USB Glossary of Terms
(A-D) (E-I) (J-P) (Q-Z)
802.3 – Short for IEEE802.3. In strictest sense, the Ethernet II (DIX) networking standards suite. In looser sense, the format for Ethernet-like frames bridged over another transport (such as a USB link). (4)
ACK – Handshake packet indicating a positive acknowledgement. (1)
Active Device – a device that is powered and is not in the Suspend state. (1)
API – Application programming interface
Anycast – Communication between a single sender and the nearest of several receivers in a group (a form of restricted broadcast). (5)
ARP – Address Resolution Protocol associates an IP address to a hardware address called Media Access Control (MAC) address. (5)
Address Resolution Protocol (ARP) – The process by which a node on an Ethernet segment discovers the MAC (Ethernet) address matching a given IP address. In the simplest case, node A issues an ARP broadcast, and the node with the IP address replies Compare with RARP (Reverse ARP) and DHCP. (5)
Bandwidth – The amount of data transmitted per unit of time, typically bits per second (b/s) or bytes per second (B/s). (1)
b/s – Transmission rate expressed in bits per second. Also bps especially with a qualifier such as Mbps (mega-bits per second) (1)
B/S – Transmission rate expressed in bytes per second. Also Bps as in MBps (mega-bytes per second) (1)
Bit Stuffing – Insertion of a ‘0’ bit into a data stream to cause an electrical transition on the data wires, allowing a PLL to remain locked. (1). USB uses NRZI signalling. A long series of 1’s is seen on the wire as no change. Therefore, after a certain number of 1’s an artificial 0 is inserted to allow the two ends to maintain timing synchronization.
BLAN MDLM – Belcarra Local Area Network protocol, a custom protocol for IP packet transfer across a USB connection. This protocol was developed by Belcarra as an alternative to CDC-ECM. BLAN streamlines and extends ECM for the case of personal area networks (PANs).
BSP – Board Support Package.
Bulk Endpoint – Bulk Endpoints are used to transfer large amounts of data between the USB Device and USB Host. Data is transferred on best efforts basis. There is a large overhead to having active IN Bulk Endpoints as the USB Host must continuously poll it to see if there is data to send. Note that the directions IN and OUT always refer to the Host. When a USB device transmits data to a USB host, it uses an IN endpoint.
Bulk Transfer – One of four USB transfer types. Bulk transfers are non-periodic, large bursty communications typically used for a transfer that can use all available bandwidth and can also be delayed until bandwidth becomes available. See also transfer type. (1)
Bus Enumeration – Detecting and identifying USB Devices. (1)
Call Management – For a communications class device (q.v.), the process of setting up and tearing down of calls (modems, ISDN, ATM, etc). The data transfer phase follows. Since the call and data transfer phase are completely different, and may (in the case of ISDN and ATM) involve accessing different hardware components, the CDC protocols sometimes use more than 1 USB interface (q.v.) to model such a device. (2)
CDC – A core communication Device Class for all Communications Device Class (q.v.) devices, together with subclass definitions matching several models (types of Communications device). These specifications were originally issued in one very large specification known as CDC1.1 (no longer available), but later broken down into a suite of documents available from usb.org. The Communications Device Class suite of protocols were created by the USB Implementers Forum to define access via USB to pre-USB communications devices such as Ethernet controllers, ISDN, ATM, and others. (http://www.usb.org/developers/devclass_docs) The suite defines a wide variety of related communications protocols, but only a few are widely used, as follows:
• Core specification – CDC120.pdf
• Ethernet Control Model (CDC-ECM). This protocol was created to model communication via USB with an external Ethernet controller or a similar device, such as Cable Modem using the same frame format. This protocol has been used for many non-Ethernet device that implement a bare minimum of the Ethernet requirements (frame format, mostly) – ecm120.pdf
• BLAN (Belcarra LAN). Belcarra’s BLAN is an extension/streamlining of CDC-ECM for personal area networks. CDC-EEM (see below), which was created later than BLAN, has a similar purpose.
• Ethernet Emulation Model (CDC-EEM). This protocol was created from the recognition that CDC-ECM was widely used for devices that are not Ethernet, but only use the Ethernet frame format to create a network consisting of the USB and the USB device. CDC-EEM removes a great deal of complexity associated with CDC-ECM and also provides enhanced buffering. http://www.usb.org/developers/devclass_docs/CDC_EEM10.pdf)
• Abstract Control Model (CDC-ACM) This strange term refers to a virtual serial port. The name refers to the fact that the serial device may in fact be a modem, requiring a control phase (dialing) followed by a data transfer phase. This protocol implements a virtual serial port or serial-controlled modem (using AT commands). The ACM protocol defines two USB interfaces to allow for this rarely used possibility. Contained inside the document PSTN120.pdf
CDC ACM - Abstract Control Model allows a USB Device to emulate a serial communications interface. That interface an be used to by any application that expects to access the embedded device using a serial connection.
CDC ECM - Ethernet Networking Control Model allows a USB Device to emulate a standard network interface card (NIC). That network interface can be used to allow networking via the device to another network.
CDC EEM - Ethernet Emulation Model allows a USB Device to emulate a reduced type of network interface suitable for allowing network traffic between two devices (the USB Host and the USB Device.)
CDC NCM - A proposed standard, under development by the Communications Device Class committee of the USB Implementers Forum to develop an enhancement of CDC-ECM suitable for high-speed environments. Among the issues being considered by the Committee is the need to encapsulate more data in a USB transfer (CDC-ECM limits this to 1500 bytes, the maximum size of 1 802.3 frame).
Client – Software resident on the host that interacts with the USB System Software to arrange data transfer between a function and the host. The client is often the data provider and consumer for transferred data. (1)
Communications device class – USB devices which offer support to external communications devices such as modems, ISDN, Asynchronous Transfer Mode (ATM), Ethernet, and others. Since these communications devices vary quite a lot, a they required a large number of subclasses (variations within the class). See the CDC entry for more information on the commonly used Communication Device Class subclass protocols in use.
Communications Interface – Refers to a USB interfaces that identifies itself as using the Communications Class definition. (2)
Composite Device – One of two mechanisms in the USB Standards for a device to offer two or more features. (c.f. compound device). The USB standards do not specify a mechanism for the partitioning of a composite device into “features”. See also Interface
Compound Device – A compound device is two or more USB devices grouped together with an integrated USB Hub. Therefore, the USB Host sees various separate USB devices and the integrated hub.
Control Endpoint - The Control Endpoint (there is only ever one) is used as the primary communication channel between the USB Device and USB Host during the enumeration and configuration phase of a USB Connection. All USB Device Requests are sent via the Control Endpoint. A USB Device must always be ready to respond to USB Device Requests sent to the Control Endpoint (1)
Control Pipe – Same as a message pipe. (1)
Control Transfer – One of four USB transfer types. Control transfers are defined to use small packet sizes, and require limited resources. This means the bulk of a system can be dedicated to bulk data transfer. Control transfers, as the name suggests, are used to implement device requests from the host to the device, especially the setup (enumeration) procedure when a device is first connected. (1)
Core – In Belcarra’s USB stack, the layer of the device driver that implements management and states of the USB stack
C/S/P – Class/SubClass/Protocol. Key fields in a USB Device Descriptor, used by host software to recognize supported types of USB device. A class code is assigned to a group of related devices by a USB Class Specification. A class of devices may be further subdivided into subclasses, each with its own class code. Finally, within a class or subclass, a protocol code may further identify how host software communicates with the device. Similar fields in Interface descriptors and Interface Association Descriptors (qqv) allow partitioning of multifunction devices into single-function devices. Examples:
• C/S/P (2,2,0) identifies an ACM device. These values will appear in the Device Descriptor of a single function device, or in an IAD descriptor of the ACM component of a multi-function device.
• C/S/P (2,0Ch,7) identifies an EEM device. These values will appear in the Device Descriptor of a single function device, or in an IAD descriptor of the ACM component of a multi-function device.
• C/S/P (0EFh,02,01) identifies an IAD (multifunction/composite) device. An IAD device will have an IAD descriptor for each component function. The IAD descriptors will contain C/S/P values which are used in the same way as C/S/P values in the device descriptors of single-function devices.
• The Mass Storage class requires that the C/S/P in the device descriptor be 0/0/0. The Mass Storage drivers are triggered depending on C/S/P values in the IAD or Interface descriptors.
Data Interface – Refers to a USB interface that identifies itself as using the Data Class definition. (2)
DCE – Data Circuit Terminating Equipment, for example, a modem or ISDN Terminal Adapter. (3)
Default Address – An address defined by the USB Specification and used by a USB device when it is first powered on or reset. The default address is 00H. (1)
Descriptor – Data structure used to describe a USB device capability or characteristic.
Device – A logical or physical entity that performs a function. The actual entry described depends on the context of the reference. At the lowest level, device may refer to a single 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 USB interface device. At an even higher level, device may refer to the function performed by an entity attached to the USB; for example;, a data/FAX modem device. Devices may be physical, electrical, addressable, and logical. (1)
Device Address – A seven-bit value representing the address of a device on the USB. The device address is the default address (00H) when the USB device is first powered or the device is reset. Devices are assigned a unique device address by the USB System Software. (1)
Device Class Specification – Any of various specifications for assorted classes of USB devices, as developed by the USB Implementers Forum (USB-IF), and approved by the USB Board, and available from http://www.usb.org/developers/devclass_docs.
Device Endpoint – A uniquely addressable portion of a USB device that is source or sink of information in a communication flow between the host and device. See also endpoint address. (1)
Device Management – Refers to requests and responses that control and configure the operational state of the device. Device management requires the use of a Communications Class interface. (2)
Device Resources – Resources provided by USB devices, such as buffer space and endpoints. See also Host Resources and the Universal Serial Bus Resources. (1)
Device Software – Software that is responsible for using a USB device. This software may or may not also be responsible for configuring the device for use. (1)
Device Subclass -- A USB specification to support a sub-category of Devices of a larger Device Class. Examples of Device Classes with many Subclasses are Communication Device Class (CDC, see above) and Mass Storage (many types of disk-like devices).
DHCP – Dynamic Host Configuration Protocol is a protocol for organizing and simplifying the administration of IP configuration for computers in a network. On a simple isolated Ethernet-like network segment, a DHCP server provides a way for transient devices (such as PC’s which connect/disconnect randomly) to be assigned a dynamic non-conflicting IP address. (5)
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. (1)
Driver – When referring to hardware, an I/O pad that drives an external load. When referring to software, a program responsible for interfacing to a hardware device, that is, a device driver. (1)
DTE – Data Terminal Equipment. In the old RS232 standard, a node controlling a serial connection (the PC or terminal). Compare DCE (Data Communications Equipment), usually identified with the modem and DCD (Data communications detect) the signal that the modem had established communication with a remote modem. In the CDC-ACM standard, these concepts are implemented in a virtual way with certain device requests and notifications (q.v.)
References
1. Universal Serial Bus Specification Revision 2.0 – Chapter 2 Terms and Abbreviations
2. Universal Serial Bus Communications Device Class – Section 1.4 Terms and Abbreviations
3. Universal Serial Bus Communications Class Subclass Specification for PSTN Devices - Section 1.4 Terms and Abbreviations
4. Universal Serial Bus Communications Class Subclass Specification for Ethernet Control Model Devices - Section 1.4 Terms and Abbreviations
5. Universal Serial Bus Communications Class Subclass Specification for Ethernet Emulation Model Devices