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
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
- ISDN bearer service
channel operating at 64 kbps, carrying user voice or data;
circuit-, packet-, or frame-mode services may be obtained on
- 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
- Bearer Service
- The basic set of services offered over the B-Channel that
provides the capability to exchange signals between two user-network
- 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
- 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
- CODEC (coder/decoder)
- Device used to convert analog signals to digital bit streams and
vice versa; used to allow voice and video communication over
- 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
- 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
- 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
- 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.
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
- 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
- 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
- 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
- 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
- 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
- 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
- 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.