7 ms·
PCI-Express 6.0 and 7.0 both abandon binary signaling and move to four voltage levels per time quantum to encode two bits at once. That lets them move twice as
by jws 4y ago
PCI-Express 6.0 and 7.0 both abandon binary signaling and move to four voltage levels per time quantum to encode two bits at once. That lets them move twice as much data in the same bandwidth, signal to noise ratio willing.
Since they will be pushing the limits of the SNR, they also build in forward error correction to keep the error rate under control.
- tomc1985 4y agoIs the PCI-E bus really so noisy?
- speed_spread 4y agoPCI-E speed is starting to push against the noise of the universe.
- crote 4y agoAt these kinds of bandwidths, it is extremely noisy. Remember socket AM4, which added support for PCIe 4.0 with chipset X570? Originally, the idea was that non-X570 boards would also have PCIe 4.0, as it doesn't actually depend on the chipset - just the CPU. Turns out most boards simply weren't capable of handling the additional bandwidth, so it was eventually restricted to boards which were designed for it. And that's at one-eighth the bandwidth of PCIe 7.0!
- alias_neo 4y agoInteresting, I could have sworn when I bought my AsRock Rack X470 server board it was advertised as PCI-E 4.0 but their site quite clearly shows 3.0 checking now.
- WithinReason 4y agoI remember for some MBs there were BIOS updates that added 4.0 support, then later BIOS updates removed it.
- Zenst 4y agoIt gets down to the differential tracks connecting the PCIe lanes. Tollerance for PCIe3 compared to 4.0 may see tracks on a motherboard tollerant for 3.0 but fall short for 4.0. Higher the speed, the smallest difference in the length of the track routing pairs can see what works become less stable. Which would explain why some can do 4.0 and others not as well.
- jsolson 4y agoIf it were truly a bus, these signaling rates would be totally impossible. Too many reflections. In practice it's a bunch of point-to-point links, but even then the path across a PCB is pretty brutal in terms of loss. Cables are (surprisingly to me, initially!) better. Overall though, sending some number of additional bits for error correction is often cheaper than driving the line harder to reduce the bit error rate. Put differently: we could manage lower noise, but it would either result in a lower bitrate or (much) higher power. Instead, pick a target goodput and design for power and error correction from both sides.
- chx 4y ago> Cables are (surprisingly to me, initially!) better. That's because the cables in these application are not single wires but complex micro-coaxial assemblies. (The industry best is 3M Twin Axial.) And that will have much more shielding than an inherently unshielded PCB trace. With an appropriate price, of course.
- jl6 4y agoCould you shield PCB traces?
- Lramseyer 4y agoYou can with enough money. Usually you see ground traces and ground planes separate high speed data buses. The problem is that the more layers you have, the more expensive it is.
- _ugfj 4y agoThat's still nowhere near https://www.i-pex.com/sites/default/files/inline-images/2_E.png https://www.i-pex.com/sites/default/files/inline-images/2_E.... this. And they come in less than 1/100 of an inch(!) thickness, look at the OD table https://www.i-pex.com/products/micro-coax-discrete-wire https://www.i-pex.com/products/micro-coax-discrete-wire. It's total bonkers. An 50cm PCIe 4.0 x16 riser from 3M sells for 160 USD retail though...
- Dylan16807 4y agoIt says "GHz" so yes it's extremely noisy. At this point the bits are millimeters long.
- derekdahmer 4y agoWhat’s the technical limitation in increasing the number of voltage levels? Why not say 64 or 128 levels?
- KZerda 4y agoNoise is a big one. The more levels you have, the more likely it is noise is going to turn a 0 to a 1 or a 1 to a 2, for example. It also requires more complicated switching transistors, because you can't rely on things being simply on or off, but rather have to generate a number of intermediary voltages.
- Zenst 4y agoDifferential signaling is one way to combat a lot of noise, which I do believe is used in the PCI spec already.
- magicalhippo 4y agoPCI[1] didn't use differential signalling, however it did use reflected-wave switching[2]. PCI express[3] does use differential signalling, as does a lot of other higher-speed busses like USB, SATA, or MIPI CSI-2/3[4]. [1]: https://en.wikipedia.org/wiki/Peripheral_Component_Interconnect#PCI_bus_signals https://en.wikipedia.org/wiki/Peripheral_Component_Interconn... [2]: https://en.wikipedia.org/wiki/Reflected-wave_switching https://en.wikipedia.org/wiki/Reflected-wave_switching [3]: https://en.wikipedia.org/wiki/PCI_Express#Lane https://en.wikipedia.org/wiki/PCI_Express#Lane [4]: https://en.wikipedia.org/wiki/Camera_Serial_Interface https://en.wikipedia.org/wiki/Camera_Serial_Interface
- willis936 4y agoIt doesn't eliminate all noise. You can play these games, but can never violate Shannon's. We hit Baud rates where channel loss was high enough that the SNR needed to be cut into to get more throughput. This cut the SNR so much that FEC was needed to maintain an acceptable error rate. PCIe lags IEEE 802.3 with these kinds of things. I'd call it bog standard but it's still bleeding edge.
- hamandcheese 4y agoThis strikes me as similar in principle to how higher density flash storage is achieved: using more voltage levels in each cell.
- rcxdude 4y agoIt's also similar to how gigabit+ ethernet, ADSL, and wireless comms work (wireless and ADSL go much further, they also vary the phase of the signal and the constellations can have >1024 points in extreme circumstances).
- oblio 4y agoSo what you're saying is that taking this to the limit, we'll end up with analog signals soon? You know, a infinite set of increments :-D
- aaaaaaaaaaab 4y agoThe physical signals are always analog.
- albrewer 4y agoI see you're getting downvoted, but that would be the ultimate goal, yes. If you can devise a circuit without anything that does state switching, it's going to be faster than any digital circuit.
- touisteur 4y agoSome modulation and multiplexing modes are so crazy their inventors get called names for years. In the optical world there's Orbital Angular Momentum and it's a very strange mechanism... https://www.degruyter.com/document/doi/10.1515/nanoph-2021-0527/html https://www.degruyter.com/document/doi/10.1515/nanoph-2021-0... I'm amazed at the progress of terabit Ethernet, soon 1.6Tbit/s in one server.
- jeffbee 4y agoPCIe will eventually adopt all of the techniques pioneered for wireless and will simply be wifi-over-copper.