USB ( Universal Serial Bus ) is a specification to establish communication between devices and a host controller (usually personal computers). USB is intended to replace many varieties of serial and parallel ports. USB can connect computer peripherals such as mice, keyboards, digital cameras, printers, personal media players, flash drives, and external hard drives. For many of those devices, USB has become the standard connection method. USB was designed for personal computers, but it has become commonplace on other devices such as smartphones, PDAs and video game consoles, and as a power cord between a device and an AC adapter plugged into a wall plug for charging. As of 2008, there are about 2 billion USB devices sold per year, and approximately 6 billion total sold to date.
The design of USB is standardized by the USB Implementers Forum (USB-IF), an industry standards body incorporating leading companies from the computer and electronics industries. Notable members have included Agere (now merged with LSI Corporation), Apple Inc., Hewlett-Packard, Intel, Microsoft and NEC.
History
The USB 1.0 specification was introduced in 1996. It was intended 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 to simplify software configuration of all devices connected to USB, as well as to permit greater bandwidth for external devices. The original USB 1.0 specification had a data transfer rate of 12 Mbit/s.
USB was created by a core group of companies that consisted of Compaq, Digital, IBM, Intel, Northern Telecom, and Microsoft. Intel produced the UHCI host controller and open software stack; Microsoft produced a USB software stack for Windows and co-authored the OHCI host controller specification with National Semiconductor and Compaq; Philips produced early USB-Audio; and TI produced the most widely used hub chips. One of the co-inventors of USB was Ajay Bhatt, who was later given credit in an Intel television advertisement, though this overstates Intel's role in the development of USB. Compaq and Microsoft played roles equal to Intel, and Microsoft in particular focused on improving the user experience and enabling hot Plug and Play of devices.
The USB 2.0 specification was released in April 2000 and was standardized by the USB-IF at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent following its merger with Alcatel in 2006), Microsoft, NEC, and Philips jointly led the initiative to develop a higher data transfer rate than the 1.0 specification (480 Mbit/s vs 12 Mbit/s).
The USB 3.0 specification was released on November 12, 2008 by the USB 3.0 Promoter Group. Its maximum transfer rate is up to 10 times faster than the USB 2.0 release. It has been dubbed the SuperSpeed USB. Equipment conforming to any version of the standard will also work with version 3.0 conforming equipment in most cases — USB 3.0 standard connectors have introduced some new incompatibilities. However, the device is designed to work with any previous specification at the maximum speed of the prior equipment, not 3.0. There is thus substantial backward compatibility.
Overview
A USB system has an asymmetric design, consisting of a host, a multitude of downstream USB ports, and multiple peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels. A USB host may have multiple host controllers and each host controller may provide one or more USB ports. Up to 127 devices, including the hub devices, may be connected to a single host controller.
USB devices are linked in series through hubs . There always exists one hub known as the root hub, which is built into the host controller. So-called sharing hubs , which allow multiple computers to access the same peripheral device(s), also exist and work by switching access between PCs, either automatically or manually. They are popular in small-office environments. In network terms, they converge rather than diverge branches.
A physical USB device may consist of several logical sub-devices that are referred to as device functions . A single device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function). Such a device is called a compound device in which each logical device is assigned a distinctive address by the host and all logical devices are connected to a built-in hub to which the physical USB wire is connected. A host assigns one and only one device address to a function.
USB device communication is based on pipes (logical channels). Pipes are connections from the host controller to a logical entity on the device named an endpoint. The term endpoint is occasionally used to incorrectly refer to the pipe because, while an endpoint exists on the device permanently, a pipe is only formed when the host makes a connection to the endpoint. Therefore, when referring to the connection between a host and an endpoint, the term pipe should be used. A USB device can have up to 32 active pipes, 16 into the host controller and 16 out of the controller.
There are two types of pipes: stream and message pipes. A stream pipe is a uni-directional pipe connected to a uni-directional endpoint that is used for bulk , interrupt , and isochronous data flow while a message pipe is a bi-directional pipe connected to a bi-directional endpoint that is exclusively used for control data flow. An endpoint is made into the USB device by the manufacturer, and therefore, exists permanently. An endpoint of a pipe is addressable with tuple (device_address, endpoint_number) as specified in a TOKEN packet that the host sends when it wants to start a data transfer session. If the direction of the data transfer is from the host to the endpoint, an OUT packet, which is a specialization of a TOKEN packet, having the desired device address and endpoint number is sent by the host. If the direction of the data transfer is from the device to the host, the host sends an IN packet instead. If the destination endpoint is a uni-directional endpoint whose manufacturer's designated direction does not match the TOKEN packet (e.g., the manufacturer's designated direction is IN while the TOKEN packet is an OUT packet), the TOKEN packet will be ignored. Otherwise, it will be 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 which is not associated with any interface. A single device function comprised 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 speed of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host, then 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 slowest device connected to a controller sets the speed of the interface. For SuperSpeed USB (USB 3.0), connected devices can request service from host, and because there are two separate controllers in each USB 3.0 host, USB 3.0 devices will transmit and receive at USB 3.0 speeds, regardless of USB 2.0 or earlier devices connected to that host. Operating speeds for them will be set in the legacy manner.
Device classes
USB defines class codes used to identify a device’s functionality and to load a device driver based on that functionality. This enables every device driver writer to support devices from different manufacturers that comply with a given class code.
Device classes include:
Note class 0: Use class information in the Interface Descriptors. This base class is defined to be used in Device Descriptors to indicate that class information should be determined from the Interface Descriptors in the device.
USB mass-storage
Main article: USB mass storage device class...
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