Universal Serial Bus (USB) is an
industry standard
''Industry Standard'' is the sixth studio album by the Dixie Dregs, released in 1982. This was the second of two albums released under the moniker The Dregs, and is their only album featuring vocals (by Alex Ligertwood of Santana and Patrick Si ...
that establishes specifications for cables, connectors and
protocols
Protocol may refer to:
Sociology and politics
* Protocol (politics), a formal agreement between nation states
* Protocol (diplomacy), the etiquette of diplomacy and affairs of state
* Etiquette, a code of personal behavior
Science and technology
...
for connection, communication and power supply (
interfacing
Interfacing is a textile used on the unseen or "wrong" side of fabrics to make an area of a garment more rigid.
Interfacings can be used to:
*stiffen or add body to fabric, such as the interfacing used in shirt collars
*strengthen a certain a ...
) between computers,
peripheral
A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...
s and other computers. A broad variety of
USB hardware
The initial versions of the USB standard specified connectors that were easy to use and that would have acceptable life spans; revisions of the standard added smaller connectors useful for compact portable devices. Higher-speed development of t ...
exists, including 14 different
connector types, of which
USB-C
USB-C (properly known as USB Type-C) is a 24-pin USB connector system with a rotationally symmetrical connector. The designation C refers only to the connector's physical configuration or form factor and should not be confused with the conne ...
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
The USB Implementers Forum (USB-IF) is a nonprofit organization created to promote and support USB (Universal Serial Bus). Its main activities are the promotion and marketing of USB, Wireless USB, USB On-The-Go, and the maintenance of the speci ...
(USB-IF). The four generations of USB are:
USB 1.''x'',
USB 2.0
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 ...
,
USB 3.''x'', and
USB4
USB4 (aka: USB 4.0) is a specification by the USB Implementers Forum (USB-IF), which was released in version 1.0 on 29 August 2019. The USB4 protocol is based on the Thunderbolt 3 protocol; the Thunderbolt 3 specification was donated to the USB- ...
.
Overview
USB was designed to standardize the connection of
peripheral
A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...
s to personal computers, both to communicate with and to supply electric power. It has largely replaced interfaces such as
serial port
In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. ...
s and
parallel port
In computing, a parallel port is a type of interface found on early computers (personal and otherwise) for connecting peripherals. The name refers to the way the data is sent; parallel ports send multiple bits of data at once ( parallel ...
s, 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.
Connector type quick reference
Objectives
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.
[Jan Axelson, ''USB Complete: The Developer's Guide, Fifth Edition'', Lakeview Research LLC, 2015, , pages 1-7]
From the computer user's perspective, the USB interface improves ease of use in several ways:
* The USB interface is self-configuring, eliminating the need for the user to adjust the device's settings for speed or data format, or configure
interrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
s, input/output addresses, or direct memory access channels.
* USB connectors are standardized at the host, so any peripheral can use most available receptacles.
* USB takes full advantage of the additional processing power that can be economically put into peripheral devices so that they can manage themselves. As such, USB devices often do not have user-adjustable interface settings.
* The USB interface is
hot-swappable
Hot swapping is the replacement or addition of components to a Computer, computer system without stopping, shutdown (computing), shutting down, or Reboot, rebooting the system; hot plugging describes the addition of components only. Components ...
(devices can be exchanged without rebooting the host computer).
* Small devices can be powered directly from the USB interface, eliminating the need for additional power supply cables.
* Because use of the USB logo is only permitted after
compliance testing, the user can have confidence that a USB device will work as expected without extensive interaction with settings and configuration.
* The USB interface defines protocols for recovery from common errors, improving reliability over previous interfaces.
[
* Installing a device that relies on the USB standard requires minimal operator action. When a user plugs a device into a port on a running computer, it either entirely automatically configures using existing ]device driver
In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and ot ...
s, or the system prompts the user to locate a driver, which it then installs and configures automatically.
The USB standard also provides multiple benefits for hardware manufacturers and software developers, specifically in the relative ease of implementation:
* The USB standard eliminates the requirement to develop proprietary interfaces to new peripherals.
* The wide range of transfer speeds available from a USB interface suits devices ranging from keyboards and mice up to streaming video interfaces.
* A USB interface can be designed to provide the best available latency for time-critical functions or can be set up to do background transfers of bulk data with little impact on system resources.
* The USB interface is generalized with no signal lines dedicated to only one function of one device.[
]
Limitations
As with all standards, USB possesses multiple limitations to its design:
* USB cables are limited in length, as the standard was intended for peripherals on the same table-top, not between rooms or buildings. However, a USB port can be connected to a gateway that accesses distant devices.
* USB data transfer rates are slower than those of other interconnects such as 100 Gigabit Ethernet
40 Gigabit Ethernet (40GbE) and 100 Gigabit Ethernet (100GbE) are groups of computer networking technologies for transmitting Ethernet frames at rates of 40 and 100 gigabits per second (Gbit/s), respectively. These technologies offer significantly ...
.
* USB has a strict tree network
A tree topology, or star-bus topology, is a hybrid network topology in which star networks are interconnected via bus networks. Tree networks are hierarchical, and each node
In general, a node is a localized swelling (a "knot") or a point of i ...
topology and master/slave protocol for addressing peripheral devices; those devices cannot interact with one another except via the host, and two hosts cannot communicate over their USB ports directly. Some extension to this limitation is possible through USB On-The-Go
USB On-The-Go (USB OTG or just OTG) is a specification first used in late 2001 that allows USB devices, such as Tablet computer, tablets or smartphones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, co ...
in, Dual-Role-Devices and protocol bridge
{{Use American English, date = March 2019
A protocol converter is a device used to convert standard or proprietary protocol of one device to the protocol suitable for the other device or tools to achieve the desired interoperability. Protocols are ...
.
* A host cannot broadcast signals to all peripherals at once—each must be addressed individually.
* While converters exist between certain legacy interfaces and USB, they might not provide a full implementation of the legacy hardware. For example, a USB-to-parallel-port converter might work well with a printer, but not with a scanner that requires bidirectional use of the data pins.
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
The USB Implementers Forum (USB-IF) is a nonprofit organization created to promote and support USB (Universal Serial Bus). Its main activities are the promotion and marketing of USB, Wireless USB, USB On-The-Go, and the maintenance of the speci ...
(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.[
]
History
A group of seven companies began the development of USB in 1995: Compaq
Compaq Computer Corporation (sometimes abbreviated to CQ prior to a 2007 rebranding) was an American information technology company founded in 1982 that developed, sold, and supported computers and related products and services. Compaq produced ...
, DEC, IBM, Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 seri ...
, Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
, NEC
is a Japanese multinational information technology and electronics corporation, headquartered in Minato, Tokyo. The company was known as the Nippon Electric Company, Limited, before rebranding in 1983 as NEC. It provides IT and network soluti ...
, and Nortel
Nortel Networks Corporation (Nortel), formerly Northern Telecom Limited, was a Canadian multinational telecommunications and data networking equipment manufacturer headquartered in Ottawa, Ontario, Canada. It was founded in Montreal, Quebec, ...
. 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
In computing, a plug and play (PnP) device or computer bus is one with a specification that facilitates the recognition of a hardware component in a system without the need for physical device configuration or user intervention in resolving resou ...
features. Ajay Bhatt
Ajay V. Bhatt is an Indian-born American computer architect who defined and developed several widely used technologies, including USB (Universal Serial Bus), Platform Power Management architecture and various chipset improvements.
Early life
Aft ...
and his team worked on the standard at Intel; the first integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s supporting USB were produced by Intel in 1995.
, about 6 billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
USB 1.x
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, 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
A legacy-free PC is a type of personal computer that lacks a floppy and/or optical disc drive, legacy ports, and an Industry Standard Architecture (ISA) bus (or sometimes, any internal expansion bus at all). According to Microsoft, "The basic goal ...
".
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
USB 2.0 was released in April 2000, adding a higher maximum signaling rate
In telecommunication, data signaling rate (DSR), also known as gross bit rate, is the aggregate rate at which data passes a point in the transmission path of a data transmission system.
# The DSR is usually expressed in bits per second.
# The d ...
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:
* ''Mini-A and Mini-B Connector''
* ''Micro-USB Cables and Connectors Specification 1.01''
* ''InterChip USB
InterChip USB (IC-USB), sometimes referred to as USB-IC or Inter-chip USB, is an addendum to the USB Implementers Forum (USB-IF) USB 2.0 specification. IC-USB is intended as a low-power variant of the standard physical USB interface, intended ...
Supplement''
* ''On-The-Go Supplement 1.3'' USB On-The-Go
USB On-The-Go (USB OTG or just OTG) is a specification first used in late 2001 that allows USB devices, such as Tablet computer, tablets or smartphones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, co ...
makes it possible for two USB devices to communicate with each other without requiring a separate USB host
* ''Battery Charging
A battery charger, recharger, or simply charger is a device that stores energy in a battery by running an electric current through it. The charging protocol (how much voltage or current for how long, and what to do when charging is complete) depen ...
Specification 1.1'' Added support for dedicated chargers, host chargers behaviour for devices with dead batteries
* ''Battery Charging Specification 1.2'': with increased current of 1.5 A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5 A
* ''Link Power Management Addendum ECN'', which adds a ''sleep'' power state
USB 3.x
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
In telecommunications, 8b/10b is a line code that maps 8-bit words to 10-bit symbols to achieve DC balance and bounded disparity, and at the same time provide enough state changes to allow reasonable clock recovery. This means that the diff ...
, 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. Communication is full-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
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.
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
In telecommunication, data signaling rate (DSR), also known as gross bit rate, is the aggregate rate at which data passes a point in the transmission path of a data transmission system.
# The DSR is usually expressed in bits per second.
# The d ...
to 10 Gbit/s, while reducing line encoding overhead to just 3% by changing the encoding scheme
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained co ...
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 new USB-C
USB-C (properly known as USB Type-C) is a 24-pin USB connector system with a rotationally symmetrical connector. The designation C refers only to the connector's physical configuration or form factor and should not be confused with the conne ...
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.
Naming scheme
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.
USB4
The USB4 specification was released on 29 August 2019 by the USB Implementers Forum.
USB4 is based on the Thunderbolt 3
Thunderbolt is the brand name of a hardware interface for the connection of external peripherals to a computer. It has been developed by Intel, in collaboration with Apple. It was initially marketed under the name Light Peak, and first sold as ...
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:
During CES 2020
CES (; formerly an initialism for Consumer Electronics Show) is an annual trade show organized by the Consumer Technology Association (CTA). Held in January at the Las Vegas Convention Center in Winchester, Nevada, United States, the event typi ...
, USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4
Thunderbolt is the brand name of a hardware interface for the connection of external peripherals to a computer. It has been developed by Intel, in collaboration with Apple. It was initially marketed under the name Light Peak, and first sold as ...
products. The first products compatible with USB4 are expected to be Intel's Tiger Lake
Tiger Lake is Intel's codename for the 11th generation Intel Core mobile processors based on the new Willow Cove Core microarchitecture, manufactured using Intel's third-generation 10 nm process node known as 10SF ("10 nm SuperFin"). Ti ...
series and AMD's Zen 3
Zen 3 is the codename for a CPU microarchitecture by AMD, released on November 5, 2020. It is the successor to Zen 2 and uses TSMC's 7 nm process, 7 nm process for the chiplets and GlobalFoundries's 14 nm process, 14 nm process for the I/O die on ...
series of CPUs. Released in 2020.
The USB4 2.0 specification was released on 1 September 2022 by the USB Implementers Forum.
Version history
Release versions
Power-related standards
System design
A USB system consists of a host with one or more downstream ports, and multiple peripherals, forming a tiered- star topology. Additional USB hub
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 v ...
s 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. 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
A webcam is a video camera which is designed to record or stream to a computer or computer network. They are primarily used in videotelephony, livestreaming and social media, and security. Webcams can be built-in computer hardware or peripheral d ...
(video device function) with a built-in microphone (audio device function). An alternative to this is a ''compound device A USB compound device contains an embedded USB hub and one or more non-removable USB devices. It may or may not have exposed downstream ports.
The internal USB hub may be a physical IC that connects to other ICs, or the hub and all functions may ...
,'' 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
An endpoint, end-point or end point may refer to:
* Endpoint (band), a hardcore punk band from Louisville, Kentucky
* Endpoint (chemistry), the conclusion of a chemical reaction, particularly for titration
* Outcome measure, a measure used as an e ...
''. 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.
* 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 for status responses from the device, 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
A sequence of events is isochronous if the events occur regularly, or at equal time intervals. The term ''isochronous'' is used in several technical contexts, but usually refers to the primary subject maintaining a constant period or interval ( ...
'', ''interrupt'', or ''bulk'' transfer:
*;Isochronous transfers: At some guaranteed data rate (for fixed-bandwidth streaming data) but with possible data loss (e.g., realtime audio or video)
*;Interrupt transfers: Devices that need guaranteed quick responses (bounded latency) such as pointing devices, mice
A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
, and keyboards
*;Bulk transfers: Large sporadic transfers using all remaining available bandwidth, but with no guarantees on bandwidth or latency (e.g., file transfers)
When a host starts a data transfer, it sends a TOKEN packet containing an endpoint specified with a tuple
In mathematics, a tuple is a finite ordered list (sequence) of elements. An -tuple is a sequence (or ordered list) of elements, where is a non-negative integer. There is only one 0-tuple, referred to as ''the empty tuple''. An -tuple is defi ...
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 driver
In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and ot ...
s 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.
Device classes
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:
USB mass storage / USB drive
The USB mass storage device class
The USB mass storage device class (also known as USB MSC or UMS) is a set of computing communications protocols, specifically a USB Device Class, defined by the USB Implementers Forum that makes a USB device accessible to a host computing device ...
(MSC or UMS) standardizes connections to storage devices. At first intended for magnetic and optical drives, it has been extended to support flash drives
A flash drive is a portable computer drive that uses flash memory. Flash drives are the larger memory modules consisting of a number of flash chips. A flash chip is used to read the contents of a single cell, but it can write entire block of cell ...
and SD card
Secure Digital, officially abbreviated as SD, is a proprietary non-volatile flash memory card format developed by the SD Association (SDA) for use in portable devices.
The standard was introduced in August 1999 by joint efforts between SanDis ...
readers. The ability to boot a write-locked SD card
Secure Digital, officially abbreviated as SD, is a proprietary non-volatile flash memory card format developed by the SD Association (SDA) for use in portable devices.
The standard was introduced in August 1999 by joint efforts between SanDis ...
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
Hot swapping is the replacement or addition of components to a computer system without stopping, shutting down, or rebooting the system; hot plugging describes the addition of components only. Components which have such functionality are said ...
, making it useful for mobile peripherals, including drives of various kinds.
Several manufacturers offer external portable USB hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
s, 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
SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard t ...
, ExpressCard, FireWire
IEEE 1394 is an interface standard for a serial bus for high-speed communications and isochronous real-time data transfer. It was developed in the late 1980s and early 1990s by Apple in cooperation with a number of companies, primarily Sony an ...
(IEEE 1394), and most recently Thunderbolt
A thunderbolt or lightning bolt is a symbolic representation of lightning when accompanied by a loud thunderclap. In Indo-European mythology, the thunderbolt was identified with the 'Sky Father'; this association is also found in later Hell ...
.
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.
Media Transfer Protocol
Media Transfer Protocol
The Media Transfer Protocol (MTP) is an extension to the Picture Transfer Protocol (PTP) communications protocol that allows media files to be transferred automatically to and from portable devices. Whereas PTP was designed for downloading pho ...
(MTP) was designed by Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
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
DRM may refer to:
Government, military and politics
* Defense reform movement, U.S. campaign inspired by Col. John Boyd
* Democratic Republic of Madagascar, a former socialist state (1975–1992) on Madagascar
* Direction du renseignement milita ...
features. MTP was designed for use with portable media player
A portable media player (PMP) (also including the related digital audio player (DAP)) is a portable consumer electronics device capable of storing and playing digital media such as audio, images, and video files. The data is typically stored o ...
s, but it has since been adopted as the primary storage access protocol of the Android operating system
Android is a mobile operating system based on a modified version of the Linux kernel and other open-source software, designed primarily for touchscreen mobile devices such as smartphones and tablets. Android is developed by a consortium of deve ...
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.
Human interface devices
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
The initial versions of the USB standard specified connectors that were easy to use and that would have acceptable life spans; revisions of the standard added smaller connectors useful for compact portable devices. Higher-speed development of t ...
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
A microcontroller (MCU for ''microcontroller unit'', often also MC, UC, or μC) is a small computer on a single VLSI integrated circuit (IC) chip. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable i ...
to perform bidirectional data translation between the two standards.
Device Firmware Upgrade mechanism
''Device Firmware Upgrade'' (DFU) is a vendor- and device-independent mechanism for upgrading the firmware
In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...
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
A promenade dance, commonly called a prom, is a dance party for high school students. It may be offered in semi-formal black tie or informal suit for boys, and evening gowns for girls. This event is typically held near the end of the school yea ...
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
BadUSB is a computer security attack using USB devices that are programmed with malicious software. For example, USB flash drives can contain a programmable Intel 8051 microcontroller, which can be reprogrammed, turning a USB flash drive into a ...
.
Audio streaming
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
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
's failure to implement UAC 2.0 for over a decade after its publication, having finally added support to Windows 10
Windows 10 is a major release of Microsoft's Windows NT operating system. It is the direct successor to Windows 8.1, which was released nearly two years earlier. It was released to manufacturing on July 15, 2015, and later to retail on J ...
through the Creators Update on 20 March 2017. UAC 2.0 is also supported by MacOS
macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and lapt ...
, iOS
iOS (formerly iPhone OS) is a mobile operating system created and developed by Apple Inc. exclusively for its hardware. It is the operating system that powers many of the company's mobile devices, including the iPhone; the term also includes ...
, and Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...
, 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:
* Asynchronous The ADC or DAC are not synced to the host computer's clock at all, operating off a free-running clock local to the device.
* Synchronous The device's clock is synced to the USB start-of-frame (SOF) or Bus Interval signals. For instance, this can require syncing an 11.2896 MHz clock to a 1 kHz SOF signal, a large frequency multiplication.
* Adaptive The device's clock is synced to the amount of data sent per frame by the host
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
Sample-rate conversion, sampling-frequency conversion or resampling is the process of changing the sampling rate or sampling frequency of a discrete signal to obtain a new discrete representation of the underlying continuous signal. Application ...
when combining multiple sources.
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''. 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
USB On-The-Go (USB OTG or just OTG) is a specification first used in late 2001 that allows USB devices, such as Tablet computer, tablets or smartphones, to act as a host, allowing other USB devices, such as USB flash drives, digital cameras, co ...
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.
Cabling
The USB 1.1 standard specifies that a standard cable can have a maximum length of with devices operating at full speed (12 Mbit/s), and a maximum length of with devices operating at low speed (1.5 Mbit/s).
USB 2.0 provides for a maximum cable length of 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 .
USB bridge cables
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
Windows Easy Transfer is a specialized file transfer program developed by Microsoft that allows users of the Windows operating system to transfer personal files and settings from a computer running an earlier version of Windows to a computer runn ...
, 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.
Dual-role USB connections
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.
Power
Upstream USB connectors supply power at a nominal 5V DC via the V_BUS pin to downstream USB devices.
Low-power and high-power 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.
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
PoweredUSB, also known as Retail USB, USB PlusPower, USB +Power, and USB Power Plus, is an addition to the Universal Serial Bus standard that allows for higher-power devices to obtain power through their USB host instead of requi ...
, developed in the 1990s, and mainly used in point-of-sale terminals such as cash registers.
Signaling
USB signals are transmitted using differential signaling
Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conducto ...
on a twisted-pair
Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...
data wires with characteristic impedance
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in ...
. USB 2.0 and earlier specifications define a single pair in half-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
(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).
* Low-speed (LS) and Full-speed (FS) modes use a single data pair, labelled D+ and D−, in half-duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
. Transmitted signal levels are for logical low, and for logical high level. The signal lines are not terminated.
* High-speed (HS) mode uses the same wire pair, but with different electrical conventions. Lower signal voltages of for low and for logical high level, and termination of 45 Ω to ground or 90 Ω differential to match the data cable impedance.
* SuperSpeed (SS) adds two additional pairs of shielded twisted wire (and new, mostly compatible expanded connectors). These are dedicated to full-duplex SuperSpeed operation. The SuperSpeed link operates independently from USB 2.0 channel, and takes a precedence on connection. Link configuration is performed using LFPS (Low Frequency Periodic Signalling, approximately at 20 MHz frequency), and electrical features include voltage de-emphasis at transmitter side, and adaptive linear equalization on receiver side to combat electrical losses in transmission lines, and thus the link introduces the concept of ''link training''.
* SuperSpeed+ (SS+) uses increased data rate (Gen 2×1 mode) and/or the additional lane in the USB-C connector (Gen 1×2 and Gen 2×2 mode).
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.
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.
Transactions
The basic transactions of USB are:
* OUT transaction
* IN transaction
* SETUP transaction
* Control transfer exchange
Related standards
Media Agnostic USB
The USB Implementers Forum introduced the Media Agnostic USB (MA-USB) v.1.0 wireless communication standard based on the USB protocol on July 29, 2015. Wireless USB
Wireless USB (Universal Serial Bus) was a short-range, high-bandwidth wireless radio communication protocol created by the Wireless USB Promoter Group which intended to increase the availability of general USB-based technologies. It was unrelat ...
is a cable-replacement technology, and uses ultra-wideband
Ultra-wideband (UWB, ultra wideband, ultra-wide band and ultraband) is a radio technology that can use a very low energy level for short-range, high-bandwidth communications over a large portion of the radio spectrum. UWB has traditional applicati ...
wireless technology
Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The most ...
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
InterChip USB is a chip-to-chip variant that eliminates the conventional transceivers found in normal USB. The HSIC physical layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
The ...
uses about 50% less power and 75% less board
Board or Boards may refer to:
Flat surface
* Lumber, or other rigid material, milled or sawn flat
** Plank (wood)
** Cutting board
** Sounding board, of a musical instrument
* Cardboard (paper product)
* Paperboard
* Fiberboard
** Hardboard, a ty ...
area compared to USB 2.0.
USB-C
USB-C (officially ''USB Type-C'') is a standard that defines a new connector, and several new connection features. Among them it supports ''Alternate Mode'', which allows transporting other protocols via the USB-C connector and cable. This is commonly used to support the DisplayPort
DisplayPort (DP) is a digital display interface developed by a consortium of PC and chip manufacturers and standardized by the Video Electronics Standards Association (VESA). It is primarily used to connect a video source to a display device su ...
or HDMI
High-Definition Multimedia Interface (HDMI) is a proprietary audio/video interface for transmitting uncompressed video data and compressed or uncompressed digital audio data from an HDMI-compliant source device, such as a display controller, ...
protocols, which allows connecting a display, such as a computer monitor
A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a visual display, support electronics, power supply, housing, electrical connectors, and external user controls.
The di ...
or television set
A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...
, via USB-C.
DisplayLink
DisplayLink is a technology which allows multiple displays to be connected to a computer via USB. It was introduced around 2006, and before the advent of Alternate Mode over USB-C it was the only way to connect displays via USB. It is a proprietary technology, not standardized by the USB Implementers Forum and typically requires a separate device driver
In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and ot ...
on the computer.
Comparisons with other connection methods
IEEE 1394
At first, USB was considered a complement to IEEE 1394
IEEE 1394 is an interface standard for a serial bus for high-speed communications and isochronous real-time data transfer. It was developed in the late 1980s and early 1990s by Apple in cooperation with a number of companies, primarily Sony an ...
(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 topology, while IEEE 1394 networks use a tree
In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
topology.
* USB 1.0, 1.1, and 2.0 use a "speak-when-spoken-to" protocol, meaning that each peripheral communicates with the host when the host specifically requests it to communicate. 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. All communications are between the host and one peripheral. In a FireWire network, any capable node can control the network.
* USB runs with a 5 V power line, while FireWire 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 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
Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU).
Without DMA, when the CPU is using programmed input/output, it is t ...
(DMA), which in turn have created opportunities for security exploits such as the DMA attack
A DMA attack is a type of side channel attack in computer security, in which an attacker can penetrate a computer or other device, by exploiting the presence of high-speed expansion ports that permit direct memory access (DMA).
DMA is included in ...
.
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.
Ethernet
The ''IEEE 802.3af'', ''802.3at'', and ''802.3bt'' Power over Ethernet
Power over Ethernet, or PoE, describes any of several standards or ad hoc systems that pass electric power along with data on twisted-pair Ethernet cabling. This allows a single cable to provide both data connection and electrical power to d ...
(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
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet t ...
telephones, security camera
A closed-circuit television camera can produce images or recordings for surveillance or other private purposes. Cameras can be either video cameras, or digital stills cameras. Walter Bruch was the inventor of the CCTV camera. The main purpose o ...
s, wireless access point
In computer networking, a wireless access point (WAP), or more generally just access point (AP), is a networking hardware device that allows other Wi-Fi devices to connect to a wired network. As a standalone device, the AP may have a wired co ...
s, and other networked devices within buildings. However, USB is cheaper than PoE provided that the distance is short and power demand is low.
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
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.
MIDI
The ''USB Device Class Definition for MIDI Devices'' transmits Music Instrument Digital Interface (MIDI
MIDI (; Musical Instrument Digital Interface) is a technical standard that describes a communications protocol, digital interface, and electrical connectors that connect a wide variety of electronic musical instruments, computers, and re ...
) 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
MIDI (; Musical Instrument Digital Interface) is a technical standard that describes a communications protocol, digital interface, and electrical connectors that connect a wide variety of electronic musical instruments, computers, and re ...
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
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
have built-in isolation to or more.
eSATA/eSATAp
The eSATA
SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard t ...
connector is a more robust SATA
SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard to ...
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
In computing, eSATAp (also known as Power over eSATA, Power eSATA, eSATA/USB Combo, eSATA USB Hybrid Port/EUHP) is a combination connection for external storage devices. An eSATA or USB device can be plugged into an eSATAp port. The socke ...
(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
Hot swapping is the replacement or addition of components to a computer system without stopping, shutting down, or rebooting the system; hot plugging describes the addition of components only. Components which have such functionality are said ...
, although this might be limited by OS drivers and device firmware.
Thunderbolt
Thunderbolt
A thunderbolt or lightning bolt is a symbolic representation of lightning when accompanied by a loud thunderclap. In Indo-European mythology, the thunderbolt was identified with the 'Sky Father'; this association is also found in later Hell ...
combines PCI Express
PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common ...
and Mini DisplayPort
The Mini DisplayPort (MiniDP or mDP) is a miniaturized and less common version of the DisplayPort audio-visual digital interface.
It was announced by Apple in October 2008, and by early 2013 all new Apple Macintosh computers had Mini DisplayP ...
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
Thunderbolt is the brand name of a Interface (computing)#Hardware interfaces, hardware interface for the connection of external peripherals to a computer. It has been developed by Intel, in collaboration with Apple Inc., Apple. It was initially ...
uses link aggregation to combine the two 10 Gbit/s channels into one bidirectional 20 Gbit/s channel.
Thunderbolt 3
Thunderbolt is the brand name of a hardware interface for the connection of external peripherals to a computer. It has been developed by Intel, in collaboration with Apple. It was initially marketed under the name Light Peak, and first sold as ...
uses the USB-C
USB-C (properly known as USB Type-C) is a 24-pin USB connector system with a rotationally symmetrical connector. The designation C refers only to the connector's physical configuration or form factor and should not be confused with the conne ...
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.
]
Interoperability
Various protocol converter
{{Use American English, date = March 2019
A protocol converter is a device used to convert standard or proprietary protocol of one device to the protocol suitable for the other device or tools to achieve the desired interoperability. Protocols are ...
s are available that convert USB data signals to and from other communications standards.
Security threats
* USB Killer
* Legacy versions of Windows would, by default, autorun USB flash drives that were inserted. This was disabled in Windows XP.
See also
USB
* USB hardware
The initial versions of the USB standard specified connectors that were easy to use and that would have acceptable life spans; revisions of the standard added smaller connectors useful for compact portable devices. Higher-speed development of t ...
* USB protocol
* USB-C
USB-C (properly known as USB Type-C) is a 24-pin USB connector system with a rotationally symmetrical connector. The designation C refers only to the connector's physical configuration or form factor and should not be confused with the conne ...
* USB hub
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 v ...
* Extensible Host Controller Interface
eXtensible Host Controller Interface (xHCI) is a computer interface specification that defines a register-level description of a host controller for Universal Serial Bus (USB), which is capable of interfacing with USB 1.x, 2.0, and 3.x compatible ...
(XHCI)
* List of device bit rates#Peripheral
* WebUSB
WebUSB is a JavaScript application programming interface (API) specification for securely providing access to USB devices from web pages.
It was published by the Web Platform Incubator Community Group. As of July 2021, it is in Draft Community s ...
Derived and related standards
* DockPort
DockPort (originally codenamed ''Lightning Bolt'') is a backward-compatible extension of DisplayPort, adding USB 3.0 and DC power, in addition to DisplayPort's video and audio signalling. Standardised by VESA, it is the first royalty-free indust ...
* Windows Easy Transfer
Windows Easy Transfer is a specialized file transfer program developed by Microsoft that allows users of the Windows operating system to transfer personal files and settings from a computer running an earlier version of Windows to a computer runn ...
* LIO Target
In computing, Linux-IO (LIO) Target is an open-source implementation of the SCSI target that has become the standard one included in the Linux kernel. Internally, LIO does not initiate sessions, but instead provides one or more Logical Unit Nu ...
* Media Transfer Protocol
The Media Transfer Protocol (MTP) is an extension to the Picture Transfer Protocol (PTP) communications protocol that allows media files to be transferred automatically to and from portable devices. Whereas PTP was designed for downloading pho ...
* Mobile High-Definition Link
Mobile High-Definition Link (MHL) is an industry standard for a mobile audio/video interface that allows the connection of smartphones, tablets, and other portable consumer electronics devices to high-definition televisions (HDTVs), audio receive ...
* Thunderbolt (interface)
Thunderbolt is the brand name of a Interface (computing)#Hardware interfaces, hardware interface for the connection of external peripherals to a computer. It has been developed by Intel, in collaboration with Apple Inc., Apple. It was initially ...
References
Further reading
*
*
*
*
*
External links
General overview
*
*
*
*
Technical documents
*
*
*
*
*
*
*
* International Electrotechnical Commission, IEC 62680 (Universal Serial Bus interfaces for data and power):
*
IEC 62680-1.1:2015 - Part 1-1: Common components - USB Battery Charging Specification, Revision 1.2
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IEC 62680-1-2:2018 - Part 1-2: Common components - USB Power Delivery specification
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IEC 62680-1-3:2018 - Part 1-3: Common components - USB Type-C Cable and Connector Specification
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IEC 62680-1-4:2018 - Part 1-4: Common components - USB Type-C Authentication Specification
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IEC 62680-2-1:2015 - Part 2-1: Universal Serial Bus Specification, Revision 2.0
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IEC 62680-2-2:2015 - Part 2-2: Micro-USB Cables and Connectors Specification, Revision 1.01
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IEC 62680-2-3:2015 - Part 2-3: Universal Serial Bus Cables and Connectors Class Document Revision 2.0
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IEC 62680-3-1:2017 - Part 3-1: Universal Serial Bus 3.1 Specification
{{Authority control
USB,
American inventions
Computer buses
Computer connectors
Computer-related introductions in 1996
Japanese inventions
Physical layer protocols
Serial buses