USB hub

[1] Physically separate USB hubs come in a wide variety of form factors: from external boxes (looking similar to an Ethernet or network hub), to small designs that can be directly plugged into a USB port (see the "compact design" picture).

Almost all modern laptop computers are equipped with USB ports, but an external USB hub can consolidate several everyday devices (like a mouse, keyboard or printer) into a single hub to enable one-step attachment and removal of all the devices.

Some USB hubs may support power delivery (PD) to charge a laptop battery, if self-powered and certified to do so, but may be referred to as a simple docking station due to the similar nature of only needing one connection to charge the battery and connect peripherals.

[citation needed] Depending on vendor and design, USB ports are often closely spaced.

External "Octopus" or "Squid" hubs (with each socket at the end of a very short cable, often around 2 inches (5 cm) long), or "star" hubs (with each port facing in a different direction, as pictured) avoid this problem completely.

Some self-powered hubs do not supply enough power to drive a 500 mA load on every port.

Designers assume the user will most likely connect many low-power devices and only one or two requiring a full 500 mA.

On the other hand, the packaging for some self-powered hubs states explicitly how many of the ports can drive a 500 mA full load at once.

It may be desirable to use a bus-powered hub with self-powered external hard-disks, as the hard-disk may not spin down when the computer turns off or enters sleep mode while using a self-powered hub since the hard disk controller would continue to see a power source on the USB ports.

All USB ports operate at 5 volts, but may draw or supply differing amounts of electric current.

If a device requires more units of current than the port it is plugged into is able to supply, the operating system usually reports this to the user.

Each transaction translator segregates lower speed traffic into its own pool, essentially creating a virtual full-speed bus.

[3] It is an important consideration that in common language (and often product marketing), USB 2.0 is used as synonymous with high-speed.

During normal transmission, hubs are essentially transparent: data received from its upstream port is broadcast to all devices attached to its downstream ports (pictorially described in the USB 2.0 specification in Figure 11–2, Hub Signalling Connectivity).

[12] Hubs are not transparent when dealing with changes in the status of downstream ports, such as insertion or removal of devices.

When someone plugs in a device, the hub detects voltage on either D+ or D− and signals the insertion to the host via this interrupt endpoint.

It then instructs the hub (via the default control pipe) to reset the port where the new device was plugged in.