Your Ultimate Guide to Understanding PCIe 6.0

With all the news surrounding PCIe 5.0 recently, there has been increasing interest in PCIe 6.0. How will it be different from PCIe 5.0? What are the specs? Keep reading to find out everything you need to know about PCIe 6.0, and check out our 60 second overview of PCIe to learn more about PCIe in general.

What is PCIe 6.0?

PCIe 6.0 (also sometimes called PCI express 6.0) is the sixth generation of PCIe (Peripheral Component Interconnect Express). Although PCI-SIG, the group who defines the PCIe standard, has released the official specs for PCIe 6.0, it will take time for PCIe gen 6 peripherals to reach the market.

A chart showing the years each PCIe generation’s specs were released

PCIe 6.0 vs 5.0: what are the key differences?

Just as PCIe 5.0 was a massive upgrade from PCIe gen 4, PCIe 6.0 improves on the speed and bandwidth of PCIe 5.0 and introduces new features. The main difference between PCIe gen 5 and 6 is the speed at which data is transferred. The data transfer speed for PCIe 5.0 is 32 GT/s (Gigatransfers per second), and PCIe 6.0 doubles this speed to 64 GT/s. 

A quick aside about the term Gigatransfers per second; GT/s indicates the raw speed at which data is transferred. GT/s differ from a bandwidth measurement like gigabytes per second (GB/s) because it takes into account both the data itself and the overhead required for it to be properly encoded, transferred and decoded by the receiver. 

So, does the increase in speed really make that much of a difference? It depends on your preferences. If you need faster data transfer speeds and want to use the latest SSDs and GPUs, these data transfer speeds can be enticing.

PCIe gen 5 has a bandwidth of 4 GB/s per lane. With 16 lanes, the maximum theoretical bandwidth is 64 GB/s. PCIe 6.0, on the other hand, has a bandwidth of 8 GB/s. This means that across 16 lanes, the maximum theoretical bandwidth is 128 GB/s.

One thing to keep in mind is that data transfer speeds as well as the bandwidth are the theoretical max. The real-world speeds and volume of data you can send may be less depending on your specific application. You can also check out the chart below to learn more about PCIe generation data transfer speeds.

PCIe 6.0: the introduction of PAM4

With all the conversations around PCIe 6.0, you may have heard the term “PAM4”. Although PAM4 has been around for a while, PCIe gen 6 is the first PCIe generation to feature PAM4 technology. But what is PAM4, and how is it used?

PAM4 Explained

In order to increase PCIe data transfer rates, you have two options; double the frequency, or keep the frequency the same and introduce 4 levels of voltage rather than 2. PCIe 6.0 has chosen the latter option.

Previous PCIe generations, all the way from PCIe 1.0 to PCIe 5.0, have utilized Non-return to Zero (NRZ) modulation for signaling. NRZ has two levels of voltage measured in 1s and 0s (1 bit). PCIe 6.0 keeps the frequency of PCIe gen 5 the same and introduces four levels of voltage (00, 01, 11, 10). This is what is referred to as PAM4, or Pulse Amplitude Modulation Four-Level. PAM4 allows voltage to be sent across four levels, transmitting data at 2 bits per UI (unit interval).

What else is new with PCIe gen 6?

One thing that is unique to PCIe 6.0 is that it uses Flow Control Unit encoding (also referred to as FLIT encoding). According to PCI-SIG, FLIT encoding supports PAM4 modulation and enables double the bandwidth gain by working in conjunction with FEC (Forward Error Correction) and CRC (Cyclic Redundancy Check).

FEC corrects any errors present during data transmission. After this step, CRC then checks if there are any errors that slipped through. Having both steps helps to ensure that all errors are corrected and CRC passes.

FEC and CRC are especially important in PCIe 6.0 because PAM4 introduces 4 voltage levels, something that increases the chance of errors occuring. Having these steps helps to catch and resolve any errors. For a deeper dive into FLIT mode, PAM4, FEC, and CRC, check out PCI-SIG’s blog.

Another unique feature of PCIe gen 6 is the introduction of L0p, a low-power state that can only be enabled in FLIT mode. L0p saves power by allowing traffic to run on fewer lanes. With this feature, at least one lane remains active to ensure traffic can flow freely at all times. You can read more about L0p in this blog by PCI-SIG.

PCIe 6.0 encoding

PCIe 1.0 and 2.0  used 8b/10b encoding, in which every 8 bits of data were encoded into a 10-bit character before being sent, which works out to a 20% overhead (you have to send 10 bits of data even though you’re only trying to send 8). PCIe 3.0, 4.0, and 5.0 all use 128b/130b encoding, dropping the overhead to just 1.5% to send those same 2 extra bits.

According to the PCI-SIG, PCIe 6.0 employs 1 bit to 1 bit (1b/1b) encoding, so the data encoded has no overhead. This means that if you want to send 8 bits over PCIe gen 6, you only need to send 8 bits. However, there is still some overhead in the form of additional FEC/CRC packets.

Is PCIe 6.0 backwards compatible?

Like all PCIe generations, PCIe 6.0 is backwards compatible with every previous PCIe generation. However, it’s important to note that the data transfer speed will be limited to the lower generation. For example, you can connect a PCIe 5.0 SSD to a gen 6 slot, but it will only run at PCIe 5.0 speeds. The inverse is also true; connecting a PCIe 6.0 SSD to a gen 5 slot will transfer data at gen 5 speeds.

PCIe 7.0: what do we know?

With humble beginnings in 2002, PCIe 1.0 started out at 2.5 GT/s. This massive leap over the past decade to 64 GT/s with PCIe 6.0 leaves the question, “How will PCIe gen 7 further improve on past generations?” Fortunately, PCI-SIG has provided a sneak peek into what we can expect from PCIe 7.0 with the release of the PCIe 7.0 specification in June of 2022.

Although the specs for PCIe gen 7 are available, its release date is set for 2025, and compatible hardware will take time after the release to reach the market.

PCIe 6.0 summary

Although PCIe gen 6 peripherals like GPUs, SSDs, and expansion cards aren’t currently available at the time of writing this blog, we’ll keep you in the loop on all the latest PCIe 6.0 updates. Subscribe to OnLogic and reach out to our technical team to have your questions answered today.