USB hub - WikiHQ

thumb|A four-port "long cable" "external box" USB hub

thumb|A four-port "compact design" USB hub: upstream and downstream ports shown

A USB hub is a device that expands a single Universal Serial Bus (USB) port into several so that there are more ports available to connect devices to a host system, similar to a power strip. All devices connected through a USB hub share the bandwidth available to that hub.

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). "Short cable" hubs typically use an integral 6-inch (15 cm) cable to slightly distance a small hub away from physical port congestion and increase the number of available ports.

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. Hubs may feature power switches for individual ports to allow conveniently power cycling unresponsive devices.

Physical layout

thumb|upright=1.136|left|A "star" "short cable" external USB with the plastic casing removed

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.

It is an important consideration that in common language (and often product marketing), USB 2.0 is used as synonymous with high-speed. However, because the USB 2.0 specification, which introduced high-speed, incorporates the USB 1.1 specification such that a USB 2.0 device is not required to operate at high speed, any compliant full-speed or low-speed device may still be labelled as a USB 2.0 device. Thus, not all USB 2.0 hubs operate at high speed.

With the introduction of USB 3.0 specification it was recommended that manufacturers distinguish USB 3.0 connectors from their USB 2.0 counterparts by using blue (Pantone 300C ) for the Standard-A receptacles and plugs, and use the USB SuperSpeed trident logo.

Protocol

Each hub has exactly one upstream port and a number of downstream ports. The upstream port connects the hub (directly or via 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 (pictorially described in the USB 2.0 specification in Figure 11–2, Hub Signalling Connectivity). 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 is sent by a device is received by the host but not by the other devices (an exception is resume signaling). Downstream routing has been changed in USB 3.0 with the addition of Point to Point routing: A route string sent in the packet header allows a USB 3.0 host to only send a downstream packet to a single destination port, decreasing congestion and power consumption.

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; with any hubs between the host and "changed hub" acting as transparent.

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.

Transaction translator

Any USB 2.0 hub that supports a higher standard than USB 1.1 (12 Mbit/s) will translate between the lower standard and the higher standard using what is called a transaction translator (TT). For example, if a USB 1.1 device is connected to a port on a USB 2.0 hub, then the TT would automatically recognize and translate the USB 1.1 signals to USB 2.0 on the uplink. However, the default design is that all lower-standard devices share the same transaction translator and thus create a bottleneck, a configuration known as the single transaction translator.

Consequently, multi transaction translators (Multi-TT) were created, which provide more transaction translators such that bottlenecks are avoided. Note that due to the nature of USB 3.0 hubs have separate logic for the USB 3.0 and 2.0 data and therefore report a HUB device for both protocols.

Electronic design

Most USB hubs use one or more integrated controllers (ICs), of which several designs are available from various manufacturers. Most support a four-port hub system, but hubs using 16-port hub controllers are also available in the industry. The USB bus allows seven cascading tiers of ports. The root hub is the first tier, and the last devices are on the seventh tier, allowing five tiers worth of hubs between them. The maximum number of user devices is reduced by the number of hubs. With 50 hubs attached, the maximum number is .

References

External links

de:Universal Serial Bus#USB-Hubs