USB

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https://en.wikipedia.org/wiki/USB

USB
Universal Serial Bus

Upper image: Certified logo. Lower image: Various USB connectors (From left to right: male Micro USB B-Type, proprietary UC-E6, male Mini USB (5-pin) B-type, female A-type, male A-type, male B-type. Shown with a centimeter ruler.)

Universal Serial Bus (USB) is an industry standard that specifies the physical interfaces and protocols for connecting, data transferring and powering of hosts, such as personal computers, peripherals, e.g. keyboards and mobile devices, and intermediate hubs. USB was designed to standardize the connection of peripherals to computers, replacing various interfaces such as serial ports, parallel ports, game ports, and ADB ports. It has become commonplace on a wide range of devices, such as keyboards, mice, cameras, printers, scanners, flash drives, smartphones, game consoles, and power banks.

As of 2023, USB consists of four generations of specifications: USB 1.‘‘x’’, USB 2.0, USB 3.‘‘x’’, and USB4. Since USB4 the specification enhances the data transfer and power supply functionality with

connection-oriented, tunneling architecture designed to combine multiple protocols onto a single physical interface, so that the total speed and performance of the USB4 Fabric can be dynamically shared.

USB4 particularly supports the tunneling of the Thunderbolt 3 protocols, namely PCI Express (PCIe, load/store interface) and DisplayPort. USB4 also adds host-to-host interfaces.

Each specification subversion supports different maximum signaling rates from 1.5 Mbit/s in USB 1.0 to 80 Gbit/s in USB4. USB also provides power supply to peripheral devices; the latest versions of the standard extend the power delivery limits for battery charging and devices requiring up to 240 watts (USB Power Delivery (USB-PD)). Over the years USB(-PD) has been adopted as the standard power supply and charging format for many mobile devices, such as mobile phones, reducing the need for proprietary chargers.

USB connector interfaces are classified into three types: A (host), B (peripheral), and C (2014, replaces A and B). The A and B types know different sizes: Standard, Mini, and Micro. The standard size is the largest and is mainly used for desktop and larger peripheral equipment. The mini size was introduced for mobile devices, but it was replaced by the thinner micro size. The micro size is nowadays the most common for smartphones and tablets. The USB Type-C connector interface is the newest and the only one applicable to USB4. It is reversible and can support various functionalities and protocols; some are mandatory, many just optional, and depending on the type of the device: host, peripheral device, or hub.

Since the USB 1.1 specification fully replaces the USB 1.0 specification, and since the USB 3.2 specification fully replaces the USB 3.1 (and therefore the USB 3.0 specification as well), and since USB 2.0 is backward-compatible with USB 1.0/1.1, and since the USB 3.x specifications include the USB 2.0 specification, and since USB4 "functionally replaces" the USB 3.2 specification "while retaining USB 2.0 bus operating in parallel", backward-compatibility is always given, but obviously always comes along with a decrease in, both, signaling rates and power rates, and less supported functionalities.

The USB 3.0 specification defined a new architecture and protocol, named SuperSpeed (aka SuperSpeed USB, marketed as SS), which included a new lane for a new signal coding scheme (8b/10b symbols, 5 Gbps; also known as Gen 1) providing full-duplex data transfers that physically required five additional wires and pins, while preserving the USB 2.0-architectur and -protocols and therefore keeping the original 4 pins/wires for the USB 2.0 backward-compatibility resulting in 9 wires (with 9 or 10 pins at connector interfaces; ID-pin is not wired) in total. The USB 3.1 specification introduced an Enhanced SuperSpeed-architecture – while preserving the SuperSpeed-architecture and -protocol – with an additional SuperSpeedPlus-architecture adding a new coding schema (128b/132b symbols, 10 Gbps; also known as Gen 2) and protocol named SuperSpeedPlus (aka SuperSpeedPlus USB, for some time period marketed as SS+). The USB 3.2 specification even added an additional second lane to the Enhanced SuperSpeed-architecture besides other enhancements, so that SuperSpeedPlus USB implements the Gen 1x2, Gen 2x1 and Gen 2x2 operation modes. The SuperSpeed-architecture and -protocol (aka SuperSpeed USB) still implements the one-lane Gen 1x1 operation mode. Therefore, two-lane operations, namely USB 3.2 Gen 1x2 (10 Gbit/s) and Gen 2x2 (20 Gbit/s), are only possible with Full-Featured USB Type-C fabrics (24 pins). As of 2023, they are hardly yet implemented by most products so far. On the other hand, USB 3.2 Gen 1(x1) (5 Gbit/s) and Gen 2(x1) (10 Gbit/s) implementations are quite common now for some years.

Overview

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.

Connector type quick reference

Main article: USB hardware § Connectors

Each USB connection is made using two connectors: a socket (or receptacle) and a plug. In the following table, schematics for only the sockets are shown, although for each there is a corresponding plug (or plugs).

Available connectors by USB standard

Standard	USB 1.0 1996	USB 1.1 1998	USB 2.0 2001	USB 2.0 Revised	USB 3.0 2008	USB 3.1 2013	USB 3.2 2017	USB4 2019	USB4 V2.0 2022
Maximum signaling rate	Low-Speed & Full-Speed		High-Speed		SS (Gen 1)	SS+ (Gen 2)	USB 3.2 Gen 2x2	USB4 Gen 3×2	USB4 Gen 4
	1.5 Mbit/s & 12 Mbit/s		480 Mbit/s		5 Gbit/s	10 Gbit/s	20 Gbit/s	40 Gbit/s	80 Gbit/s
Standard A connector							[rem 1]	—
Standard B connector							[rem 1]	—
Mini-A connector	—				—
Mini-B connector	—				—
Mini-AB connector	—				—
Micro-A connector	—							—
Micro-B connector	—							—
Micro-AB connector	—							—
Type-C connector	Backward compatibility given by USB 2.0 implementation		(Enlarged to show detail)

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.

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 interrupts, 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 (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 drivers, 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.
• USB has a strict tree network 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 in, Dual-Role-Devices and protocol bridge.
• 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 (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

The basic USB trident logo

USB logo on the head of a standard USB-A plug

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.

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".

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 miniaturized 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

The Hi-Speed USB logo

A USB 2.0 PCI expansion card

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:

• Mini-A and Mini-B Connector
• Micro-USB Cables and Connectors Specification 1.01
• InterChip USB Supplement
• On-The-Go Supplement 1.3 USB On-The-Go makes it possible for two USB devices to communicate with each other without requiring a separate USB host
• Battery Charging Specification 1.1 Added support for dedicated chargers, host chargers behavior 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

Main article: USB 3.0

The SuperSpeed USB logo

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 operation 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 an operation mode at a rate of 5.0 Gbit/s, in addition to the three existing operation 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. Communication is full-duplex in SuperSpeed operation mode; earlier modes are half-duplex, arbitrated by the host.

USB-A 3.1 Gen 1 (formerly known as USB 3.0; later renamed USB 3.2 Gen 1x1) ports

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 operation mode and is labeled USB 3.1 Gen 1, and the second version introduces a new SuperSpeed+ operation 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+ operation modes but introduces two new SuperSpeed+ operation modes with the new USB-C Fabrics with signaling 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 USB Attached SCSI protocol (UASP), 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 operation modes, USB-IF recommended branding the 5, 10, and 20 Gbit/s operation modes as SuperSpeed USB 5Gbps, SuperSpeed USB 10Gbps, and SuperSpeed USB 20Gbps, respectively. As of September 2022, this naming scheme is deprecated.

USB4

Main article: USB4

 

The certified USB4 40Gbps logo and trident logo

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:

Connection	Mandatory for			Remarks
	host	hub	device
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.
Tunneled USB 3.2 Gen 2x1 (10 Gbit/s)	Yes	Yes	No
Tunneled USB 3.2 Gen 2x2 (20 Gbit/s)	No	No	No
Tunneled USB3 Gen T (5-80 Gbit/s)	No	No	No	A type of USB3 Tunneling architecture where the Enhanced SuperSpeed protocol is extended to allow operation at the maximum bandwidth available on the USB4 Link.
USB4 Gen 2 (10 or 20 Gbit/s)	Yes	Yes	Yes	Either one or two lanes
USB4 Gen 3 (20 or 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 CPU, released in 2020.

The USB4 2.0 specification was released on 1 September 2022 by the USB Implementers Forum.

September 2022 naming scheme

An overview of USB naming scheme that was put in place in September 2022.
(A mix of USB specifications and their marketing names are being displayed, because specifications are sometimes wrongly used as marketing names)^{[disputed (for: USB4 20Gbps does not exist; USB4 2x2 is not interchangeable with USB 3.2 2x2 as indicated by the logo; logos for USB 3.x and USB are different)  – discuss]}

Because of the previous confusing naming schemes, USB-IF decided to change it once again. As of 2 September 2022, marketing names follow the syntax "USB XGbps", where X is the speed of transfer in Gb/s. Overview of the updated names and logos can be seen in the adjacent table.

The operation modes USB 3.2 Gen 2x2 and USB4 Gen 2x2 – or: USB 3.2 Gen 2x1 and USB4 Gen 2x1 – are not interchangeable or compatible; all participating controllers must operate with the same mode.

Version history

Release versions

Name	Release date	Maximum signaling rate	Note
USB 0.7	November 1994	?	Pre-release
USB 0.8	December 1994	?	Pre-release
USB 0.9	April 1995	12 Mbit/s : Full Speed (FS)	Pre-release
USB 0.99	August 1995	?	Pre-release
USB 1.0-RC	November 1995	?	Release Candidate
USB 1.0	January 1996	1.5 Mbit/s : Low Speed (LS) 12 Mbit/s : Full Speed (FS)
USB 1.1	August 1998
USB 2.0	April 2000	480 Mbit/s : High Speed (HS)
USB 3.0	November 2008	5 Gbit/s : SuperSpeed (SS)	Also referred to as USB 3.1 Gen 1 and USB 3.2 Gen 1×1.
USB 3.1	July 2013	10 Gbit/s : SuperSpeed+ (SS+)	Includes new USB 3.1 Gen 2, also named USB 3.2 Gen 2×1 in later specification.
USB 3.2	August 2017	20 Gbit/s : SuperSpeed+ two-lane	Includes new USB 3.2 Gen 1×2 and Gen 2×2 multi-link modes. Requires USB-C Fabrics.
USB4	August 2019	40 Gbit/s : two-lane	Includes new USB4 Gen 2×2 (64b/66b encoding) and Gen 3×2 (128b/132b encoding) modes and introduces USB4 routing for tunneling of USB 3.2, DisplayPort 1.4a and PCI Express traffic and host-to-host transfers, based on the Thunderbolt 3 protocol; requires USB-C Fabrics.
USB4 2.0	September 2022	120 Gbit/s	Includes new 80 and 120 Gbit/s modes over type C connector. Requires USB-C Fabrics.

Power-related standards

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, 1.5 A 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: Clarify use of Retries Battery Capabilities FRS timing problem PPS power rule clarifications Peak current support for EPR AVS APDO

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 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. 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 endpoints reside on the connected device: the channels to the host are referred to as pipes.

USB device communication is based on pipes (logical channels). A pipe is a connection from the host controller to a logical entity 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.

• 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, 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, 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 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.

Two USB 3.0 Standard-A receptacles (left) and two USB 2.0 Standard-A receptacles (right) on a computer's front panel

Endpoints are grouped into interfaces and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and is not associated with any interface. A single device function composed of independently controlled interfaces is called a composite device. A composite device only has a single device address because the host only assigns a device address to a function.

When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The signaling 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 signaling rates regardless of USB 2.0 or earlier devices connected to that host. Operating signaling 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:

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
06h	Interface	Media (PTP/MTP)	Scanner, Camera
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
0Eh	Interface	Video	Webcam
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
EFh	Both	Miscellaneous	ActiveSync device
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 / USB drive

See also: USB mass storage device class, Disk enclosure, and External hard disk drive

A flash drive, a typical USB mass-storage device

An M.2 (2242) solid-state-drive (SSD) connected into USB 3.0 adapter and connected to computer.

The 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 and SD card readers. 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.

Media Transfer Protocol

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.

Human interface devices

Main article: USB human interface device class

A USB mouse or keyboard can usually be used with older computers that have PS/2 ports with the aid of a small USB-to-PS/2 adapter. For mice and keyboards with dual-protocol support, a passive adapter 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. Active converters that connect USB keyboards and mice (usually one of each) to PS/2 ports also exist.

Device Firmware Upgrade mechanism

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.

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: USB 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 only implements a subset of the UAC 1.0 specification.

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.e 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 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.

Connectors

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. The official USB specification documents also periodically define the term male to represent the plug, and female to represent the receptacle.

The standard USB Type-A plug. This is one of many types of USB connector.

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.

Cabling

Main article: USB hardware § Cabling

A variety of USB cables for sale in Hong Kong

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

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.

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

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 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. Multi-lane operations (USB-C) support low power devices with a unit load of 250 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. Multi-lane operations (USB-C) support high power devices with up to 1500 mA (6x250mA).

USB power standards

Specification	Current	Voltage	Power (max.)
Low-power device	100 mA	5 V	0.50 W
Low-power SuperSpeed (USB 3.0) device	150 mA	5 V	0.75 W
High-power device	500 mA	5 V	2.5 W
High-power SuperSpeed (USB 3.0) device	900 mA	5 V	4.5 W
Multi-lane SuperSpeed (USB 3.2 Gen x2) device	1.5 A	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 (multi-lane)	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	20 V	100 W
Power Delivery 3.1 Type-C	5 A	48 V	240 W

The V_BUS supply from a low-powered hub port may drop to 4.40 V.

Up to five unit loads; with non-SuperSpeed devices, one unit load is 100 mA.

Up to six unit loads; with SuperSpeed devices, one unit load is 150 mA.

Up to six unit loads; with multi-lane devices, one unit load is 250 mA.

>3 A (>60 W) operation requires an electronically marked cable rated at 5 A.

>20 V (>100 W) operation requires an electronically marked Extended Power Range (EPR) cable.

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.

Signaling

Main article: USB (Communications) § Signaling (USB PHY)

USB signals are transmitted using differential signaling on 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 dedicated pair for USB 2.0 compatibility and two or four pairs for data transfer: two pairs in full-duplex (FDx) for single lane variants require SuperSpeed connectors; four pairs in full-duplex for two lane (×2) variants require USB-C connectors. USB4 Gen 4 requires the use of all four pairs but allow for asymmetrical pairs configuration, in this case one lane is used for the upstream data and the other tree for the downstream data or vice-versa. USB4 Gen 4 use pulse amplitude modulation on 3 levels, providing a trit of information every baud transmited, the transmission frequency of 12,8GHz translate to a transmission rate of 25,6 Gigabaud and the 11 bit to 7 trit translation provides a theorical maximum transmision speed just over 40,2 Gbit/s.

Operation mode	Old name	Introduced in	Encoding	Data wire pairs	Nominal rate	USB-IF marketing name	Logo
Low-Speed		USB 1.0	NRZI	1 HDx	1.5 Mbit/s	Low-Speed USB
Full-Speed					12 Mbit/s
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 (+ 1 HDx)	5 Gbit/s	SuperSpeed USB 5Gbps
USB 3.2 Gen 2×1	USB 3.1 Gen 2	USB 3.1	128b/132b	2 FDx (+ 1 HDx)	10 Gbit/s	SuperSpeed USB 10Gbps
USB 3.2 Gen 1×2		USB 3.2	8b/10b	4 FDx (+ 1 HDx)	10 Gbit/s	—
USB 3.2 Gen 2×2			128b/132b	4 FDx (+ 1 HDx)	20 Gbit/s	SuperSpeed USB 20Gbps
USB4 Gen 2×1		USB4	64b/66b	2 FDx (+ 1 HDx)	10 Gbit/s	—
USB4 Gen 2×2			64b/66b	4 FDx (+ 1 HDx)	20 Gbit/s	USB4 20Gbps
USB4 Gen 3×1			128b/132b	2 FDx (+ 1 HDx)	20 Gbit/s	—
USB4 Gen 3×2			128b/132b	4 FDx (+ 1 HDx)	40 Gbit/s	USB4 40Gbps
USB4 Gen 4		USB4 version 2.0	PAM-3 11b/7t	4 FDx (+ 1 HDx)	80 Gbit/s	USB4 80Gbps
					40 Gbit/s up 120 Gbit/s down
					120 Gbit/s up 40 Gbit/s down

 

USB 2.0 implementation

2. USB4 can use optional Reed–Solomon forward error correction (RS FEC). In this mode, 12 × 16 B (128 bit) symbols are assembled together with 2 B (12 bit + 4 bit reserved) synchronization bits indicating the respective symbol types and 4 B of RS FEC to allow to correct up to 1 B of errors anywhere in the total 198 B block.

• Low-speed (LS) and Full-speed (FS) modes use a single data wire pair, labeled D+ and D−, in half-duplex. Transmitted signal levels are 0.0–0.3 V for logical low, and 2.8–3.6 V for logical high level. The signal lines are not terminated.
• High-speed (HS) uses the same wire pair, but with different electrical conventions. Lower signal voltages of −10 to 10 mV for low and 360 to 440 mV 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 data wires (and new, mostly compatible expanded connectors) besides another grounding wire. 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 Signaling, 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 a new coding scheme with an increased signaling rate (Gen 2×1 mode) and/or the additional lane of USB-C (Gen 1×2 and Gen 2×2 modes).

A USB connection is always between a host or hub at the A connector (or USB-C) end, and a device or hub's "upstream" port at the other end.

Protocol layer

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.

Transactions

Main article: USB (Communications) § Transaction

The basic transactions of USB are:

• OUT transaction
• IN transaction
• SETUP transaction
• Control transfer exchange

Related standards

The Wireless USB logo

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 29 July 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 use MA-USB will be branded as 'Powered by MA-USB', provided the product qualifies its certification program.

InterChip USB

Main article: InterChip USB

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. It is an alternative standard to SPI and I2C.

USB-C

Main article: 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 or HDMI protocols, which allows connecting a display, such as a computer monitor or television set, via USB-C.

All other connectors are not capable of two-lane operations (Gen 1x2 and Gen 2x2) in USB 3.2, but can be used for one-lane operations (Gen1x1 and Gen2x1).

DisplayLink

Main article: 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 on the computer.

Comparisons with other connection methods

FireWire (IEEE 1394)

At first, USB was considered a complement to FireWire (IEEE 1394) 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 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 signaling 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.

Ethernet

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.

MIDI

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.

eSATA/eSATAp

The eSATA connector is a more robust SATA connector, intended for connection to external hard drives and SSDs. eSATA's transfer rate (up to 6 Gbit/s) is similar to that of USB 3.0 (up to 5 Gbit/s) 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

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 USB-C. 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 Fabrics, making a Thunderbolt 3 port a superset of a USB 3.1 Gen 2 port with DisplayPort alternate mode.

DisplayPort Alt Mode 2.0: USB4 supports DisplayPort 2.0 over its alternate 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

Main article: USB adapter

Various protocol converters are available that convert USB data signals to and from other communications standards.

Security threats

Due to the prevalency of the USB standard, there are many exploits using the USB standard. One of the biggest instances of this today is known as the USB Killer, a device which damages devices by sending high voltage pulses across the data lines.

In versions of Microsoft Windows before Windows XP, Windows would automatically run a script (if present) on certain devices via autorun, one of which are USB mass storage devices, which may contain malicious software.

Date rape

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Date_rape

Date rape is a form of acquaintance rape and dating violence. The two phrases are often used interchangeably, but date rape specifically refers to a rape in which there has been some sort of romantic or potentially sexual relationship between the two parties. Acquaintance rape also includes rapes in which the victim and perpetrator have been in a non-romantic, non-sexual relationship, for example as co-workers or neighbors.

Since the 1980s, date rape has constituted the majority of rapes in some countries. It is particularly prevalent on college campuses, and frequently involves consumption of alcohol or other date rape drugs. The peak age for date rape victims is from the late teens to early twenties.

Overview

A feature of date rape is that in most cases the victim is female, knows the perpetrator, and the rape takes place in the context of an actual or potential romantic or sexual relationship between the parties, or when that relationship has come to an end. The perpetrator may use physical or psychological intimidation to force a victim to have sex against their will, or when the perpetrator has sex with a victim who is incapable of giving consent, for example, because they have been incapacitated by alcohol or other drug.

According to the United States Bureau of Justice Statistics (BJS), date rapes are among the most common forms of rape cases. Date rape most commonly takes place among college students when alcohol is involved or date rape drugs are taken. One of the most targeted groups are women between the ages of 16 and 24.

The phenomenon of date rape is relatively new. Historically, date rape has been considered less serious than rape by a stranger. Since the 1980s, it has constituted the majority of rapes in some countries. It has been increasingly seen as a problem involving society's attitude towards women and as a form of violence against women. It is controversial, however, with some people believing the problem is overstated and that many date rape victims are actually willing, consenting participants, and others believing that date rape is seriously underreported and almost all women who claim date rape were actually raped.

American researcher Mary Koss describes date rape as a specific form of acquaintance rape, in which there has been some level of romantic interest between the perpetrator and the victim, and in which sexual activity would have been generally seen as appropriate, if consensual. Acquaintance rape is a broader category than date rape, that can include many types of relationships including employer-employee, landlord-tenant, service provider-consumer, driver-hitchhiker, and rape among people who have a family relationship or who are neighbours.

In his 1992 book Sex and Reason American jurist, legal theorist and economist Richard Posner characterized the increased attention being given to date rape as a sign of the changing status of women in American society, pointing out that dating itself is a feature of modern societies and that date rape can be expected to be frequent in a society in which sexual morals vary between the permissive and the repressive. In Sara Alcid's 2013 article "Navigating Consent: Debunking the 'Gray Area' Myth", she argues that dating is incorrectly believed to mean "a permanent state of consenting to sex".

History

Since the final decades of the 20th century, in much of the world, rape has come to be broadly regarded as sexual intercourse (including anal or oral penetration) without a person's immediate consent, making rape illegal, including among people who know each other or who have previously had consensual sex. Some jurisdictions have specified that people debilitated by alcohol or other drugs are incapable of consenting to sex. Courts have also disagreed on whether consent, once given, can later be withdrawn. "Cultural and legal definitions of rape are always shaped by the relationships and status of those involved, a premise that holds both historically and cross-culturally."

Many societies rank the seriousness of a rape based on the relationship between the perpetrator and the victim. "An assault by a stranger is more likely to be seen as a 'real rape' than one by some-one known to the victim." Because of this cultural conception, many date rapes are considered to be less serious than stranger rapes because the nature of the perpetrator-victim relationship, especially for those who have had a prior or current sexual relationship.

Use of term

The first appearance of the term date rape in a book was in 1975, in Against Our Will: Men, Women and Rape by American feminist journalist, author and activist Susan Brownmiller. The phrase appears in a few newspapers and journal articles earlier, but these had a more limited readership. The prominent feminist American-British lawyer Ann Olivarius helped popularize "date rape" in a series of public lectures at Yale University when she was an undergraduate to describe the strangulation and rape of a woman by a now-prominent gerontologist in California, Dr. Calvin Hirsch, to Yale's police department. In 1980 it was used in Mademoiselle magazine, in 1982 Ms. magazine published an article titled "Date Rape: A Campus Epidemic?", and in 1984 English novelist Martin Amis used the term in his novel Money: A Suicide Note. One of the earliest and most prominent date rape researchers is Mary Koss, who in 1987 conducted the first large-scale nationwide study on rape in the United States, surveying 7,000 students at 25 schools, and who is sometimes credited with originating the phrase date rape.

Prevalence

See also: Rape statistics

The concept of date rape originated in the United States, where most of the research on date rape has been carried out. One out of every five teens are victims of date rape. Rape prevalence among women in the U.S. (the percentage of women who experienced rape at least once in their lifetime so far) is in the range of 15–20%, with different studies disagreeing with each other. An early 1987 study found that one in four American women will be the victim of a rape or attempted rape in her lifetime, and 84% of those will know their attacker. However, only 27% of American women whose sexual assault met the legal definition of rape think of themselves as rape victims, and only about 5% report their rape. One study of rape on American college campuses found that 13% of acquaintance rapes, and 35% of attempted acquaintance rapes, took place during a date, and another found that 22% of female rape victims had been raped by a current or former date, boyfriend or girlfriend, and another 20% by a spouse or former spouse.^[26] A 2007 American study found black non-Hispanic students were likeliest to be victims of dating violence, followed by Hispanic students and then white non-Hispanic students.

Rates of date rape are relatively low in Europe compared with the United States.

The rate of reported rapes is much lower in Japan than the United States,. In a 1993 paper German sociologist and criminologist Joachim Kersten suggested date rape may be less prevalent in Japan compared with the United States because Japanese culture puts a lesser emphasis on romantic love and dating, and because young Japanese people have less physical privacy than their American counterparts, and in her 2007 book Kickboxing Geishas: How Modern Japanese Women Are Changing Their Nation, American feminist Veronica Chambers questions whether date rape is under-reported in Japan because it isn't yet understood there to be rape. In the 2011 book Transforming Japan: How Feminism and Diversity Are Making a Difference Japanese feminist Masaki Matsuda argued that date rape was becoming an increasing problem for Japanese college and high school students.

A 2007 study of attitudes towards rape among university students in South Korea found that date rape was "rarely recognized" as a form of rape, and that forced sex by a date was not viewed as traumatizing or criminal.

Date rape is generally underreported in Vietnam.

In 2012, 98% of reported rapes in India were committed by someone known to the victim.

Victims

Researcher Mary Koss says the peak age for women being date raped is from their late teens to early twenties.

Even though date rape is considered a hurtful, destructive and life-changing experience, research done by Mufson and Kranz showed that lack of support is a factor that determines the fragmented recovery of victims. They refused to disclose any information about the sexual assault to others, especially if they have experienced date or acquaintance rape due to self-humiliation and self-blame feelings.

However, there are several situational contexts where victims are able to seek for help or reveal the sexual assaults they have experienced. One act for disclosure can be provoked from the willing of preventing other people from being raped, in other words, speaking out. Also, a concern transmitted by the people surrounding the victim can lead into a confession of the assault, or within a situation in which alcohol is involved and that leads to recount the experience.

Minority group victims

Most of the research on sexual assault victims has been carried out with White-middle class population. However, the scale of date and acquaintance rape among the Black and Hispanic youth population is higher, and has its particular risk factors. A study conducted in 2013 indicated that sexual assault situations were greater among Hispanic (12.2%) and Black (11.5%) female high-school students than whites (9.1%).

Effects

Date rape affects victims similarly to stranger rape, although the failure of others to acknowledge and take the rape seriously can make it harder for victims to recover.

Rape crimes are more frequently perpetrated by people that the victims have confidence with and have known for quite some time. Nevertheless, some people's beliefs do not fit within the date rape scenario paradigm because they firmly prejudiced and stereotyped rape, victims and perpetrators. They tend to justify date rape and blame victims, particularly women victims, for the sexual assault by emphasizing the wearing of provocative clothing or the existence of a romantic relationship.

One of the main problems of date rape attributions is the type of relationship that the victim and the offender shared. The more intimate the relationship between both partners, the more probable that witnesses will consider the sexual assault as consensual rather than a serious incident.

Perpetrators and motivations

A 2002 landmark study of undetected date rapists in Boston found that compared with non-rapists, rapists are measurably more angry at women and more motivated by a desire to dominate and control them, are more impulsive, disinhibited, antisocial, hypermasculine, and less empathic. The study found the rapists were extremely adept at identifying potential victims and testing their boundaries, and that they planned their attacks and used sophisticated strategies to isolate and groom victims, used violence instrumentally in order to terrify and coerce, and used psychological weapons against their victims including power, manipulation, control and threats. Date rapists target vulnerable victims, such as female freshmen who have less experience with drinking and are more likely to take risks, or people who are already intoxicated; they use alcohol as a weapon, as it makes the victim more vulnerable and impairs their credibility with the justice system should they choose to report the rape.

American clinical psychologist David Lisak, the study's author and an expert in date rape, says that serial rapists account for 90% of all campus rapes, with an average of six rapes each. Lisak argues that this and similar findings conflict sharply with the widely held view that college rapes are typically perpetrated by "a basically 'decent' young man who, were it not for too much alcohol and too little communication, would never do such a thing", with the evidence actually suggesting that the vast majority of rapes, including date rapes, are committed by serial, violent predators.

Punishment

Date rape has a particular dynamic: the sexual assault happens on a date type of setting. Therefore, date rapes trials are considered inconclusive by nature and are charged with social concerns (e.g. gender roles, sexuality, body-shape). The criminal justice system urges the victim to describe the sexual assault in detail in order to be able to make a decision in court, ignoring the possibility that cross-examination can be a hostile and disturbing moment for the victim. Jurors’ personal beliefs and rape myth acceptance can be influential in their decision when it comes to evaluating the scenery, evidence, and making a sentence.

Research has found that jurors are more likely to convict in stranger rape cases than in date rape cases. Often, even in cases in which sufficient physical evidence is present to support conviction, juries have reported being influenced by irrelevant factors related to the female victim such as whether she used birth control, engaged in non-marital sex, was perceived by jurors as sexually dressed, or had engaged in alcohol or other drug use. Researchers have noted that because date rape by definition occurs in the context of a dating relationship, jurors' propensity to discount the likelihood of rape having occurred based on date-like behaviors is problematic. A 1982 American study of assignment of responsibility for rape found respondents were more likely to assign greater responsibility to a rape victim if she was intoxicated at the time of the rape; however, when her assailant was intoxicated, respondents assigned him less responsibility.

Some critics of the term date rape believe the distinction between stranger rape and date rape seems to position date rape as a lesser offence, which is insulting to date rape victims and could partly explain the lower conviction rates and lesser punishments of date rape cases.

Prevention

David Lisak argues that prevention efforts aimed at persuading men not to rape are unlikely to work, and universities should instead focus on helping non-rapists to identify rapists and intervene in high-risk situations to stop them. Lisak also argues that whenever a nonstranger sexual assault is reported, it represents a window of opportunity for law enforcement to comprehensively investigate the alleged offender, rather than "putting blinders on looking solely on the alleged 45-minute interaction between these two people". Lisak believes rape victims should be treated with respect, and that every report of an alleged rape should trigger two simultaneous investigations: one into the incident itself, and a second into the alleged perpetrator to determine whether they are a serial offender.

Education programs are one way to prevent, protect, and raise awareness about rape and acquaintance rape. But these prevention programs don't have a huge impact. The combination of sexual harassment prevention tips, survival information and the psychosocial data gathered from women's assessment of date risks, make these programs focus on broad topics and don't emphasize specific and particular areas of date rape prevention.

Future prevention programs should focus on engaging men, creating an open space for conversation and the possible recognition of holding gender bias beliefs and sexual behavior myths, which can lead them to promote sexual harassment behavior.

In media and popular culture

Date rape was widely discussed on college campuses in North America during the 1980s but first attracted significant media attention in 1991, when an unnamed 29-year-old woman accused William Kennedy Smith, a nephew of former President John F. Kennedy, Senator Robert F. Kennedy, and Senator Ted Kennedy, of raping her on a nearby beach after meeting in a Florida bar. Millions of people watched the trial on television. Also in 1991, Katie Koestner came forward publicly about her own experience with date rape. Koestner was featured on the cover of Time magazine, appeared on shows such as Larry King Live and The Oprah Winfrey Show. Her efforts helped bring a human face to victims of date rape and helped bring the term into common use. Koestner was featured in a 1993 HBO special, No Visible Bruises: The Katie Koestner Story as part of the series, Lifestories: Families in Crisis.

British ska band The Special AKA, with Rhoda Dakar, released the single The Boiler in 1982, which follows a woman who recounts being date raped by a boiler; the single reached No. 35 in the UK charts. Hip hop band A Tribe Called Quest has a song titled "The Infamous Date Rape", included in their album The Low End Theory, which was released shortly after the William Kennedy Smith incident. American ska punk band Sublime released a humorous song called "Date Rape" in 1991; the song ends with the date rapist being sent to prison and being anally raped by a fellow inmate.

Date rape received more media attention in 1992, when former boxer Mike Tyson was convicted of rape after inviting 18-year-old Desiree Washington to a party and then raping her in his hotel room.

Controversies

In her 1994 book The Morning After: Sex, Fear, and Feminism, American author Katie Roiphe wrote about attending Harvard and Princeton in the late 1980s and early 1990s, amid what she described as a "culture captivated by victimization", and argued "If a woman's 'judgment is impaired' and she has sex, it isn't always the man's fault; it isn't necessarily always rape."

In 2007, American journalist Laura Sessions Stepp wrote an article for Cosmopolitan magazine titled "A New Kind of Date Rape", in which she popularized the term "gray rape" to refer to "sex that falls somewhere between consent and denial". The term was afterwards picked up and discussed by The New York Times, Slate, and PBS, and was criticized by many feminists, including Bitch founding editor Lisa Jervis, who argued that gray rape and date rape "are the same thing", and that the popularization of gray rape constituted a backlash against women's sexual empowerment and risked rolling back the gains women had made in having rape taken seriously.

Method (computer programming)

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Method_(computer_programming) 

A method in object-oriented programming (OOP) is a procedure associated with an object, and generally also a message. An object consists of state data and behavior; these compose an interface, which specifies how the object may be utilized by any of its various consumers. A method is a behavior of an object parametrized by a consumer.

Data is represented as properties of the object, and behaviors are represented as methods. For example, a Window object could have methods such as open and close, while its state (whether it is open or closed at any given point in time) would be a property.

In class-based programming, methods are defined within a class, and objects are instances of a given class. One of the most important capabilities that a method provides is method overriding - the same name (e.g., area) can be used for multiple different kinds of classes. This allows the sending objects to invoke behaviors and to delegate the implementation of those behaviors to the receiving object. A method in Java programming sets the behavior of a class object. For example, an object can send an area message to another object and the appropriate formula is invoked whether the receiving object is a rectangle, circle, triangle, etc.

Methods also provide the interface that other classes use to access and modify the properties of an object; this is known as encapsulation. Encapsulation and overriding are the two primary distinguishing features between methods and procedure calls.

Overriding and overloading

Method overriding and overloading are two of the most significant ways that a method differs from a conventional procedure or function call. Overriding refers to a subclass redefining the implementation of a method of its superclass. For example, findArea may be a method defined on a shape class, triangle, etc. would each define the appropriate formula to calculate their area. The idea is to look at objects as "black boxes" so that changes to the internals of the object can be made with minimal impact on the other objects that use it. This is known as encapsulation and is meant to make code easier to maintain and re-use.

Method overloading, on the other hand, refers to differentiating the code used to handle a message based on the parameters of the method. If one views the receiving object as the first parameter in any method then overriding is just a special case of overloading where the selection is based only on the first argument. The following simple Java example illustrates the difference:

Accessor, mutator and manager methods

Accessor methods are used to read the data values of an object. Mutator methods are used to modify the data of an object. Manager methods are used to initialize and destroy objects of a class, e.g. constructors and destructors.

These methods provide an abstraction layer that facilitates encapsulation and modularity. For example, if a bank-account class provides a getBalance() accessor method to retrieve the current balance (rather than directly accessing the balance data fields), then later revisions of the same code can implement a more complex mechanism for balance retrieval (e.g., a database fetch), without the dependent code needing to be changed. The concepts of encapsulation and modularity are not unique to object-oriented programming. Indeed, in many ways the object-oriented approach is simply the logical extension of previous paradigms such as abstract data types and structured programming.

Constructors

Main article: Constructor (computer science)

A constructor is a method that is called at the beginning of an object's lifetime to create and initialize the object, a process called construction (or instantiation). Initialization may include an acquisition of resources. Constructors may have parameters but usually do not return values in most languages. See the following example in Java:

public class Main {
    String _name;
    int _roll;

    Main(String name, int roll) { // constructor method
        this._name = name;
        this._roll = roll;
    }
}

Destructors

Main article: Destructor (computer science)

A destructor is a method that is called automatically at the end of an object's lifetime, a process called destruction. Destruction in most languages does not allow destructor method arguments nor return values. Destruction can be implemented so as to perform cleanup chores and other tasks at object destruction.

Finalizers

In garbage-collected languages, such as Java, C#, and Python, destructors are known as finalizers. They have a similar purpose and function to destructors, but because of the differences between languages that utilize garbage-collection and languages with manual memory management, the sequence in which they are called is different.

Abstract methods

An abstract method is one with only a signature and no implementation body. It is often used to specify that a subclass must provide an implementation of the method. Abstract methods are used to specify interfaces in some programming languages.

Example

The following Java code shows an abstract class that needs to be extended:

abstract class Shape {
    abstract int area(int h, int w); // abstract method signature
}

The following subclass extends the main class:

public class Rectangle extends Shape {
    @Override
    int area(int h, int w) {
        return h * w;
    }
}

Reabstraction

If a subclass provides an implementation for an abstract method, another subclass can make it abstract again. This is called reabstraction.

In practice, this is rarely used.

Example

In C#, a virtual method can be overridden with an abstract method. (This also applies to Java, where all non-private methods are virtual.)

class IA
{
    public virtual void M() { }
}
abstract class IB : IA
{
    public override abstract void M(); // allowed
}

Interfaces' default methods can also be reabstracted, requiring subclasses to implement them. (This also applies to Java.)

interface IA
{
    void M() { }
}
interface IB : IA
{
    abstract void IA.M();
}
class C : IB { } // error: class 'C' does not implement 'IA.M'.

Class methods

Class methods are methods that are called on a class rather than an instance. They are typically used as part of an object meta-model. I.e, for each class, defined an instance of the class object in the meta-model is created. Meta-model protocols allow classes to be created and deleted. In this sense, they provide the same functionality as constructors and destructors described above. But in some languages such as the Common Lisp Object System (CLOS) the meta-model allows the developer to dynamically alter the object model at run time: e.g., to create new classes, redefine the class hierarchy, modify properties, etc.

Special methods

Special methods are very language-specific and a language may support none, some, or all of the special methods defined here. A language's compiler may automatically generate default special methods or a programmer may be allowed to optionally define special methods. Most special methods cannot be directly called, but rather the compiler generates code to call them at appropriate times.

Static methods

See also: Static member function

Static methods are meant to be relevant to all the instances of a class rather than to any specific instance. They are similar to static variables in that sense. An example would be a static method to sum the values of all the variables of every instance of a class. For example, if there were a Product class it might have a static method to compute the average price of all products.

A static method can be invoked even if no instances of the class exist yet. Static methods are called "static" because they are resolved at compile time based on the class they are called on and not dynamically as in the case with instance methods, which are resolved polymorphically based on the runtime type of the object.

Examples

In Java

In Java, a commonly used static method is:

Math.max(double a, double b)

This static method has no owning object and does not run on an instance. It receives all information from its arguments.

Copy-assignment operators

Copy-assignment operators define actions to be performed by the compiler when a class object is assigned to a class object of the same type.

Operator methods

Operator methods define or redefine operator symbols and define the operations to be performed with the symbol and the associated method parameters. C++ example:

#include <string>

class Data {
 public:
  bool operator<(const Data& data) const { return roll_ < data.roll_; }
  bool operator==(const Data& data) const {
    return name_ == data.name_ && roll_ == data.roll_;
  }

 private:
  std::string name_;
  int roll_;
};

Member functions in C++

Some procedural languages were extended with object-oriented capabilities to leverage the large skill sets and legacy code for those languages but still provide the benefits of object-oriented development. Perhaps the most well-known example is C++, an object-oriented extension of the C programming language. Due to the design requirements to add the object-oriented paradigm on to an existing procedural language, message passing in C++ has some unique capabilities and terminologies. For example, in C++ a method is known as a member function. C++ also has the concept of virtual functions which are member functions that can be overridden in derived classes and allow for dynamic dispatch.

Virtual functions

Virtual functions are the means by which a C++ class can achieve polymorphic behavior. Non-virtual member functions, or regular methods, are those that do not participate in polymorphism.

C++ Example:

#include <iostream>
#include <memory>

class Super {
 public:
  virtual ~Super() = default;

  virtual void IAm() { std::cout << "I'm the super class!\n"; }
};

class Sub : public Super {
 public:
  void IAm() override { std::cout << "I'm the subclass!\n"; }
};

int main() {
  std::unique_ptr<Super> inst1 = std::make_unique<Super>();
  std::unique_ptr<Super> inst2 = std::make_unique<Sub>();

  inst1->IAm();  // Calls |Super::IAm|.
  inst2->IAm();  // Calls |Sub::IAm|.
}