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    What is a hub?


    The easiest way to do this is to think of a USB hub as a continuum of splitters and repeaters.The hub provides an electronic interface between the USB device and the host.The hub is directly responsible for supporting the many properties of USB, providing users with a friendly interface and avoiding complex usage problems.The hub is responsible for detecting connection and disconnection events on downstream ports and reporting relevant information to the host.The hub must be able to support all types of USB peripherals of any speed specification connected on the downstream port (see below for details).The specification specifies two types of hub power supplies, one for bus power and one for self-power.The bus-powered hub gets all the power from the upstream port, but only gets 100mA from the upstream connection when powering up, and the configured current does not exceed 500mA.
    Since each port has a current of 100mA and has four ports, the available current for the downstream connection is 400mA.If you add a fifth downstream port, this port also needs 100mA, which causes the hub itself to have no current available.Its power does not come from upstream connections.The self-powered hub can draw up to 100mA from the upstream port, which also works when the rest of the hub is powered down.The self-powered hub should supply 500mA to each downstream port.


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