IBM Books

MAS V3.4 Protocol Reference V2

Glossary

This glossary includes terms and definitions from:

The following cross-references are used in this glossary:

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Synonymous with:
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See:
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A

AAL
ATM Adaptation Layer, the layer that adapts user data to/from the ATM network by adding/removing headers and segmenting/reassembling the data into/from cells.

AAL-5
ATM Adaptation Layer 5, one of several standard AALs. AAL-5 was designed for data communications, and is used by LAN Emulation and Classical IP.

abstract syntax
A data specification that includes all distinctions that are needed in data transmissions, but that omits (abstracts) other details such as those that depend on specific computer architectures. See also abstract syntax notation 1 (ASN.1) and basic encoding rules (BER).

abstract syntax notation 1 (ASN.1)
The Open Systems Interconnection (OSI) method for abstract syntax specified in the following standards:

See also basic encoding rules (BER).

ACCESS
In the Simple Network Management Protocol (SNMP), the clause in a Management Information Base (MIB) module that defines the minimum level of support that a managed node provides for an object.

acknowledgment
(1) The transmission, by a receiver, of acknowledge characters as an affirmative response to a sender. (T)  
(2) An indication that an item sent was received.

active
(1) Operational.
(2) Pertaining to a node or device that is connected or is available for connection to another node or device.

active monitor
In a token-ring network, a function performed at any one time by one ring station that initiates the transmission of tokens and provides token error recovery facilities. Any active adapter on the ring has the ability to provide the active monitor function if the current active monitor fails.

address
In data communication, the unique code assigned to each device, workstation, or user connected to a network.

address mapping table (AMT)
A table, maintained within the AppleTalk router, that provides a current mapping of node addresses to hardware addresses.

address mask
For internet subnetworking, a 32-bit mask used to identify the subnetwork address bits in the host portion of an IP address. Synonymous with subnet mask and subnetwork mask.

address resolution
(1) A method for mapping network-layer addresses to media-specific addresses.
(2) See also Address Resolution Protocol (ARP) and AppleTalk Address Resolution Protocol (AARP).

Address Resolution Protocol (ARP)
(1) In the Internet suite of protocols, the protocol that dynamically maps an IP address to an address used by a supporting metropolitan or local area network such as Ethernet or token-ring.
(2) See also Reverse Address Resolution Protocol (RARP).

addressing
In data communication, the way in which a station selects the station to which it is to send data.

adjacent nodes
Two nodes connected together by at least one path that connects no other node. (T)

Administrative Domain
A collection of hosts and routers, and the interconnecting networks, managed by a single administrative authority.

Advanced Peer-to-Peer Networking (APPN)
An extension to SNA featuring (a) greater distributed network control that avoids critical hierarchical dependencies, thereby isolating the effects of single points of failure; (b) dynamic exchange of network topology information to foster ease of connection, reconfiguration, and adaptive route selection; (c) dynamic definition of network resources; and (d) automated resource registration and directory lookup. APPN extends the LU 6.2 peer orientation for end-user services to network control and supports multiple LU types, including LU 2, LU 3, and LU 6.2.

Advanced Peer-to-Peer Networking (APPN) end node
A node that provides a broad range of end-user services and supports sessions between its local control point (CP) and the CP in an adjacent network node. It uses these sessions to dynamically register its resources with the adjacent CP (its network node server), to send and receive directory search requests, and to obtain management services. An APPN end node can also attach to a subarea network as a peripheral node or to other end nodes.

Advanced Peer-to-Peer Networking (APPN) network
A collection of interconnected network nodes and their client end nodes.

Advanced Peer-to-Peer Networking (APPN) network node
A node that offers a broad range of end-user services and that can provide the following:

Advanced Peer-to-Peer Networking (APPN) node
An APPN network node or an APPN end node.

agent
A system that assumes an agent role.

alert
A message sent to a management services focal point in a network to identify a problem or an impending problem.

all-stations address
In communications, synonym for broadcast address.

American National Standards Institute (ANSI)
An organization consisting of producers, consumers, and general interest groups, that establishes the procedures by which accredited organizations create and maintain voluntary industry standards in the United States. (A)  

analog
(1) Pertaining to data consisting of continuously variable physical quantities. (A)
(2) Contrast with digital.

AppleTalk
A network protocol developed by Apple Computer, Inc. This protocol is used to interconnect network devices, which can be a mixture of Apple and non-Apple products.

AppleTalk Address Resolution Protocol (AARP)
In AppleTalk networks, a protocol that (a) translates AppleTalk node addresses into hardware addresses and (b) reconciles addressing discrepancies in networks that support more than one set of protocols.

AppleTalk Transaction Protocol (ATP)
In AppleTalk networks, a protocol that provides client/server request and response functions for hosts accessing the Zone Information Protocol (ZIP) for zone information.

APPN network
See Advanced Peer-to-Peer Networking (APPN) network.

APPN network node
See Advanced Peer-to-Peer Networking (APPN) network node.

arbitrary MAC addressing (AMA)
In DECnet architecture, an addressing scheme used by DECnet Phase IV-Prime that supports universally administered addresses and locally administered addresses.

area
In Internet and DECnet routing protocols, a subset of a network or gateway grouped together by definition of the network administrator. Each area is self-contained; knowledge of an area's topology remains hidden from other areas.

asynchronous (ASYNC)
Pertaining to two or more processes that do not depend upon the occurrence of specific events such as common timing signals. (T)

ATM
Asynchronous Transfer Mode, a connection-oriented, high-speed networking technology based on cell switching.

ATMARP
ARP in Classical IP.

attachment unit interface (AUI)
In a local area network, the interface between the medium attachment unit and the data terminal equipment within a data station. (I)  (A)  

Attribute Value Pair (AVP)
A uniform method of encoding message types and bodies. This method maximizes the extensibility while permitting interoperability of L2TP.

authentication failure
In the Simple Network Management Protocol (SNMP), a trap that may be generated by an authentication entity when a requesting client is not a member of the SNMP community.

autonomous system
In TCP/IP, a group of networks and routers under one administrative authority. These networks and routers cooperate closely to propagate network reachability (and routing) information among themselves using an interior gateway protocol of their choice.

autonomous system number
In TCP/IP, a number assigned to an autonomous system by the same central authority that also assigns IP addresses. The autonomous system number makes it possible for automated routing algorithms to distinguish autonomous systems.

B

backbone
(1) In a local area network multiple-bridge ring configuration, a high-speed link to which the rings are connected by means of bridges or routers. A backbone may be configured as a bus or as a ring.
(2) In a wide area network, a high-speed link to which nodes or data switching exchanges (DSEs) are connected.

backbone network
A central network to which smaller networks, normally of lower speed, connect. The backbone network usually has a much higher capacity than the networks it helps interconnect or is a wide-area network (WAN) such as a public packet-switched datagram network.

backbone router
(1) A router used to transmit data between areas.
(2) One in a series of routers that is used to interconnect networks into a larger internet.

Bandwidth
The bandwidth of an optical link designates the information-carrying capacity of the link and is related to the maximum bit rate that a fiber link can support.

basic transmission unit (BTU)
In SNA, the unit of data and control information passed between path control components. A BTU can consist of one or more path information units (PIUs).

baud
In asynchronous transmission, the unit of modulation rate corresponding to one unit interval per second; that is, if the duration of the unit interval is 20 milliseconds, the modulation rate is 50 baud. (A)  

bootstrap
(1) A sequence of instructions whose execution causes additional instructions to be loaded and executed until the complete computer program is in storage. (T)
(2) A technique or device designed to bring itself into a desired state by means of its own action, for example, a machine routine whose first few instructions are sufficient to bring the rest of itself into the computer from an input device. (A)

Border Gateway Protocol (BGP)
An Internet Protocol (IP) routing protocol used between domains and autonomous systems.

border router
In Internet communications, a router, positioned at the edge of an autonomous system, that communicates with a router that is positioned at the edge of a different autonomous system.

bridge
A functional unit that interconnects multiple LANs (locally or remotely) that use the same logical link control protocol but that can use different medium access control protocols. A bridge forwards a frame to another bridge based on the medium access control (MAC) address.

bridge identifier
An 8-byte field, used in a spanning tree protocol, composed of the MAC address of the port with the lowest port identifier and a user-defined value.

bridging
In LANs, the forwarding of a frame from one LAN segment to another. The destination is specified by the medium access control (MAC) sublayer address encoded in the destination address field of the frame header.

broadcast
(1) Transmission of the same data to all destinations. (T)
(2) Simultaneous transmission of data to more than one destination.
(3) Contrast with multicast.

broadcast address
In communications, a station address (eight 1's) reserved as an address common to all stations on a link. Synonymous with all-stations address.

C

cache
(1) A special-purpose buffer storage, smaller and faster than main storage, used to hold a copy of instructions and data obtained from main storage and likely to be needed next by the processor. (T)  
(2) A buffer storage that contains frequently accessed instructions and data; it is used to reduce access time.
(3) An optional part of the directory database in network nodes where frequently used directory information may be stored to speed directory searches.
(4) To place, hide, or store in a cache.

call request packet
(1) A call supervision packet that a data terminal equipment (DTE) transmits to ask that a connection for a call be established throughout the network.
(2) In X.25 communications, a call supervision packet transmitted by a DTE to ask for a call establishment through the network.

canonical address
In LANs, the IEEE 802.1 format for the transmission of medium access control (MAC) addresses for token-ring and Ethernet adapters. In canonical format, the least significant (rightmost) bit of each address byte is transmitted first. Contrast with noncanonical address.

carrier
An electric or electromagnetic wave or pulse train that may be varied by a signal bearing information to be transmitted over a communication system. (T)

carrier detect
Synonym for received line signal detector (RLSD).

carrier sense
In a local area network, an ongoing activity of a data station to detect whether another station is transmitting. (T)

carrier sense multiple access with collision detection (CSMA/CD)
A protocol that requires carrier sense and in which a transmitting data station that detects another signal while transmitting, stops sending, sends a jam signal, and then waits for a variable time before trying again. (T)  (A)  

CCITT
International Telegraph and Telephone Consultative Committee. This was an organization of the International Telecommunication Union (ITU). On 1 March 1993 the ITU was reorganized, and responsibilities for standardization were placed in a subordinate organization named the Telecommunication Standardization Sector of the Telecommunication Union (ITU-TS). "CCITT" continues to be used for recommendations that were approved before the reorganization.

channel
(1) A path along which signals can be sent, for example, data channel, output channel. (A)  
(2) A functional unit, controlled by the processor, that handles the transfer of data between processor storage and local peripheral equipment.

channel service unit (CSU)
A unit that provides the interface to a digital network. The CSU provides line conditioning (or equalization) functions, which keep the signal's performance consistent across the channel bandwidth; signal reshaping, which constitutes the binary pulse stream; and loopback testing, which includes the transmission of test signals between the CSU and the network carrier's office channel unit. See also data service unit (DSU).

channelization
The process of breaking the bandwidth on a communication line into a number of channels, possibly of different size. Also called time division multiplexing (TDM).

checksum
(1) The sum of a group of data associated with the group and used for checking purposes. (T)
(2) In error detection, a function of all bits in a block. If the written and calculated sums do not agree, an error is indicated.
(3) On a diskette, data written in a sector for error detection purposes; a calculated checksum that does not match the checksum of data written in the sector indicates a bad sector. The data are either numeric or other character strings regarded as numeric for the purpose of calculating the checksum.

circuit switching
(1) A process that, on demand, connects two or more data terminal equipment (DTEs) and permits the exclusive use of a data circuit between them until the connection is released. (I)  (A)  
(2) Synonymous with line switching.

class A network
In Internet communications, a network in which the high-order (most significant) bit of the IP address is set to 0 and the host ID occupies the three low-order octets.

class B network
In Internet communications, a network in which the two high-order (most significant and next-to-most significant) bits of the IP address are set to 1 and 0, respectively, and the host ID occupies the two low-order octets.

class of service (COS)
A set of characteristics (such as route security, transmission priority, and bandwidth) used to construct a route between session partners. The class of service is derived from a mode name specified by the initiator of a session.

client
(1) A functional unit that receives shared services from a server. (T)  
(2) A user.

client/server
In communications, the model of interaction in distributed data processing in which a program at one site sends a request to a program at another site and awaits a response. The requesting program is called a client; the answering program is called a server.

clocking
(1) In binary synchronous communication, the use of clock pulses to control synchronization of data and control characters.
(2) A method of controlling the number of data bits sent on a telecommunication line in a given time.

collision
An unwanted condition that results from concurrent transmissions on a channel. (T)

collision detection
In carrier sense multiple access with collision detection (CSMA/CD), a signal indicating that two or more stations are transmitting simultaneously.

Committed information rate
The maximum amount of data in bits that the network agrees to deliver.

community
In the Simple Network Management Protocol (SNMP), an administrative relationship between entities.

community name
In the Simple Network Management Protocol (SNMP), a string of octets identifying a community.

compression
(1) The process of eliminating gaps, empty fields, redundancies, and unnecessary data to shorten the length of records or blocks.
(2) Any encoding to reduce the number of bits used to represent a given message or record.

configuration
(1) The manner in which the hardware and software of an information processing system are organized and interconnected. (T)  
(2) The devices and programs that make up a system, subsystem, or network.

configuration database (CDB)
A database that stores the configuration parameters of one or several devices. It is prepared and updated using the configuration program.

configuration file
A file that specifies the characteristics of a system device or network.

configuration parameter
A variable in a configuration definition, the values of which can characterize the relationship of a product to other products in the same network or can define characteristics of the product itself.

configuration report server (CRS)
In the IBM Token-Ring Network Bridge Program, the server that accepts commands from the LAN Network Manager (LNM) to get station information, set station parameters, and remove stations on its ring. This server also collects and forwards configuration reports generated by stations on its ring. The configuration reports include the new active monitor reports and the nearest active upstream neighbor (NAUN) reports.

congestion
See network congestion.

connection
In data communication, an association established between functional units for conveying information. (I) (A)

control point (CP)
(1) A component of an APPN or LEN node that manages the resources of that node. In an APPN node, the CP is capable of engaging in CP-CP sessions with other APPN nodes. In an APPN network node, the CP also provides services to adjacent end nodes in the APPN network.
(2) A component of a node that manages resources of that node and optionally provides services to other nodes in the network. Examples are a system services control point (SSCP) in a type 5 subarea node, a network node control point (NNCP) in an APPN network node, and an end node control point (ENCP) in an APPN or LEN end node. An SSCP and an NNCP can provide services to other nodes.

control point management services (CPMS)
A component of a control point, consisting of management services function sets, that provides facilities to assist in performing problem management, performance and accounting management, change management, and configuration management. Capabilities provided by the CPMS include sending requests to physical unit management services (PUMS) to test system resources, collecting statistical information (for example, error and performance data) from PUMS about the system resources, and analyzing and presenting test results and statistical information collected about the system resources. Analysis and presentation responsibilities for problem determination and performance monitoring can be distributed among multiple CPMSs.

control point management services unit (CP-MSU)
The message unit that contains management services data and flows between management services function sets. This message unit is in general data stream (GDS) format. See also management services unit (MSU) and network management vector transport (NMVT).

CU Logical Address
The Control Unit address defined in the host for the 2216. This value is defined in the host Input/Output Configuration Program (IOCP) by the CUADD statement on the CNTLUNIT macro instruction. The Control Unit Address must be unique for each logical partition defined on the same host.

D

D-bit
Delivery-confirmation bit. In X.25 communications, the bit in a data packet or call-request packet that is set to 1 if end-to-end acknowledgment (delivery confirmation) is required from the recipient.

daemon
A program that runs unattended to perform a standard service. Some daemons are triggered automatically to perform their task; others operate periodically.

data carrier detect (DCD)
Synonym for received line signal detector (RLSD).

data circuit
(1) A pair of associated transmit and receive channels that provide a means of two-way data communication. (I)
(2) In SNA, synonym for link connection.
(3) See also physical circuit and virtual circuit.

Notes:

  1. Between data switching exchanges, the data circuit may include data circuit-terminating equipment (DCE), depending on the type of interface used at the data switching exchange.

  2. Between a data station and a data switching exchange or data concentrator, the data circuit includes the data circuit-terminating equipment at the data station end, and may include equipment similar to a DCE at the data switching exchange or data concentrator location.

data circuit-terminating equipment (DCE)
In a data station, the equipment that provides the signal conversion and coding between the data terminal equipment (DTE) and the line. (I)  

Notes:

  1. The DCE may be separate equipment or an integral part of the DTE or of the intermediate equipment.

  2. A DCE may perform other functions that are usually performed at the network end of the line.

data link connection identifier (DLCI)
The numeric identifier of a frame-relay subport or PVC segment in a frame-relay network. Each subport in a single frame-relay port has a unique DLCI. The following table, excerpted from the American National Standards Institute (ANSI) Standard T1.618 and the International Telegraph and Telephone Consultative Committee (ITU-T/CCITT) Standard Q.922, indicates the functions associated with certain DLCI values:
DLCI Values Function
0 in-channel signaling
1-15 reserved
16-991 assigned using frame-relay connection procedures
992-1007 Layer 2 management of frame-relay bearer service
1008-1022 reserved
1023 in-channel layer management

data link control (DLC)
A set of rules used by nodes on a data link (such as an SDLC link or a token ring) to accomplish an orderly exchange of information.

data link control (DLC) layer
In SNA, the layer that consists of the link stations that schedule data transfer over a link between two nodes and perform error control for the link. Examples of data link control are SDLC for serial-by-bit link connection and data link control for the System/370 channel.
Note:The DLC layer is usually independent of the physical transport mechanism and ensures the integrity of data that reaches the higher layers.

data link layer
In the Open Systems Interconnection reference model, the layer that provides services to transfer data between entities in the network layer over a communication link. The data link layer detects and possibly corrects errors that may occur in the physical layer. (T)

data link level
(1) In the hierarchical structure of a data station, the conceptual level of control or processing logic between high level logic and the data link that maintains control of the data link. The data link level performs such functions as inserting transmit bits and deleting receive bits; interpreting address and control fields; generating, transmitting, and interpreting commands and responses; and computing and interpreting frame check sequences. See also packet level and physical level.
(2) In X.25 communications, synonym for frame level.

data link switching (DLSw)
A method of transporting network protocols that use IEEE 802.2 logical link control (LLC) type 2. SNA and NetBIOS are examples of protocols that use LLC type 2. See also encapsulation and spoofing.

data packet
In X.25 communications, a packet used for the transmission of user data on a virtual circuit at the DTE/DCE interface.

data service unit (DSU)
A device that provides a digital data service interface directly to the data terminal equipment. The DSU provides loop equalization, remote and local testing capabilities, and a standard EIA/CCITT interface.

data set ready (DSR)
Synonym for DCE ready.

data switching exchange (DSE)
The equipment installed at a single location to provide switching functions, such as circuit switching, message switching, and packet switching. (I)

data terminal equipment (DTE)
That part of a data station that serves as a data source, data sink, or both. (I)  (A)  

data terminal ready (DTR)
A signal to the modem used with the EIA 232 protocol.

data transfer rate
The average number of bits, characters, or blocks per unit time passing between corresponding equipment in a data transmission system. (I)

Notes:

  1. The rate is expressed in bits, characters, or blocks per second, minute, or hour.

  2. Corresponding equipment should be indicated; for example, modems, intermediate equipment, or source and sink.

datagram
(1) In packet switching, a self-contained packet, independent of other packets, that carries information sufficient for routing from the originating data terminal equipment (DTE) to the destination DTE without relying on earlier exchanges between the DTEs and the network. (I)
(2) In TCP/IP, the basic unit of information passed across the Internet environment. A datagram contains a source and destination address along with the data. An Internet Protocol (IP) datagram consists of an IP header followed by the transport layer data.
(3) See also packet and segment.

Datagram Delivery Protocol (DDP)
In AppleTalk networks, a protocol that provides network connectivity by means of connectionless socket-to-socket delivery service on the internet layer.

DCE ready
In the EIA 232 standard, a signal that indicates to the data terminal equipment (DTE) that the local data circuit-terminating equipment (DCE) is connected to the communication channel and is ready to send data. Synonymous with data set ready (DSR).

DECnet
A network architecture that defines the operation of a family of software modules, databases, and hardware components typically used to tie Digital Equipment Corporation systems together for resource sharing, distributed computation, or remote system configuration. DECnet network implementations follow the Digital Network Architecture (DNA) model.

default
Pertaining to an attribute, condition, value, or option that is assumed when none is explicitly specified. (I)  

dependent LU requester (DLUR)
An APPN end node or an APPN network node that owns dependent LUs, but requests that a dependent LU server provide the SSCP services for those dependent LUs.

designated router
A router that informs end nodes of the existence and identity of other routers. The selection of the designated router is based upon the router with the highest priority. When several routers share the highest priority, the router with the highest station address is selected.

destination node
The node to which a request or data is sent.

destination port
The 8-port asynchronous adapter that serves as a connection point with a serial service.

destination service access point (DSAP)
In SNA and TCP/IP, a logical address that allows a system to route data from a remote device to the appropriate communications support. Contrast with source service access point (SSAP).

device
A mechanical, electrical, or electronic contrivance with a specific purpose.

device address
The unit address transmitted on the channel path to select a 2216 device. It is also referred to as subchannel number in S/370 I/O architecture. This value is defined in the host IOCP by the UNITADD statement on the CNTLUNIT macro instruction for the real device.

digital
(1) Pertaining to data that consist of digits. (T)
(2) Pertaining to data in the form of digits. (A)
(3) Contrast with analog.

Digital Network Architecture (DNA)
The model for all DECnet hardware and software implementations.

direct memory access (DMA)
The system facility that allows a device on the Micro Channel bus to get direct access to the system or bus memory without the intervention of the system processor.

directory
A table of identifiers and references to the corresponding items of data. (I)  (A)  

directory service (DS)
An application service element that translates the symbolic names used by application processes into the complete network addresses used in an OSI environment. (T)

directory services (DS)
A control point component of an APPN node that maintains knowledge of the location of network resources.

disable
To make nonfunctional.

disabled
(1) Pertaining to a state of a processing unit that prevents the occurrence of certain types of interruptions.
(2) Pertaining to the state in which a transmission control unit or audio response unit cannot accept incoming calls on a line.

domain
(1) That part of a computer network in which the data processing resources are under common control. (T)
(2) In Open Systems Interconnection (OSI), a part of a distributed system or a set of managed objects to which a common policy applies.
(3) See Administrative Domain and domain name.

domain name
In the Internet suite of protocols, a name of a host system. A domain name consists of a sequence of subnames separated by a delimiter character. For example, if the fully qualified domain name (FQDN) of a host system is ralvm7.vnet.ibm.com, each of the following is a domain name:

domain name server
In the Internet suite of protocols, a server program that supplies name-to-address translation by mapping domain names to IP addresses. Synonymous with name server.

Domain Name System (DNS)
In the Internet suite of protocols, the distributed database system used to map domain names to IP addresses.

dotted decimal notation
The syntactical representation for a 32-bit integer that consists of four 8-bit numbers written in base 10 with periods (dots) separating them. It is used to represent IP addresses.

dump
(1) Data that has been dumped. (T)  
(2) To copy the contents of all or part of virtual storage for the purpose of collecting error information.

dynamic reconfiguration (DR)
The process of changing the network configuration (peripheral PUs and LUs) without regenerating complete configuration tables or deactivating the affected major node.

Dynamic Routing
Routing using learned routes rather than routes statically configured at initialization.

E

echo
In data communication, a reflected signal on a communications channel. For example, on a communications terminal, each signal is displayed twice, once when entered at the local terminal and again when returned over the communications link. This allows the signals to be checked for accuracy.

EIA 232
In data communication, a specification of the Electronic Industries Association (EIA) that defines the interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE), using serial binary data interchange.

Electronic Industries Association (EIA)
An organization of electronics manufacturers that advances the technological growth of the industry, represents the views of its members, and develops industry standards.

EIA unit
A unit of measure, established by the Electronic Industries Association, equal to 44.45 millimeters (1.75 inches).

encapsulation
(1) In communications, a technique used by layered protocols by which a layer adds control information to the protocol data unit (PDU) from the layer it supports. In this respect, the layer encapsulates the data from the supported layer. In the Internet suite of protocols, for example, a packet would contain control information from the physical layer, followed by control information from the network layer, followed by the application protocol data.
(2) See also data link switching.

encode
To convert data by the use of a code in such a manner that reconversion to the original form is possible. (T)  

end node (EN)
(1) See Advanced Peer-to-Peer Networking (APPN) end node and low-entry networking (LEN) end node.
(2) In communications, a node that is frequently attached to a single data link and cannot perform intermediate routing functions.

entry point (EP)
In SNA, a type 2.0, type 2.1, type 4, or type 5 node that provides distributed network management support. It sends network management data about itself and the resources it controls to a focal point for centralized processing, and it receives and executes focal-point initiated commands to manage and control its resources.

Ethernet
A 10-Mbps baseband local area network that allows multiple stations to access the transmission medium at will without prior coordination, avoids contention by using carrier sense and deference, and resolves contention by using collision detection and delayed retransmission. Ethernet uses carrier sense multiple access with collision detection (CSMA/CD).

exception
An abnormal condition such as an I/O error encountered in processing a data set or a file.

exception response (ER)
In SNA, a protocol requested in the form-of-response-requested field of a request header that directs the receiver to return a response only if the request is unacceptable as received or cannot be processed; that is, a negative response, but not a positive response, can be returned. Contrast with definite response and no response.

exchange identification (XID)
A specific type of basic link unit that is used to convey node and link characteristics between adjacent nodes. XIDs are exchanged between link stations before and during link activation to establish and negotiate link and node characteristics, and after link activation to communicate changes in these characteristics.

explicit route (ER)
In SNA, a series of one or more transmission groups that connect two subarea nodes. An explicit route is identified by an origin subarea address, a destination subarea address, an explicit route number, and a reverse explicit route number. Contrast with virtual route (VR).

explorer frame
See explorer packet.

explorer packet
In LANs, a packet that is generated by the source host and that traverses the entire source routing part of a LAN, gathering information on the possible paths available to the host.

exterior gateway
In Internet communications, a gateway on one autonomous system that communicates with another autonomous system. Contrast with interior gateway.

Exterior Gateway Protocol (EGP)
In the Internet suite of protocols, a protocol, used between domains and autonomous systems, that enables network reachability information to be advertised and exchanged. IP network addresses in one autonomous system are advertised to another autonomous system by means of EGP-participating routers. An example of an EGP is the Border Gateway Protocol (BGP). Contrast with Interior Gateway Protocol (IGP).

F

fax
Hardcopy received from a facsimile machine. Synonymous with telecopy.

File Transfer Protocol (FTP)
In the Internet suite of protocols, an application layer protocol that uses TCP and Telnet services to transfer bulk-data files between machines or hosts.

flash memory
A data storage device that is programmable, erasable, and does not require continuous power. The chief advantage of flash memory over other programmable and erasable data storage devices is that it can be reprogrammed without being removed from the circuit board.

flow control
(1) In SNA, the process of managing the rate at which data traffic passes between components of the network. The purpose of flow control is to optimize the rate of flow of message units with minimum congestion in the network; that is, to neither overflow the buffers at the receiver or at intermediate routing nodes, nor leave the receiver waiting for more message units.
(2) See also pacing.

fragment
See fragmentation.

fragmentation
(1) The process of dividing a datagram into smaller parts, or fragments, to match the capabilities of the physical medium over which it is to be transmitted.
(2) See also segmenting.

frame
(1) In Open Systems Interconnection architecture, a data structure pertaining to a particular area of knowledge and consisting of slots that can accept the values of specific attributes and from which inferences can be drawn by appropriate procedural attachments. (T)
(2) The unit of transmission in some local area networks, including the IBM Token-Ring Network. It includes delimiters, control characters, information, and checking characters.
(3) In SDLC, the vehicle for every command, every response, and all information that is transmitted using SDLC procedures.

frame level
Synonymous with data link level. See link level.

frame relay
(1) An interface standard describing the boundary between a user's equipment and a fast-packet network. In frame-relay systems, flawed frames are discarded; recovery comes end-to-end rather than hop-by-hop.
(2) A technique derived from the integrated services digital network (ISDN) D channel standard. It assumes that connections are reliable and dispenses with the overhead of error detection and control within the network.

front-end processor
A processor such as the IBM 3745 or 3174, that relieves a main frame from the communication control tasks.

G

gateway
(1) A functional unit that interconnects two computer networks with different network architectures. A gateway connects networks or systems of different architectures. A bridge interconnects networks or systems with the same or similar architectures. (T)  
(2) In the IBM Token-Ring Network, a device and its associated software that connect a local area network to another local area network or a host that uses different logical link protocols.
(3) In TCP/IP, synonym for router.

general data stream (GDS)
The data stream used for conversations in LU 6.2 sessions.

general data stream (GDS) variable
A type of RU substructure that is preceded by an identifier and a length field and includes either application data, user control data, or SNA-defined control data.

H

header
(1) System-defined control information that precedes user data.
(2) The portion of a message that contains control information for the message such as one or more destination fields, name of the originating station, input sequence number, character string indicating the type of message, and priority level for the message.

heap memory
The amount of RAM used to dynamically allocate data structures.

Hello
A protocol used by a group of cooperating, trusting routers to allow them to discover minimal delay routes.

hello message
(1) A message sent periodically to establish and test reachability between routers or between routers and hosts.
(2) In the Internet suite of protocols, a message defined by the Hello protocol as an Interior Gateway Protocol (IGP).

heuristic
Pertaining to exploratory methods of problem solving in which solutions are discovered by evaluation of the progress made toward the final result.

high-level data link control (HDLC)
In data communication, the use of a specified series of bits to control data links in accordance with the International Standards for HDLC: ISO 3309 Frame Structure and ISO 4335 Elements of Procedures.

high-performance routing (HPR)
An addition to the Advanced Peer-to-Peer Networking (APPN) architecture that enhances data routing performance and reliability, especially when using high-speed links.

hop
(1) In APPN, a portion of a route that has no intermediate nodes. It consists of only a single transmission group connecting adjacent nodes.
(2) To the routing layer, the logical distance between two nodes in a network.

hop count
(1) A metric or measure of distance between two points.
(2) In Internet communications, the number of routers that a datagram passes through on its way to its destination.
(3) In SNA, a measure of the number of links to be traversed in a path to a destination.

host
In the Internet suite of protocols, an end system. The end system can be any workstation; it does not have to be a mainframe.

hot pluggable
Refers to an hardware component that can be installed or removed without disturbing the operation of any other resource that is not connected to, or dependant on, this component.

hub (intelligent)
A wiring concentrator, such as the IBM 8260, that provides bridging and routing functions for LANs with different cables and protocols.

hysteresis
The amount the temperature must change past the set alert threshold before the alert condition is cleared.

I

I-frame
Information frame.

information (I) frame
A frame in I format used for numbered information transfer.

input/output channel
In a data processing system, a functional unit that handles transfer of data between internal and peripheral equipment. (I) (A)

Integrated Digital Network Exchange (IDNX)
A processor integrating voice, data, and image applications. It also manages the transmission resources, and connects to multiplexers and network management support systems. It allows integration of equipment from different vendors.

integrated services digital network (ISDN)
A digital end-to-end telecommunication network that supports multiple services including, but not limited to, voice and data.
Note:ISDNs are used in public and private network architectures.

interface
(1) A shared boundary between two functional units, defined by functional characteristics, signal characteristics, or other characteristics, as appropriate. The concept includes the specification of the connection of two devices having different functions. (T)  
(2) Hardware, software, or both, that links systems, programs, or devices.

interior gateway
In Internet communications, a gateway that communicates only with its own autonomous system. Contrast with exterior gateway.

Interior Gateway Protocol (IGP)
In the Internet suite of protocols, a protocol used to propagate network reachability and routing information within an autonomous system. Examples of IGPs are Routing Information Protocol (RIP) and Open Shortest Path First (OSPF).

interleaving
(1) The alternating of two or more operations or functions through the overlapped use of a computer facility.
(2) In data transmission, the alternating of packets from one data stream with packets from another.

intermediate node
A node that is at the end of more than one branch. (T)

intermediate session routing (ISR)
A type of routing function within an APPN network node that provides session-level flow control and outage reporting for all sessions that pass through the node but whose end points are elsewhere.

International Organization for Standardization (ISO)
An organization of national standards bodies from various countries established to promote development of standards to facilitate international exchange of goods and services, and develop cooperation in intellectual, scientific, technological, and economic activity.

International Telecommunication Union (ITU)
The specialized telecommunication agency of the United Nations, established to provide standardized communication procedures and practices, including frequency allocation and radio regulations worldwide.

internet
A collection of networks interconnected by a set of routers that allow them to function as a single, large network. See also Internet.

Internet
The internet administered by the Internet Architecture Board (IAB), consisting of large national backbone networks and many regional and campus networks all over the world. The Internet uses the Internet suite of protocols.

Internet address
See IP address.

Internet Architecture Board (IAB)
The technical body that oversees the development of the Internet suite of protocols that are known as TCP/IP.

Internet Control Message Protocol (ICMP)
The protocol used to handle errors and control messages in the Internet Protocol (IP) layer. Reports of problems and incorrect datagram destinations are returned to the original datagram source. ICMP is part of the Internet Protocol.

Internet Control Protocol (ICP)
The VIrtual NEtworking System (VINES) protocol that provides exception notifications, metric notifications, and PING support. See also RouTing update Protocol (RTP).

Internet Engineering Task Force (IETF)
The task force of the Internet Architecture Board (IAB) that is responsible for solving the short-term engineering needs of the Internet.

Internetwork Packet Exchange (IPX)
(1) The network protocol used to connect Novell's servers, or any workstation or router that implements IPX, with other workstations. Although similar to the Internet Protocol (IP), IPX uses different packet formats and terminology.
(2) See also Xerox Network Systems (XNS).

Internet Protocol (IP)
A connectionless protocol that routes data through a network or interconnected networks. IP acts as an intermediary between the higher protocol layers and the physical network. However, this protocol does not provide error recovery and flow control and does not guarantee the reliability of the physical network.

interoperability
The capability to communicate, execute programs, or transfer data among various functional units in a way that requires the user to have little or no knowledge of the unique characteristics of those units. (T)  

intra-area routing
In Internet communications, the routing of data within an area.

Inverse Address Resolution Protocol (InARP)
In the Internet suite of protocols, the protocol used for locating a protocol address through the known hardware address. In a frame-relay context, the data link connection identifier (DLCI) is synonymous with the known hardware address.

IPPN
The interface that other protocols can use to transport data over IP.

IP address
The 32-bit address defined by the Internet Protocol, standard 5, Request for Comments (RFC) 791. It is usually represented in dotted decimal notation.

IP datagram
In the Internet suite of protocols, the fundamental unit of information transmitted through an internet. It contains source and destination addresses, user data, and control information such as the length of the datagram, the header checksum, and flags indicating whether the datagram can be or has been fragmented.

IP router
A device in an IP internet that is responsible for making decisions about the paths over which network traffic will flow. Routing protocols are used to gain information about the network and to determine the best route over which the datagram should be forwarded toward the final destination. The datagrams are routed based on IP destination addresses.

IPXWAN
A Novell protocol that is used to exchange router-to-router information before exchanging standard Internetwork Packet Exchange (IPX) routing information and traffic over wide area networks (WANs).

J

jitter
(1) Short-term non-cumulative variations of the significant instants of a digital signal from their ideal positions in time.
(2) Undesirable variations of a transmitted digital signal.
(3) Variations in the network delay.

L

L2TP Access Concentrator (LAC)
A device attached to one or more public service telephone network (PSTN) or ISDN lines capable of handling both PPP operation and of the L2TP protocol. The LAC implements the media over which L2TP operates. L2TP passes the traffic to one or more L2TP Network Servers (LNS). L2TP can tunnel any protocol carried by the PPP network.

L2TP Network Server (LNS)
An LNS operates on any platform capable that can be a PPP end station. The LNS handles the server side of the L2TP protocol. Since L2TP relies only on the single media over which L2TP tunnels arrive, the LNS has only a single LAN or WAN interface, yet is still able to terminate calls arriving from any the full range of PPP interfaces supported by a LAC. These include asynchronous ISDN, synchronous ISDN, V.120, and other types of connections.

LAN bridge server (LBS)
In the IBM Token-Ring Network Bridge Program, the server that keeps statistical information about frames forwarded between two or more rings (through a bridge). The LBS sends these statistics to the appropriate LAN managers through the LAN reporting mechanism (LRM).

LAN Emulation (LE)
An ATM Forum standard that supports legacy LAN applications over ATM networks.

LAN Emulation Client (LEC)
A LAN Emulation component that represents users of the Emulated LAN.

LAN Emulation Configuration Server (LECS)
A LAN Emulation Service component that centralizes and disseminates configuration data.

LAN Emulation Server (LES)
A LAN Emulation Service component that resolves LAN Destinations to ATM Addresses.

LAN Network Manager (LNM)
An IBM licensed program that enables a user to manage and monitor LAN resources from a central workstation.

LAN segment
(1) Any portion of a LAN (for example, a bus or ring) that can operate independently, but that is connected to other parts of the network by means of bridges.
(2) A ring or bus network without bridges.

layer
(1) In network architecture, a group of services that is complete from a conceptual point of view, that is one out of a set of hierarchically arranged groups, and that extends across all systems that conform to the network architecture. (T)
(2) In the Open Systems Interconnection reference model, one of seven conceptually complete, hierarchically arranged groups of services, functions, and protocols, that extend across all open systems. (T)
(3) In SNA, a grouping of related functions that are logically separate from the functions in other groups. Implementation of the functions in one layer can be changed without affecting functions in other layers.

LE
LAN Emulation. An ATM Forum standard that supports legacy LAN applications over ATM networks.

LEC
LAN Emulation Client. A LAN Emulation component that represents users of the Emulated LAN.

LECS
LAN Emulation Configuration Server. A LAN Emulation Service component that centralizes and disseminates configuration data.

LES
LAN Emulation Server. A LAN Emulation Service component that resolves LAN Destinations to ATM Addresses.

line switching
Synonym for circuit switching.

link
The combination of the link connection (the transmission medium) and two link stations, one at each end of the link connection. A link connection can be shared among multiple links in a multipoint or token-ring configuration.

link access protocol balanced (LAPB)
A protocol used for accessing an X.25 network at the link level. LAPB is a duplex, asynchronous, symmetric protocol, used in point-to-point communication.

Link Address
For the 2216 with an ESCON Channel Adapter, a port number determined as follows: If one ESCD is in the communication path, it is the ESCON Director (ESCD) port number that is attached to the host. If two ESCDs are in the path, it is the host-side port number of the ESCD defined with the dynamic connection. When no ESCD is in the communication path, this value must be set to X'01'.

link-attached
(1) Pertaining to devices that are connected to a controlling unit by a data link.
(2) Contrast with channel-attached.
(3) Synonymous with remote.

link connection
(1) The physical equipment providing two-way communication between one link station and one or more other link stations; for example, a telecommunication line and data circuit-terminating equipment (DCE).
(2) In SNA, synonymous with data circuit.

link level
(1) A part of Recommendation X.25 that defines the link protocol used to get data into and out of the network across the full-duplex link connecting the subscriber's machine to the network node. LAP and LAPB are the link access protocols recommended by the CCITT.
(2) See data link level.

link-state
In routing protocols, the advertised information about the usable interfaces and reachable neighbors of a router or network. The protocol's topological database is formed from the collected link-state advertisements.

link station
(1) The hardware and software components within a node representing a connection to an adjacent node over a specific link. For example, if node A is the primary end of a multipoint line that connects to three adjacent nodes, node A will have three link stations representing the connections to the adjacent nodes.
(2) See also adjacent link station (ALS).

local
(1) Pertaining to a device accessed directly without use of a telecommunication line.
(2) Contrast with remote.
(3) Synonym for channel-attached.

local area network (LAN)
(1) A computer network located on a user's premises within a limited geographical area. Communication within a local area network is not subject to external regulations; however, communication across the LAN boundary may be subject to some form of regulation. (T)  
(2) A network in which a set of devices are connected to one another for communication and that can be connected to a larger network.
(3) See also Ethernet and token ring.
(4) Contrast with metropolitan area network (MAN) and wide area network (WAN).

local bridging
A function of a bridge program that allows a single bridge to connect multiple LAN segments without using a telecommunication link. Contrast with remote bridging.

local management interface (LMI)
See local management interface (LMI) protocol.

local management interface (LMI) protocol
In NCP, a set of frame-relay network management procedures and messages used by adjacent frame-relay nodes to exchange line status information over DLCI X'00'. NCP supports both the American National Standards Institute (ANSI) and International Telegraph and Telephone Consultative Committee (ITU-T/CCITT) versions of LMI protocol. These standards refer to LMI protocol as link integrity verification tests (LIVT).

locally administered address
In a local area network, an adapter address that the user can assign to override the universally administered address. Contrast with universally administered address.

logical channel
In packet mode operation, a sending channel and a receiving channel that together are used to send and receive data over a data link at the same time. Several logical channels can be established on the same data link by interleaving the transmission of packets.

logical link
A pair of link stations, one in each of two adjacent nodes, and their underlying link connection, providing a single link-layer connection between the two nodes. Multiple logical links can be distinguished while they share the use of the same physical media connecting two nodes. Examples are 802.2 logical links used on local area network (LAN) facilities and LAP E logical links on the same point-to-point physical link between two nodes. The term logical link also includes the multiple X.25 logical channels that share the use of the access link from a DTE to an X.25 network.

logical link control (LLC)
The data link control (DLC) LAN sublayer that provides two types of DLC operation for the orderly exchange of information. The first type is connectionless service, which allows information to be sent and received without establishing a link. The LLC sublayer does not perform error recovery or flow control for connectionless service. The second type is connection-oriented service, which requires establishing a link prior to the exchange of information. Connection-oriented service provides sequenced information transfer, flow control, and error recovery.

logical link control (LLC) protocol
In a local area network, the protocol that governs the exchange of transmission frames between data stations independently of how the transmission medium is shared. (T)   The LLC protocol was developed by the IEEE 802 committee and is common to all LAN standards.

logical link control (LLC) protocol data unit
A unit of information exchanged between link stations in different nodes. The LLC protocol data unit contains a destination service access point (DSAP), a source service access point (SSAP), a control field, and user data.

logical partition
A number assigned to a partition in a host that can operate in logically partitioned (LPAR) mode. In LPAR mode, the ESCON adapter can share a physical fiber connection with multiple host partitions.

Logically Partitioned (LPAR) mode
A function of some host processors in which processing is divided into logical partitions (LPs) to provide the appearance of multiple processors. In LPAR mode, the ESCON adapter can share a physical fiber connection with multiple host partitions.

LP
logical partition

LP number
Logical partition number. This allows multiple logical host partitions, LPs, to share one ESCON fiber. This value is defined in the host Input/Output Configuration Program (IOCP) by the RESOURCE macro instruction. If the host is not using EMIF, use the default of 0 for the LP number.

LPAR
logically partitioned

LPAR mode
Logically Partitioned (LPAR) mode.

logical unit (LU)
A type of network accessible unit that enables users to gain access to network resources and communicate with each other.

loopback test
A test in which signals from a tester are looped at a modem or other network element back to the tester for measurements that determine or verify the quality of the communications path.

low-entry networking (LEN)
A capability of nodes to attach directly to one another using basic peer-to-peer protocols to support multiple and parallel sessions between logical units.

low-entry networking (LEN) end node
A LEN node receiving network services from an adjacent APPN network node.

low-entry networking (LEN) node
A node that provides a range of end-user services, attaches directly to other nodes using peer protocols, and derives network services implicitly from an adjacent APPN network node, that is, without the direct use of CP-CP sessions.

M

Management Information Base (MIB)
(1) A collection of objects that can be accessed by means of a network management protocol.
(2) A definition for management information that specifies the information available from a host or gateway and the operations allowed.
(3) In OSI, the conceptual repository of management information within an open system.

management station
In Internet communications, the system responsible for managing all, or a portion of, a network. The management station communicates with network management agents that reside in the managed node by means of a network management protocol, such as the Simple Network Management Protocol (SNMP).

mapping
The process of converting data that is transmitted in one format by the sender into the data format that can be accepted by the receiver.

mask
(1) A pattern of characters used to control retention or elimination of portions of another pattern of characters. (I) (A)
(2) To use a pattern of characters to control retention or elimination of portions of another pattern of characters. (I) (A)

maximum transmission unit (MTU)
In LANs, the largest possible unit of data that can be sent on a given physical medium in a single frame. For example, the MTU for Ethernet is 1500 bytes.

medium access control (MAC)
In LANs, the sublayer of the data link control layer that supports medium-dependent functions and uses the services of the physical layer to provide services to the logical link control (LLC) sublayer. The MAC sublayer includes the method of determining when a device has access to the transmission medium.

medium access control (MAC) protocol
In a local area network, the protocol that governs access to the transmission medium, taking into account the topological aspects of the network, in order to enable the exchange of data between data stations. (T)  

medium access control (MAC) sublayer
In a local area network, the part of the data link layer that applies a medium access method. The MAC sublayer supports topology-dependent functions and uses the services of the physical layer to provide services to the logical link control sublayer. (T)

metric
In Internet communications, a value, associated with a route, which is used to discriminate between multiple exit or entry points to the same autonomous system. The route with the lowest metric is preferred.

metropolitan area network (MAN)
A network formed by the interconnection of two or more networks which may operate at higher speed than those networks, may cross administrative boundaries, and may use multiple access methods. (T) Contrast with local area network (LAN) and wide area network (WAN).

MIB
(1) MIB module.
(2) Management Information Base.

MIB object
Synonym for MIB variable.

MIB variable
In the Simple Network Management Protocol (SNMP), a specific instance of data defined in a MIB module. Synonymous with MIB object.

MIB view
In the Simple Network Management Protocol (SNMP), the collection of managed objects, known to the agent, that is visible to a particular community.

MILNET
The military network that was originally part of ARPANET. It was partitioned from ARPANET in 1984. MILNET provides a reliable network service for military installations.

modem (modulator/demodulator)
(1) A functional unit that modulates and demodulates signals. One of the functions of a modem is to enable digital data to be transmitted over analog transmission facilities. (T) (A)
(2) A device that converts digital data from a computer to an analog signal that can be transmitted on a telecommunication line, and converts the analog signal received to data for the computer.

module
In the Nways Switch, a packaged functional hardware unit containing logic cards, connectors, and lights. The modules are used to package adapters, line interface couplers, voice server extensions, and other components. All modules are hot pluggable in the logic subracks.

modulo
(1) Pertaining to a modulus; for example, 9 is equivalent to 4 modulo 5.
(2) See also modulus.

modulus
A number, such as a positive integer, in a relationship that divides the difference between two related numbers without leaving a remainder; for example, 9 and 4 have a modulus of 5 (9 - 4 = 5; 4 - 9 = -5; and 5 divides both 5 and -5 without leaving a remainder).

monitor
(1) A device that observes and records selected activities within a data processing system for analysis. Possible uses are to indicate significant departure from the norm, or to determine levels of utilization of particular functional units. (T)  
(2) Software or hardware that observes, supervises, controls, or verifies operations of a system. (A)  
(3) The function required to initiate the transmission of a token on the ring and to provide soft-error recovery in case of lost tokens, circulating frames, or other difficulties. The capability is present in all ring stations.

multicast
(1) Transmission of the same data to a selected group of destinations. (T)
(2) A special form of broadcast in which copies of a packet are delivered to only a subset of all possible destinations.

multipath channel (MPC)
A channel protocol that uses multiple unidirectional subchannels for VTAM-to-VTAM bidirectional communication.

multiple-domain support (MDS)
A technique for transporting management services data between management services function sets over LU-LU and CP-CP sessions. See also multiple-domain support message unit (MDS-MU).

multiple-domain support message unit (MDS-MU)
The message unit that contains management services data and flows between management services function sets over the LU-LU and CP-CP sessions used by multiple-domain support. This message unit, as well as the actual management services data that it contains, is in general data stream (GDS) format. See also control point management services unit (CP-MSU), management services unit (MSU), and network management vector transport (NMVT).

N

Name Binding Protocol (NBP)
In AppleTalk networks, a protocol that provides name translation function from the AppleTalk entity (resource) name (character string) into an AppleTalk IP address (16-bit number) on the transport layer.

name resolution
In Internet communications, the process of mapping a machine name to the corresponding Internet Protocol (IP) address. See also Domain Name System (DNS).

name server
In the Internet suite of protocols, synonym for domain name server.

nearest active upstream neighbor (NAUN)
In the IBM Token-Ring Network, the station sending data directly to a given station on the ring.

neighbor
A router on a common subnetwork that has been designated by a network administrator to receive routing information.

NetBIOS
Network Basic Input/Output System. A standard interface to networks, IBM personal computers (PCs), and compatible PCs, that is used on LANs to provide message, print-server, and file-server functions. Application programs that use NetBIOS do not need to handle the details of LAN data link control (DLC) protocols.

network
(1) A configuration of data processing devices and software connected for information interchange.
(2) A group of nodes and the links interconnecting them.

Network Access Server (NAS)
A device providing temporary, on-demand network access to users. This access is point-to-point using PSTN or ISDN lines.

network accessible unit (NAU)
A logical unit (LU), physical unit (PU), control point (CP), or system services control point (SSCP). It is the origin or the destination of information transmitted by the path control network. Synonymous with network addressable unit.

network address
According to ISO 7498-3, a name, unambiguous within the OSI environment, that identifies a set of network service access points.

network addressable unit (NAU)
Synonym for network accessible unit.

network architecture
The logical structure and operating principles of a computer network. (T)
Note:The operating principles of a network include those of services, functions, and protocols.

network congestion
An undesirable overload condition caused by traffic in excess of what a network can handle.

network identifier
(1) In TCP/IP, that part of the IP address that defines a network. The length of the network ID depends on the type of network class (A, B, or C).
(2) A 1- to 8-byte customer-selected name or an 8-byte IBM -registered name that uniquely identifies a specific subnetwork.

Network Information Center (NIC)
In Internet communications, local, regional, and national groups throughout the world who provide assistance, documentation, training, and other services to users.

network layer
In Open Systems Interconnection (OSI) architecture, the layer that is responsible for routing, switching, and link-layer access across the OSI environment.

network management
The process of planning, organizing, and controlling a communication-oriented data processing or information system.

network management station
In the Simple Network Management Protocol (SNMP), a station that executes management application programs that monitor and control network elements.

network management vector transport (NMVT)
A management services request/response unit (RU) that flows over an active session between physical unit management services and control point management services (SSCP-PU session).

network manager
A program or group of programs that is used to monitor, manage, and diagnose the problems of a network.

network node (NN)
See Advanced Peer-to-Peer Networking (APPN) network node.

network support station
The processor used to locally operate and service the Nways Switch. It is used by the Nways Switch administrator or service personnel.

network user address (NUA)
In X.25 communications, the X.121 address containing up to 15 binary code digits.

node
(1) In a network, a point at which one or more functional units connect channels or data circuits. (I)  
(2) Any device, attached to a network, that transmits and receives data.

noncanonical address
In LANs, a format for the transmission of medium access control (MAC) addresses for token-ring adapters. In noncanonical format, the most significant (leftmost) bit of each address byte is transmitted first. Contrast with canonical address.

Non-Return-to-Zero Changes-on-Ones Recording (NRZ-1)
A recording method in which the ones are represented by a change in the condition of magnetization, and zeros are represented by the absence of change. Only the one signals are explicitly recorded. (Previously called non-return-to-zero inverted, NRZI, recording.)

nonseed router
In AppleTalk networks, a router that acquires network number range and zone list information from a seed router attached to the same network.

Nways Switch
Synonymous with IBM 2220 Nways BroadBand Switch.

Nways Switch configuration station
A dedicated OS/2 station running a stand-alone version of the Nways Switch Configuration Tool (NCT). It is used to generate a network configuration database and should be installed as a remote console.

O

Open Shortest Path First (OSPF)
In the Internet suite of protocols, a function that provides intradomain information transfer. An alternative to the Routing Information Protocol (RIP), OSPF allows the lowest-cost routing and handles routing in large regional or corporate networks.

Open Systems Interconnection (OSI)
(1) The interconnection of open systems in accordance with standards of the International Organization for Standardization (ISO) for the exchange of information. (T)  (A)  
(2) The use of standardized procedures to enable the interconnection of data processing systems.
Note:OSI architecture establishes a framework for coordinating the development of current and future standards for the interconnection of computer systems. Network functions are divided into seven layers. Each layer represents a group of related data processing and communication functions that can be carried out in a standard way to support different applications.

Open Systems Interconnection (OSI) architecture
Network architecture that adheres to that particular set of ISO standards that relates to Open Systems Interconnection. (T)  

Open Systems Interconnection (OSI) reference model
A model that describes the general principles of the Open Systems Interconnection, as well as the purpose and the hierarchical arrangement of its seven layers. (T)  

origin
An external logical unit (LU) or application program from which a message or other data originates. See also destination.

orphan circuit
A non-configured circuit whose availability is learned dynamically.

P

pacing
(1) A technique by which a receiving component controls the rate of transmission of a sending component to prevent overrun or congestion.
(2) See also flow control, receive pacing, send pacing, session-level pacing, and virtual route (VR) pacing.

packet
In data communication, a sequence of binary digits, including data and control signals, that is transmitted and switched as a composite whole. The data, control signals, and, possibly, error control information are arranged in a specific format. (I)  

packet internet groper (PING)
(1) In Internet communications, a program used in TCP/IP networks to test the ability to reach destinations by sending the destinations an Internet Control Message Protocol (ICMP) echo request and waiting for a reply.
(2) In communications, a test of reachability.

packet loss ratio
The probability that a packet will not reach its destination or not reach it within a specified time.

packet mode operation
Synonym for packet switching.

packet switching
(1) The process of routing and transferring data by means of addressed packets so that a channel is occupied only during transmission of a packet. On completion of the transmission, the channel is made available for transfer of other packets. (I)  
(2) Synonymous with packet mode operation. See also circuit switching.

parallel bridges
A pair of bridges connected to the same LAN segment, creating redundant paths to the segment.

parallel transmission groups
Multiple transmission groups between adjacent nodes, with each group having a distinct transmission group number.

path
(1) In a network, any route between any two nodes. A path may include more than one branch. (T)  
(2) The series of transport network components (path control and data link control) that are traversed by the information exchanged between two network accessible units. See also explicit route (ER), route extension, and virtual route (VR).

path control (PC)
The function that routes message units between network accessible units in the network and provides the paths between them. It converts the basic information units (BIUs) from transmission control (possibly segmenting them) into path information units (PIUs) and exchanges basic transmission units containing one or more PIUs with data link control. Path control differs by node type: some nodes (APPN nodes, for example) use locally generated session identifiers for routing, and others (subarea nodes) use network addresses for routing.

path cost
In link-state routing protocols, the sum of the link costs along the path between two nodes or networks.

path information unit (PIU)
A message unit consisting of a transmission header (TH) alone, or a TH followed by a basic information unit (BIU) or a BIU segment.

pattern-matching character
A special character such as an asterisk (*) or a question mark (?) that can be used to represent one or more characters. Any character or set of characters can replace a pattern-matching character. Synonymous with global character and wildcard character.

permanent virtual circuit (PVC)
In X.25 and frame-relay communications, a virtual circuit that has a logical channel permanently assigned to it at each data terminal equipment (DTE). Call-establishment protocols are not required. Contrast with switched virtual circuit (SVC).

physical circuit
A circuit established without multiplexing. See also data circuit. Contrast with virtual circuit.

physical layer
In the Open Systems Interconnection reference model, the layer that provides the mechanical, electrical, functional, and procedural means to establish, maintain, and release physical connections over the transmission medium. (T)

physical unit (PU)
(1) The component that manages and monitors the resources (such as attached links and adjacent link stations) associated with a node, as requested by an SSCP via an SSCP-PU session. An SSCP activates a session with the physical unit in order to indirectly manage, through the PU, resources of the node such as attached links. This term applies to type 2.0, type 4, and type 5 nodes only.
(2) See also peripheral PU and subarea PU.

ping command
The command that sends an Internet Control Message Protocol (ICMP) echo-request packet to a gateway, router, or host with the expectation of receiving a reply.

Point-to-Point Protocol (PPP)
A protocol that provides a method for encapsulating and transmitting packets over serial point-to-point links.

polling
(1) On a multipoint connection or a point-to-point connection, the process whereby data stations are invited, one at a time, to transmit. (I)  
(2) Interrogation of devices for such purposes as to avoid contention, to determine operational status, or to determine readiness to send or receive data. (A)  

port
(1) An access point for data entry or exit.
(2) A connector on a device to which cables for other devices such as display stations and printers are attached.
(3) The representation of a physical connection to the link hardware. A port is sometimes referred to as an adapter; however, there can be more than one port on an adapter. There may be one or more ports controlled by a single DLC process.
(4) In the Internet suite of protocols, a 16-bit number used to communicate between TCP or the User Datagram Protocol (UDP) and a higher-level protocol or application. Some protocols, such as File Transfer Protocol (FTP) and Simple Mail Transfer Protocol (SMTP), use the same well-known port number in all TCP/IP implementations.
(5) An abstraction used by transport protocols to distinguish among multiple destinations within a host machine.
(6) Synonymous with socket.

port number
In Internet communications, the identification of an application entity to the transport service.

private branch exchange (PBX)
A private telephone exchange for transmission of calls to and from the public telephone network.

problem determination
The process of determining the source of a problem; for example, a program component, machine failure, telecommunication facilities, user or contractor-installed programs or equipment, environmental failure such as a power loss, or user error.

program temporary fix (PTF)
A temporary solution or bypass of a problem diagnosed by IBM in a current unaltered release of the program.

protocol
(1) A set of semantic and syntactic rules that determine the behavior of functional units in achieving communication. (I)  
(2) In Open Systems Interconnection architecture, a set of semantic and syntactic rules that determine the behavior of entities in the same layer in performing communication functions. (T)  
(3) In SNA, the meanings of, and the sequencing rules for, requests and responses used for managing the network, transferring data, and synchronizing the states of network components. Synonymous with line control discipline and line discipline. See bracket protocol and link protocol.

protocol data unit (PDU)
A unit of data specified in a protocol of a given layer and consisting of protocol control information of this layer, and possibly user data of this layer. (T)  

pulse code modulation (PCM)
A standard adopted for the digitalization of an analog voice signal. In PCM, the voice is sampled at a rate of eight kHz and each sample is coded in an 8-bit frame.

R

Rapid Transport Protocol (RTP) connection
In high-performance routing (HPR), the connection established between the endpoints of the route to transport session traffic.

reachability
The ability of a node or a resource to communicate with another node or resource.

read-only memory (ROM)
Memory in which stored data cannot be modified by the user except under special conditions.

real-time processing
The manipulation of data that are required, or generated, by some process while the process is in operation. Usually the results are used to influence the process, and perhaps related processes, while it is occurring.

reassembly
In communications, the process of putting segmented packets back together after they have been received.

receive not ready (RNR)
In communications, a data link command or response that indicates a temporary condition of being unable to accept incoming frames.

receive not ready (RNR) packet
See RNR packet.

received line signal detector (RLSD)
In the EIA 232 standard, a signal that indicates to the data terminal equipment (DTE) that it is receiving a signal from the remote data circuit-terminating equipment (DCE). Synonymous with carrier detect and data carrier detect (DCD).

Recognized Private Operating Agency (RPOA)
Any individual, company, or corporation, other than a government department or service, that operates a telecommunication service and is subject to the obligations undertaken in the Convention of the International Telecommunication Union and in the Regulations; for example, a communication common carrier.

reduced instruction-set computer (RISC)
A computer that uses a small, simplified set of frequently used instructions for rapid execution.

remote
(1) Pertaining to a system, program, or device that is accessed through a telecommunication line.
(2) Synonym for link-attached.
(3) Contrast with local.

remote bridging
The function of a bridge that allows two bridges to connect multiple LANs using a telecommunication link. Contrast with local bridging.

remote console
A station running OS/2, TCP/IP, and the remote Nways Switch Resource Control program. It can be connected to any network support station to operate and service the Nways Switch remotely.

The connection may be through:

Any network support station can be used as a remote console of another network support station.

Remote Execution Protocol (REXEC)
A protocol that allows the execution of a command or program on any host in the network. The local host receives the results of the command execution.

Request for Comments (RFC)
In Internet communications, the document series that describes a part of the Internet suite of protocols and related experiments. All Internet standards are documented as RFCs.

reset
On a virtual circuit, reinitialization of data flow control. At reset, all data in transit are eliminated.

reset request packet
In X.25 communications, a packet transmitted by the data terminal equipment (DTE) to the data circuit-terminating equipment (DCE) to request that a virtual call or a permanent virtual circuit be reset. The reason for the request can also be specified in the packet.

resource
In the Nways Switch, an hardware element or a logical entity created by the Control Program. For example, the adapters, LICs, and lines are physical resources. The control points and connections are logical resources.

ring
See ring network.

ring network
(1) A network in which every node has exactly two branches connected to it and in which there are exactly two paths between any two nodes. (T)  
(2) A network configuration in which devices are connected by unidirectional transmission links to form a closed path.

ring segment
A section of a ring that can be isolated (by unplugging connectors) from the rest of the ring. See LAN segment.

rlogin (remote login)
A service, offered by Berkeley UNIX-based systems, that allows authorized users of one machine to connect to other UNIX systems across an internet and interact as if their terminals were connected directly. The rlogin software passes information about the user's environment (for example, terminal type) to the remote machine.

RNR packet
A packet used by a data terminal equipment (DTE) or by a data circuit-terminating equipment (DCE) to indicate a temporary inability to accept additional packets for a virtual call or permanent virtual circuit.

root bridge
The bridge that is the root of a spanning tree formed between other active bridges in the bridging network. The root bridge originates and transmits bridge protocol data units (BPDUs) to other active bridges to maintain the spanning tree topology. It is the bridge with the highest priority in the network.

route
(1) An ordered sequence of nodes and transmission groups (TGs) that represent a path from an origin node to a destination node traversed by the traffic exchanged between them.
(2) The path that network traffic uses to get from source to destination.

route bridge
A function of an IBM bridge program that allows two bridge computers to use a telecommunication link to connect two LANs. Each bridge computer is connected directly to one of the LANs, and the telecommunication link connects the two bridge computers.

route extension (REX)
In SNA, the path control network components, including a peripheral link, that make up the portion of a path between a subarea node and a network addressable unit (NAU) in an adjacent peripheral node. See also explicit route (ER), path, and virtual route (VR).

Route Selection control vector (RSCV)
A control vector that describes a route within an APPN network. The RSCV consists of an ordered sequence of control vectors that identify the TGs and nodes that make up the path from an origin node to a destination node.

router
(1) A computer that determines the path of network traffic flow. The path selection is made from several paths based on information obtained from specific protocols, algorithms that attempt to identify the shortest or best path, and other criteria such as metrics or protocol-specific destination addresses.
(2) An attaching device that connects two LAN segments, which use similar or different architectures, at the reference model network layer.
(3) In OSI terminology, a function that determines a path by which an entity can be reached.
(4) In TCP/IP, synonymous with gateway.
(5) Contrast with bridge.

routing
(1) The assignment of the path by which a message is to reach its destination.
(2) In SNA, the forwarding of a message unit along a particular path through a network, as determined by parameters carried in the message unit, such as the destination network address in a transmission header.

routing domain
In Internet communications, a group of intermediate systems that use a routing protocol so that the representation of the overall network is the same within each intermediate system. Routing domains are connected to each other by exterior links.

Routing Information Protocol (RIP)
In the Internet suite of protocols, an interior gateway protocol used to exchange intradomain routing information and to determine optimum routes between internet hosts. RIP determines optimum routes on the basis of route metrics, not link transmission speed.

routing loop
A situation that occurs when routers circulate information among themselves until convergence occurs or until the networks involved are considered unreachable.

routing protocol
A technique used by a router to find other routers and to remain up to date about the best way to get to reachable networks.

routing table
A collection of routes used to direct datagram forwarding or to establish a connection. The information is passed among routers to identify network topology and destination feasibility.

Routing Table Maintenance Protocol (RTMP)
In AppleTalk networks, a protocol that provides routing information generation and maintenance on the transport layer by means of the AppleTalk routing table. The AppleTalk routing table directs packet transmission through the internet from source socket to destination socket.

RouTing update Protocol (RTP)
The VIrtual NEtworking System (VINES) protocol that maintains the routing database and allows the exchange of routing information between VINES nodes. See also Internet Control Protocol (ICP).

rsh
A variant of the rlogin command that invokes a command interpreter on a remote UNIX machine and passes the command-line arguments to the command interpreter, skipping the login step completely.

S

SAP
See service access point.

seed router
In AppleTalk networks, a router that maintains configuration data (network range numbers and zone lists, for example) for the network. Each network must have at least one seed router. The seed router must be initially set up using the configurator tool. Contrast with nonseed router.

segment
(1) A section of cable between components or devices. A segment may consist of a single patch cable, several patch cables that are connected, or a combination of building cable and patch cables that are connected.
(2) In Internet communications, the unit of transfer between TCP functions in different machines. Each segment contains control and data fields; the current byte-stream position and actual data bytes are identified along with a checksum to validate received data.

segmenting
In OSI, a function performed by a layer to map one protocol data unit (PDU) from the layer it supports into multiple PDUs.

sequence number
In communications, a number assigned to a particular frame or packet to control the transmission flow and receipt of data.

Serial Line Internet Protocol (SLIP)
A protocol used over a point-to-point connection between two IP hosts over a serial line, for example, a serial cable or an RS232 connection into a modem, over a telephone line.

server
A functional unit that provides shared services to workstations over a network; for example, a file server, a print server, a mail server. (T)  

service access point (SAP)
(1) In Open Systems Interconnection (OSI) architecture, the point at which the services of a layer are provided by an entity of that layer to an entity of the next higher layer. (T)  
(2) A logical point made available by an adapter where information can be received and transmitted. A single service access point can have many links terminating in it.

Service Advertising Protocol (SAP)
In Internetwork Packet Exchange (IPX), a protocol that provides the following:

session
(1) In network architecture, for the purpose of data communication between functional units, all the activities which take place during the establishment, maintenance, and release of the connection. (T)  
(2) A logical connection between two network accessible units (NAUs) that can be activated, tailored to provide various protocols, and deactivated, as requested. Each session is uniquely identified in a transmission header (TH) accompanying any transmissions exchanged during the session.
(3) In L2TP, L2TP creates a session when an end-to-end PPP connection is attempted between a dial user and the LNS; regardless of whether the user initiates the session or the LNS initiates an outbound call. The datagrams for th session are sent over the tunnel between the LAC and LNS. The LNS and LAC maintain the state information for each user attached to an LAC.

Simple Network Management Protocol (SNMP)
In the Internet suite of protocols, a network management protocol that is used to monitor routers and attached networks. SNMP is an application layer protocol. Information on devices managed is defined and stored in the application's Management Information Base (MIB).

SNA management services (SNA/MS)
The services provided to assist in management of SNA networks.

socket
(1) An endpoint for communication between processes or application programs.
(2) The abstraction provided by the University of California's Berkeley Software Distribution (commonly called Berkeley UNIX or BSD UNIX) that serves as an endpoint for communication between processes or applications.

source route bridging
In LANs, a bridging method that uses the routing information field in the IEEE 802.5 medium access control (MAC) header of a frame to determine which rings or token-ring segments the frame must transit. The routing information field is inserted into the MAC header by the source node. The information in the routing information field is derived from explorer packets generated by the source host.

source routing
In LANs, a method by which the sending station determines the route the frame will follow and includes the routing information with the frame. Bridges then read the routing information to determine whether they should forward the frame.

source service access point (SSAP)
In SNA and TCP/IP, a logical address that allows a system to send data to a remote device from the appropriate communications support. Contrast with destination service access point (DSAP).

spanning tree
In LAN contexts, the method by which bridges automatically develop a routing table and update that table in response to changing topology to ensure that there is only one route between any two LANs in the bridged network. This method prevents packet looping, where a packet returns in a circuitous route back to the sending router.

sphere of control (SOC)
The set of control point domains served by a single management services focal point.

sphere of control (SOC) node
A node directly in the sphere of control of a focal point. A SOC node has exchanged management services capabilities with its focal point. An APPN end node can be a SOC node if it supports the function to exchange management services capabilities.

split horizon
A technique for minimizing the time to achieve network convergence. A router records the interface over which it received a particular route and does not propagate its information about the route back over the same interface.

spoofing
For data links, a technique in which a protocol initiated from an end station is acknowledged and processed by an intermediate node on behalf of the final destination. In IBM 6611 data link switching, for example, SNA frames are encapsulated into TCP/IP packets for transport across a non-SNA wide area network, unpacked by another IBM 6611, and passed to the final destination. A benefit of spoofing is the prevention of end-to-end session timeouts.

standard MIB
In the Simple Network Management Protocol (SNMP), a MIB module that is located under the management branch of the Structure of Management Information (SMI) and that is considered a standard by the Internet Engineering Task Force (IETF).

static route
The route between hosts, networks, or both that is manually entered into a routing table.

station
An input or output point of a system that uses telecommunication facilities; for example, one or more systems, computers, terminals, devices, and associated programs at a particular location that can send or receive data over a telecommunication line.

StreetTalk
In the VIrtual NEtworking System (VINES), a unique network-wide naming and addressing system that allows users to locate and access any resource on the network without knowing the network topology. See also Internet Control Protocol (ICP) and RouTing update Protocol (RTP).

Structure of Management Information (SMI)
(1) In the Simple Network Management Protocol (SNMP), the rules used to define the objects that can be accessed by means of a network management protocol.
(2) In OSI, the set of standards relating to management information. The set includes the Management Information Model and the Guidelines for the Definition of Managed Objects

subarea
A portion of the SNA network consisting of a subarea node, attached peripheral nodes, and associated resources. Within a subarea node, all network accessible units (NAUs), links, and adjacent link stations (in attached peripheral or subarea nodes) that are addressable within the subarea share a common subarea address and have distinct element addresses.

subnet
(1) In TCP/IP, a part of a network that is identified by a portion of the IP address.
(2) Synonym for subnetwork.

subnet address
In Internet communications, an extension to the basic IP addressing scheme where a portion of the host address is interpreted as the local network address.

subnet mask
Synonym for address mask.

subnetwork
(1) Any group of nodes that have a set of common characteristics, such as the same network ID.
(2) Synonymous with subnet.

Subnetwork Access Protocol (SNAP)
In LANs, a 5-byte protocol discriminator that identifies the non-IEEE standard protocol family to which a packet belongs. The SNAP value is used to differentiate between protocols that use $AA as their service access point (SAP) value.

subnetwork mask
Synonym for address mask.

subsystem
A secondary or subordinate system, usually capable of operating independently of, or asynchronously with, a controlling system. (T)  

switched virtual circuit (SVC)
An X.25 circuit that is dynamically established when needed. The X.25 equivalent of a switched line. Contrast with permanent virtual circuit (PVC).

synchronous
(1) Pertaining to two or more processes that depend upon the occurrence of specific events such as common timing signals. (T)  
(2) Occurring with a regular or predictable time relationship.

Synchronous Data Link Control (SDLC)
(1) A discipline conforming to subsets of the Advanced Data Communication Control Procedures (ADCCP) of the American National Standards Institute (ANSI) and High-level Data Link Control (HDLC) of the International Organization for Standardization, for managing synchronous, code-transparent, serial-by-bit information transfer over a link connection. Transmission exchanges may be duplex or half-duplex over switched or nonswitched links. The configuration of the link connection may be point-to-point, multipoint, or loop. (I)
(2) Contrast with binary synchronous communication (BSC).

synchronous optical network (SONET)
A US standard for transmitting digital information over optical interfaces. It is closely related to the synchronous digital hierarchy (SDH) recommendation.

SYNTAX
In the Simple Network Management Protocol (SNMP), a clause in the MIB module that defines the abstract data structure that corresponds to a managed object.

system
In data processing, a collection of people, machines, and methods organized to accomplish a set of specific functions. (I) (A)

system configuration
A process that specifies the devices and programs that form a particular data processing system.

system services control point (SSCP)
A component within a subarea network for managing the configuration, coordinating network operator and problem determination requests, and providing directory services and other session services for users of the network. Multiple SSCPs, cooperating as peers with one another, can divide the network into domains of control, with each SSCP having a hierarchical control relationship to the physical units and logical units within its own domain.

Systems Network Architecture (SNA)
The description of the logical structure, formats, protocols, and operational sequences for transmitting information units through, and controlling the configuration and operation of, networks. The layered structure of SNA allows the ultimate origins and destinations of information, that is, the users, to be independent of and unaffected by the specific SNA network services and facilities used for information exchange.

T

TCP/IP
(1) Transmission Control Protocol/Internet Protocol.
(2) A UNIX-like/Ethernet-based system-interconnect protocol originally developed by the US Department of Defense. TCP/IP facilitated ARPANET (Advanced Research Projects Agency Network), a packet-switched research network for which Layer 4 was TCP and Layer 3, IP.

Telnet
In the Internet suite of protocols, a protocol that provides remote terminal connection service. It allows users of one host to log on to a remote host and interact as directly attached terminal users of that host.

threshold
(1) In IBM bridge programs, a value set for the maximum number of frames that are not forwarded across a bridge due to errors, before a "threshold exceeded" occurrence is counted and indicated to network management programs.
(2) An initial value from which a counter is decremented to 0, or a value to which a counter is incremented or decremented from an initial value.

throughput class
In packet switching, the speed at which data terminal equipment (DTE) packets travel through the packet switching network.

time division multiplexing (TDM)
See channelization.

time to live (TTL)
A technique used by best-effort delivery protocols to inhibit endlessly looping packets. The packet is discarded if the TTL counter reaches 0.

timeout
(1) An event that occurs at the end of a predetermined period of time that began at the occurrence of another specified event. (I)
(2) A time interval allotted for certain operations to occur; for example, response to polling or addressing before system operation is interrupted and must be restarted.

token
(1) In a local area network, the symbol of authority passed successively from one data station to another to indicate the station temporarily in control of the transmission medium. Each data station has an opportunity to acquire and use the token to control the medium. A token is a particular message or bit pattern that signifies permission to transmit. (T)  
(2) In LANs, a sequence of bits passed from one device to another along the transmission medium. When the token has data appended to it, it becomes a frame.

token ring
(1) According to IEEE 802.5, network technology that controls media access by passing a token (special packet or frame) between media-attached stations.
(2) A FDDI or IEEE 802.5 network with a ring topology that passes tokens from one attaching ring station (node) to another.
(3) See also local area network (LAN).

token-ring network
(1) A ring network that allows unidirectional data transmission between data stations, by a token passing procedure, such that the transmitted data return to the transmitting station. (T)
(2) A network that uses a ring topology, in which tokens are passed in a circuit from node to node. A node that is ready to send can capture the token and insert data for transmission.

topology
In communications, the physical or logical arrangement of nodes in a network, especially the relationships among nodes and the links between them.

topology database update (TDU)
A message about a new or changed link or node that is broadcast among APPN network nodes to maintain the network topology database, which is fully replicated in each network node. A TDU contains information that identifies the following:

trace
(1) A record of the execution of a computer program. It exhibits the sequences in which the instructions were executed. (A)  
(2) For data links, a record of the frames and bytes transmitted or received.

transceiver (transmitter-receiver)
In LANs, a physical device that connects a host interface to a local area network, such as Ethernet. Ethernet transceivers contain electronics that apply signals to the cable and that sense collisions.

Transmission Control Protocol (TCP)
A communications protocol used in the Internet and in any network that follows the U.S. Department of Defense standards for internetwork protocol. TCP provides a reliable host-to-host protocol between hosts in packet-switched communications networks and in interconnected systems of such networks. It uses the Internet Protocol (IP) as the underlying protocol.

Transmission Control Protocol/Internet Protocol (TCP/IP)
A set of communications protocols that support peer-to-peer connectivity functions for both local and wide area networks.

transmission group (TG)
(1) A connection between adjacent nodes that is identified by a transmission group number.
(2) In a subarea network, a single link or a group of links between adjacent nodes. When a transmission group consists of a group of links, the links are viewed as a single logical link, and the transmission group is called a multilink transmission group (MLTG). A mixed-media multilink transmission group (MMMLTG) is one that contains links of different medium types (for example, token-ring, switched SDLC, nonswitched SDLC, and frame-relay links).
(3) In an APPN network, a single link between adjacent nodes.
(4) See also parallel transmission groups.

transmission header (TH)
Control information, optionally followed by a basic information unit (BIU) or a BIU segment, that is created and used by path control to route message units and to control their flow within the network. See also path information unit.

transparent bridging
In LANs, a method for tying individual local area networks together through the medium access control (MAC) level. A transparent bridge stores the tables that contain MAC addresses so that frames seen by the bridge can be forwarded to another LAN if the tables indicate to do so.

transport layer
In the Open Systems Interconnection reference model, the layer that provides a reliable end-to-end data transfer service. There may be relay open systems in the path. (T) See also Open Systems Interconnection reference model.

trap
In the Simple Network Management Protocol (SNMP), a message sent by a managed node (agent function) to a management station to report an exception condition.

trunk line
A high-speed line connecting two Nways Switches. It can be a coaxial cable, fiber cable, or radio wave, for example, and may be leased from telecommunication companies.

Tunnel
A tunnel is defined by an LNS-LAC pair. The tunnel carries PPP datagrams between the LAC and the LNS. A single tunnel can multiplex many sessions. A control connection operating over the same tunnel controls the establishment, release, and maintenance of all sessions and of the tunnel itself.

tunneling
To treat a transport network as though it were a single communication link or LAN. See also encapsulation.

T1
In the United States, a 1.544-Mbps public access line. It is available in twenty-four 64-Kbps channels. The European version (E1) transmits 2.048 Mbps.

U

universally administered address
In a local area network, the address permanently encoded in an adapter at the time of manufacture. All universally administered addresses are unique. Contrast with locally administered address.

User Datagram Protocol (UDP)
In the Internet suite of protocols, a protocol that provides unreliable, connectionless datagram service. It enables an application program on one machine or process to send a datagram to an application program on another machine or process. UDP uses the Internet Protocol (IP) to deliver datagrams.

V

V.24
In data communication, a specification of the CCITT that defines the list of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE).

V.25
In data communication, a specification of the CCITT that defines the automatic answering equipment and parallel automatic calling equipment on the General Switched Telephone Network, including procedures for disabling of echo controlled devices for both manually and automatically established calls.

V.34
An ITU-T Recommendation for modem communication over standard commercially available voice-grade 33.6-Kbps (and slower) channels.

V.35
In data communication, a specification of the CCITT that defines the list of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) at various data rates.

V.36
In data communication, a specification of the CCITT that defines the list of definitions for interchange circuits between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) at rates of 48, 56, 64, or 72 kilobits per second.

version
A separately licensed program that usually has significant new code or new function.

VINES
VIrtual NEtworking System.

virtual circuit
(1) In packet switching, the facilities provided by a network that give the appearance to the user of an actual connection. (T)   See also data circuit. Contrast with physical circuit.
(2) A logical connection established between two DTEs.

virtual connection
In frame relay, the return path of a potential connection.

virtual link
In Open Shortest Path First (OSPF), a point-to-point interface that connects border routers that are separated by a non-backbone transit area. Because area routers are part of the OSPF backbone, the virtual link connects the backbone. The virtual links ensure that the OSPF backbone does not become discontinuous.

VIrtual NEtworking System (VINES)
The network operating system and network software from Banyan Systems, Inc. In a VINES network, virtual linking allows all devices and services to appear to be directly connected to each other, when they may actually be thousands of miles apart. See also StreetTalk.

virtual route (VR)
(1) In SNA, either (a) a logical connection between two subarea nodes that is physically realized as a particular explicit route or (b) a logical connection that is contained wholly within a subarea node for intranode sessions. A virtual route between distinct subarea nodes imposes a transmission priority on the underlying explicit route, provides flow control through virtual route pacing, and provides data integrity through sequence numbering of path information units (PIUs).
(2) Contrast with explicit route (ER). See also path and route extension (REX).

W

wide area network (WAN)
(1) A network that provides communication services to a geographic area larger than that served by a local area network or a metropolitan area network, and that may use or provide public communication facilities. (T)  
(2) A data communication network designed to serve an area of hundreds or thousands of miles; for example, public and private packet-switching networks, and national telephone networks.
(3) Contrast with local area network (LAN) and metropolitan area network (MAN).

wildcard character
Synonym for pattern-matching character.

X

X.21
An International Telegraph and Telephone Consultative Committee (CCITT) recommendation for a general-purpose interface between data terminal equipment and data circuit-terminating equipment for synchronous operations on a public data network.

X.25
(1) An International Telegraph and Telephone Consultative Committee (CCITT) recommendation for the interface between data terminal equipment and packet-switched data networks.
(2) See also packet switching.

Xerox Network Systems (XNS)
The suite of internet protocols developed by the Xerox Corporation. Although similar to TCP/IP protocols, XNS uses different packet formats and terminology. See also Internetwork Packet Exchange (IPX).

Z

zone
In AppleTalk networks, a subset of nodes within an internet.

Zone Information Protocol (ZIP)
In AppleTalk networks, a protocol that provides zone management service by maintaining a mapping of the zone names and network numbers across the internet on the session layer.

zone information table (ZIT)
A listing of network numbers and their associated zone name mappings in the internet. This listing is maintained by each internet router in an AppleTalk internet.

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