| The USB-C connector dedicates 4 pairs of pins (8 total) to power transmission (VBUS and GND). It only does that to give enough contact area and enough conductor circumference in the cable to support 100w transmission. It also has two pairs of unshielded twisted pair for non-superspeed (USB 2.0) though only one pair is used in the cable apparently; which USB 2 channel you get depends on which way the cable is plugged in I guess. It has a pair for sideband use. A pair for configuration and control. The data primarily travels over the four high-speed differential signal pairs; two TX pairs and two RX pairs. All USB 3 cables must have a chip in them. It communicates over the C&C channel to select how the high-speed pairs are used. When a display is connected, one TX/RX set are used for USB 3 and the other for DisplayPort. USB 3 also supports "alternate" modes, so the C&C can negotiate that all high-speed pairs are used for Thunderbolt which is just the PCI-Express bus. Intel added the ability to interleave DisplayPort packets with the PCI-Express data. I assume that displays with USB 3 built-in are simply connecting a PCI-E USB hub to the Thunderbolt interface since in that mode there is no actual USB signal in the cable. USB 2 can use the dedicated signal pair for that and not interfere with DisplayPort or Thunderbolt traffic. Thunderbolt is a nicer standard in some ways - interleaving means if you aren't using a 16bpp 5K display at max refresh rate you have more bandwidth available for other devices. With USB 3 you get half the available bandwidth if you connect a display. Thunderbolt is also old-school in the sense that it projects the CPU's bus to external peripherals... something all early computers used to do. Everything old is new again! It is obvious USB-3 was very forward-looking. The C&C channel means some future version of the standard can drop USB-2 support and start connecting the unconnected pair if both ends negotiate for it. Now you have an extra TX/RX channel to give you +50% bandwidth. You can also imagine taking over the side channel pins. If you assume the next-gen standard can double USB-3's 10Gbps and add in double the data pairs that would be 40Gbps which matches Thunderbolt 3. Apply the same math to Thunderbolt and that's 160Gbps. |