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USB Terminology

The USB terminology used in this document is defined here:

asynchronous data	Data transferred at irregular intervals with no specific latency requirements.
bandwidth	The amount of data capable of being transmitted per unit of time, typically bits per second (bps) or bytes per second (Bps).
big endian	A method of storing data that places the most significant byte of multiple byte values at a lower storage address. For example, a word stored in big endian format places the least significant byte at the higher address and the most significant byte at the lower address. See also, little endian.
bps	Transmission rate expressed in bits per second.
buffer	Storage used to compensate for a difference in data rates or time of occurrence of events, when transmitting data from one device to another. The area in memory where data is either stored or retrieved programmatically.
bulk transfer	Nonperiodic, large bursts of communication typically used for a data transfer that can use any available bandwidth and also be delayed until bandwidth is available.
bus enumeration	Detecting and identifying Universal Serial Bus devices.
class	A group of devices or interfaces that have a set of attributes or functions in common.
client	Software resident on the host that interacts with host software to arrange data transfer between a function in a device and the host. The client is often the data provider and consumer for transferred data.
configuration	One of possibly several settings a device can be programmed into. Configurations may be constrained by available power or bandwidth, or may be differentiated by function. See also, function.
configuring software	The host software responsible for configuring a Universal Serial Bus device. This may be a system configurator or software specific to the device.
control pipe	Same as a message pipe.
control transfer	One of four Universal Serial Bus Transfer Types. Control transfers support configuration/command/status type communications between client and function.
default address	An address defined by the Universal Serial Bus Specification and used by a Universal Serial Bus device when it is first powered or reset. The default address is 0x0.
default pipe	The message pipe created by Universal Serial Bus system software to pass control and status information between the host and a Universal Serial Bus device's Endpoint 0. See also, pipe.
device	A logical or physical entity that performs a function. The actual entity described depends on the context of the reference. At the lowest level, device may refer to a single physical hardware component, as in a memory device. At a higher level, it may refer to a collection of hardware components that perform a particular function, such as a Universal Serial Bus interface device. At an even higher level, device may refer to the function performed by an entity attached to the Universal Serial Bus; for example, a data/FAX modem device. Devices may be physical, electrical, addressable, and logical. When used as a non-specific reference, a Universal Serial Bus device is either a hub or a function.
device address	The address of a device on the Universal Serial Bus. The device address is the default address when the Universal Serial Bus device is first powered or reset. Hubs and functions are assigned a unique device address by Universal Serial Bus software. See also, hub.
device driver	A program responsible for interfacing to a hardware device.
device endpoint	A uniquely identifiable portion of a Universal Serial Bus device that is the source or sink of information in a communication flow between the host and device. See also, isochronous sink endpoint, and isochronous source endpoint.
downstream	The direction of data flow from the host or away from the host. A downstream port is the port on a hub electrically farthest from the host that generates downstream data traffic from the hub. Downstream ports receive upstream data traffic.
endpoint	See device endpoint.
endpoint address	The combination of a Device Address and an Endpoint Number on a Universal Serial Bus device.
endpoint number	A number that identifies a unique pipe endpoint on a Universal Serial Bus device.
frame	The time from the start of one start of frame (SOF) token to the start of the subsequent SOF token. A frame is the master clock of the USB, and is typically 1ms long. See also, SOF.
function	A capability provided to the host by a Universal Serial Bus device. For example, an ISDN connection, a digital microphone, or speakers. A device may provide one or more functions.
host	The computer system in which the Universal Serial Bus host controller is installed. This includes the host hardware platform (CPU, bus, etc.) and the operating system in use.
host controller	The host's Universal Serial Bus interface.
host controller driver	The Universal Serial Bus software layer that abstracts the host controller hardware. Host Controller Driver provides an SPI for interaction with a host controller. Host Controller Driver hides the specifics of the host controller hardware implementation. On the Macintosh this is the Universal Serial Bus interface module (UIM), which is pronounced whim .
hub	A Universal Serial Bus device that provides additional attachment points to the Universal Serial Bus.
interface	A collection of pipes which form a logical interface to part or all of a device. USB devices all have an interface or interfaces. Interfaces provide the definitions of the functions available within a device. The device's function or functions are defined by the interfaces it supports. See also, pipe.
isochronous data	A stream of data whose timing is implied by its delivery rate.
isochronous device	An entity with isochronous endpoints, as defined in the USB specification, that sources or sinks sampled analog streams or synchronous data streams.
isochronous sink endpoint	An endpoint that is capable of consuming an isochronous data stream.
isochronous source endpoint	An endpoint that is capable of producing an isochronous data stream.
Isochronous transfer	One of four Universal Serial Bus transfer types. Isochronous transfers are used when working with isochronous data. Isochronous transfers provide periodic, continuous communication between host and device.
little endian	Method of storing data that places the least significant byte of multiple byte values at lower storage addresses. For example, a word stored in little endian format places the least significant byte at the lower address and the most significant byte at the higher address. The USB standard uses little-endian format for multi-byte fields. See also big endian.
message pipe	A pipe that transfers data using a request/data/status paradigm. The data has an imposed structure which allows requests to be reliably identified and communicated. See also, pipe.
packet	Data organized in a group for transmission. Packets typically contain three elements: control information (source, destination, and length), the data to be transferred, and error detection and correction bits.
packet buffer	The logical buffer used by a Universal Serial Bus device for sending or receiving a single packet. This determines the maximum packet size the device can send or receive.
packet ID (PID)	A field in a Universal Serial Bus packet that indicates the type of packet, and by inference the format of the packet and the type of error detection applied to the packet.
physical device	A device that has a physical implementation; for example, speakers, microphones, and CD players.
pipe	A logical abstraction representing the association between an endpoint on a device and software on the host. A pipe has several attributes; for example, a pipe may transfer data as streams (stream pipe) or messages (message pipe).
port	Point of access to or from a system or circuit. For Universal Serial Bus, the point where a Universal Serial Bus device is attached.
root hub	A Universal Serial Bus hub attached directly to the host controller. The root hub is the origin (tier 0) of the USB, and is a software simulation of a standard USB hub device.
root port	The upstream port on a hub.
SOF	An acronym for Start of Frame. The SOF is the first transaction token in each frame. SOF allows endpoints to identify the start of frame and synchronize internal endpoint clocks to the host.
stream pipe	A pipe that transfers data as a stream of samples with no defined Universal Serial Bus structure.
synchronization type	A classification that characterizes an isochronous endpoint's capability to connect to other isochronous endpoints.
transaction	The delivery of service to an endpoint; a complete logical transfer with a beginning and end, consists of a token packet, optional data packet, and optional handshake packet. Specific packets are allowed/required based on the transaction type.
transfer	One or more bus transactions to move information between a software client and its function.
transfer type	Determines the characteristics of the data flow between a software client and its function. Four transfer types are defined: control, interrupt, bulk, and isochronous.
UIM	The Universal Serial Bus Interface Module (UIM); the low-level (controller specific) software that provides the upper layers of the USB management software with a hardware abstraction layer to the USB host controller interface hardware.
Universal Serial Bus (USB)	A collection of Universal Serial Bus devices and the software and hardware that allow them to connect the capabilities provided by functions to the host.
USB software	The host-based software responsible for managing the interactions between the host and the attached Universal Serial Bus devices. The USB drivers, USB Manager, and UIM provide these software services on the Macintosh computer.
USB driver	The host-resident software entity responsible for providing common services to clients that are manipulating one or more functions on one or more host controllers, hubs or devices.
upstream	The direction of data flow towards the host. An upstream port is the port on a device electrically closest to the host that generates upstream data traffic from the hub. Upstream ports receive downstream data traffic.

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