Glossary

agent

A process, usually corresponding to a particular managed object, that carries out request from a manager.

Bits per second (bps)

The number of bits passed per second.

Call

Communication between two DTEs using a virtual circuit.

CCITT

Consultative Committee for International Telephony and Telegraphy. Previous name of the ITU-T.

Closed User Group (CUG)

A PSDN-provided service that permits the DTEs belonging to the group to communicate with each other, but precludes communication with other DTEs. A single DTE can belong to multiple CUGs. The PSDN, not the caller or called parties, enforce the limitation inherent in a CUG.

Data Circuit-terminating Equipment (DCE)

The network side of the user-to-network interface. Commonly corresponds to a modem or other device used to connect to a PSDN.

Data Network Identification Code (DNIC)

A four-digit number that identifies a specific PSDN. The DNIC is the first component of a complete X.121 address and is comparable to the exchange portion of a telephone number used for switched telephone service. In a DNIC, the first three digits make up a data country code, which identify a country, while the remaining digit can be used to distinguish up to 10 different networks within the specified country.

Data Terminal Equipment (DTE)

The device at the user's side of a user-to-network interface. This might be a computer system (packet-mode DTE) or a character-mode terminal (DTE-C), and is both a source and destination for data.

Data Terminal Equipment-Character Mode (DTE-C)

A character-mode (asynchronous) terminal that uses a PAD to connect to a remote host across a PSDN. Part of the Solstice X.25 software plays the role of a DTE-C when you use the PAD application that is shipped with the product.

Defense Data Network (DDN)

A network administered by the US Department of Defense, that uses a form of X.121 to IP address mapping defined in RFC 1236.

Duplex

Circuit or modem that allows for simultaneous two way data transmission.

Facility

An optional service offered by a public network's administration. Usually requested by the user when subscribing.

Flag

A sequence of bits that signifies the beginning and end of a frame.

Frame

The unit of information used at layer 2 of the X.25 Recommendations.

I-Frame

A frame containing data--an Information Frame.

International Telecommunication Union--Telecommunication Standardization Sector (ITU-T)

An international organization of communication carriers, most of which are government telephone and telegraph agencies. The ITU-T develops telecommunication standards through the use of their recommendations. The X.25, X.3, and X.29 standards are ITU-T Recommendations.

LAPB

The datalink layer protocol specified by the X.25 Recommendations.

Logical Channel

An association between two DTEs through which they exchange data.

MLP

MultiLink Procedure. This provides the datalink layer allowing a DTE and a DCE to connect using more than one WAN interface.

MIB

Management information base. A collection of objects that can be accessed through the SNMP agent.

Octet

The ITU-T's term for an 8-bit byte.

Packet

A sequence of bits representing data and associated control information. Is self-contained in that it has routing and packet-sequence information. Commonly used to refer to the structure and format defined by the X.25 recommendation.

Packet switching

A data transmission technique in which user information is broken up into discrete, self-contained units called packets. Packet switching has two distinctive characteristics: 1) it allows a communication channel to be shared by many users, each using the circuit only for the time required to transmit a single packet and 2) it allows for the individuals packets that make up a message to be routed over the optimal path of a given moment. See packet.

Packet Assembler/Disassembler (PAD)

A device that resides between one or more character-mode devices, such as asynchronous terminals (or terminal emulation software), and a PSDN. On the terminal side, the PAD accepts asynchronous characters, assembles packets from these characters, and forwards the packets to the PSDN. On the PSDN side, the PAD accepts packets, disassembles the packets into asynchronous characters, and forwards the characters to the terminal(s).

Packet-Switched Data Network (PSDN)

A more general term than Public Data Network, refers to any public or private packet-switching network that provides X.25-compliant interfaces to its users.

Permanent virtual circuit (PVC)

A permanent, logical association between two physically separate DTEs that does not require call set-up or clearing procedures. Analogous to a leased line in a circuit-switched telephone network.

Public Data Network (PDN)

A data communications network whose services are available to any user willing to pay for them. Tymnet and Telnet are examples of PDNs in the United States; the public telephone and telegraph agencies of European and Asian nations are also examples of PDNs. Solstice X.25 product documentation usually uses the term "Packet-Switched Data Networks" in preference to "Public Data Network", as the former is more general than the latter.

Recognized Private Operating Agency (RPOA)

An X.25 user facility that provides for user specification of a particular RPOA transit network through which a call is to be routed internationally when more than one RPOA transit network exists at an international gateway.

SNMP

Simple Network Management Protocol, the standard network management protocol used in TCP/IP networks.

SNMP agent

The process that carries out requests from SNMP.

Subaddress

An optional component of an X.121 address that identifies a specific application on a DTE.

Switched virtual circuit (SVC)

A temporary logical association between two physically separate DTEs that exists only for the duration of the data transfer. Call setup and call clearing procedures are required with a switched virtual circuit. Analogous to an everyday phone call on a circuit-switched telephone network.

Throughput

The rate at which data is passed through a particular system.

V25bis

The recommendation that defines dial-up X.25.

Defines the interface between PADs and packet-mode DTEs or other PADs.

X.3

Describes the functions of the PAD and the various parameters used to specify its mode of operation.

X.21bis

A set of CCITT recommendations that define the physical interface between a DTE and a DCE of a public data network. Access to the DCE is through synchronous modems and voice-band lines. Equivalent to RS-232-C and V.24/V.28.

X.25 client

An application layered above X.25.

X.28

Defines the format of the terminal user's instructions to the PAD--referred to as PAD command signals--and the format of the PAD's responses to the terminal--referred to as PAD service signals.

X.29

Defines the interface between PADs and packet-mode DTEs or other PADs.

X.32

The recommendation that defines the security procedures used for dial-up access to public PSDNs.