--- Begin Message ---On May 8, 2022, at 1:30 PM, Michael Richardson <m...@sandelman.ca> wrote:> I guess I would have thought that a physical bus could have a mix of > different devices which operate at different speeds. As such, I wondered if > you really needed pcapng to be able to mix LINKTYPES in the same file, or > a different bit of meta-data to indicate bus speed for each frame captured. > > But, maybe I'm wrong and that actually requires there to be a USB hub out > there. "Bus" is a bit weird here. To quote section 4.1.1 "Bus Topology" of the USB 2.0 pec: The USB connects USB devices with the USB host. The USB physical interconnect is a tiered star topology. A hub is at the center of each star. Each wire segment is a point-to-point connection between the host and a hub or function, or a hub connected to another hub or function. Figure 4-1 illustrates the topology of the USB. and Figure 5-6 "Multiple Full-speed Buses in a High-speed System" seems to use the term "bus" to refer to wire segments. I think a point-to-point connection between the host and another entity may always run at a single speed, as well as a connection between a hub and a function. It might also be the case that a hub-to-hub connection also runs at a single speed. Section 11.14 "Transaction Translator" says: A hub has a special responsibility when it is operating in high-speed and has full-/low-speed devices connected on downstream facing ports. In this case, the hub must isolate the high-speed signaling environment from the full-/low-speed signaling environment. This function is performed by the Transaction Translator (TT) portion of the hub. so if you have a full-speed or low-speed device plugged into a USB 2.0 hub, and that hub is connected to a host, the host-to-hub link is high-speed, and the hub-to-device link is full-speed or low-speed, and the hub does the translation. That way, you can plug a high-speed device and a full-speed or low-speed device into the hub, and the host will be able to talk at high speed to the high-speed device. USB isn't a shared bus like non-switched Ethernet; it's more like switched Ethernet or point-to-point Ethernet, with links being point-to-point, either a direct connection between end nodes or connections to a switching device that handles speed translation if two end nodes of different speed capabilities are communicating.
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