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How USB HUB (hub) works

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Author : PURPLELEC
Update time : 2024-02-29 10:18:53
  USB HUB is used for device expansion connection, and all USB DEVICE are connected to the USB HUB port. A USB HOST is always connected to a root HUB (USB ROOT HUB). USB HUB provides 100mA current for each of its ports for devices to use. At the same time, USB HUB can diagnose the plugging and unplugging operations of the device through the electrical changes of the port, and report the port status to the USB HOST by responding to the USB HOST data packet. Generally speaking, the length of the connection between the USB device and the USB HUB does not exceed 5m, and the cascading of the USB system cannot exceed 5 levels (including ROOT HUB).
  Each downstream interface of the USB Hub will equally share the bandwidth of the upstream interface, and the USB Hub itself also needs to transmit control signals, so the bandwidth of each downstream device must be smaller than the total bandwidth of the upstream device.
USB HUB
  USB Hub is responsible for integrating various transmission signals of downstream devices. There are certain limits on synchronous transmission. If it exceeds a certain proportion of the total bandwidth (20% in memory), the synchronous transmission exceeding the proportion will be discarded; interrupt transmission and control transmission What is occupied is reserved bandwidth and is not allowed to be discarded; Bulk transmission occupies the remaining bandwidth. If there is no synchronous transmission, then Bulk transmission can account for more than 90% of the total bandwidth.
  The USB Hub integrates the data and sends it to the upstream device. If there is a USB Hub upstream, the upstream device will continue to perform the same operation. In addition: It should be noted that generally speaking, the USB interface on the host is itself a Root Hub. If there are multiple interfaces on a PC, it may be that multiple interfaces share a controller, and each controller has a Root Hub. .
  Therefore, USB Hub is equivalent to re-encapsulating various data packets, similar to a router in network equipment, and is not as simple as a switch or hub (Hub).