|Produced||Since May 1996|
|Superseded||Serial port, parallel port, game port, Apple Desktop Bus, PS/2 port, and FireWire (IEEE 1394)|
Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad variety of USB hardware exists, including 14 different connector types, of which USB-C is the most recent and the only one not currently deprecated.
First released in 1996, the USB standards are maintained by the USB Implementers Forum (USB-IF). The four generations of USB are: USB 1.x, USB 2.0, USB 3.x, and USB4.
USB was designed to standardize the connection of peripherals to personal computers, both to communicate with and to supply electric power. It has largely replaced interfaces such as serial ports and parallel ports, and has become commonplace on a wide range of devices. Examples of peripherals that are connected via USB include computer keyboards and mice, video cameras, printers, portable media players, mobile (portable) digital telephones, disk drives, and network adapters.
USB connectors have been increasingly replacing other types as charging cables of portable devices.
Main article: USB hardware § Connectors
|Maximum transfer rate||12 Mbps||480 Mbps||5 Gbps||10 Gbps||20 Gbps||40 Gbps|
|Type A connector||Deprecated|
|Type B connector||Deprecated|
|Type C connector||Backwards compatibility only|
(Enlarged to show detail)
The Universal Serial Bus was developed to simplify and improve the interface between personal computers and peripheral devices, such as cell phones, computer accessories, and monitors, when compared with previously existing standard or ad hoc proprietary interfaces.
From the computer user's perspective, the USB interface improves ease of use in several ways:
The USB standard also provides multiple benefits for hardware manufacturers and software developers, specifically in the relative ease of implementation:
As with all standards, USB possesses multiple limitations to its design:
For a product developer, using USB requires the implementation of a complex protocol and implies an "intelligent" controller in the peripheral device. Developers of USB devices intended for public sale generally must obtain a USB ID, which requires that they pay a fee to the USB Implementers Forum (USB-IF). Developers of products that use the USB specification must sign an agreement with the USB-IF. Use of the USB logos on the product requires annual fees and membership in the organization.
A group of seven companies began the development of USB in 1995: Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel. 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 transfer rates for external devices and Plug and Play features. Ajay Bhatt and his team worked on the standard at Intel; the first integrated circuits supporting USB were produced by Intel in 1995.
As of 2008[update], about 6 billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
Released in January 1996, USB 1.0 specified signaling rates of 1.5 Mbit/s (Low Bandwidth or Low Speed) and 12 Mbit/s (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. USB 1.1 was the earliest revision that was widely adopted and led to what Microsoft designated the "Legacy-free PC".
Neither USB 1.0 nor 1.1 specified a design for any connector smaller than the standard type A or type B. Though many designs for a miniaturised type B connector appeared on many peripherals, conformity to the USB 1.x standard was hampered by treating peripherals that had miniature connectors as though they had a tethered connection (that is: no plug or receptacle at the peripheral end). There was no known miniature type A connector until USB 2.0 (revision 1.01) introduced one.
USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s (maximum theoretical data throughput 53 MByte/s) named High Speed or High Bandwidth, in addition to the USB 1.x Full Speed signaling rate of 12 Mbit/s (maximum theoretical data throughput 1.2 MByte/s).
Modifications to the USB specification have been made via engineering change notices (ECNs). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:
Main article: USB 3.0
The USB 3.0 specification was released on 12 November 2008, with its management transferring from USB 3.0 Promoter Group to the USB Implementers Forum (USB-IF), and announced on 17 November 2008 at the SuperSpeed USB Developers Conference.
USB 3.0 adds a SuperSpeed transfer mode, with associated backward compatible plugs, receptacles, and cables. SuperSpeed plugs and receptacles are identified with a distinct logo and blue inserts in standard format receptacles.
The SuperSpeed bus provides for a transfer mode at a nominal rate of 5.0 Gbit/s, in addition to the three existing transfer modes. Its efficiency is dependent on a number of factors including physical symbol encoding and link level overhead. At a 5 Gbit/s signaling rate with 8b/10b encoding, each byte needs 10 bits to transmit, so the raw throughput is 500 MB/s. When flow control, packet framing and protocol overhead are considered, it is realistic for 400 MB/s (3.2 Gbit/s) or more to transmit to an application.: 4–19 Communication is full-duplex in SuperSpeed transfer mode; earlier modes are half-duplex, arbitrated by the host.
Low-power and high-power devices remain operational with this standard, but devices using SuperSpeed can take advantage of increased available current of between 150 mA and 900 mA, respectively.: 9–9
USB 3.1, released in July 2013 has two variants. The first one preserves USB 3.0's SuperSpeed transfer mode and is labeled USB 3.1 Gen 1, and the second version introduces a new SuperSpeed+ transfer mode under the label of USB 3.1 Gen 2. SuperSpeed+ doubles the maximum data signaling rate to 10 Gbit/s, while reducing line encoding overhead to just 3% by changing the encoding scheme to 128b/132b.
USB 3.2, released in September 2017, preserves existing USB 3.1 SuperSpeed and SuperSpeed+ data modes but introduces two new SuperSpeed+ transfer modes over the USB-C connector with data rates of 10 and 20 Gbit/s (1.25 and 2.5 GB/s). The increase in bandwidth is a result of multi-lane operation over existing wires that were intended for flip-flop capabilities of the USB-C connector.
USB 3.0 also introduced the UASP protocol, which provides generally faster transfer speeds than the BOT (Bulk-Only-Transfer) protocol.
Starting with the USB 3.2 standard, USB-IF introduced a new naming scheme. To help companies with branding of the different transfer modes, USB-IF recommended branding the 5, 10, and 20 Gbit/s transfer modes as SuperSpeed USB 5Gbps, SuperSpeed USB 10Gbps, and SuperSpeed USB 20Gbps, respectively.
Main article: USB4
The USB4 specification was released on 29 August 2019 by the USB Implementers Forum.
USB4 is based on the Thunderbolt 3 protocol. It supports 40 Gbit/s throughput, is compatible with Thunderbolt 3, and backward compatible with USB 3.2 and USB 2.0. The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application.
The USB4 specification states that the following technologies shall be supported by USB4:
|USB 2.0 (480 Mbit/s)||Yes||Yes||Yes||Contrary to other functions—which use the multiplexing of high-speed links—USB 2.0 over USB-C utilizes its own differential pair of wires.|
|USB4 Gen 2×2 (20 Gbit/s)||Yes||Yes||Yes||A USB 3.0-labelled device still operates via a USB4 host or hub as a USB 3.0 device. The device requirement of Gen 2x2 applies only to the newcoming USB4-labelled devices.|
|USB4 Gen 3×2 (40 Gbit/s)||No||Yes||No|
|DisplayPort||Yes||Yes||No||The specification requires that hosts and hubs support the DisplayPort Alternate Mode.|
|Host-to-Host communications||Yes||Yes||—||A LAN-like connection between two peers.|
|PCI Express||No||Yes||No||The PCI Express function of USB4 replicates the functionality of previous versions of the Thunderbolt specification.|
|Thunderbolt 3||No||Yes||No||Thunderbolt 3 uses USB-C cables; the USB4 specification allows hosts and devices and requires hubs to support interoperability with the standard using the Thunderbolt 3 Alternate Mode.|
|Other Alternate Modes||No||No||No||USB4 products may optionally offer interoperability with the HDMI, MHL, and VirtualLink Alternate Modes.|
During CES 2020, USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4 products. The first products compatible with USB4 are expected to be Intel's Tiger Lake series and AMD's Zen 3 series of CPUs. Released in 2020.
USB4 Version 2.0
|Name||Release date||Maximum transfer rate||Note|
|USB 0.7||11 November 1994||?||Pre-release|
|USB 0.8||December 1994||?||Pre-release|
|USB 0.9||13 April 1995||Full Speed (12 Mbit/s)||Pre-release|
|USB 0.99||August 1995||?||Pre-release|
|USB 1.0-RC||November 1995||?||Release Candidate|
|USB 1.0||15 January 1996||Full Speed (12 Mbit/s),
Low Speed (1.5 Mbit/s)
|USB 1.1||August 1998|
|USB 2.0||April 2000||High Speed (480 Mbit/s)|
|USB 3.0||November 2008||SuperSpeed USB (5 Gbit/s)||Also referred to as USB 3.1 Gen 1 and USB 3.2 Gen 1 × 1|
|USB 3.1||July 2013||SuperSpeed+ USB (10 Gbit/s)||Includes new USB 3.1 Gen 2, also named USB 3.2 Gen 2 × 1 in later specifications. Last version to support Type A connector.|
|USB 3.2||August 2017||SuperSpeed+ USB dual-lane (20 Gbit/s)||Includes new USB 3.2 Gen 1 × 2 and Gen 2 × 2 multi-link modes. Requires Type C connector.|
|USB4||August 2019||40 Gbit/s (2-lane)||Includes new USB4 Gen 2 × 2 (64b/66b encoding) and Gen 3 × 2 (128b/132b encoding) modes and introduces USB4 routing for tunnelling of USB3.x, DisplayPort 1.4a and PCI Express traffic and host-to-host transfers, based on the Thunderbolt 3 protocol|
|Release name||Release date||Max. power||Note|
|USB Battery Charging Rev. 1.0||2007-03-08||7.5 W (5 V, 1.5 A)|
|USB Battery Charging Rev. 1.1||2009-04-15||7.5 W (5 V, 1.5 A)||Page 28, Table 5–2, but with limitation on paragraph 3.5. In ordinary USB 2.0's standard-A port, 1.5A only.|
|USB Battery Charging Rev. 1.2||2010-12-07||7.5 W (5 V, 1.5 A)|||
|USB Power Delivery Rev. 1.0 (V. 1.0)||2012-07-05||100 W (20 V, 5 A)||Using FSK protocol over bus power (VBUS)|
|USB Power Delivery Rev. 1.0 (V. 1.3)||2014-03-11||100 W (20 V, 5 A)|
|USB Type-C Rev. 1.0||2014-08-11||15 W (5 V, 3 A)||New connector and cable specification|
|USB Power Delivery Rev. 2.0 (V. 1.0)||2014-08-11||100 W (20 V, 5 A)||Using BMC protocol over communication channel (CC) on USB-C cables.|
|USB Type-C Rev. 1.1||2015-04-03||15 W (5 V, 3 A)|
|USB Power Delivery Rev. 2.0 (V. 1.1)||2015-05-07||100 W (20 V, 5 A)|
|USB Type-C Rev. 1.2||2016-03-25||15 W (5 V, 3 A)|
|USB Power Delivery Rev. 2.0 (V. 1.2)||2016-03-25||100 W (20 V, 5 A)|
|USB Power Delivery Rev. 2.0 (V. 1.3)||2017-01-12||100 W (20 V, 5 A)|
|USB Power Delivery Rev. 3.0 (V. 1.1)||2017-01-12||100 W (20 V, 5 A)|
|USB Type-C Rev. 1.3||2017-07-14||15 W (5 V, 3 A)|
|USB Power Delivery Rev. 3.0 (V. 1.2)||2018-06-21||100 W (20 V, 5 A)|
|USB Type-C Rev. 1.4||2019-03-29||15 W (5 V, 3 A)|
|USB Type-C Rev. 2.0||2019-08-29||15 W (5 V, 3 A)||Enabling USB4 over USB Type-C connectors and cables.|
|USB Power Delivery Rev. 3.0 (V. 2.0)||2019-08-29||100 W (20 V, 5 A)|||
|USB Power Delivery Rev. 3.1 (V. 1.0)||2021-05-24||240 W (48 V, 5 A)|
|USB Type-C Rev. 2.1||2021-05-25||15 W (5 V, 3 A)|||
|USB Power Delivery Rev. 3.1 (V. 1.1)||2021-07-06||240 W (48 V, 5 A)|||
|USB Power Delivery Rev. 3.1 (V. 1.2)||2021-10-26||240 W (48 V, 5 A)||Including errata through October 2021
This version incorporates the following ECNs:
A USB system consists of a host with one or more downstream ports, and multiple peripherals, forming a tiered-star topology. Additional USB hubs may be included, allowing up to five tiers. A USB host may have multiple controllers, each with one or more ports. Up to 127 devices may be connected to a single host controller.: 8–29 USB devices are linked in series through hubs. The hub built into the host controller is called the root hub.
A USB device may consist of several logical sub-devices that are referred to as device functions. A composite device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function). An alternative to this is a compound device, in which the host assigns each logical device a distinct address and all logical devices connect to a built-in hub that connects to the physical USB cable.
USB device communication is based on pipes (logical channels). A pipe is a connection from the host controller to a logical entity within a device, called an endpoint. Because pipes correspond to endpoints, the terms are sometimes used interchangeably. Each USB device can have up to 32 endpoints (16 in and 16 out), though it is rare to have so many. Endpoints are defined and numbered by the device during initialization (the period after physical connection called "enumeration") and so are relatively permanent, whereas pipes may be opened and closed.
There are two types of pipe: stream and message.
When a host starts a data transfer, it sends a TOKEN packet containing an endpoint specified with a tuple of (device_address, endpoint_number). If the transfer is from the host to the endpoint, the host sends an OUT packet (a specialization of a TOKEN packet) with the desired device address and endpoint number. If 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.
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. There may be one translator per hub or per port.
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.
The functionality of a USB device is defined by a class code sent to a USB host. This allows the host to load software modules for the device and to support new devices from different manufacturers.
Device classes include:
|Class||Usage||Description||Examples, or exception|
|00h||Device||Unspecified||Device class is unspecified, interface descriptors are used to determine needed drivers|
|01h||Interface||Audio||Speaker, microphone, sound card, MIDI|
|02h||Both||Communications and CDC control||UART and RS-232 serial adapter, Modem, Wi-Fi adapter, Ethernet adapter. Used together with class 0Ah (CDC-Data) below|
|03h||Interface||Human interface device (HID)||Keyboard, mouse, joystick|
|05h||Interface||Physical interface device (PID)||Force feedback joystick|
|07h||Interface||Printer||Laser printer, inkjet printer, CNC machine|
|08h||Interface||USB mass storage, USB Attached SCSI||USB flash drive, memory card reader, digital audio player, digital camera, external drive|
|09h||Device||USB hub||High speed USB hub|
|0Ah||Interface||CDC-Data||Used together with class 02h (Communications and CDC Control) above|
|0Bh||Interface||Smart Card||USB smart card reader|
|0Dh||Interface||Content security||Fingerprint reader|
|0Fh||Interface||Personal healthcare device class (PHDC)||Pulse monitor (watch)|
|10h||Interface||Audio/Video (AV)||Webcam, TV|
|11h||Device||Billboard||Describes USB-C alternate modes supported by device|
|DCh||Both||Diagnostic device||USB compliance testing device|
|E0h||Interface||Wireless Controller||Bluetooth adapter, Microsoft RNDIS|
|FEh||Interface||Application-specific||IrDA Bridge, Test & Measurement Class (USBTMC), USB DFU (Device Firmware Upgrade)|
|FFh||Both||Vendor-specific||Indicates that a device needs vendor-specific drivers|
USB mass storage device class (MSC or UMS) standardizes connections to storage devices. At first intended for magnetic and optical drives, it 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. 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 personal computers since early 2005 can boot from USB mass storage devices, USB is not intended as a primary bus for a computer's internal storage. However, USB has the advantage of allowing hot-swapping, making it useful for mobile peripherals, including drives of various kinds.
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 number and types of attached USB devices, and by the upper limit of the USB interface. Other competing standards for external drive connectivity include eSATA, ExpressCard, 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.
See also: Picture 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 – 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.
Main article: USB human interface device class
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 USB hardware in the 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 that connect PS/2 keyboards and mice (usually one of each) to a USB port also exist. 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 (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 to deploy firmware bug fixes. 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.
DFU can also give the user the freedom to flash USB devices with alternative firmware. One consequence of this is that USB devices after being re-flashed may act as various unexpected device types. For example, a USB device that the seller intends to be just a flash drive can "spoof" an input device like a keyboard. See BadUSB.
The USB Device Working Group has laid out specifications for audio streaming, and specific standards have been developed and implemented for audio class uses, such as microphones, speakers, headsets, telephones, musical instruments, etc. The working group has published three versions of audio device specifications: Audio 1.0, 2.0, and 3.0, referred to as "UAC" or "ADC".
UAC 3.0 primarily introduces improvements for portable devices, such as reduced power usage by bursting the data and staying in low power mode more often, and power domains for different components of the device, allowing them to be shut down when not in use.
UAC 2.0 introduced support for High Speed USB (in addition to Full Speed), allowing greater bandwidth for multi-channel interfaces, higher sample rates, lower inherent latency, and 8× improvement in timing resolution in synchronous and adaptive modes. UAC2 also introduced the concept of clock domains, which provides information to the host about which input and output terminals derive their clocks from the same source, as well as improved support for audio encodings like DSD, audio effects, channel clustering, user controls, and device descriptions.
UAC 1.0 devices are still common, however, due to their cross-platform driverless compatibility, and also partly due to Microsoft's failure to implement UAC 2.0 for over a decade after its publication, having finally added support to Windows 10 through the Creators Update on 20 March 2017. UAC 2.0 is also supported by MacOS, iOS, and Linux, however Android also only implements a subset of UAC 1.0.
USB provides three isochronous (fixed-bandwidth) synchronization types, all of which are used by audio devices:
While the USB spec originally described asynchronous mode being used in "low cost speakers" and adaptive mode in "high-end digital speakers", the opposite perception exists in the hi-fi world, where asynchronous mode is advertised as a feature, and adaptive/synchronous modes have a bad reputation. In reality, all the types can be high-quality or low-quality, depending on the quality of their engineering and the application. Asynchronous has the benefit of being untied from the computer's clock, but the disadvantage of requiring sample rate conversion when combining multiple sources.
Main article: USB hardware § Connectors
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 female connector mounted on the host or device is called the receptacle, and the male connector attached to the cable is called the plug.: 2–5 – 2–6 The official USB specification documents also periodically define the term male to represent the plug, and female to represent the receptacle.
The design is intended to make it difficult to insert a USB plug into its receptacle incorrectly. The USB specification requires that the cable plug and receptacle be marked so the user can recognize the proper orientation. The USB-C plug however is reversible. USB cables and small USB devices are held in place by the gripping force from the receptacle, with no screws, clips, or thumb-turns as some connectors use.
The different A and B plugs prevent accidentally connecting two power sources. 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.
USB connector types multiplied as the specification progressed. The original USB specification detailed standard-A and standard-B plugs and receptacles. The connectors were different so that users could not connect one computer receptacle to another. The data pins in the standard plugs are recessed compared to the power pins, so that the device can power up before establishing a data connection. Some devices operate in different modes depending on whether the data connection is made. Charging docks supply power and do not include a host device or data pins, allowing any capable USB device to charge or operate from a standard USB cable. Charging cables provide power connections, but not data. In a charge-only cable, the data wires are shorted at the device end, otherwise the device may reject the charger as unsuitable.
Main article: USB hardware § Cabling
The USB 1.1 standard specifies that a standard cable can have a maximum length of 5 meters (16 ft 5 in) with devices operating at full speed (12 Mbit/s), and a maximum length of 3 meters (9 ft 10 in) with devices operating at low speed (1.5 Mbit/s).
USB 2.0 provides for a maximum cable length of 5 meters (16 ft 5 in) for devices running at high speed (480 Mbit/s).
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 ft 10 in).
USB bridge cables, or data transfer cables can be found within the market, offering direct PC to PC connections. A bridge cable is a special cable with a chip and active electronics in the middle of the cable. The chip in the middle of the cable acts as a peripheral to both computers, and allows for peer-to-peer communication between the computers. The USB bridge cables are used to transfer files between two computers via their USB ports.
Popularized by Microsoft as Windows Easy Transfer, the Microsoft utility used a special USB bridge cable to transfer personal files and settings from a computer running an earlier version of Windows to a computer running a newer version. In the context of the use of Windows Easy Transfer software, the bridge cable can sometimes be referenced as Easy Transfer cable.
Many USB bridge / data transfer cables are still USB 2.0, but there are also a number of USB 3.0 transfer cables. Despite USB 3.0 being 10 times faster than USB 2.0, USB 3.0 transfer cables are only 2 - 3 times faster given their design.
The USB 3.0 specification introduced an A-to-A cross-over cable without power for connecting two PCs. These are not meant for data transfer but are aimed at diagnostic uses.
USB bridge cables have become less important with USB dual-role-device capabilities introduced with the USB 3.1 specification. Under the most recent specifications, USB supports most scenarios connecting systems directly with a Type-C cable. For the capability to work, however, connected systems must support role-switching. Dual-role capabilities requires there be two controllers within the system, as well as a role controller. While this can be expected in a mobile platform such as a tablet or a phone, desktop PCs and laptops often will not support dual roles.
Main article: USB hardware § Power
Upstream USB connectors supply power at a nominal 5V DC via the V_BUS pin to downstream USB devices.
Low-power devices may draw at most 1 unit load, and all devices must act as low-power devices when starting out as unconfigured. 1 unit load is 100 mA for USB devices up to USB 2.0, while USB 3.0 defines a unit load as 150 mA.
High-power devices (such as a typical 2.5-inch USB hard disk drive) draw at least 1 unit load and at most 5 unit loads (5x100mA = 500 mA) for devices up to USB 2.0 or 6 unit loads (6x150mA= 900 mA) for SuperSpeed (USB 3.0 and up) devices.
|Low-power device||100 mA||5 V[a]||0.50 W|
|Low-power SuperSpeed (USB 3.0) device||150 mA||5 V[a]||0.75 W|
|High-power device||500 mA[b]||5 V||2.5 W|
|High-power SuperSpeed (USB 3.0) device||900 mA[c]||5 V||4.5 W|
|Multi-lane SuperSpeed (USB 3.2 Gen 2) device||1.5 A[d]||5 V||7.5 W|
|Battery Charging (BC) 1.1||1.5 A||5 V||7.5 W|
|Battery Charging (BC) 1.2||1.5 A||5 V||7.5 W|
|USB-C||1.5 A||5 V||7.5 W|
|3 A||5 V||15 W|
|Power Delivery 1.0/2.0/3.0 Type-C||5 A[e]||20 V||100 W|
|Power Delivery 3.1 Type-C||5 A[e]||48 V[f]||240 W|
To recognize Battery Charging mode, a dedicated charging port places a resistance not exceeding 200 Ω across the D+ and D− terminals. Shorted or near-shorted data lanes with less than 200 Ω of resistance across the "D+" and "D−" terminals signify a dedicated charging port (DCP) with indefinite charging rates.
In addition to standard USB, there is a proprietary high-powered system known as PoweredUSB, developed in the 1990s, and mainly used in point-of-sale terminals such as cash registers.
Main article: USB (Communications) § Signaling (USB PHY)
USB signals are transmitted using differential signaling on a twisted-pair data wires with 90 Ω ± 15% characteristic impedance. USB 2.0 and earlier specifications define a single pair in half-duplex (HDx). USB 3.0 and later specifications define one pair for USB 2.0 compatibility and two or four pairs for data transfer: two pairs in full-duplex (FDx) for single lane variants (requires SuperSpeed connectors); four pairs in full-duplex for dual lane (×2) variants (requires USB-C connector).
|Rate Name||Old Name||First publication (Standard)||Encoding||Data pairs||Nominal
|Low-Speed||Does not appear||USB 1.0||NRZI||1 HDx||1.5 Mbit/s||Basic-Speed USB|
|High-Speed||USB 2.0||480 Mbit/s||Hi-Speed USB|
|USB 3.2 Gen 1×1||USB 3.0;
USB 3.1 Gen 1
|USB 3.0||8b/10b||2 FDx||5 Gbit/s||SuperSpeed USB 5Gbps|
|USB 3.2 Gen 2×1||USB 3.1 Gen 2||USB 3.1||128b/132b||2 FDx||10 Gbit/s||SuperSpeed USB 10Gbps|
|USB 3.2 Gen 1×2||Does not appear||USB 3.2||8b/10b||4 FDx ×2||10 Gbit/s||—|
|USB 3.2 Gen 2×2||128b/132b||4 FDx ×2||20 Gbit/s||SuperSpeed USB 20Gbps|
|USB4 Gen 2×1||USB4||64b/66b[a]||2 FDx||10 Gbit/s||—|
|USB4 Gen 2×2||64b/66b[a]||4 FDx ×2||20 Gbit/s||USB4 20Gbps|
|USB4 Gen 3×1||128b/132b[a]||2 FDx||20 Gbit/s||—|
|USB4 Gen 3×2||128b/132b[a]||4 FDx ×2||40 Gbit/s||USB4 40Gbps|
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.
Main article: USB (Communications) § Protocol layer
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.
Main article: USB (Communications) § Transaction
The basic transactions of USB are:
The USB Implementers Forum introduced the Media Agnostic USB v.1.0 wireless communication standard based on the USB protocol on July 29, 2015. Wireless USB is a cable-replacement technology, and uses ultra-wideband wireless technology for data rates of up to 480 Mbit/s.
The USB-IF used WiGig Serial Extension v1.2 specification as its initial foundation for the MA-USB specification, and is compliant with SuperSpeed USB (3.0 and 3.1) and Hi-Speed USB (USB 2.0). Devices that uses MA-USB will be branded as 'Powered by MA-USB', provided the product qualifies its certification program.
InterChip USB is a chip-to-chip variant that eliminates the conventional transceivers found in normal USB. The HSIC physical layer uses about 50% less power and 75% less board area compared to USB 2.0.
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:
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 high-bandwidth in real-use, especially in high-bandwidth use such as external hard drives. The newer FireWire 800 standard is twice as fast as FireWire 400 and faster than USB 2.0 high-bandwidth both theoretically and practically. 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.
The IEEE 802.3af, 802.3at, and 802.3bt Power over Ethernet (PoE) standards specify more elaborate power negotiation schemes than powered USB. They operate at 48 V DC and can supply more power (up to 12.95 W for 802.3af, 25.5 W for 802.3at aka PoE+, 71 W for 802.3bt aka 4PPoE) 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. 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.
The USB Device Class Definition for MIDI Devices transmits Music Instrument Digital Interface (MIDI) music data over USB. The MIDI capability is extended to allow up to sixteen simultaneous virtual MIDI cables, each of which can carry the usual MIDI sixteen channels and clocks.
USB is competitive for low-cost and physically adjacent 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.
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) and USB 3.1 (up to 10 Gbit/s). 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, backward 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 the motherboard SATA, power, and USB resources.
eSATA, like USB, supports hot plugging, although this might be limited by OS drivers and device firmware.
Thunderbolt combines PCI Express and Mini DisplayPort into a new serial data interface. Original Thunderbolt implementations have two channels, each with a transfer speed of 10 Gbit/s, resulting in an aggregate unidirectional bandwidth of 20 Gbit/s.
Thunderbolt 2 uses link aggregation to combine the two 10 Gbit/s channels into one bidirectional 20 Gbit/s channel.
Thunderbolt 3 uses the USB-C connector. Thunderbolt 3 has two physical 20 Gbit/s bi-directional channels, aggregated to appear as a single logical 40 Gbit/s bi-directional channel. Thunderbolt 3 controllers can incorporate a USB 3.1 Gen 2 controller to provide compatibility with USB devices. They are also capable of providing DisplayPort alternate mode over the USB-C connector, making a Thunderbolt 3 port a superset of a USB 3.1 Gen 2 port with DisplayPort alternate mode.
DisplayPort Alt Mode 2.0: USB 4 supports DisplayPort 2.0 over its alternative mode. DisplayPort 2.0 can support 8K resolution at 60 Hz with HDR10 color. DisplayPort 2.0 can use up to 80 Gbit/s, which is double the amount available to USB data, because it sends all the data in one direction (to the monitor) and can thus use all eight data lanes at once.
After the specification was made royalty-free and custodianship of the Thunderbolt protocol was transferred from Intel to the USB Implementers Forum, Thunderbolt 3 has been effectively implemented in the USB4 specification—with compatibility with Thunderbolt 3 optional but encouraged for USB4 products.
Main article: USB adapter
Various protocol converters are available that convert USB data signals to and from other communications standards.
In applications where streaming latency is important, UAC2 offers up to an 8x reduction over UAC1. ... Each clocking method has pros and cons and best-fit applications.
ADC-2 refers to the USB Device Class Definition for Audio Devices, Release 2.0.
All operating systems (Win, OSX, and Linux) support USB Audio Class 1 natively. This means you don't need to install drivers, it is plug&play.
Note that Full Speed USB has a much higher intrinsic latency of 2ms
Class 2 support enables much higher sample rates such as PCM 24 bit / 384 kHz and DSD (DoP) up through DSD256.
We now have native support for USB Audio 2.0 devices with an inbox class driver! This is an early version of the driver that does not have all features enabled
Synchronous sub-mode is not commonly used with audio because both host and peripheral are at the mercy of the USB clock.
The PCM2906C employs SpAct™ architecture, TI's unique system that recovers the audio clock from USB packet data.
Early USB replay interfaces used synchronous mode but acquired a reputation for poor quality of the recovered clock (and resultant poor replay quality). This was primarily due to deficiencies of clocking implementation rather than inherent shortcomings of the approach.
The fact that there is no clock line within the USB cable leads to a thinner cable, which is an advantage. But, no matter how good the crystal oscillators are at the send and receive ends, there will always be some difference between the two...
Synchronous USB DAC is the lowest quality of the three ... Adaptive ... means that there is no continuous, accurate master clock in the DAC, which causes jitter in the audio stream. ... Asynchronous – this is the most complex to implement but it is a huge improvement on the other types.
Synchronous is not used in a quality DAC as it is very jittery. ... asynchronous is the better of these modes.
Some manufacturers may lead you to believe that Asynchronous USB transfers are superior to Adaptive USB transfers and that therefore you must believe in the asynchronous solution. This no more true than saying that you "must" hold the fork in your left hand. In fact, if you know what you are doing, you will feed yourself with either hand. The issue is really about good engineering practices.