Ref: 99980085
Title: X.25 Terminology
Date: 6/1/87

Copyright 3Com Corporation, 1991.  All rights reserved.

When you manage a network, it pays to know the language.  Much of
the nomenclature behind X.25 is cryptic.  This is  a brief
explanation of some of the terms.

CCITT       Comite Consultatif International pour le Telegraphe
            et le Telephone, which published the X.25
            recommendation.

CRC         Cyclic Redundancy Check.  Method using polynomial for
            detecting and possibly correcting errors in a block
            of data.

Datagram    Data communication mode allowing user to send data
            without any connection between the sender and the
            addressee.

DATAPAC     Canadian Public Data Network.

DATEX-P     West German Public Data Network.

DCE         Data Circuit Terminating Equipment or Data
            Communications Equipment.

DDXP        Japanese Public Data Network.

DTE         Data Terminal Equipment is a device at user's end of
            an X.25 network.  It can be a computer, a terminal,
            or the GS/1.

Facility    Optional service offered by the PDN and requested by
            the user either at subscription time or at connection
            time.  A facilities field is contained in an X.25
            packet.

Fast Select Optional network service that allows the user to send
            data as part of a control packet which establishes a
            virtual connection.  The response to this packet
            (call accepted or call cleared) can also contain
            data.  It is not supported on the GS/1.

HDLC        High-level Data Link Control is a bit-oriented
            protocol defined by the International Standards
            Organization, used by X.25, and implemented on the GS/
            1.

LAP         Link Access Protocol is a data-link protocol used in
            older versions of X.25.

LAPB        Link Access Protocol Balanced refers to a peer-to-
            peer data link protocol that is a subset of HDLC
            implemented on the GS/1.

PAD         Packet Assembler/Disassembler is a communications
            device designed to provide an interface betwen a
            terminal and an X.25 network.  The exchange of
            control information between a DTE and a host attached
            to the X.25 network is regulated by the X.29
            recommendation.  PAD parameters are called X.3
            parameters.

PDN         Public Data Networks outside the United States are
            usually administered by governments's PTT.  In the
            U.S., PDNs are privately owned; e.g., Tymnet,
            Telenet.

PSS         British Public Data Network.

Telenet     American VAN (Value Added Network).

TRANSPAC    French Public Data Network.

Tymnet      American Value Added Network.

V-series    CCITT recommendations.

V.24        CCITT recommendation defining an electrical and
            physical interface.  It is equivalent to RS-232-C.

X.1         CCITT recommendation that defines the classes of
            service which may be offered by an international PDN.

X.2         CCITT recommendation that defines the international
            user services and facilities which may be supported
            by a PDN.

X.3         CCITT recommendation that defines the PAD (Packet
            Assembly/Disassembly) facility which may be offered
            by a Public Data Network.  It is supported on the GS/
            1.

X.21        CCITT recommendation that defines the interface
            between a DTE and a DCE of synchronous transmission.

X.21-bis    CCITT recommendation that defines the interim (bis)
            interface between a DTE and a DCE for synchronous
            transmission.  It is equivalent to CCITT V.24 and RS-
            232-C and is supported on the GS/1.

X.25        CCITT recommendation that defines the interface
            between a DTE and a DCE for packet mode operation on
            a packet switching network.

X.28        CCITT recommendation that defines the interface
            between a start-stop mode DTE and a PAD.

X.29        CCITT recommendation that defines the interface for
            the exchange of control information and user data
            between a packet mode DTE (X.25 host) and a PAD.  It
            is supported on the GS/1.

X.75        CCITT recommendation that defines the interface for
            the connection of two Public Data Networks.
