In Guangdong Dongguan, there are a couple of factories for several industries, that is the reason why the city called the factory of the world. Among these factories, the USB hub factory is one of the automatic product ones, which means they apply the newest technology to improve the producing issues.
A USB network is built from USB hubs connected downstream to USB ports, which themselves may stem from USB hubs. USB hubs can extend a USB network to a maximum of 127 ports. The USB specification requires that bus-powered (passive) hubs are not connected in series to other bus-powered hubs.
Depending on vendor and design, USB ports are often closely spaced. Consequently, plugging a device into one port may physically block an adjacent port, particularly when the plug is not part of a cable but is integral to a device such as a USB flash drive. A horizontal array of horizontal sockets may be easy to fabricate, but may cause only two out of four ports to be usable (depending on plug width).
Port arrays in which the port orientation is perpendicular to the array orientation generally have fewer blockage problems. 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.
Length limitations USB cables are limited to 3 metres (10 feet) for low-speed USB 1.1 devices. A hub can be used as an active USB repeater to extend cable length for up to 5 metre (16 feet) lengths at a time. Active cables (specialized connector-embedded one-port hubs) perform the same function, but since they are strictly bus-powered, externally powered USB hubs would likely be required for some of the segments.
PowerA bus-powered hub (passive hub) is a hub that draws all its power from the host computer’s USB interface. It does not need a separate power connection. However, many devices require more power than this method can provide and will not work in this type of hub. 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.
A USB’s electric current is allocated in units of 100 mA up to a maximum total of 500 mA per port. Therefore, a compliant bus powered hub can have no more than four downstream ports and cannot offer more than four 100 mA units of current in total to downstream devices (since the hub needs one unit for itself). 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.
In contrast, a self-powered hub (active hub) takes its power from an external power supply unit and can therefore provide full power (up to 500 mA) to every port. Many hubs can operate as either bus powered or self powered hubs.
However, there are many non-compliant hubs on the market which announce themselves to the host as self-powered despite really being bus-powered. Equally, there are plenty of non-compliant devices that use more than 100 mA without announcing this fact. These hubs and devices do allow more flexibility in the use of power (in particular, many devices use far less than 100 mA and many USB ports can supply more than 500 mA before going into overload shut-off), but they are likely to make power problems harder to diagnose.
Some self-powered hubs do not supply enough power to drive a 500 mA load on every port. For example, many seven port hubs have a 1 A power supply, when in fact seven ports could draw a maximum of 7 x 0.5 = 3.5 A, plus power for the hub itself. 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. For example, the packaging on a seven-port hub might claim to support a maximum of four full-load devices.
Dynamic-powered hubs are hubs which can work as bus-powered as well as self-powered hubs. They can automatically switch between modes depending on whether a separate power supply is available or not. While switching from bus-powered to self-powered operation does not necessarily require immediate renegotiations with the host, switching from self-powered to bus-powered operation may cause USB connections to be reset if connected devices previously requested more power than available in bus-powered mode.