsets supporting PCIe 5.0, just remember that you can use PCIe 4.0 endpoints for now, and upgrade when PCIe 5.0 endpoints are available. Remember that a PCIe 4.0 GPU (or any other PCIe Gen 4.0 endpoint) will perform like a PCIe 4.0 GPU even if connected to a PCIe 5.0 slot.

What Processors Support PCIe 5.0?

As of this writing, Intel’s 12th generation Core processors, code-named “Alder Lake” and the newly announced AMD Ryzen 7000 Desktop Processors support PCIe Gen 5.0.  

Our newest line of rugged computers, the Karbon 800 Series is built upon Intel’s 12th gen Core processors and they will support PCIe 5.0 on the x16 slot. However, we are currently configuring Karbon 800 systems with PCIe 4.0 riser cables. Once additional PCIe Gen 5 devices become available, we will update to PCIe 5.0 riser cables in line with demand. 

What Processors Support PCIe 4.0?

As of this writing, PCIe 4.0 technology is available on the following platforms, including:

  • Intel’s 11th Generation “Rocket Lake” Core processors 
  • Intel’s “Ice Lake” Scalable Xeon CPU
  • AMD Ryzen 3000 and 5000 Series 
  • The AMD Threadripper 3000 Series
  • AMD EPYC Rome server platform

Should I wait for PCIe 5.0 or PCIe 6.0?

One common misconception is that PCIe 5.0 is already “widely available” and that 6.0 devices are “just around the corner”. Unfortunately, as we outlined above, while the specs for PCIe 5.0 and PCIe 6.0 are currently available, PCIe 5.0 devices are just beginning to be made available, and devices that use PCIe 6.0 are unlikely to be commercially available for months or even years. Luckily, you can expect some OnLogic products to support PCIe 5.0 in the very near future.

To learn more about PCIe in general, be sure to check out our blog and associated video, PCIe Basics in 60 Seconds.

Wondering what the best configuration is for your solution? Talk to one of our experts today. 

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About the Author: Sarah Lavoie

Sarah Lavoie is a content creator for OnLogic. When not writing, she can usually be found exploring the Vermont landscape with her camera looking to photograph something amazing.
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