Glossary Content

American Standard Code for Information Interchange (ASCII)
ASCII defines values, in the range of 0 to 127, that represent individual characters of the Western alphabet.

API
See Application Programming Interface.

Application Programming Interface (API)
An Application Programming Interface is a set of subroutines or parameter blocks that is used by a software program to control or pass data to or from another software subsystem.

ASCII
See American Standard Code for Information Interchange.

asynchronous communication
When transmitted using asynchronous communications, a byte of data is always preceded by a "start bit" and followed by one or two "stop bits". When no data is being transmitted, the line is in a steady state. This steady state is broken at any time with a start bit, so data may be started without reference to any other source of clocking, hence the term "asynchronous". The opposite of asynchronous is synchronous, in which an idle line is filled with pad bytes. In synchronous communication, a data byte can only be transmitted in place of a pad byte, so all data is "synchronized" to start at predetermined intervals.

Autonomous System
An autonomous system is a group of networks and gateways controlled by a single administrative authority.

Call ID
A feature offered by many telecommunications companies which displays or provides the telephone number of the location originating the call.

Call User Data
This is a field in an X.25 SVC call setup packet that may contain data defined by the caller or call receiver.

Calling Address
The Data Network Address (DNA) of the DTE that is placing the X.25 SVC Call Request.

CCITT
See International Telegraph and Telephone Consultative Committee.

CRC
See Cyclic Redundancy Check.

Cyclic Redundancy Check (CRC)
A Cyclic Redundancy Check sequence of bytes is a checksum that is calculated by rotating successive bytes of a data block. It is a more reliable indication of errors than a horizontal checksum, which does not rotate bytes.

D-bit
The Delivery Confirmation bit in an X.25 Packet Layer packet. It requires the network to withhold acknowledgement for the data packet in which it is set until the packet is transmitted to the remote DTE and DTE acknowledges receipt.

daemon
This term originated with UNIX, and refers to a program that runs in the computer without being attached to a user or a user terminal. For example, a line printer daemon spools printout to a printer independently of the user program that generated the output.

Data Network Address (DNA)
An X.25 host address. See X.121.

datagram
A packet of data that contains an address sufficient to define the destination globally. Therefore, a virtual circuit does not need to be set up to provide a route for the packet; the packet may be freely transmitted through whatever gateways are available to reach its destination.

Data Link Connection Identifier (DLCI)
The address within a Frame Relay network. DLCIs only have local significance.

Datapac
The public X.25 data network operated in Canada.

DCE and DTE
The distinction between the two entities communicating on an X.25 link. One end is the Data Terminal Equipment (DTE, i.e. customer-provied equipment), the other the Data Communications Equipment (DCE, i.e. network switch).

DDN-style addressing
DDN-style addressing defines an algorithm for determining an X.25 address via numeric operations on an IP address, as opposed to the arbitrary IP/X.25 address mapping which needs to be used when a public data network is used for transporting IP data.

DISC
This is the name of an X.25 Frame Layer response frame.

Disconnect Mode (DM)
This is the name of a mode-setting and reporting X.25 Frame Layer supervisory frame.

DLCI
See Data Link Connection Identifier.

DM
See Disconnect Mode.

DNA
See Data Network Address

DXE
A term used to denote one side of an X.25 link that can be either a DTE or DCE.

EGP
See Exterior Gateway Protocol.

EIA
See Electronic Industries Association.

Electronic Industries Association (EIA)
The term "EIA" is typically used in data communications to refer to the electrical signals of the RS-232 standard.

Exterior Gateway Protocol (EGP)
This term refers to the protocol used by gateways to autonomous systems to inform other gateways of the ability to reach the core system of Internet gateways.

FEP
See Front End Processor.

Front End Processor (FEP)
The processor card that is plugged into the PC chassis that runs the actual serial communications line.

Frame Relay
Frame Relay is a specification designation defined by both ANSI and CCITT.

Gateway
A computer which attaches to two or more networks in order to pass packets between the networks.

HDLC
See High Level Data Link Control.

High Level Data Link Control (HDLC)
This is an ISO standard data communications protocol that can recover from data transmission errors, and accommodate two-way simultaneous traffic.

International Telegraph and Telephone Consultative Committee (CCITT)
In French, the words are changed around so that the acronym becomes CCITT. This international body sets standards for telephone equipment.

internet
A collection of packet-switching networks interconnected by gateways. Because of the protocols used in internets, the entire collection appears as one logical network, although elements of the network may be widely geographically scattered.

Internet
The collection of networks and gateways which uses the TCP/IP protocol suite to function as a single cooperative virtual network.

Internet Format
Internet format, or "dot format", is a way of representing the four bytes of an internet address. Rather than a huge decimal number, addresses are written as a string of four numbers in the range 0 to 255, separated by periods.

Internet Protocol (IP)
The lower level protocol in TCP/IP, a protocol suite defined by the United States Department of Defense, and originally used in the DARPA (Defense Advanced Research Project Agency) network, now called the Internet. IP looks after moving packets from one node in a network to another.

intranet
A network within an organization that uses all the protocols and software that are used on the Internet, but the only hosts that are communicated with are within the organization.

IP
See Internet Protocol.

IP address
Internet Protocol address. A 32 bit number identifying the host to which the datagram in which it appears is to be delivered.

kernel
The term "kernel" originated with UNIX, but it can be used with any operating system. It denotes the software that is linked into and resides with the operating system. It is distinct from applications programs that are loaded and run by the operating system.

LAN
See Local Area Network.

LAPB
See Link Access Procedure B.

LCI
See Logical Channel Identification.

Link Access Procedure B (LAPB)
This is the name given to the HDLC variant used by all X.25 Frame Layers since 1984.

Link Layer Interface API (LLI)
As used by The Software Group, this refers to the Link Layer Interface API. This is the definition of streams messages passed between a link layer user and a link layer provider. NetcomRouter is a link layer provider.

LLI
See Link Layer Interface.

LMI
See Local Management Interface.

Local Area Network (LAN)
A collection of computers connected by communications media. The maximum distance between hosts and the speed at which the networks operate depend on the type of LAN.

Local Management Interface (LMI)
A Frame Relay Consortium implementation of PVC management functions.

Logical Channel Identification (LCI)
An element of an X.25 Packet Layer packet that identifies the virtual circuit the packet is for.

Management Information Base (MIB)
A Management Information Base is the definition of information that must be maintained by an Simple Network Management Protocol (SNMP) managed object, and the data that may be set by a manager.

Maximum Received Units

The Maximum Received Units is the largest physical packet size, measured in bytes, that a network can receive.

MIB
See Management Information Base.

MRU

See Maximum Received Units

Network Packet Interface (NPI)
This Streams-based API is a standard for interfacing to X.25 Packet Layers.

Network User Identifier (NUI)
The Network User Identifier is a billing number to which the charges for an X.25 SVC are billed, as opposed to the originator or the receiver.

NPI
See Network Packet Interface.

NRZ
Non-return to Zero is a data encoding scheme.

NUI
See Network User Identifier.

Operating System (OS)
The basic control program that accepts user commands to load and run applications programs. For example, the DOS operating system loads and controls the operation of the Word application program.

OS
See Operating System.

Packet Assembler/Dissembler (PAD)
A Packet Assembler/Dissembler is any device or program that converts a user data stream into X.25 data packets and vice versa.

PAD
See Packet Assembler/Dissembler.

permanent virtual circuit (PVC)
A permanent virtual circuit is a circuit where the destination is fixed at network configuration time. The other type of virtual circuit is a switched virtual circuit (SVC), which is an X.25 virtual circuit that is established with a Call Request packet on an X.25 link. See also virtual circuit.

PTT
A European telephone company, usually the only one in a particular country.

PVC
See permanent virtual circuit.

Request For Comments (RFC)
Specifications of Internet-related protocols and facilities are described in Request for Comments documents published by the DDN Network Information Center, at SRI in Menlo Park, California.

RFC
See Request For Comments.

RIP
See Routing Information Protocol.

Router
A system which makes decisions about which path network traffic should take. See also Gateway.

Routing Information Protocol (RIP)
Routing Information Protocol is part of the Berkeley 4.3 BSD UNIX suite of communications software that provides a mechanism for exchanging routing information among a set of computers.

RPOA
Defines the public data networks through which an X.25 SVC should transit in order to reach the destination.

RR
X.25 term Received Ready, meaning it has received the last packet sent and is ready to receive the next.

RNR
X.25 term Received Not Ready, meaning it has received the last packet sent and is NOT ready to receive the next.

SABM
See Start Asynchronous Balanced Mode.

SNA
See Sub Network Address.

Start Asynchronous Balanced Mode (SABM)
A command in the X.25 Frame Layer. In this case, Asynchronous means the style of HDLC being used, not the electrical line utilization technique.

Sub Network Address
The last few digits of a Data Network Address (DNA) that is not used by the public network to identify a DTE. It is used within a DTE to select internal facilities, such as different applications.

SyncPPP

Synchronous PPP (SyncPPP) is used for for transferring TCP/IP data over leased lines or other dedicated connections. IP data is sent in frames using a bit-oriented protocol and synchronous clocking. When the connection is first established, Link Configuration packets (LCP), Authentication packets (such as PAP or CHAP) and Internet Protocol Control Protocol (IPCP) packets are exchanged with the remote. These packets automatically negotiate features for the connection (such as data compression), verify passwords and configure the IP addresses. This has the benefit of requiring less manual configuration and setup.

SVC
See Switched Virtual Circuit.

switched virtual circuit (SVC)
A Switched Virtual Circuit is an X.25 virtual circuit that is established with a Call Request packet on an X.25 link. The Call Request specifies the destination with an X.121 Data Network Address. The other type of virtual circuit is a permanent virtual circuit (PVC), where the destination is fixed at network configuration time. See also virtual circuit.

TCP
See Transmission Control Protocol.

Transmission Control Protocol (TCP)
The higher level protocol of TCP/IP, a protocol suite defined by the United States Department of Defense, and originally used in the Defense Advanced Research Project Agency network, now called the Internet. TCP looks after error detection and correction, in order to provide its users with an error-free path between two points.

TSG
An abbreviation for "The Software Group."

tty
An abbreviation for "teletype", the old terminals used by the telex network.

tty device
Keyboard and printer units in a telex network. These units preceded glass screens. However, the term has continued to be used to designate any character based input/output device.

UNIX kernel
See kernel.

VC
See virtual circuit.

virtual circuit (VC)
Virtual circuits are alternatives to datagrams. A Switched Virtual Circuit employs the use of a call establishment phase that addresses the destination. The network then provides a very short virtual circuit number (in X.25, the Logical Channel Identifier) that identifies the destination of packets to the network. Then data packets do not need to include the overhead of a global address - just the short LCI. A Permanent Virtual Circuit sets an LCI's destination at configuration time; it cannot be changed dynamically. See also switched virtual circuit.

X.121
Recommendation X.121 is the ITU designation for public packet-switched data network addressing.

X.25
X.25 is a specification designation. The ITU (formerly CCITT), a worldwide standards development/approval body, issues Recommendations which define how various aspects of telecommunications devices work. Recommendation X.25 specifies the communications protocol a customer's host uses to communicate with a common carrier packet-switched network.

X.25 Interface Package (XIP)
An implementation of the X.25 communications protocol developed by The Software Group Limited.

X.25 protocol
This popular computer communications protocol, defined by the CCITT, was developed to allow computers to access shared public data networks. There are two main layers to X.25: the HDLC, or Frame Layer, assures that any transmission errors are recovered. Above the Frame Layer is the Packet Layer. It is organized into multiple virtual circuits between this computer and up to 4096 simultaneous destination computers.

X.32
X.32 is the standard outlining the establishment of an X.25 link over public telephone network dial-up facilities, as opposed to dedicated facilities.

Revision 6.3.0 (April 2004)