USB hub

A USB hub is a device that expands a single USB port into several so that there are more ports available to connect devices to a host system.

USB hubs are often built into equipment such as computers, keyboards, monitors, or printers. When such a device has many USB ports they all usually stem from one or two internal USB hubs rather than each port having independent USB circuitry.

Physically separate USB hubs come in a wide variety of form factors: from external boxes (looking similar to a network hub) connectable with a long cable, to small designs that can be directly plugged into a USB port (see the 'compact design' picture). In the middle case, there are "short cable" hubs which typically use an integral 6 inch cable to slightly distance a small hub away from physical port congestion and of course increase the number of available ports.

Laptop computers may be equipped with many USB ports, but an external USB hub can consolidate several everyday devices (like a mouse and a printer) into a single hub to enable one-step attachment and removal of all the devices.

Physical layout

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 hubs may not be connected in series to other bus-powered hubs.

USB ports are often closely spaced. Consequentially 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 maybe 2 inches 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 3m for low-speed USB 1.1 device. A hub can be used as an active USB repeater to extend cable length for up to 5m 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 (non-bus-powered) USB hubs would likely be required for some of the segments.

Power

A bus-powered 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.

USB current (related to power) 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 can supply, the operating system usually reports this to the user.

In contrast a self-powered hub is one that 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 (or indeed sometimes without identifying themselves as USB devices at all). 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 500mA load on every port. For example, many seven port hubs have a 1A power supply, when in fact seven ports could draw a maximum of 7 x 0.5 = 3.5A, 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 500mA.

Protocol

Each hub has exactly one upstream port and a number of downstream ports. The upstream port connects the hub (directly or through other hubs) to the host. Other hubs or devices can be attached to the downstream ports. During normal transmission, hubs are essentially transparent: data received from its upstream port is broadcast to all devices attached to its downstream ports; data received from a downstream port is generally forwarded to the upstream port only. This way, what is sent by the host is received by all hubs and devices, and what sent by a device is received by the host but not by the other devices (an exception is resume signaling).

Hubs are not transparent when dealing with changes in the status of downstream ports, such as insertion or removal of devices. In particular, if a downstream port of a hub changes status, this change is dealt with in an interaction between the host and this hub; the hubs between them act as transparent in this case.

To this aim, each hub has a single interrupt endpoint "1 IN" (endpoint address 1, hub-to-host direction) used to signal changes in the status of the downstream ports. 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. When the host polls this interrupt endpoint, it learns that the new device is present. It then instructs the hub (via the default control pipe) to reset the port where the new device was plugged in. This reset makes the new device assume address 0, and the host can then interact with it directly; this interaction will result in the host assigning a new (non-zero) address to the device.

Specially designed for PC's equipped with a USB port, the USB four-port hub offers instant connectivity for up to four USB devices. Plug in peripherals like digital cameras, Ethernet adapters, modems, printers, mice, keyboards, scanners, and portable data storage devices in seconds.

Ready to run, right out of the box, this compact USB four-port hub makes high-speed, low-cost expansion a snap. The hub allows four USB devices to be configured and ready for use as soon as you plug them in. Equipped with advanced hot-swap technology, the hub even lets you attach or detach USB devices while your computer is on, making it possible to switch devices on the fly!

For added expandability, you can daisy chain the hub to additional USB hubs. Use up to 127 USB devices at the same time.

---

Product Description

Specially designed for PCs equipped with a USB port, theCompact USB 4-Port offers instant connectivity for up to four USB devices. Plug in peripherals like digital cameras, Ethernet adapters, modems, printers, mice, keyboards,scanners, and portable data storage devices in seconds.Ready to run right out of the box, the Compact USB 4-Port Hub makes high-speed, low-cost expansion a snap. The Compact USB4-Port Hub allows four USB devices to be configured and ready for use as soon as you plug them in. Equipped with advanced hotswap technology, the hub even lets you attach or detach USB devices while your computer is on, making it possible to switch devices on the fly, The Compact USB 4-Port Hub offers data transfer rates of up to 12 million bits per second, which makes it perfect for speed-hungry multimedia, gaming, and Ethernet networking applications. For added expandability, you can daisy-chain the 4-Port Hub to additional USB hubs -- you can use up to 127 USB devices at the same time Expand the capabilities of your PC with one easy step.its never been easier.