4 ms·

Isn't Thunderbolt also "just" PCIe over USB-C cabling? :) PCIe seems to be a well designed standard all things considered - very fast and very robust.

by izacus 4y ago

wtallis 4y ago
Thunderbolt is quite different at the lower layers. It's more accurate to think of Thunderbolt as its own connection technology that can carry both PCIe and DisplayPort data packets multiplexed on one cable, with the DisplayPort packets getting priority.
- jjoonathan 4y ago
  I've heard that Thunderbolt and USB4 "merged" -- does this mean it's Thunderbolt at the lowest layers and USB4 is encapsulated? Or are there two different signaling standards sharing the same PHY?
  - wtallis 4y ago
    When USB is involved, you can always assume they chose the more complicated option. USB4 is a competing, incompatible method of encapsulating and multiplexing PCIe, DisplayPort and USB data packets over one cable. But the USB4 spec incorporates Thunderbolt as an alternate mode. Thunderbolt support is required for USB4 hubs but optional for USB4 hosts and devices. So you will often (maybe even always, in practice) be able to use a USB4 host port as a Thunderbolt port.
    - izacus 4y ago
      It's worth mentioning that Microsoft seems to demand support for Thunderbolt mode for Win11 certification.
  - deleted 4y ago
    [deleted]
  - rektide 4y ago
    Don't let Usb4 hear you saying that, it'll be mad. USB4 is usb4. Which... very closely resembles thunderbolt. Thunderbolt 4 is PCIe tunneling over USB4. Note also Display Port now works similarly: instead of switching to an alt-mode (dedicating half the lanes to a video signal), in USB4 Display Port packets are encapsulated in usb4 packets. So now you can attach a variety of displays, to a tree of hubs, provided you have bandwidth, whereas this was a cluster fuck for a while... it's part of the reason usb-c output hubs were so rare, no one wanted to explain to consumers why a couple monitors with usb-c cables wouldn't work with their hub. Now, they will (provided there's sufficient bandwidth).
jjoonathan 4y ago
In college I read (parts of) the big computer standards -- USB, PCIe, SATA (rip), and Bluetooth. PCIe was consistently head and shoulders above the rest. This was true on an editorial level and on a structural level. They defined terms before they used them, they didn't use the same term to mean two different things, they foresaw and sidestepped problems that other standards blundered into, they didn't go on architecture astronaut tangents, they weren't stingy with bits when they could buy simplicity/clarity/separation of concerns but they spared no effort when performance was on the line. These were all frequent sins in the other standards. I've always wondered what human factors led to this. Good leadership? Fewer cooks in the kitchen? More time? There is probably a lesson in there, but I didn't stay in computer engineering long enough to gain industry contacts and figure it out.
- Arrath 4y ago
  This is something I would be very interested in reading in more detail, with examples from multiple specs pointing out pitfalls or successes. Dammit I'm supposed to be working not diving down a rabbit hole.
- AdamH12113 4y ago
  I suspect it was a combination of technical limitations and the nature of the developers. The USB 1.0 standard was finalized in 1996, seven years before PCIe 1.0 (2003), and was one of the first (maybe the first?) consumer protocols to fully support automatically-enumerated hot-swappable devices. (Having worked on USB drivers a bit, I still kind of hate the protocol, though.) Keep in mind too that there is a massive difference in computing power, memory, etc. between 1996 and 2003. Similarly, I get the impression that "low-power wireless" was a herculean effort when Bluetooth was developed in ~1999. It looks like PCIe 1.0 was designed solely by Intel, which probably accounts for a lot of the coherence and editing. SATA (2003) was designed by committee.
jeffbee 4y ago
Thunderbolt is confusing because of the many roles it can adopt. When Thunderbolt is used to connect a PCIe peripheral to a host, it is tunneling PCIe inside packets on Thunderbolt lower layers. The reason this is confusing is because Thunderbolt can also encapsulate DisplayPort traffic, and it can encapsulate DisplayPort and PCIe traffic at the same time, but Thunderbolt ports can also just be DisplayPort ports, electrically, but they cannot be PCIe ports. If you plug a Thunderbolt ports into a DisplayPort display, the Thunderbolt interface will switch modes and simply become a DisplayPort interface. It has the same ability to become a USB port, if it thinks it is connected to a USB peer. But there is no such way for Thunderbolt to be connected to a PCIe device and act like it is just pure PCIe.

Isn't Thunderbolt also "just" PCIe over USB-C cabling? :) PCIe seems to be a well designed standard all things considered - very fast and very robust.

by izacus 4y ago

Isn't Thunderbolt also "just" PCIe over USB-C cabling? :) PCIe seems to be a well designed standard all things considered - very fast and very robust.

wtallis 4y ago

Thunderbolt is quite different at the lower layers. It's more accurate to think of Thunderbolt as its own connection technology that can carry both PCIe and DisplayPort data packets multiplexed on one cable, with the DisplayPort packets getting priority.

jjoonathan 4y ago
I've heard that Thunderbolt and USB4 "merged" -- does this mean it's Thunderbolt at the lowest layers and USB4 is encapsulated? Or are there two different signaling standards sharing the same PHY?
- wtallis 4y ago
  When USB is involved, you can always assume they chose the more complicated option. USB4 is a competing, incompatible method of encapsulating and multiplexing PCIe, DisplayPort and USB data packets over one cable. But the USB4 spec incorporates Thunderbolt as an alternate mode. Thunderbolt support is required for USB4 hubs but optional for USB4 hosts and devices. So you will often (maybe even always, in practice) be able to use a USB4 host port as a Thunderbolt port.
  - izacus 4y ago
    It's worth mentioning that Microsoft seems to demand support for Thunderbolt mode for Win11 certification.
- deleted 4y ago
  [deleted]
- rektide 4y ago
  Don't let Usb4 hear you saying that, it'll be mad. USB4 is usb4. Which... very closely resembles thunderbolt. Thunderbolt 4 is PCIe tunneling over USB4. Note also Display Port now works similarly: instead of switching to an alt-mode (dedicating half the lanes to a video signal), in USB4 Display Port packets are encapsulated in usb4 packets. So now you can attach a variety of displays, to a tree of hubs, provided you have bandwidth, whereas this was a cluster fuck for a while... it's part of the reason usb-c output hubs were so rare, no one wanted to explain to consumers why a couple monitors with usb-c cables wouldn't work with their hub. Now, they will (provided there's sufficient bandwidth).

jjoonathan 4y ago

In college I read (parts of) the big computer standards -- USB, PCIe, SATA (rip), and Bluetooth. PCIe was consistently head and shoulders above the rest. This was true on an editorial level and on a structural level. They defined terms before they used them, they didn't use the same term to mean two different things, they foresaw and sidestepped problems that other standards blundered into, they didn't go on architecture astronaut tangents, they weren't stingy with bits when they could buy simplicity/clarity/separation of concerns but they spared no effort when performance was on the line. These were all frequent sins in the other standards. I've always wondered what human factors led to this. Good leadership? Fewer cooks in the kitchen? More time? There is probably a lesson in there, but I didn't stay in computer engineering long enough to gain industry contacts and figure it out.

Arrath 4y ago
This is something I would be very interested in reading in more detail, with examples from multiple specs pointing out pitfalls or successes. Dammit I'm supposed to be working not diving down a rabbit hole.
AdamH12113 4y ago
I suspect it was a combination of technical limitations and the nature of the developers. The USB 1.0 standard was finalized in 1996, seven years before PCIe 1.0 (2003), and was one of the first (maybe the first?) consumer protocols to fully support automatically-enumerated hot-swappable devices. (Having worked on USB drivers a bit, I still kind of hate the protocol, though.) Keep in mind too that there is a massive difference in computing power, memory, etc. between 1996 and 2003. Similarly, I get the impression that "low-power wireless" was a herculean effort when Bluetooth was developed in ~1999. It looks like PCIe 1.0 was designed solely by Intel, which probably accounts for a lot of the coherence and editing. SATA (2003) was designed by committee.

jeffbee 4y ago

Thunderbolt is confusing because of the many roles it can adopt. When Thunderbolt is used to connect a PCIe peripheral to a host, it is tunneling PCIe inside packets on Thunderbolt lower layers. The reason this is confusing is because Thunderbolt can also encapsulate DisplayPort traffic, and it can encapsulate DisplayPort and PCIe traffic at the same time, but Thunderbolt ports can also just be DisplayPort ports, electrically, but they cannot be PCIe ports. If you plug a Thunderbolt ports into a DisplayPort display, the Thunderbolt interface will switch modes and simply become a DisplayPort interface. It has the same ability to become a USB port, if it thinks it is connected to a USB peer. But there is no such way for Thunderbolt to be connected to a PCIe device and act like it is just pure PCIe.