HP OpenVMS Systems Documentation

area address: Address prefix, which consists of the inital domain part (IDP) through to and including the LOC-AREA field of an NSAP. A DECnet-Plus system can have more than one area address, but the systems making up an area must have at least one area address in common with each of their neighbors.

area router: See level 2 router, DECnet-Plus level 2 router, and DECnet Phase IV level 2 router.

area routing: Forwarding of data packets from one network area to another by the intermediate systems in the level 2 network.

ASE: See application service element.

ASN.1 (Abstract Syntax Notation One): Widely-used abstract syntax notation that uses standard Backus-Naur Form notation to describe application syntaxes. Defined in ISO 8224. See also basic encoding rules.

associating with a TSAP: Assigning a tsap-id as a transport address for a given transport user, so it can receive a connection request.

association: Connection between two application entity invocations. Communications channel for data transfer that is established at the Application layer between two peer service users.

Association Control Service Element (ACSE): Application service element (ASE) that provides association setup and release services. Element of the Application layer that manages associations for applications, such as FTAM, using a presentation connection. Negotiates and establishes all the services of the upper three layers.

asynchronous balance mode (ABM): See link access protocol balanced.

asynchronous connection: Connection between two DECnet-Plus systems with a low-cost, low-speed asynchronous line; a full-duplex connection that can be used for remote asynchronous communications over a telephone line using a modem. There are two types of asynchronous DECnet-Plus connections---static asynchronous connections (See also permanent virtual circuit) and dynamic asynchronous connections (See also switched virtual circuit).

asynchronous response mode (ARM): See link access protocol.

asynchronous transmission: Data transmission in which the time intervals between character transmissions differ. Each character is surrounded by a start bit and stop bits to allow the receiving device to recognize the beginning and end of each character (also called start-stop transmission). Most common over terminal lines and lines connecting other inexpensive devices, such as personal computers. Contrast with synchronous transmission.

attribute: Defined as:

• Network management --- Piece of information, or characteristic, that describes part of an entity; also called characteristic attribute. Has a value that is generally contained within the entity, and can usually be examined, and possibly modified. Categories of attributes are: status, identifier, characteristic, and counter.
  Controllable or observable part of an entity; variable that network managers and applications programmers can manipulate for optimal performance.
• FTAM --- Information that states a property of something as one of a set of values, each of which has a defined meaning. See also file attribute and activity attribute.
• DECdns --- Piece of information associated with a DECdns name or entity. DECdns creates some attributes for its own internal use. Other attributes are created and used by client applications interacting with DECdns. Some attributes consist of a set of values, such as the access control set (ACS), and others are single-valued. An object entry's attributes can describe its class, its network address, and its ACS, among other things. DECdns attributes can be one of four types: characteristics, counters, identifiers, or status attributes.

attribute group: Defined as:

• Network management --- Architecturally defined, named collection of attributes for an entity class, grouped together, such as all information relating to errors. Identifiers of an entity that uniquely define that entity within its class. Also called entity identifier.
• DECdns --- Particular category of DECdns attribute. DECdns attributes can be one of four groups: characteristics, counters, identifiers, or status attributes.

attribute set: Defined as:

• Network management --- Value of an attribute that actually consists of a set of values.
• DECdns --- Set of attributes maintained by a particular DECdns entity.

Contrast with single-valued attribute and set-valued attribute.

audit trail: Recording of all of the directives issued to an entity.

authentication: Security check to ensure that the requested action can be performed on a remote system by matching valid user name and password combinations; used by FTAM software and other applications.

authority and format identifier (AFI): Part of the initial domain part (IDP) of an NSAP address that indicates the addressing authority responsible for the assignment of the IDP and its format, as well as the format (binary or decimal) of the domain-specific part (DSP).

availability: Proportion of time a specific piece of equipment, system, or network is usable, compared to the total time it is expected to be.

backbone: Name sometimes given to the level 2 routers that tie together all the areas of a network.

background skulk time: In DECdns, automatic timer that guarantees a maximum lapse of time between skulks of a directory, regardless of other factors such as DECdns namespace management activities and user-initiated skulks.

balanced mode: HDLC operational mode used over full-duplex links. Contrast with normal mode.

bandwidth: Range between the highest and lowest frequencies at which signals can pass through a receiver without the signal being distorted beyond recognition; reflects the medium's information-carrying capability. Usually expressed in terms of its signaling rate in Hertz (Hz) or bits per second.

baseband technology: Network technology such as Ethernet that uses one carrier frequency and requires all network stations to participate in every transmission. See also broadband technology.

basic encoding rules (BER): Set of rules for encoding any language-specific data type (defined in ASN.1) into transfer syntax and for decoding from transfer syntax back into the language-specific data type; uses the style of encoding known as tag-length-value encoding. Sometimes incorrectly lumped under the term ASN.1, which properly refers only to the abstract syntax description language, not the encoding technique.

basic encoding rules of a single ASN.1 type: Transfer syntax that is obtained by applying the basic encoding rules of the ISO standard abstract syntax notation (ASN.1) defined by ISO 8825.

BER: See basic encoding rules.

bilateral closed user group (BCUG): Closed user group consisting of two DTEs.

binary timestamp: Opaque, 128-bit (16-octet) binary sequence that represents a DECdts time value.

block: Contiguous unit of user information that is grouped together for transmission, such as the user data within a packet, excluding the protocol overhead.

bottleneck: Point in the network where traffic is delayed or blocked. Bottlenecks are the limiting factors in network performance.

bridge: Device that expands the extent of a LAN by connecting it to another LAN or physical link. Data Link layer relay for interconnecting LANs, used to increase the maximum number of stations, maximum distance, and total available bandwidth. Only data destined for different LANs passes through the bridge; thus a bridge improves performance by reducing traffic between LANs. See also repeater and router.

broadband technology: Network technology that multiplexes multiple, independent network carriers onto a single cable, accomplished with frequency-division multiplexing. Allows several networks to coexist on one cable; traffic from one network does not interfere with traffic from another because each uses a different frequency. See also baseband technology.

broadcast: Sending a single message that will be received by all nodes in a subnetwork for example, the Ethernet. See also multicast.

broadcast addressing: Type of multicast addressing in which all nodes receive the same message.

broadcast circuit: Circuit on which multiple nodes are connected. A message can be transmitted to multiple receivers, and all nodes are adjacent. Examples: Ethernet, ISO 8802-3, CSMA-CD.

broadcast end node adjacency (BEA): End node connected to the same broadcast circuit as the local node. See also adjacency.

broadcast router adjacency (BRA): Intermediate system (router) connected to the same broadcast circuit as the local node. See also adjacency.

broadcast subnetwork: CSMA-CD LAN functioning as a subnetwork. See also subnetwork.

buffer: Device or an area of memory used for temporary storage when transmitting data to compensate for a difference in rate of data flow or in time of occurrence of events between sender and receiver; used on intermediate systems to temporarily store data that is to be forwarded.

buffering level: Number of buffers provided at one time by the network software to hold data.

Single buffering tends to be less efficient than multibuffering but uses less memory on the local system. Multibuffering provides better performance if sufficient memory is available. With multibuffering, a network can send or process several buffers of data in quick succession.

bus: Defined as:

• LAN topology in which all nodes connect to a single transmission medium. All nodes are equal, and all nodes hear all transmissions on the medium. Bus topologies are reliable because failure of a node does not affect the ability of other nodes to transmit and receive.
• Flat, flexible cable consisting of many transmission lines or wires used to interconnect computer system components to provide communication paths for addresses, data, and control information.

cache: Process of temporarily storing new information so it will be quickly accessible for future use; used to minimize physical transfer of data between mass storage devices and memory; also is very fast memory used in combination with slower, large-capacity memories.

call: To make, or attempt to make, a connection with a remote DTE across a virtual circuit by sending call request packets.

call accept: Packet returned to the local DTE by a remote DTE that has received an incoming call packet and agrees to accept the requested virtual circuit connection.

call reference: Unique value used locally to identify a Modem Connect or X.21 call.

call request: Packet sent by a DTE to initiate setting up a switched virtual circuit with another DTE; results in the remote DCE sending the remote DTE an incoming call packet.

call sharing: Form of switched line sharing in which many clients have access to the same call on that line. See also line sharing.

carrier sense: Signal provided by the Physical layer to indicate that one or more nodes are transmitting on the Ethernet channel.

Carrier Sense, Multiple Access: See CSMA.

Carrier Sense, Multiple Access with Collision Detect (CSMA-CD) protocol: See CSMA-CD.

CCITT (International Telegraph and Telephone Consultative Committee): Technical committee of the International Telecommunications Union (ITU) of the United Nations; also the international standards body for the international authorities that provide telecommunications, including the Post, Telephone and Telegraph Authorities (PTTs). Produces technical standards, known as Recommendations, for all internationally controlled aspects of analog and digital communications. See also X recommendations.

CCR (Commitment, Concurrency, and Recovery): OSI application service element (ASE) used to create atomic operations across distributed systems; used primarily to implement two-phase commit for transactions and nonstop operations.

CEN/CENELEC: Union of the European Committee for Standardization (CEN) and the European Committee for Electrotechnical Standardization (CENELEC); makes up the Joint European Standards Institution; primary European standards body.

centralized management: Form of network management in which management is performed from a single point in the network.

channel: Defined as:

• Routing --- Data path between two or more stations, including the communications control capability of the associated stations.
• X.25 --- Logical path between a DTE and a DCE over which data is transmitted. Each channel is identified by a unique reference number, called a logical channel number (LCN).

characteristic: Attribute of a manageable entity that helps specify how it will function, for example, router type level 1 or level 2. As specified by the Enterprise Management Architecture (EMA), an entity has the following types of attributes: characteristics, status, and counters.

Characteristics are the tunable attributes of an entity. If modifiable, can be modified only through the direct result of a manager's directive.

character mode DTE: DTE that is unable to handle data in packet form; must interface through a packet assembler/disassembler (PAD) to connect to a PSDN; also known as a Remote X.29 Terminal.

checksum: Parameter that is carried in a block of data, and whose value can be used to determine whether the data was corrupted during transmission.

child directory: DECdns directory that has one or more levels of directories above it in a DECdns namespace. A directory is the child of a directory immediately above it. Any directory other than the root is a child of either the root or of some other directory in the namespace.

child entity: Lower class of entity that receives directives forwarded from its parent entity as defined by the management access relationship; is named by appending its subclass name and its entity identifier to the parent's entity name.

child pointer: Pointer that connects a DECdns directory to a directory immediately below it in a DECdns namespace; automatically created by DECdns when a manager creates a new directory. DECdns stores the child pointer in the directory that is the parent of the new directory.

circuit: Logical (virtual) link that provides a communications connection between adjacent nodes. Examples: point-to-point circuit, broadcast circuit, dynamically established link.

circuit switching: Technique that dynamically establishes a physical connection before information exchange and releases the connection following the exchange.

class name: Name of an entity class. For example, node is the global entity class.

class of protocol: Type of transport protocol. OSI Transport supports three transport protocol classes: class 0, class 2, and class 4. Each class defines a transport service with specific characteristics.

class-specific attribute: DECdns --- Attribute that has meaning only to a particular class of DECdns object and to the application using that object class. A DECdns object's class is defined in the DNS$Class attribute.

clear: Stop a connection across a virtual circuit to a remote DTE by sending clear request packets.

clear confirmation: Packet sent under the following circumstances:

• To the local DCE by a DTE that has received a clear indication packet (a DTE clear confirmation packet).
• To a DTE that has requested clearing when a DCE receives a DTE clear confirmation packet through the network (a DCE clear confirmation packet).

clear indication: Packet sent by a DCE to a DTE when it is clearing a virtual circuit.

clearinghouse: Collection of directory replicas on one DECdns server. A clearinghouse takes the form of a database file. It can exist only on a DECdns server. Usually only one clearinghouse exists on a server, but there may be special cases when more than one exists.

clearinghouse object entry: Special class of object entry that describes a clearinghouse; pointer to the node address of a clearinghouse, which enables DECdns to find a clearinghouse and use and manage its contents; has the same name as the clearinghouse. A clearinghouse modifies and manages its own object entry when necessary; normally DECdns managers do not need to maintain it.

clear request: Packet sent by a DTE to initiate the clearing of a switched virtual circuit; can also be sent by a destination DTE that has received an Incoming Call packet and is unable to accept the requested virtual circuit connection.

clerk: Defined as:

• DECdns --- Software that provides an interface between client applications and DECdns servers. The clerk receives a request from an application, sends the request to a DECdns server, and returns any resulting information to the application.
• DECdts --- Software that synchronizes the clock for its client system by requesting time values from DECdts servers, computing a new time from the values, and supplying the computed time to client applications, such as the operating system.

client: Defined as:

• X.25 --- Access system.
• Distributed processing --- See client/server model.

client application: Defined as:

• DECdns --- Any application that interacts with a DECdns server through the DECdns clerk.
• DECdts --- Any application that interacts with a DECdts server through the DECdts clerk.

client entity: Entity that constitutes the client side of a client/server relationship.

client/server model: Model of interaction used in DIGITAL's distributed processing products. A program at one system sends a request to a program at another system and awaits a response; the requesting system is the client; the system satisfying the request is the server.

CLNP: See Connectionless Network Protocol.

CLNS: See Connectionless-Mode Network Service.

CLNS with Internet/ES-IS: Connectionless network service used for transport connections in which the two end systems are on different subnetworks; provided by the implementation of the ES-IS and Internet routing protocols. The intervening subnetworks can be a mixture of technologies.

CLNS with null Internet: Connectionless network service used for transport connections in which both end systems are on the same 802.3 LAN. This network service is provided by the inactive subset of the Internet routing protocol.

clock: Combined hardware interrupt timer and software register that maintain system time. In many systems, the hardware timer sends interrupts to the operating system; at each interrupt, the operating system adds an increment to a software register which contains the time value.

closed user group (CUG): Group of DTEs restricted to communicating with each other.

CLTP: See Connectionless Transport Protocol.

cluster alias: Optional node name and address used by some or all nodes in an OpenVMS cluster, allowing any of these nodes to be reachable on the network with the same address.

CMIP: See Common Management Information Protocol.

collision: Condition in which two packets are transmitted over a medium at the same time, making both unintelligible.

collision detect: Signal provided by the Physical layer to the Data Link layer to indicate that one or more nodes are contending with the local node's transmission. Stations detect the collision and retry the transmission.

Comite Consultatif Internationale Telegraphique et Telephonique (CCITT): See CCITT.

Commitment, Concurrency, and Recovery: See CCR.

common carrier: Organization in the United States that offers standard and consistent communications services within a country. See Postal, Telegraph and Telephone Authority.

Common Management Information Protocol (CMIP): ISO protocol that describes the exchange of network management information.

Common Trace Facility (CTF): DECnet-Plus for OpenVMS problem-solving facility for looking at and analyzing data generated by events on a network.

communications buffers: Buffers allocated at each node for temporary storage of communications data.

communications link: Physical medium connecting two systems.

communications server: Special-purpose standalone system dedicated to managing communications activities for other computer systems.

computed time: Result of the synchronization process used to adjust the system clock time; the time interval that the DECdts clerk or DECdts server computes according to the intersections of the DECdts server time values it receives.

concatenation: Process of joining two or more items together, as when input files are appended to a new output file.

conceptual communications area: Data store used in virtual terminal associations; contains various data structures that model a terminal's behavior. The systems in a virtual terminal association maintain a separate copy of the conceptual communications area and share information about changes to these data structures.

confirm primitive: Primitive generated by a service provider to pass an incoming response to a service user.

confirmed event report: Report in which the event sink responds to a request after the event record has been stored in the event log.

confirmed service: Service whose requested parameter values are acknowledged and often negotiated by a two-way exchange of service primitives.

congestion: Condition in which a network or part of a network is overloaded and has insufficient communication resources for the volume of traffic.

congestion avoidance: DECnet-Plus routing mechanism that adjusts the load on the network to prevent congestion.

congestion loss: Condition in which data packets are lost when routing is unable to buffer them.

connection: Logical link between two open systems.

connection control: DNA session control function that controls system-dependent functions that create, maintain, and end transport connections.

Connectionless-Mode Network Service (CLNS): Connectionless network service provided by OSI transport; operates according to a datagram model. Each message is routed and delivered to its destination independently of others. For example, the DNA Network (Routing) layer provides this type of service.

See also CLNS with Internet/ES-IS and CLNS with null Internet.

Connectionless Network Protocol (CLNP): OSI protocol for providing CLNS (datagram service). A "gateway" protocol that creates an Internet header, allows dynamic routing, and is critical for X.25 communications.

OSI equivalent to the Internet Protocol (IP), sometimes called ISO IP.

connectionless service: Service in which no permanent connection is set up between the two communicating ends. Instead, each service data unit contains the addressing information that identifies its destination. Examples: LANs, Internet IP, OSI CLNP, and UDP.

Connectionless Transport Protocol (CLTP): OSI protocol that describes end-to-end transport data addressing (via transport selector) and error control (via checksum), with no guarantee of delivery and no flow control.

Connection-Oriented Network Service (CONS): Connection-orientated network service provided to OSI transport; operates according to a connection-oriented model. A connection is set up between two communicating end users, is used for data exchange, and is then broken by either end. Service data units sent over the connection do not have to contain a destination address. X.25, for example, provides this type of service.

connection-oriented service: Service in which communication proceeds through three well-defined phases: connection establishment, data transfer, connection release. A logical connection, such as virtual circuits, is established between end points. Examples: X.25 and OSI TP4.

connection request: Request for a connection to a peer entity or user.

connectivity: Degree to which network nodes are interconnected. Full connectivity means all nodes have links to every other node.

connector system: DECnet-Plus system that connects to PSDNs and also provides indirect connection to PSDNs for other systems connected to this system.

CONS: See Connection-Oriented Network Service.

console carrier: Maintenance Operations Protocol (MOP) function that provides access to the remote console subsystem of a network server on a LAN.

constraint set: Set of related statements limiting the range of structures allowed for a given type of file, and specifying how file-access actions can modify those structures without changing their essential nature.

contention control: Scheme of access control used by many networks. Control is distributed among the nodes of the network. Any node wanting to transmit can do so, accessing the network on a first-come, first-served basis. However, it is possible that two nodes are in contention, or start transmitting at the same time, in which case a collision occurs. Each node must then back off and retransmit after waiting a random period of time.

contents type: FTAM file attribute that defines file structure and contents, can be expressed as an abstract syntax/constraint set pair or as a document type. DECnet-Plus for OpenVMS and DECnet-Plus for DIGITAL UNIX FTAM use only the document-type form of contents type.

control access: DECdns access right that permits users to change the access control on a name and do other powerful management tasks, such as replicate a directory or move a clearinghouse.

control object: Data structure in the conceptual communications area that models a terminal's control sequences, such as the control sequence that rings a terminal's bell.