Universal Serial Bus (
USB) is a
serial bus standard to interface devices to a host computer. USB was designed to allow many peripherals to be connected using a single
standardized interface socket and to improve the Plug and play capabilities by allowing hot swapping, that is, by allowing devices to be connected and disconnected without rebooting the computer or turning off the device. Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.
USB is intended to replace many legacy varieties of serial and parallel ports. USB can connect computer peripherals such as mice, keyboards, PDAs, gamepads and joysticks, scanners, 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 originally designed for personal computers, 
but it has become commonplace on other devices such as PDAs and video game consoles, and as a bridging power cord between a device and an AC adapter plugged into a wall plug for charging purposes. As of 2008[update], there are about 2 billion USB devices sold per year, and about 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, NEC, and Microsoft.
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).
USB endpoints actually reside on the connected device: the channels to the host are referred to as pipes.
USB device communication is based on pipes (logical channels). 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. A USB device can have up to 32 active pipes, 16 into the host controller and 16 out of the controller.
Each endpoint can transfer data in one direction only, either into or out of the device, so each pipe is uni-directional. 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.
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, host controller polls the bus for traffic, usually in a round-robin fashion. In SuperSpeed USB, connected device can request service from host
USB packets
USB communication takes the form of packets. Initially, all packets are sent from the host, via the root hub and possibly more hubs, to devices. Some of those packets direct a device to send some packets in reply.
After the sync field described above, all packets are made of 8-bit bytes, transmitted least-significant bit first. The first byte is a packet identifier (PID) byte. The PID is actually 4 bits; the byte consists of the 4-bit PID followed by its bitwise complement. This redundancy helps detect errors. (Note also that a PID byte contains at most four consecutive 1 bits, and thus will never need bit-stuffing, even when combined with the final 1 bit in the sync byte. However, the OUT PID byte ends with three consecutive 1 bits, so if the following USB device address begins with three 1 bits, bit-stuffing will be required.)
Packets come in three basic types, each with a different format and CRC (cyclic redundancy check):
Handshake packets
Handshake packets consist of nothing but a PID byte, and are generally sent in response to data packets. The three basic types are ACK, indicating that data was successfully received, NAK, indicating that the data cannot be received at this time and should be retried, and STALL, indicating that the device has an error and will never be able to successfully transfer data until some corrective action (such as device initialization) is performed.
USB 2.0 added two additional handshake packets, NYET which indicates that a split transaction is not yet complete, and an ERR handshake to indicate that a split transaction failed.
The only handshake packet the USB host may generate is ACK; if it is not ready to receive data, it should not instruct a device to send any.
Token packets
Token packets consist of a PID byte followed by 11 bits of address and a 5-bit CRC. Tokens are only sent by the host, never a device.--
IN and OUT tokens contain a 7-bit device number and 4-bit function number (for multifunction devices) and command the device to transmit DATAx packets, or receive the following DATAx packets, respectively.
An IN token expects a response from a device. The response may be a NAK or STALL response, or a DATAx frame. In the latter case, the host issues an ACK handshake if appropriate.
An OUT token is followed immediately by a DATAx frame. The device responds with ACK, NAK, or STALL, as appropriate.
SETUP operates much like an OUT token, but is used for initial device setup.
Every millisecond (12000 full-speed bit times), the USB host transmits a special SOF (start of frame) token, containing an 11-bit incrementing frame number in place of a device address. This is used to synchronize isochronous data flows. High-speed USB 2.0 devices receive 7 additional duplicate SOF tokens per frame, each introducing a 125 µs "microframe".
USB 2.0 added a PING token, which asks a device if it is ready to receive an OUT/DATA packet pair. The device responds with ACK, NAK, or STALL, as appropriate. This avoids the need to send the DATA packet if the device knows that it will just respond with NAK.
USB 2.0 also added a larger SPLIT token with a 7-bit hub number, 12 bits of control flags, and a 5-bit CRC. This is used to perform split transactions. Rather than tie up the high-speed USB bus sending data to a slower USB device, the nearest high-speed capable hub receives a SPLIT token followed by one or two USB packets at high speed, performs the data transfer at full or low speed, and provides the response at high speed when prompted by a second SPLIT token. The details are complex; see the USB specification.
Data packets
There are two basic data packets, DATA0 and DATA1. Both consist of a DATAx PID field, 0–1023 bytes of data payload (up to 1024 in high speed, at most 8 at low speed), and a 16-bit CRC. They must always be preceded by an address token, and are usually followed by a handshake token from the receiver back to the transmitter. The two packet types provide the 1-bit sequence number required by Stop-and-wait ARQ. If a USB host does not receive a response (such as an ACK) for data it has transmitted, it does not know if the data was received or not; the data might have been lost in transit, or it might have been received but the handshake response was lost.
To solve this problem, the device keeps track of the type of DATAx packet it last accepted. If it receives another DATAx packet of the same type, it is acknowledged but ignored as a duplicate. Only a DATAx packet of the opposite type is actually received.
When a device is reset with a SETUP packet, it expects a DATA0 packet next.
USB 2.0 added DATA2 and MDATA packet types as well. They are used only by high-speed devices doing high-bandwidth isochronous transfers which need to transfer more than 1024 bytes per 125 µs "microframe" (8192 kB/s).
PRE "packet"
Low-speed devices are supported with a special PID value, PRE. This marks the beginning of a low-speed packet, and is used by hubs which normally do not send full-speed packets to low-speed devices. Since all PID bytes include four 0 bits, they leave the bus in the full-speed K state, which is the same as the low-speed J state. It is followed by a brief pause during which hubs enable their low-speed outputs, already idling in the J state, then a low-speed packet follows, beginning with a sync sequence and PID byte, and ending with a brief period of SE0. Full-speed devices other than hubs can simply ignore the PRE packet and its low-speed contents, until the final SE0 indicates that a new packet follows.
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