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Introduction

A USB hub is a device that allows many USB devices to be connected to a single USB port on a host computer or another hub.

USB hubs are often built into equipment, such as keyboards, monitors, printers, or computers. When a computer has several USB ports they might all be part of one internal USB hub rather than each port having independent USB hardware.

Physically separate USB hubs come in a wide variety of form factors: from boxes (looking similar to a network hub as shown above) connectible 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.

Inverse or Sharing Hubs

Also available are so-called "sharing hubs", which effectively are the reverse of a USB hub, allowing several PCs to access (usually) a single peripheral. They can either be manual, effectively a simple switch-box, or automatic, incorporating a mechanism that recognises which PC wishes to use the peripheral and switches accordingly. They cannot grant both PCs access at once. Some models, however, have the ability to control multiple peripherals separately (e.g. 2 PCs and 4 peripherals, assigning access separately). Only the simpler switches tend to be automatic, and this feature generally places them at a higher price point too.

Physical layout

A USB network with many devices requires one or more hubs connected to each other. USB hubs can extend a USB network a maximum of five times. 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, so that 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 can sometimes cause say 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. 'Octopus' hubs (with each socket at the end of a very short cable), or 'star' hubs (with each port pointing in a different direction, as pictured) avoid this problem completely.

Aside from practical layouts, novelty USB hubs have also been produced, such as one shaped like the TARDIS, a fictional time-travelling space ship from the BBC science fiction series Doctor Who, or another shaped like a nuclear missile launch console complete with a big red button (which shuts down the PC).

Laptop computers may come with many USB ports built in, but an external USB hub can consolidate several everyday devices (like a mouse and a printer) into a single port for quick attachment and removal.

Length limitations

USB cables are limited to 5m. 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 one unit is needed for the hub itself). If more units of current are required by a device than can be supplied by the port it is plugged into, 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 500mA) 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 powered hubs do not supply enough power to support a 500mA load on every port. For example, many 7 port hubs come with a 1A power adapter, when in fact seven ports could draw a maximum of 7 x 0.5 = 3.5A, plus power for the hub itself. The assumption is that the user will most likely connect many low power devices and only one or two requiring a full 500mA.

Operation

To allow high-speed devices to operate in their fastest mode all hubs between the devices and the computer must be high speed. High-speed devices should fall back to full-speed when plugged in to a full-speed hub (or connected to an older full-speed computer port). While high-speed hubs support all device speeds, low and full-speed traffic is combined and segregated from high-speed traffic through a transaction translator. Each transaction translator segregates lower speed traffic into its own pool, essentially creating a virtual full-speed bus. Some designs use a single transaction translator, while other designs have multiple translators. Having multiple translators is only a significant benefit when connecting multiple high-bandwidth full-speed devices.

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 and supersedes the USB 1.1 specification, any compliant full-speed or low-speed device is still a USB 2.0 device. Thus, not all USB 2.0 hubs operate at high-speed.

Types

Digital audio players are generally categorized by storage media:

• Flash-based Players: These are non-mechanical solid state devices that hold digital audio files on internal flash memory or removable flash media called memory cards. Due to technological advancements in flash memory, these originally low-storage devices are now available commercially ranging up to 32 GB.^[10] Because they are solid state and do not have moving parts they require less battery power and may be more resilient to hazards such as dropping or fragmentation than hard disk-based players. Basic MP3 player functions are commonly integrated into USB flash drives.
• Hard drive-based Players or Digital Jukeboxes: Devices that read digital audio files from a hard disk drive (HDD). These players have higher capacities currently ranging up to 250 GB.^[11] At typical encoding rates, this means that tens of thousands of songs can be stored on one player.
• MP3 CD Players: Portable CD players that can decode and play MP3 audio files stored on CDs.
• Networked audio players: Players that connect via (WiFi) network to receive and play audio.

Common audio formats

MP3 is the dominant format, and is nearly universally supported. The main alternative formats are AAC and WMA. Unlike MP3, these formats support DRM restrictions that are often implemented into files from paid download services. Open source formats, which are completely patent-free, are available - though less widely supported. Examples include Ogg Vorbis, FLAC, and Speex. Most players can also play uncompressed PCM in a container such as WAV or AIFF.

Controversy

Although these issues aren't usually controversial within digital audio players, they are matters of continuing controversy and litigation, including but not limited to content distribution and protection, and digital rights management (DRM).

Lawsuit with RIAA

The Recording Industry Association of America (RIAA) filed a lawsuit with Diamond Multimedia for its Rio players, alleging that the device encouraged copying music illegally. But Diamond won a legal victory on the shoulders of the Sony Corp. v. Universal City Studios case and DAPs were legally ruled as electronic devices.

Risk of hearing damage

According to SCENIHR, the risk of hearing damage from digital audio players depends on both sound level and listening time. The listening habits of most users are unlikely to cause hearing loss, but some people are putting their hearing at risk, because they set the volume control very high or listen to music at high levels for many hours per day. Such listening habits may result in temporary or permanent hearing loss, tinnitus, and difficulties understanding speech in noisy environments.