USB - Çfar eshte dhe si perdoret - Detyre Kursi Anglisht

Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the
cables, connectors and communications protocols used in a bus for connection, communication, and power supply between computers and electronic devices.[2]
USB was designed to standardize the connection of computer peripherals (including keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters) to personal computers, both to communicate and to supply electric power. It has become
commonplace on other devices, such as smartphones, PDAs and video game consoles.[3] USB has effectively replaced a variety of earlier interfaces, such as serial and parallel ports, as well as separate power chargers for portable devices.
Overview
In general, there are three basic kinds or sizes related to the USB connectors and types of established connection: the older "standard" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on USB flash drives), the "mini" size (primarily for the B connector end, such as on many cameras), and the "micro" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on most modern cellphones).
Unlike other data cables (Ethernet, HDMI etc.), each end of a USB cable uses a different kind of connector; an A-type or a B-type. This kind of design was chosen to prevent electrical overloads and damaged equipment, as only the A-type socket provides power. There are cables with A-type connectors on both ends, but they should be used carefully.[4] Therefore, in general, each of the different "sizes" requires four different connectors; USB cables have the A-type and B-type plugs, and the corresponding receptacles are on the computer or electronic device. In common practice, the A-type connector is usually the full size, and the B-type side can vary as needed.
The mini and micro sizes also allow for a reversible AB-type receptacle, which can accept either an A-type or a B-type plug. This scheme, known as "USB On-The-Go", allows one receptacle to perform its double duty in space-constrained applications.
Counter-intuitively, the "micro" size is the most durable from the point of designed insertion lifetime. The standard and mini connectors were designed for less than daily connections, with a design lifetime of 1,500 insertion/removal cycles.[5] (Improved mini-B connectors have reached 5,000-cycle lifetimes.) Micro connectors were designed with frequent charging of portable devices in mind; not only is design lifetime of the connector improved to 10,000 cycles,[5] but it was also redesigned to place the flexible contacts, which wear out sooner, on the easily replaced cable, while the more durable rigid contacts are located in the micro-USB receptacles. Likewise, the springy part of the retention mechanism (parts that provide required gripping force) were also moved into plugs on the cable side.[6]
USB connections also come in four data transfer speeds: Low Speed, Full Speed, High Speed and SuperSpeed. High Speed is only supported by specifically designed USB 2.0 High Speed interfaces (that is, USB 2.0 controllers without the High Speed designation do not support it), as well as by USB 3.0 interfaces. SuperSpeed is supported only by USB 3.0 interfaces.
History

The basic USB trident logo[7]

USB sign is on the head of a USB Cable
A group of seven companies began the development of USB in 1994: Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel.[8] The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data rates for external devices. A team including Ajay Bhatt worked on the standard at Intel;[9][10] the first integrated circuits supporting USB were produced by Intel in 1995.[11]
The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5 Mbit/s "Low Speed" and 12 Mbit/s "Full Speed".[11] The first widely used version of USB was 1.1, which was released in September 1998. The 12 Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as joysticks.[12]

A USB Standard Type A plug, the most common USB plug
The USB 2.0 specification was released in April 2000 and was ratified by the USB Implementers Forum (USB-IF) at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent), NEC and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, a 40-times increase over the original USB 1.1 specification.
The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5 Gbit/s), decrease power consumption, increase power output, and be backwards-compatible with USB 2.0.[13] USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus.[14] For this reason, the new version is also called SuperSpeed.[15] The first USB 3.0 equipped devices were presented in January 2010.[15][16]
As of 2008, approximately six billion USB ports and interfaces were in the global marketplace, and about two billion were being sold each year.[17]
Version history

A PCI USB 2.0 card for a computer motherboard
Prereleases
The USB standard evolved through several versions before its official release in 1996:
USB 0.7 – released in November 1994
USB 0.8 – released in December 1994
USB 0.9 – released in April 1995
USB 0.99 – released in August 1995
USB 1.0 Release Candidate – released in November 1995
USB 1.x
Released in January 1996, USB 1.0 specified data rates of 1.5 Mbit/s (Low Bandwidth or Low Speed) and 12 Mbit/s (Full Bandwidth or Full Speed). It did not allow for extension cables or pass-through monitors, due to timing and power limitations. Few USB devices made it to the market until USB 1.1 was released in August 1998, fixing problems identified in 1.0, mostly related to using hubs. USB 1.1 was the earliest revision that was widely adopted.
USB 2.0

The Hi-Speed USB Logo
USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s called High Speed, in addition to the USB 1.x Full Speed signaling rate of 12 Mbit/s. Due to bus access constraints, the effective throughput of the High Speed signaling rate is limited to 35 MB/s or 280 Mbit/s.[18][19]
Further modifications to the USB specification have been made via Engineering Change Notices (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:[20]
Mini-A and Mini-B Connector ECN: Released in October 2000.
Specifications for Mini-A and B plug and receptacle. Also receptacle that accepts both plugs for On-The-Go. These should not be confused with Micro-B plug and receptacle.
Pull-up/Pull-down Resistors ECN: Released in May 2002
Interface Associations ECN: Released in May 2003.
New standard descriptor was added that allows associating multiple interfaces with a single device function.
Rounded Chamfer ECN: Released in October 2003.
A recommended, compatible change to Mini-B plugs that results in longer lasting connectors.
Unicode ECN: Released in February 2005.
This ECN specifies that strings are encoded using UTF-16LE. USB 2.0 specified Unicode, but did not specify the encoding.
Inter-Chip USB Supplement: Released in March 2006
On-The-Go Supplement 1.3: Released in December 2006.
USB On-The-Go makes it possible for two USB devices to communicate with each other without requiring a separate USB host. In practice, one of the USB devices acts as a host for the other device.
Battery Charging Specification 1.1: Released in March 2007 (Updated 15 Apr 2009).
Adds support for dedicated chargers (power supplies with USB connectors), host chargers (USB hosts that can act as chargers) and the No Dead Battery provision, which allows devices to temporarily draw 100 mA current after they have been attached. If a USB device is connected to dedicated charger, maximum current drawn by the device may be as high as 1.8 A. (Note that this document is not distributed with USB 2.0 specification package only USB 3.0 and USB On-The-Go.)
Micro-USB Cables and Connectors Specification 1.01: Released in April 2007.
Link Power Management Addendum ECN: Released in July 2007.
This adds "sleep", a new power state between enabled and suspended states. Device in this state is not required to reduce its power consumption. However, switching between enabled and sleep states is much faster than switching between enabled and suspended states, which allows devices to sleep while idle.
Battery Charging Specification 1.2:[21] Released in December 2010.
Several changes and increasing limits including allowing 1.5 A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5 A and allowing a maximum current of 5 A.
USB 3.0
Main article: USB 3.0

The SuperSpeed USB Logo
USB 3.0 standard was released in November 2008, defining a new SuperSpeed mode. A USB 3.0 port is usually colored blue and is backward-compatible with USB 2.0 devices and cables.
The USB 3.0 Promoter Group announced on 17 November 2008 that the specification of version 3.0 had been completed and had made the transition to the USB Implementers Forum (USB-IF), the managing body of USB specifications.[22] This move effectively opened the specification to hardware developers for implementation in products.
The new SuperSpeed bus provides a fourth transfer mode at 5.0 Gbit/s (raw data rate), in addition to the modes supported by earlier versions. The payload throughput is 4 Gbit/s (due to the overhead induced by used 8b/10b encoding), and the specification considers it reasonable to achieve around 3.2 Gbit/s (0.4 GB/s or 400 MB/s), which should increase with future hardware advances. Communication is full-duplex in SuperSpeed transfer mode; in the modes supported previously, by 1.x and 2.0, communication is half-duplex, with direction controlled by the host.[23]
As with previous USB versions, USB 3.0 ports come in low-power and high-power variants, providing 150 mA and 900 mA respectively, while simultaneously transmitting data at SuperSpeed rates.[24] Additionally, there is a Battery Charging Specification (Version 1.2 – December 2010), which increases the power handling capability to 1.5 A but does not allow concurrent data transmission.[21] The Battery Charging Specification requires that the physical ports themselves be capable of handling 5 A of current[citation needed] but limits the maximum current drawn to 1.5 A.
USB 3.1
A January 2013 press release from the USB group revealed plans to update USB 3.0 to 10 Gbit/s, effectively putting it on par with one first-generation Thunderbolt channel.[25] The USB 3.1 specification was released on 31 July 2013,[26] introducing a faster transfer mode called "SuperSpeed USB 10 Gbps"; its logo features a Superspeed+ (stylized as SUPERSPEED+) caption. The USB 3.1 standard increases the signalling rate to 10 Gbit/s, double that of USB 3.0, and reduces line encoding overhead to just 3% by changing the encoding scheme to 128b/132b.[27] However, some initial tests demonstrated usable transfer speeds of only 7.2 Gbit/s, suggesting a 30% overall overhead.[28]
The USB 3.1 standard is backward compatible with USB 3.0 and USB 2.0. Using three power profiles of those defined in the USB Power Delivery Specification, it lets devices with larger energy demands request higher currents and supply voltages from compliant hosts – up to 2 A at 5 V (for a power consumption of up to 10 W), and optionally up to 5 A at either 12 V (60 W) or 20 V (100 W).[29]
System design
The design architecture of USB is asymmetrical in its topology, consisting of a host, a multitude of downstream USB ports, and multiple peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels. A USB host may implement multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including hub devices if present, may be connected to a single host controller.[30][31] USB devices are linked in series through hubs. One hub—built into the host controller—is the root hub.
A physical USB device may consist of several logical sub-devices that are referred to as device functions. A single device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function). This kind of device is called a composite device. An alternative to this is compound device, in which the host assigns each logical device a distinctive address and all logical devices connect to a built-in hub that connects to the physical USB cable.

USB endpoints actually reside on the connected device: the channels to the host are referred to as pipes
USB device communication is based on pipes (logical channels). A pipe is a connection from the host controller to a logical entity, found on a device, and named an endpoint. Because pipes correspond 1-to-1 to endpoints, the terms are sometimes used interchangeably. A USB device could have up to 32 endpoints (16 IN, 16 OUT), though it's rare to have so many. An endpoint is defined and numbered by the device during initialization (the period after physical connection called "enumeration") and so is relatively permanent, whereas a pipe may be opened and closed.
There are two types of pipe: stream and message. A message pipe is bi-directional and is used for control transfers. Message pipes are typically used for short, simple commands to the device, and a status response, used, for example, by the bus control pipe number 0. A stream pipe is a uni-directional pipe connected to a uni-directional endpoint that transfers data using an isochronous,[32] interrupt, or bulk transfer:
isochronous transfers: at some guaranteed data rate (often, but not necessarily, as fast as possible) but with possible data loss (e.g., realtime audio or video).
interrupt transfers: devices that need guaranteed quick responses (bounded latency) (e.g., pointing devices and keyboards).
bulk transfers: large sporadic transfers using all remaining available bandwidth, but with no guarantees on bandwidth or latency (e.g., file transfers).
An endpoint of a pipe is addressable with a tuple (device_address, endpoint_number) as specified in a TOKEN packet that the host sends when it wants to start a data transfer session. If the direction of the data transfer is from the host to the endpoint, an OUT packet (a specialization of a TOKEN packet) having the desired device address and endpoint number is sent by the host. If the direction of the data transfer is from the device to the host, the host sends an IN packet instead. If the destination endpoint is a uni-directional endpoint whose manufacturer's designated direction does not match the TOKEN packet (e.g., the manufacturer's designated direction is IN while the TOKEN packet is an OUT packet), the TOKEN packet is ignored. Otherwise, it is accepted and the data transaction can start. A bi-directional endpoint, on the other hand, accepts both IN and OUT packets.

Two USB 3.0 standard-A sockets (left) and two USB 2.0 sockets (right) on a computer's front panel
Endpoints are grouped into interfaces and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and is not associated with any interface. A single device function composed of independently controlled interfaces is called a composite device. A composite device only has a single device address because the host only assigns a device address to a function.
When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The data rate of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host and the device is assigned a unique 7-bit address. If the device is supported by the host, the device drivers needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.
The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB 2.0, the host controller polls the bus for traffic, usually in a round-robin fashion. The throughput of each USB port is determined by the slower speed of either the USB port or the USB device connected to the port.
High-speed USB 2.0 hubs contain devices called transaction translators that convert between high-speed USB 2.0 buses and full and low speed buses. When a high-speed USB 2.0 hub is plugged into a high-speed USB host or hub, it operates in high-speed mode. The USB hub then uses either one transaction translator per hub to create a full/low-speed bus routed to all full and low speed devices on the hub, or uses one transaction translator per port to create an isolated full/low-speed bus per port on the hub.
Because there are two separate controllers in each USB 3.0 host, USB 3.0 devices transmit and receive at USB 3.0 data rates regardless of USB 2.0 or earlier devices connected to that host. Operating data rates for earlier devices are set in the legacy manner.
USB mass storage / USB drive

A flash drive, a typical USB mass-storage device

Circuit board from a USB 3.0 external 2.5-inch SATA HDD enclosure
See also: USB mass storage device class, Disk enclosure and External hard disk drive
USB implements connections to storage devices using a set of standards called the USB mass storage device class (MSC or UMS). This was at first intended for traditional magnetic and optical drives and has been extended to support flash drives. It has also been extended to support a wide variety of novel devices as many systems can be controlled with the familiar metaphor of file manipulation within directories. The process of making a novel device look like a familiar device is also known as extension.[citation needed] The ability to boot a write-locked SD card with a USB adapter is particularly advantageous for maintaining the integrity and non-corruptible, pristine state of the booting medium.
Though most post-Summer 2004 computers can boot from USB mass storage devices, USB is not intended as a primary bus for a computer's internal storage. Buses such as Parallel ATA (PATA or IDE), Serial ATA (SATA), or SCSI fulfill that role in PC class computers. However, USB has one important advantage, in that it is possible to install and remove devices without rebooting the computer (hot-swapping), making it useful for mobile peripherals, including drives of various kinds.
Firstly conceived and still used today for optical storage devices (CD-RW drives, DVD drives, etc.), several manufacturers offer external portable USB hard disk drives, or empty enclosures for disk drives. These offer performance comparable to internal drives, limited by the current number and types of attached USB devices, and by the upper limit of the USB interface (in practice about 30 MB/s for USB 2.0 and potentially 400 MB/s or more[37] for USB 3.0). These external drives typically include a "translating device" that bridges between a drive's interface to a USB interface port. Functionally, the drive appears to the user much like an internal drive. Other competing standards for external drive connectivity include eSATA, ExpressCard (now at version 2.0), FireWire (IEEE 1394), and most recently Thunderbolt.
Another use for USB mass storage devices is the portable execution of software applications (such as web browsers and VoIP clients) with no need to install them on the host computer.[38][39]
Media Transfer Protocol
Media Transfer Protocol (MTP) was designed by Microsoft to give higher-level access to a device's filesystem than USB mass storage, at the level of files rather than disk blocks. It also has optional DRM features. MTP was designed for use with portable media players, but it has since been adopted as the primary storage access protocol of the Android operating system from the version 4.1 Jelly Bean as well as Windows Phone 8 (Windows Phone 7 devices had used the Zune protocol which was an evolution of MTP). The primary reason for this is that MTP does not require exclusive access to the storage device the way UMS does, alleviating potential problems should an Android program request the storage while it is attached to a computer. The main drawback is that MTP is not as well supported outside of Windows operating systems.
Human interface devices
Main article: USB human interface device class
Joysticks, keypads, tablets and other human-interface devices (HIDs) are also progressively migrating from MIDI, and PC game port connectors to USB.[citation needed]
USB mice and keyboards can usually be used with older computers that have PS/2 connectors with the aid of a small USB-to-PS/2 adapter. For mice and keyboards with dual-protocol support, an adaptor that contains no logic circuitry may be used: the hardware in the USB keyboard or mouse is designed to detect whether it is connected to a USB or PS/2 port, and communicate using the appropriate protocol. Converters also exist that connect PS/2 keyboards and mice (usually one of each) to a USB port.[40] These devices present two HID endpoints to the system and use a microcontroller to perform bidirectional data translation between the two standards.
Device Firmware Upgrade
Device Firmware Upgrade (DFU) is a vendor- and device-independent mechanism for upgrading the firmware of USB devices with improved versions provided by their manufacturers, offering (for example) a way for firmware bugfixes to be deployed. During the firmware upgrade operation, USB devices change their operating mode effectively becoming a PROM programmer. Any class of USB device can implement this capability by following the official DFU specifications.[36][41][42]
In addition to its intended legitimate purposes, DFU can also be exploited by uploading maliciously crafted firmwares that cause USB devices to spoof various other device types; one of such exploiting approaches is known as BadUSB.[43]
Connectors and plugs
     This section may contain too much repetition or redundant language. Please help improve it by merging similar text or removing repeated statements. (December 2013)

Connectors properties

Standard type A plug and receptacle
The connectors the USB committee specifies support a number of USB's underlying goals, and reflect lessons learned from the many connectors the computer industry has used. The connector mounted on the host or device is called the receptacle, and the connector attached to the cable is called the plug.[44] The standard purposely defines this to prevent the use of extension cables.[citation needed] The official USB specification documents also periodically define the term male to represent the plug, and female to represent the receptacle.[citation needed]
Usability and orientation

USB extension cord
By design, it is difficult to insert a USB plug into its receptacle incorrectly. The USB specification states that the required USB icon must be embossed on the "topside" of the USB plug, which "...provides easy user recognition and facilitates alignment during the mating process." The specification also shows that the "recommended" "Manufacturer's logo" ("engraved" on the diagram but not specified in the text) is on the opposite side of the USB icon. The specification further states, "The USB Icon is also located adjacent to each receptacle. Receptacles should be oriented to allow the icon on the plug to be visible during the mating process." However, the specification does not consider the height of the device compared to the eye level height of the user, so the side of the cable that is "visible" when mated to a computer on a desk can depend on whether the user is standing or kneeling.[44]
While it would have been better for usability if the cable could be plugged in with either side up, the original design left this out to make manufacturing as inexpensive as possible. Ajay Bhatt, who was involved in the original USB design team, is working on a new design to make the cable insertable either side up.[45] The new reversible plug is also much smaller than the current USB 3.0 Micro-B connector. It is called Type-C, and should be introduced as an addition to the existing USB 3.1 specification.[46]
Only moderate force is needed to insert or remove a USB cable. USB cables and small USB devices are held in place by the gripping force from the receptacle (without need of the screws, clips, or thumb-turns other connectors have required).
Power-use topology
The standard connectors were deliberately intended to enforce the directed topology of a USB network: type A connectors on host devices that supply power and type B connectors on target devices that draw power. This is intended to prevent users from accidentally connecting two USB power supplies to each other, which could lead to short circuits and dangerously high currents, circuit failures, or even fire. USB does not support cyclic networks and the standard connectors from incompatible USB devices are themselves incompatible.[4]
However, some of this directed topology is lost with the advent of multi-purpose USB connections (such as USB On-The-Go in smartphones, and USB-powered Wi-Fi routers), which require A-to-A, B-to-B, and sometimes Y/splitter cables. See the USB On-The-Go connectors section below, for a more detailed summary description.
Durability
The standard connectors were designed to be robust. Because USB is hot-pluggable, the connectors would be used more frequently, and perhaps with less care, than other connectors. Many previous connector designs were fragile, specifying embedded component pins or other delicate parts that were vulnerable to bending or breaking. The electrical contacts in a USB connector are protected by an adjacent plastic tongue, and the entire connecting assembly is usually protected by an enclosing metal sheath.[47]
The connector construction always ensures that the external sheath on the plug makes contact with its counterpart in the receptacle before any of the four connectors within make electrical contact. The external metallic sheath is typically connected to system ground, thus dissipating damaging static charges. This enclosure design also provides a degree of protection from electromagnetic interference to the USB signal while it travels through the mated connector pair (the only location when the otherwise twisted data pair travels in parallel). In addition, because of the required sizes of the power and common connections, they are made after the system ground but before the data connections. This type of staged make-break timing allows for electrically safe hot-swapping.[47]
The newer Micro-USB receptacles are designed for up to 10,000 cycles of insertion and removal between the receptacle and plug, compared to 1,500 for the standard USB and 5,000 for the Mini-USB receptacle. This is accomplished by adding a locking device and by moving the leaf-spring connector from the jack to the plug, so that the most-stressed part is on the cable side of the connection. This change was made so that the connector on the less expensive cable would bear the most wear instead of the more expensive micro-USB device.[6][47]
Compatibility
The USB standard specifies relatively loose tolerances for compliant USB connectors to minimize physical incompatibilities in connectors from different vendors. To address a weakness present in some other connector standards, the USB specification also defines limits to the size of a connecting device in the area around its plug. This was done to prevent a device from blocking adjacent ports due to the size of the cable strain relief mechanism (usually molding integral with the cable outer insulation) at the connector. Compliant devices must either fit within the size restrictions or support a compliant extension cable that does.
In general, USB cables have only plugs on their ends, while hosts and devices have only receptacles. Hosts almost universally have type-A receptacles, while devices have one or another type-B variety. Type-A plugs mate only with type-A receptacles, and the same applies to their type-B counterparts; they are deliberately physically incompatible. However, an extension to the USB standard specification called USB On-The-Go (OTG) allows a single port to act as either a host or a device, what is selectable by the end of the cable that plugs into the receptacle on the OTG-enabled unit. Even after the cable is hooked up and the units are communicating, the two units may "swap" ends under program control. This capability is meant for units such as PDAs in which the USB link might connect to a PC's host port as a device in one instance, yet connect as a host itself to a keyboard and mouse device in another instance.
USB 3.0 connectors
Main article: USB 3.0 backward compatibility
Type A plugs and receptacles from both USB 3.0 and USB 2.0 are designed to interoperate. Type B plugs and receptacles in USB 3.0 are somewhat larger than those in USB 2.0; thus, USB 2.0 Type B plugs can fit into USB 3.0 Type B receptacles, while the opposite is not possible.
Connector types

Types of USB connector left to right (ruler in centimeters): Micro-B plug, UC-E6 proprietary (non-USB) plug, Mini-B plug, Standard-A receptacle (upside down), Standard-A plug, Standard-B plug
There are several types of USB connector, including some that have been added while the specification progressed. The original USB specification detailed Standard-A and Standard-B plugs and receptacles; the B connector was necessary so that cabling could be plug ended at both ends and still prevent users from connecting one computer receptacle to another. The first engineering change notice to the USB 2.0 specification added Mini-B plugs and receptacles.
The data connectors in the Standard-A plug are actually recessed in the plug as compared to the outside power connectors. This permits the power to connect first, which prevents data errors by allowing the device to power up first and then transfer the data. Some devices operate in different modes depending on whether the data connection is made. This difference in connection can be exploited by inserting the connector only partially. For example, some battery-powered MP3 players switch into file transfer mode and cannot play MP3 files while a USB plug is fully inserted, but can be operated in MP3 playback mode using USB power by inserting the plug only part way so that the power slots make contact while the data slots do not. This enables those devices to be operated in MP3 playback mode while getting power from the cable.[original research?]
To reliably enable a charge-only feature, modern USB accessory peripherals now include charging cables that provide power connections to the host port but no data connections, and both home and vehicle charging docks are available that supply power from a converter device and do not include a host device and data pins, allowing any capable USB device to charge or operate from a standard USB cable.
Standard connectors

Pin configuration of the USB connectors Standard A/B, viewed looking into face/end of plug
The USB 2.0 Standard-A type of USB plug is a flattened rectangle that inserts into a "downstream-port" receptacle on the USB host, or a hub, and carries both power and data. This plug is frequently seen on cables that are permanently attached to a device, such as one connecting a keyboard or mouse to the computer via USB connection.
USB connections eventually wear out as the connection loosens through repeated plugging and unplugging. The lifetime of a USB-A male connector is approximately 1,500 connect/disconnect cycles.[48]
A Standard-B plug—which has a square shape with beveled exterior corners—typically plugs into an "upstream receptacle" on a device that uses a removable cable, e.g., a printer. On some devices, the Type B receptacle has no data connections, being used solely for accepting power from the upstream device. This two-connector-type scheme (A/B) prevents a user from accidentally creating an electrical loop.[49]
Mini and Micro connectors

Micro A USB

Micro B USB

USB Mini A (left) and USB Mini B (right) plugs
Various connectors have been used for smaller devices such as digital cameras, smartphones, and tablet computers. These include the now-deprecated[50] (i.e. de-certified but standardized) Mini-A and Mini-AB connectors (Mini-B connectors are still supported but not OTG (On The Go, i.e. mobile) compliant).[51] The Mini-B USB connector was standard for transferring data to and from the early smartphones and PDAs.
The Mini-A and Mini-B plugs are approximately 3 by 7 mm. The micro-USB plugs have a similar width and approximately half the thickness, enabling their integration into thinner portable devices. The micro-A connector is 6.85 by 1.8 mm with a maximum overmold size of 11.7 by 8.5 mm. The micro-B connector is 6.85 by 1.8 mm with a maximum overmold size of 10.6 by 8.5 mm.[52]
The Micro-USB connector was announced by the USB-IF on 4 January 2007.[53][5] The Mini-A connector and the Mini-AB receptacle connector were deprecated on 23 May 2007.[54] While many currently available devices and cables still use Mini plugs, the newer Micro connectors are being widely adopted and as of December 2010, they are the most widely used.[citation needed] The thinner micro connectors are intended to replace the Mini plugs in new devices including smartphones, personal digital assistants, and cameras.[55]
The Micro plug design is rated for at least 10,000 connect-disconnect cycles, which is more than the Mini plug design.[5][56] It is also designed to reduce the mechanical wear on the device; instead the easier-to-replace cable is designed to bear the mechanical wear of connection and disconnection. The Universal Serial Bus Micro-USB Cables and Connectors Specification details the mechanical characteristics of Micro-A plugs, Micro-AB receptacles (which accept both Micro-A and Micro-B plugs), and Micro-B plugs and receptacles,[56] along with a Standard-A receptacle to Micro-A plug adapter.
The cellular phone carrier group, Open Mobile Terminal Platform (OMTP) in 2007 endorsed Micro-USB as the standard connector for data and power on mobile devices[57] In addition, on 22 October 2009 the International Telecommunication Union (ITU) has also announced that it had embraced micro-USB as the Universal Charging Solution its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger."[58]
The European Standardisation Bodies CEN, CENELEC and ETSI (independent of the OMTP/GSMA proposal) defined a common External Power Supply (EPS) for use with smartphones sold in the EU based on micro-USB.[59] 14 of the world's largest mobile phone manufacturers signed the EU's common EPS Memorandum of Understanding (MoU).[60][61] Apple Inc., one of the original MoU signers, makes micro-USB adapters available – as permitted in the Common EPS MoU – for its iPhones equipped with Apple's proprietary 30 pin dock connector or (later) "Lightning" connector.[62][63]
USB On-The-Go connectors
Main article: USB On-The-Go

Each PHY has its own clock domain with Spread Spectrum Clocking (SSC) modulation. The USB 3.1 cable does not include a reference clock so the clock domains on each end of the physical connection are not explicitly connected. Bit-level timing synchronization relies on the local receiver aligning its bit recovery clock to the remote transmitter’s clock by phase-locking to the signal transitions in the received bit stream. The receiver needs to reliably recover clock and data from the bit stream. For Gen 1 operation the transmitter encodes data and control characters into symbols, see section 3.2.1.1.Control symbols are used to achieve byte alignment and are used for framing data and managing the link. Special characteristics make control symbols uniquely identifiable from data symbols. For Gen 2 operation the transmitter block encodes the data and control bytes, see section 3.2.1.2. Special control blocks are used to achieve block alignment in the receiver and for managing the link. A number of techniques are employed to improve channel performance. For example, to avoid overdriving and improve eye margin at the receiver, transmitter de-emphasis may be applied when multiple bits of the same polarity are sent. Also, equalization may be used in the receiver with the characteristics of the equalization profile being established adaptively as part of link training. Signal (timing, jitter tolerance, etc.) and electrical (DC characteristics, channel capacitance, etc.) performance of Gen X links are defined with compliance requirements specified in terms of transmit and receive signaling eyes. The specific Gen X physical layers are summarized in the following sections.
All current USB On-The-Go (OTG) devices are required to have one, and only one, USB connector: a Micro-AB receptacle. Non-OTG compliant devices are not allowed to use the micro-AB receptacle, due to power supply shorting hazards on the VBUS line. The micro-AB receptacle is capable of accepting both Micro-A and Micro-B plugs, attached to any of the legal cables and adapters as defined in revision 1.01 of the Micro-USB specification. Prior to the development of Micro-USB, USB On-The-Go devices were required to use Mini-AB receptacles to perform the equivalent job.
To enable type-AB receptacles to distinguish which end of a cable is plugged in, mini and micro plugs have an "ID" pin in addition to the four contacts found in standard-size USB connectors. This ID pin is connected to GND in type-A plugs, and left unconnected in type-B plugs. Typically, a pull-up resistor in the device is used to detect the presence or absence of an ID connection.
The OTG device with the A-plug inserted is called the A-device and is responsible for powering the USB interface when required and by default assumes the role of host. The OTG device with the B-plug inserted is called the B-device and by default assumes the role of peripheral. An OTG device with no plug inserted defaults to acting as a B-device. If an application on the B-device requires the role of host, then the Host Negotiation Protocol (HNP) is used to temporarily transfer the host role to the B-device.
OTG devices attached either to a peripheral-only B-device or a standard/embedded host have their role fixed by the cable, since in these scenarios it is only possible to attach the cable one way.[citation needed]
Host and device interface receptacles
Connectors (receptacles and plugs) mating matrix is displayed below. As a note, the assignment of functions (VCC, D+, D−, GND and ID) to pin numbers marked below is mostly consistent, with the exception of Mini and Micro connectors. When compared to standard-sized connectors (Type-A and Type-B), Mini and Micro connectors have their GND connections moved from pin #4 to pin #5, while their pin #4 serves as an ID pin for the On-The-Go host/client identification.[64]
Cabling

A USB twisted pair, where the "Data +" and "Data −" conductors are twisted together in a double helix. The wires are enclosed in a further layer of shielding.
The data cables for USB 1.x and USB 2.x use a twisted pair to reduce noise and crosstalk. USB 3.0 cables contain twice as many wires as USB 2.x to support SuperSpeed data transmission, and are thus larger in diameter.[68]
The USB 1.1 Standard specifies that a standard cable can have a maximum length of 5 meters with devices operating at Full Speed (12 Mbit/s), and a maximum length of 3 meters with devices operating at Low Speed (1.5 Mbit/s).[69][70]
USB 2.0 provides for a maximum cable length of 5 meters for devices running at Hi Speed (480 Mbit/s). The primary reason for this limit is the maximum allowed round-trip delay of about 1.5 μs. If USB host commands are unanswered by the USB device within the allowed time, the host considers the command lost. When adding USB device response time, delays from the maximum number of hubs added to the delays from connecting cables, the maximum acceptable delay per cable amounts to 26 ns.[71] The USB 2.0 specification requires that cable delay be less than 5.2 ns per meter (192 000 km/s, which is close to the maximum achievable transmission speed for standard copper wire).
The USB 3.0 standard does not directly specify a maximum cable length, requiring only that all cables meet an electrical specification: for copper cabling with AWG 26 wires the maximum practical length is 3 meters (9.8 ft).[
Power
The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire to power connected USB devices. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V ± 5%) between the positive and negative bus power lines (VBUS voltage). For USB 3.0, the voltage supplied by low-powered hub ports is 4.45–5.25 V.[73]
A unit load is defined as 100 mA in USB 1.x and 2.0, and 150 mA in USB 3.0. A device may draw a maximum of five unit loads (500 mA) from a port in USB 1.x and 2.0, or six unit loads (900 mA) in USB 3.0. There are two types of device: low-power and high-power. A low-power device (such as a USB HID) draws at most one-unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws, at most, the maximum number of unit loads the standard permits. Every device functions initially as low-power (including high-power functions during their low-power enumeration phases), but may request high-power, and get it if available on the providing bus.[74][75][76][77]
Some devices, such as high-speed external disk drives, require more than 500 mA of current[78] and therefore may have power issues if powered from just one USB 2.0 port: erratic function, failure to function, or overloading/damaging the port. Such devices may come with an external power source or a Y-shaped cable that has two USB connectors (one for power and data, the other for power only) to plug into a computer. With such a cable, a device can draw power from two USB ports simultaneously.[79] However, USB compliance specification states that "use of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB peripheral", meaning that "if a USB peripheral requires more power than allowed by the USB specification to which it is designed, then it must be self-powered."[80]
A bus-powered hub initializes itself at one-unit load and transitions to maximum unit loads after it completes hub configuration. Any device connected to the hub draws one-unit load regardless of the current draw of devices connected to other ports of the hub (i.e., one device connected on a four-port hub draws only one-unit load despite the fact that more unit loads are being supplied to the hub).[74][full citation needed]
A self-powered hub supplies maximum supported unit loads to any device connected to it. In addition, the VBUS presents one-unit load upstream for communication if parts of the Hub are powered down
Charging ports

A small gadget that provides voltage and current readouts for devices charged over USB.

A small gadget that provides voltage, current and charge (mAh) readouts and logging for charging of devices over USB.
The USB Battery Charging Specification Revision 1.1 (released in 2007) defines a new type of USB port, called the charging port. Contrary to the standard downstream port, for which current draw by a connected portable device can exceed 100 mA only after digital negotiation with the host or hub, a charging port can supply currents between 500 mA and 1.5 A without the digital negotiation. A charging port supplies up to 500 mA at 5 V, up to the rated current at 3.6 V or more, and drops its output voltage if the portable device attempts to draw more than the rated current. The charger port may shut down if the load is too high.[81]
Two types of charging port exist: the charging downstream port (CDP), supporting data transfers as well, and the dedicated charging port (DCP), without data support. A portable device can recognize the type of USB port; on a dedicated charging port, the D+ and D− pins are shorted with a resistance not exceeding 200 ohms, while charging downstream ports provide additional detection logic so their presence can be determined by attached devices.[81]
With charging downstream ports, current passing through the thin ground wire may interfere with high-speed data signals; therefore, current draw may not exceed 900 mA during high-speed data transfer. A dedicated charge port may have a rated current between 500 and 1,500 mA. For all charging ports, there is maximum current of 5 A, as long as the connector can handle the current (standard USB 2.0 A-connectors are rated at 1.5 A).[81]
Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's iPod and iPhone chargers indicate the available current by voltages on the D− and D+ lines. When D+ = D− = 2.0 V, the device may pull up to 500 mA. When D+ = 2.0 V and D− = 2.8 V, the device may pull up to 1 A of current.[82] When D+ = 2.8 V and D− = 2.0 V, the device may pull up to 2 A of current.[83]
Dedicated charging ports can be found on USB power adapters that convert utility power or another power source (e.g., a car's electrical system) to run attached devices and battery packs. On a host (such as a laptop computer) with both standard and charging USB ports, the charging ports should be labeled as such.[81]
To support simultaneous charge and data communication, even if the communication port does not support charging a demanding device, so-called accessory charging adapters (ACA) are introduced. By using an accessory charging adapter, a device providing a single USB port can be attached to both a charger, and another USB device at the same time.[81]
The USB Battery Charging Specification Revision 1.2 (released in 2010) makes clear that there are safety limits to the rated current at 5 A coming from USB 2.0. On the other hand, several changes are made and limits are increasing including allowing 1.5 A on charging downstream ports for unconfigured devices, allowing high speed communication while having a current up to 1.5 A, and allowing a maximum current of 5 A. Also, revision 1.2 removes support for USB ports type detection via resistive detection mechanisms.[21]
Sleep-and-charge ports

A yellow USB port denoting sleep-and-charge
Sleep-and-charge USB ports can be used to charge electronic devices even when the computer is switched off. Normally, when a computer is powered off, the USB ports are powered down. This prevents phones and other devices from being able to charge unless the computer is powered on. Sleep-and-charge USB ports remain powered even when the computer is off. On laptops, charging devices from the USB port when it is not being powered from AC drains the laptop battery faster; most laptops have a facility to stop charging if their own battery charge level gets too low.[84] Desktop machines need to remain plugged into AC power for sleep-and-charge to work as they do not contain batteries like laptops.
These ports are found colored differently (mostly red or yellow). On Dell laptops, the port is marked with the standard USB symbol with an added lightning bolt icon on the right side. Dell calls this feature "PowerShare."[85]
On Acer Inc. and Packard Bell laptops, sleep-and-charge USB ports are marked with a non-standard symbol (the letters "USB" over a drawing of a battery); the feature is simply called "Power-off USB".[86]
On Apple MacBook models, it is possible to plug a device in, close the laptop (putting it into sleep mode) and have the device continue to charge. However, one cannot have the device charge if plugged in after the computer has already gone into sleep mode.[citation needed]
Mobile device charger standards
In China

The Micro-USB interface is commonly found on chargers for mobile phones
As of 14 June 2007, all new mobile phones applying for a license in China are required to use a USB port as a power port for battery charging.[87][88] This was the first standard to use the convention of shorting D+ and D-.[89]
OMTP/GSMA Universal Charging Solution
In September 2007, the Open Mobile Terminal Platform group (a forum of mobile network operators and manufacturers such as Nokia, Samsung, Motorola, Sony Ericsson and LG) announced that its members had agreed on micro-USB as the future common connector for mobile devices.[90][91]
The GSM Association (GSMA) followed suit on 17 February 2009,[92][92][93][94][95] and on 22 April 2009, this was further endorsed by the CTIA – The Wireless Association,[96] with the International Telecommunication Union (ITU) announcing on 22 October 2009 that it had also embraced the Universal Charging Solution as its "energy-efficient one-charger-fits-all new mobile phone solution", and added: "Based on the Micro-USB interface, UCS chargers will also include a 4-star or higher efficiency rating—up to three times more energy-efficient than an unrated charger."[97]
EU Smartphone Power Supply Standard
Main article: Common External Power Supply
In June 2009, many of the world's largest mobile phone manufacturers signed an EC-sponsored Memorandum of Understanding (MoU), agreeing to make most data-enabled mobile phones marketed in the European Union compatible with a common External Power Supply (EPS). The EU's common EPS specification (EN 62684:2010) references the USB Battery Charging standard and is similar to the GSMA/OMTP and Chinese charging solutions.[98][99] In January 2011, the International Electrotechnical Commission (IEC) released its version of the (EU's) common EPS standard as IEC 62684:2011.[100]
Non-standard devices
     This section does not cite any references or sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2011)

USB-powered mini fans

USB vacuum cleaner novelty device
Some USB devices require more power than is permitted by the specifications for a single port. This is common for external hard and optical disc drives, and generally for devices with motors or lamps. Such devices can use an external power supply, which is allowed by the standard, or use a dual-input USB cable, one input of which is used for power and data transfer, the other solely for power, which makes the device a non-standard USB device. Some USB ports and external hubs can, in practice, supply more power to USB devices than required by the specification but a standard-compliant device may not depend on this.
In addition to limiting the total average power used by the device, the USB specification limits the inrush current (i.e., that used to charge decoupling and filter capacitors) when the device is first connected. Otherwise, connecting a device could cause problems with the host's internal power. USB devices are also required to automatically enter ultra low-power suspend mode when the USB host is suspended. Nevertheless, many USB host interfaces do not cut off the power supply to USB devices when they are suspended.[citation needed]
Some non-standard USB devices use the 5 V power supply without participating in a proper USB network, which negotiates power draw with the host interface. These are usually called USB decorations.[citation needed] Examples include USB-powered keyboard lights, fans, mug coolers and heaters, battery chargers, miniature vacuum cleaners, and even miniature lava lamps. In most cases, these items contain no digital circuitry, and thus are not standard compliant USB devices. This may cause problems with some computers, such as drawing too much current and damaging circuitry. Prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and communicate their current requirements to the host, which then permits the device to switch into high-power mode.
Some devices, when plugged into charging ports, draw even more power (10 watts or 2.1 amperes) than the Battery Charging Specification allows. The iPad and MiFi 2200 are two such devices.[101] Barnes & Noble NOOK Color devices also require a special charger that runs at 1.9 amperes.[102]
USB Power Delivery
In July 2012, the USB Promoters Group announced the finalization of the USB Power Delivery ("PD") specification, an extension that specifies using certified "PD aware" USB cables with standard USB type A and B connectors to deliver up to 100 W of power at 20 V. For a PD-aware cable with a Micro-USB connector the maximum power supported is up to 60 W at 20 V, 36 W at 12 V and 10 W at 5 V. In all cases, both host-to-device and device-to-host configurations are supported.[103]
The intent is to permit uniformly charging laptops, tablets, USB-powered disks and similarly higher power consumer electronics, as a natural extension of existing European and Chinese mobile telephone charging standards. This may also affect the way electric power used for small devices is transmitted and used in both residential and public buildings.[45][104]
The USB Power Delivery revision 2.0 specification has been released as part of the USB 3.1 specification,[105] which also covers the type C cable and connector. Among other changes, the new specification allows for an alternate physical layer that is not RF-based, therefore reducing the possibilities for RF interference; see USB 3.0 RF interference for more details.[106]
PoweredUSB
Main article: PoweredUSB
PoweredUSB is a proprietary extension that adds four additional pins supplying up to 6 A at 5 V, 12 V, or 24 V. It is commonly used in point of sale systems to power peripherals such as barcode readers, credit card terminals, and printers.
Signaling
USB allows the following signaling rates (the terms speed and bandwidth are used interchangeably, while "high-" is alternatively written as "hi-"):
A low-speed (USB 1.0) rate of 1.5 Mbit/s is defined by USB 1.0. It is very similar to full-bandwidth operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks.
The full-speed (USB 1.1) rate of 12 Mbit/s is the basic USB data rate defined by USB 1.0. All USB hubs can operate at this speed.
A high-speed (USB 2.0) rate of 480 Mbit/s was introduced in 2001. All hi-speed devices are capable of falling back to full-bandwidth operation if necessary; i.e., they are backward compatible with USB 1.1. Connectors are identical for USB 2.0 and USB 1.x.
A SuperSpeed (USB 3.0) rate of 5.0 Gbit/s. The written USB 3.0 specification was released by Intel and its partners in August 2008. The first USB 3.0 controller chips were sampled by NEC in May 2009,[107] and the first products using the USB 3.0 specification arrived in January 2010.[108] USB 3.0 connectors are generally backwards compatible, but include new wiring and full duplex operation.
USB signals are transmitted on a twisted-pair data cable with 90Ω ±15% characteristic impedance,[109] labeled D+ and D−. Prior to USB 3.0, these collectively use half-duplex differential signaling to reduce the effects of electromagnetic noise on longer lines. Transmitted signal levels are 0.0 to 0.3 volts for low and 2.8 to 3.6 volts for high in full-bandwidth and low-bandwidth modes, and −10 to 10 mV for low and 360 to 440 mV for high in hi-bandwidth mode. In FS mode, the cable wires are not terminated, but the HS mode has termination of 45 Ω to ground, or 90 Ω differential to match the data cable impedance, reducing interference due to signal reflections. USB 3.0 introduces two additional pairs of shielded twisted wire and new, mostly interoperable contacts in USB 3.0 cables, for them. They permit the higher data rate, and full duplex operation.
A USB connection is always between a host or hub at the "A" connector end, and a device or hub's "upstream" port at the other end. Originally, this was a "B" connector, preventing erroneous loop connections, but additional upstream connectors were specified, and some cable vendors designed and sold cables that permitted erroneous connections (and potential damage to circuitry). USB interconnections are not as fool-proof or as simple as originally intended.
The host includes 15 kΩ pull-down resistors on each data line. When no device is connected, this pulls both data lines low into the so-called "single-ended zero" state (SE0 in the USB documentation), and indicates a reset or disconnected connection.
A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the pull-down resistors in the host and leaves the data lines in an idle state called "J". For USB 1.x, the choice of data line indicates of what signal rates the device is capable; full-bandwidth devices pull D+ high, while low-bandwidth devices pull D− high. The "k" state is just the opposite polarity to the "j" state.

Example of a Negative Acknowledge packet transmitted by USB 1.1 Full-speed device when there is no more data to read. It consists of the following fields: clock synchronization byte, type of packet and end of packet. Data packets would have more information between the type of packet and end of packet.
USB data is transmitted by toggling the data lines between the J state and the opposite K state. USB encodes data using the NRZI line coding; a 0 bit is transmitted by toggling the data lines from J to K or vice-versa, while a 1 bit is transmitted by leaving the data lines as-is. To ensure a minimum density of signal transitions remains in the bitstream, USB uses bit stuffing; an extra 0 bit is inserted into the data stream after any appearance of six consecutive 1 bits. Seven consecutive received 1 bits is always an error. USB 3.0 has introduced additional data transmission encodings.
A USB packet begins with an 8-bit synchronization sequence '00000001'. That is, after the initial idle state J, the data lines toggle KJKJKJKK. The final 1 bit (repeated K state) marks the end of the sync pattern and the beginning of the USB frame. For high bandwidth USB, the packet begins with a 32-bit synchronization sequence.
A USB packet's end, called EOP (end-of-packet), is indicated by the transmitter driving 2 bit times of SE0 (D+ and D− both below max) and 1 bit time of J state. After this, the transmitter ceases to drive the D+/D− lines and the aforementioned pull up resistors hold it in the J (idle) state. Sometimes skew due to hubs can add as much as one bit time before the SE0 of the end of packet. This extra bit can also result in a "bit stuff violation" if the six bits before it in the CRC are '1's. This bit should be ignored by receiver.
A USB bus is reset using a prolonged (10 to 20 milliseconds) SE0 signal.
USB 2.0 devices use a special protocol during reset, called "chirping", to negotiate the high bandwidth mode with the host/hub. A device that is HS capable first connects as an FS device (D+ pulled high), but upon receiving a USB RESET (both D+ and D− driven LOW by host for 10 to 20 ms) it pulls the D− line high, known as chirp K. This indicates to the host that the device is high bandwidth. If the host/hub is also HS capable, it chirps (returns alternating J and K states on D− and D+ lines) letting the device know that the hub operates at high bandwidth. The device has to receive at least three sets of KJ chirps before it changes to high bandwidth terminations and begins high bandwidth signaling. Because USB 3.0 uses wiring separate and additional to that used by USB 2.0 and USB 1.x, such bandwidth negotiation is not required.
Clock tolerance is 480.00 Mbit/s ±500 ppm, 12.000 Mbit/s ±2500 ppm, 1.50 Mbit/s ±15000 ppm.
Though high bandwidth devices are commonly referred to as "USB 2.0" and advertised as "up to 480 Mbit/s", not all USB 2.0 devices are high bandwidth. The USB-IF certifies devices and provides licenses to use special marketing logos for either "basic bandwidth" (low and full) or high bandwidth after passing a compliance test and paying a licensing fee. All devices are tested according to the latest specification, so recently compliant low bandwidth devices are also 2.0 devices.
USB 3 uses tinned copper stranded AWG-28 cables with 90±7 Ω impedance for its high-speed differential pairs and linear feedback shift register and 8b/10b encoding sent with a voltage of 1 V nominal with a 100 mV receiver threshold; the receiver uses equalization.[110] SSC clock and 300 ppm precision is used. Packet headers are protected with CRC-16, while data payload is protected with CRC-32.[111] Power up to 3.6 W may be used. One unit load in superspeed mode is equal to 150 mA.[111]
Transmission rates
The theoretical maximum data rate in USB 2.0 is 480 Mbit/s (60 MB/s) per controller and is shared amongst all attached devices. Some chipset manufacturers overcome this bottleneck by providing multiple USB 2.0 controllers within the southbridge.
According to routine testing performed by CNet, write operations to typical Hi-Speed (USB 2.0) hard drives can sustain rates of 25–30 MB/s, while read operations are at 30–42 MB/s;[112] this is 70% of the total available bus bandwidth. For USB 3.0, typical write speed is 70-90 MB/s, while read speed is 90-110 MB/s.[112] Mask Tests, also known as Eye Diagram Tests, are used to determine the quality of a signal in the time domain. They are defined in the referenced document as part of the electrical test description for the high-speed (HS) mode at 480 Mbit/s.[113]
According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-Speed USB goes to overhead—the communication protocol between the card and the peripheral. Overhead is a component of all connectivity standards".[114] Tables illustrating the transfer limits are shown in Chapter 5 of the USB spec.
For isochronous devices like audio streams, the bandwidth is constant, and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the "speed" or latency of the transmission.
Latency
For USB1 low-speed (1.5 Mbit/s) and full-speed (12 Mbit/s) devices the shortest time for a transaction in one direction is 1 ms.[115] USB2 high-speed (480 Mbit/s) uses transactions within each micro frame (125 µs)[116] where using 1-byte interrupt packet results in a minimal response time of 940 ns. 4-byte interrupt packet results in 984 ns.[117]
Communication
During USB communication data is transmitted as packets. Initially, all packets are sent from the host, via the root hub and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.
After the sync field, all packets are made of 8-bit bytes, transmitted least-significant bit first. The first byte is a packet identifier (PID) byte. The PID is actually 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. This redundancy helps detect errors. (Note also that a PID byte contains at most four consecutive 1 bits, and thus never needs bit-stuffing, even when combined with the final 1 bit in the sync byte. However, trailing 1 bits in the PID may require bit-stuffing within the first few bits of the payload.)
Handshake packets
Handshake packets consist of only a single PID byte, and are generally sent in response to data packets. Error detection is provided by transmitting four bits that represent the packet type twice, in a single PID byte using complemented form. Three basic types are ACK, indicating that data was successfully received, NAK, indicating that the data cannot be received and should be retried, and STALL, indicating that the device has an error condition and cannot transfer data until some corrective action (such as device initialization) occurs.[118][119]
USB 2.0 added two additional handshake packets: NYET and ERR. NYET indicates that a split transaction is not yet complete, while ERR handshake indicates that a split transaction failed. A second use for a NYET packet is to tell the host that the device has accepted a data packet, but cannot accept any more due to full buffers. This allows a host to switch to sending small PING tokens to inquire about the device's readiness, rather than sending an entire unwanted DATA packet just to elicit a NAK.[118][119]
The only handshake packet the USB host may generate is ACK. If it is not ready to receive data, it should not instruct a device to send.
Token packets
Token packets consist of a PID byte followed by two payload bytes: 11 bits of address and a five-bit CRC. Tokens are only sent by the host, never a device.
IN and OUT tokens contain a seven-bit device number and four-bit function number (for multifunction devices) and command the device to transmit DATAx packets, or receive the following DATAx packets, respectively. An IN token expects a response from a device. The response may be a NAK or STALL response, or a DATAx frame. In the latter case, the host issues an ACK handshake if appropriate. An OUT token is followed immediately by a DATAx frame. The device responds with ACK, NAK, NYET, or STALL, as appropriate.
SETUP operates much like an OUT token, but is used for initial device setup. It is followed by an eight-byte DATA0 frame with a standardized format.
Every millisecond (12000 full-bandwidth bit times), the USB host transmits a special SOF (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous and interrupt data transfers. High-bandwidth USB 2.0 devices receive seven additional SOF tokens per frame, each introducing a 125 µs "microframe" (60000 high-bandwidth bit times each).
USB 2.0 added PING token, which asks a device if it is ready to receive an OUT/DATA packet pair. PING is usually sent by a host when polling a device that most recently responded with NAK or NYET. This avoids the need to send a large data packet to a device that the host suspects to be unwilling to accept it.[120] The device responds with ACK, NAK or STALL, as appropriate.
USB 2.0 also added a larger three-byte SPLIT token with a seven-bit hub number, 12 bits of control flags, and a five-bit CRC. This is used to perform split transactions. Rather than tie up the high-bandwidth USB bus sending data to a slower USB device, the nearest high-bandwidth capable hub receives a SPLIT token followed by one or two USB packets at high bandwidth, performs the data transfer at full or low bandwidth, and provides the response at high bandwidth when prompted by a second SPLIT token.
Data packets
A data packet consists of the PID followed by 0–1,024 bytes of data payload (up to 1,024 bytes for high-speed devices, up to 64 bytes for full-speed devices, and at most eight bytes for low-speed devices),[121] and a 16-bit CRC.
There are two basic forms of data packet, DATA0 and DATA1. A data packet must always be preceded by an address token, and is usually followed by a handshake token from the receiver back to the transmitter. The two packet types provide the 1-bit sequence number required by Stop-and-wait ARQ. If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.
To solve this problem, the device keeps track of the type of DATAx packet it last accepted. If it receives another DATAx packet of the same type, it is acknowledged but ignored as a duplicate. Only a DATAx packet of the opposite type is actually received.
If the data is corrupted while transmitted or received, the CRC check fails. When this happens, the receiver does not generate an ACK, which makes the sender resend the packet.[122]
When a device is reset with a SETUP packet, it expects an 8-byte DATA0 packet next.
USB 2.0 added DATA2 and MDATA packet types as well. They are used only by high-bandwidth devices doing high-bandwidth isochronous transfers that must transfer more than 1024 bits per 125 µs microframe (8,192 kB/s).
PRE packet
Low-bandwidth devices are supported with a special PID value, PRE. This marks the beginning of a low-bandwidth packet, and is used by hubs that normally do not send full-bandwidth packets to low-bandwidth devices. Since all PID bytes include four 0 bits, they leave the bus in the full-bandwidth K state, which is the same as the low-bandwidth J state. It is followed by a brief pause, during which hubs enable their low-bandwidth outputs, already idling in the J state. Then a low-bandwidth packet follows, beginning with a sync sequence and PID byte, and ending with a brief period of SE0. Full-bandwidth devices other than hubs can simply ignore the PRE packet and its low-bandwidth contents, until the final SE0 indicates that a new packet follows.
Comparisons with other connection methods

A variety of USB cables for sale in Hong Kong
FireWire
At first, USB was considered a complement to IEEE 1394 (FireWire) technology, which was designed as a high-bandwidth serial bus that efficiently interconnects peripherals such as disk drives, audio interfaces, and video equipment. In the initial design, USB operated at a far lower data rate and used less sophisticated hardware. It was suitable for small peripherals such as keyboards and pointing devices.
The most significant technical differences between FireWire and USB include:
USB networks use a tiered-star[clarification needed] topology, while IEEE 1394 networks use a tree topology.
USB 1.0, 1.1 and 2.0 use a "speak-when-spoken-to" protocol; peripherals cannot communicate with the host unless the host specifically requests communication. USB 3.0 allows for device-initiated communications towards the host. A FireWire device can communicate with any other node at any time, subject to network conditions.
A USB network relies on a single host at the top of the tree to control the network. In a FireWire network, any capable node can control the network.
USB runs with a 5 V power line, while FireWire in current implementations supplies 12 V and theoretically can supply up to 30 V.
Standard USB hub ports can provide from the typical 500 mA/2.5 W of current, only 100 mA from non-hub ports. USB 3.0 and USB On-The-Go supply 1.8 A/9.0 W (for dedicated battery charging, 1.5 A/7.5 W Full bandwidth or 900 mA/4.5 W High Bandwidth), while FireWire can in theory supply up to 60 watts of power, although 10 to 20 watts is more typical.
These and other differences reflect the differing design goals of the two buses: USB was designed for simplicity and low cost, while FireWire was designed for high performance, particularly in time-sensitive applications such as audio and video. Although similar in theoretical maximum transfer rate, FireWire 400 is faster than USB 2.0 Hi-Bandwidth in real-use,[123] especially in high-bandwidth use such as external hard-drives.[124][125][126][127] The newer FireWire 800 standard is twice as fast as FireWire 400 and faster than USB 2.0 Hi-Bandwidth both theoretically and practically.[128] However, Firewire's speed advantages rely on low-level techniques such as direct memory access (DMA), which in turn have created opportunities for security exploits such as the DMA attack.
The chipset and drivers used to implement USB and FireWire have a crucial impact on how much of the bandwidth prescribed by the specification is achieved in the real world, along with compatibility with peripherals.[129]
Ethernet
The IEEE 802.3af Power over Ethernet (PoE) standard specifies a more elaborate power negotiation scheme than powered USB. It operates at 48 V DC and can supply more power (up to 12.95 W, PoE+ 25.5 W) over a cable up to 100 meters compared to USB 2.0, which provides 2.5 W with a maximum cable length of 5 meters. This has made PoE popular for VoIP telephones, security cameras, wireless access points and other networked devices within buildings. However, USB is cheaper than PoE provided that the distance is short, and power demand is low.
Ethernet standards require electrical isolation between the networked device (computer, phone, etc.) and the network cable up to 1500 V AC or 2250 V DC for 60 seconds.[130] USB has no such requirement as it was designed for peripherals closely associated with a host computer, and in fact it connects the peripheral and host grounds. This gives Ethernet a significant safety advantage over USB with peripherals such as cable and DSL modems connected to external wiring that can assume hazardous voltages under certain fault conditions.[131]
MIDI
Digital musical instruments are another example where USB is competitive in low-cost devices. However Power over Ethernet and the MIDI plug standard have an advantage in high-end devices that may have long cables. USB can cause ground loop problems between equipment, because it connects ground references on both transceivers. By contrast, the MIDI plug standard and Ethernet have built-in isolation to 500V or more.
eSATA/eSATAp
The eSATA connector is a more robust SATA connector, intended for connection to external hard drives and SSDs. eSATA's transfer rate (up to 6 Gbit/s) is similar to that of USB 3.0 (up to 5 Gbit/s on current devices; 10 Gbit/s speeds via USB 3.1, announced on July 31, 2013). A device connected by eSATA appears as an ordinary SATA device, giving both full performance and full compatibility associated with internal drives.
eSATA does not supply power to external devices. This is an increasing disadvantage compared to USB. Even though USB 3.0's 4.5 W is sometimes insufficient to power external hard drives, technology is advancing and external drives gradually need less power, diminishing the eSATA advantage. eSATAp (power over eSATA; aka ESATA/USB) is a connector introduced in 2009 that supplies power to attached devices using a new, backwards-compatible, connector. On a notebook eSATAp usually supplies only 5 V to power a 2.5-inch HDD/SSD; on a desktop workstation it can additionally supply 12 V to power larger devices including 3.5-inch HDD/SSD and 5.25-inch optical drives.
eSATAp support can be added to a desktop machine in the form of a bracket connecting to motherboard SATA, power, and USB resources.
eSATA, like USB, supports hot plugging, although this might be limited by OS drivers and device firmware.
Thunderbolt
Thunderbolt combines PCI Express and Mini DisplayPort into a new serial data interface. Current Thunderbolt implementations have two channels, each with a transfer speed of 10 Gbit/s, resulting in an aggregate unidirectional bandwidth of 20 Gbit/s.[132]