Appendix B - Glossary

Advanced Relay Corporation, creators of LayGO.
American National Standards Institute (ANSI)
Coordinator of U.S. national standards activity and the U.S. representative to ISO.
Application Layer
Applications to the user, such as message Layer 7 of the OSI Reference Model; provides specific network services and handling and electronic mail, directory services, file transfer, virtual terminal emulation, and network management.
Asynchronous Transfer Mode (ATM)
A cell relay transmission scheme that will be used for B-ISDN (Broadband ISDN) applications; ATM uses a 53-octet cell, where the first 5 octets convey header information and the remaining 48 octets are payload; the ATM protocol architecture comprises a physical layer, ATM Layer, and ATM Adaptation Layer.
Asynchronous Transmission
Transmission scheme where each octet is preceded by a single START bit and followed by a STOP interval lasting for at least 1 bit time. Typically used in terminal-to-computer communications. The term asynchronous refers to the variable timing between characters and is sometimes used for any time-insensitive application.
ISDN bearer service channel operating at 64 kbps, carrying user voice or data; circuit-, packet-, or frame-mode services may be obtained on this channel.
The width of a channel's passband (e.g. the bandwidth of a channel with a 300- to 3400-Hz passband is 3100 Hz, or 3.1 kHz)
Basic Rate Interface (BRI)
One of the access methods to an ISDN, comprising two B-Channels and one D-Channel (2B+D); described in CCITT Recommendation I.430.
Bearer Service
The basic set of services offered over the B-Channel that provides the capability to exchange signals between two user-network interfaces.
Bit-oriented Protocol
A type of a data link layer protocol commonly used in ISDN applications, based upon ISO's High-level Data Link Control (HDLC) protocol. Bit-oriented protocols usually use this frame format, or a variant; a Flag (01111110) to indicate the beginning of a frame, an Address field to indicate the sender or intended receiver, a Control field to indicate the frame type and carry sequence numbers, an Information field to carry the data, a Frame Check Sequence field for bit error detection, and another Flag to indicate the end of the frame; to insure that an unwanted Flag bit pattern does not occur, zero-bit insertion is used for transparency. LAPB (X.25), LAPD (ISDN), LAPF (frame relay), the Signaling Link protocol (SS7), and V.120 (rate adaptation) are all HDLC-like bit-oriented protocols.
See Zero-bit Insertion and Removal.
In ISDN, channels supporting rates above the primary rate (1.544 or 2.048 MBPS). In general data communications, usually refers to analog, modulated signals and a bandwidth greater than that of the 3.1-Khz voiceband.
Unidirectional transmission from a single point to one or more subscribers.
Bits per second. A data rate defining the number of binary digits (1 or 0) passing a given point per second.
Call Request Packet
In the X.25 protocol, a call packet is a specially formatted block of data which is transmitted to a receiving station in order to request establishment of a virtual circuit. The call packet contains the X.25 network address of the calling and called stations (i.e. the source and destination), call facilities which define the nature of the virtual circuit, and sometimes a user name and password for using the services of a PSDN.
Circuit switching
A switching procedure where two devices are connected by a physical resource that is dedicated to the parties for the duration of the call.
CODEC (coder/decoder)
Device used to convert analog signals to digital bit streams and vice versa; used to allow voice and video communication over digital networks.
See Datagram
See Virtual Circuit
CRC (cycling redundancy check)
Mathematical algorithm used to detect bit errors in data transmission; implemented in chips with shift registers and exclusive-OR gates, thus adding no processing delay.
CCITT (International Telegraph and Telephone Consultative Committee)
A committee of the ITU, creating recommendations regarding public telegraph, telephone, and data networks; originally organized in 1865, the CCITT is the oldest intergovernmental agency. Renamed ITSS in March 1993. This standard is normally updated every four years, using different colors for the documentation set. The standards are therefore also sometimes referred to as the Red Book (1984) or the Blue-Book (1988).
DCE (data circuit-terminating equipment)
This term is used in conjunction with DTE to define the relationship between two end-points in a data communications circuit. A DCE is typically a computer or other device to which a DTE, or terminal, is attached. For example, a Video Display Terminal (DTE) can be attached to a modem (DCE) or to a host computer (DCE). In this sort of physical connection, each end-point generates certain electrical signals. DTE/DCE is also used to refer to end-points of a communications circuit realized through a protocol, such as HDLC/LAPB or X.25. In this case, the terms DTE/DCE refer not to electrical signals, but to protocol features or capabilities supported by an end-point. For example, a PSDN functions as an X.25 DCE to provide data communication and routing services to X.25 DTEs which attach to the PSDN.
The ISDN out-of-band signaling channel, carrying ISDN user-network messages; it can also be used to carry packet- or frame-mode user data. The D-channel operates at 16 kbps in the BRI and 64 kbps in the PRI.
DLCI (Data Link Connection Identifier)
A portion of the address field that identifies a data link layer logical channel; in LAPD, the DLCI is the 13-bit data link identifier comprising the Service Access Point Identifier (SAPI) and Terminal Endpoint Identifier (TEI) subfields; in LAPF, the DLCI refers to the frame-mode virtual circuit.
Data Link Layer
Part of the multi-layer OSI protocol model defined by the International Standards Organization. The link layer provides for error free data transfer between directly connected systems. The X.25 protocol typically used HDLC/LAPB as its link layer protocol. The X.25 terminology uses Frame Level for X.25 Link Level.
Datagram Services
A network service that does not require that an end-to-end connection be established prior to the exchange of information; each datagram must contain the complete destination address for routing, and there is no guarantee of delivery or of sequential delivery; also called a connectionless service.
Signals or data are discrete (i.e., they may only take on specified values within a range of values, such as binary data streams containing only 0s and 1s).
In the world of computer software, a driver usually refers to a software module which is an add-on part of a computer operating system and is responsible for interfacing the operating system to a physical device, such as a disk drive or data communications processor.
Data Service Unit/Channel Service Unit.
DTE (data terminal equipment)
This term is used in conjunction with DCE to define the relationship between end-points in a data communications circuit. See DCE.
See Full-duplex
FCS (frame check sequence)
The portion in a data link frame containing the remainder from the Cyclic Redundancy Check (CRC), or other bit-error detection calculation.
In bit-oriented protocols the unit of transmission at the physical or data link layer; physical layer frames are fixed-size blocks of transmitted signals and contain some sort of frame delimiters; data link layer frames are variable-length groups of octets, often delimited by a special 8-bit pattern called a Flag (01111110).
Frame Relay
An HDLC frame-mode service based on fast packet switching technology. Frame relay uses a minimal set of data link layer procedures across the user-network interface, providing unacknowledged transfer of variable-length frames between users. Additional layer 2 and 3 functions must be provided by the end user. The layer 2 protocol uses LAPF core procedures, and the network discards any frame with bit errors.
Bi-directional communications facility where transmissions may travel in both directions simultaneously. Also called duplex.
Bi-directional communications facility where transmissions may travel in either one direction or the other at any given time, but not both. Sometimes called simplex.
HDLC (High-level Data Link Control)
The ISO standard for bit-oriented protocols: LAPB, LAPD, LAPF, LAPM (V.42), V.120 and the SS7 data link layer are all based on HDLC.
An end-communicating system in a network (e.g., a telephone in the telephone network or a computer in a data network).
ISDN (Integrated Service Digital Network)
A digital network that provides a wide variety of communication services, a standard set of user-network messages, and integrated access to the network.
ISO (International Standard Organization)
International standards organizations that comprise national standards bodies. ANSI, for example, is the U.S. representative to ISO. ISO's data communications standards include HDLC, the OSI Reference Model, and the OSI protocols.
ITU (International Telecommunication Union)
An agency of the United Nations, the parent organization of the CCITT (now called ITSS).
ITTS (ITU Telecommunications Standardization Sector)
The new name of the CCITT after the ITU's reorganization in 1993.
LAPB (Link Access Procedures Balanced)
The X.25 data link layer protocol.
LAPD (Link Access Procedures on the D-channel)
The ISDN data link layer protocol specified for the D-channel; described in CCITT Recommendation Q.920 (I.440) and Q.921 (I.441).
LAPF (Link Access Procedures to Frame-Mode Bearer Services)
The data link layer protocol for frame relay and frame switching described in CCITT Recommendation Q.922.
Kilobits per second. 1 kbps = 1000 bps.
Local Area Network. A local area network is a physical communication medium (such as a cable) combined with communication hardware and protocol software, used to interconnect computers and peripheral devices to facilitate the exchange of data over short distances, such as within an office building.
In the LayGO API, a state to await an incoming connect request. When the connect request is received, it is directed to the application program, which may then send a connect accepted in reply, thus forming a connection.
A mechanism to logically group together sets of individual physical layer frames, allowing for the definition of channels for physical layer signaling; ANSI T1.601 and CCITT Recommendation I.430 (BRI) and I.431 (PRI) define multiframing formats.
The ability of many users to share a single communications facility. Common approaches are frequency division multiplexing, time compression multiplexing, and time division multiplexing.
National ISDN
Defined by Bellcore, a process of identifying and implementing ISDN service features in a consistent way across different vendors' switches and CPE, resulting in a more consistent service definition and CPE portability. National ISDN-1 (1991) focused primarily on the BRI and National ISDN-2 (1992) added PRI features.
A collection of communicating devices, switches, and links that are interconnected and autonomous.
Network Layer
Layer 3 of the OSI Reference Model; primarily responsible for congestion control, routing, and network accounting.
An 8-bit block.
OSI (Open System Interconnection) Reference Model
A seven-layer model architecture for open systems, allowing communication between computers from different vendors using different network architectures; initially proposed by ISO, it has been adopted by CCITT and all major computer manufacturers around the world; the model and protocols are defined in CCITT X.200-series Recommendations.
PAD (Packet Assembler/Disassambler) facility
In X.25, provides access to a PSPDN by an asynchronous terminal; at the sender's side, the PAD collects a group of characters and builds an X.25 packet; at the receiver's side the PAD accepts an X.25 packet and delivers individual characters to the terminal; the PAD is based upon CCITT X.3, X.28, X.29 (Triple X) Recommendation.
Packet Layer Protocol (PLP)
The X.25 layer 3 protocol.
The frequency spectrum that can pass through a channel, which may be limited by the characteristics of the medium or by filters; the passband of a telephone local loop is 300 to 3400 Hz.
PVC (Permanent Virtual Circuit)
A virtual circuit service that is established at subscription time and remains available for a predetermined amount of time. No call establishment or termination procedure is required to use a PVC. PVCs are supported by both X.25 and Frame Relay.
Physical Layer Protocol
Layer 1 of the OSI Reference Model; primarily responsible for the transport of bits between adjacent devices in a network, describing electrical and mechanical characteristics of a connection and media.
PSPDN (Packet Switched Public Data Network)
A public data network using packet switching technology; commonly supports X.25 interface.
Virtual Circuit
In a store-and-forward network, a logical end-to-end connection between two hosts; the VC must be established at service subscription time (PVC) or on demand by the user (SVC), but the network does not dedicate a transmission facility to this connection.
WAN (Wide Area Network)
A network that spans a large geographic scope, such as a national or international telephone or data network.
A CCITT recommendation describing layers 1 through 3 of the user-network interface for PSPDNs.
Zero-bit Insertion and Removal
The transparency technique used in bit-oriented protocols; to ensure that an unwanted Flag bit pattern (01111110) does not occur, the transmitter will automatically insert a 0 after any string of five contiguous 1 bits while sending data; conversely the receiver will automatically remove a 0 after any string of five contiguous 1 bits while receiving data.