D

D CHANNEL

In an ISDN interface, the data or D channel is used to carry control signals and customer call data in a packet switched mode. In the BRI (Basic Rate Interface) the D channel operates at 16Kbps, part of which will handle setup, teardown, and other characteristics of the call. 9,600 bps will be free for a separate conversation by the user. In the PRI (Primary Rate Interface), the D channel runs at 64Kbps. The D channel is sometimes referred to as the delta Channel. See also BASIC RATE INTERFACE, PRIMARY RATE INTERFACE, and ISDN.

D-INSIDE WIRE

Typically DIW is 4 pairs of copper wire in the same sheath. Also called unshielded twisted pair.

DAC

National Semiconductor part prefix that indicates the device is in device family: Data Conversion

DAEMON

A system process that runs in the background. It has no attached terminal and never exits.

DAP

Data Access Protocol

DARK CURRENT

Current flowing or signal amplitude generated in a photosensor placed in total darkness; dark noise expressed as current flow.

DARKAVENGER

Each time the virus infects 16 files, it writes garbage to a random disk sector on the workstation.

DARPA

See Defense Advanced Research Projects Agency

DAT

Duplicate Address Test OR See Digital Audio Tape.

DATA CARRIER DETECT

See DCD.

DATA COMMUNICATION EQUIPMENT

The DCE actually transmits the data. A DCE consists of two types: modems for analog data and Digital Service Unit/Channel Service Unit (DSU/CSU) for digital data.

DATA COMPRESSION

The encoding of text or data so that it takes up less space (fewer bits). CCITT V.42bis and MNP class 5 are two different data compression protocols. Data compression allows your modem to transmit more information in a shorter period of time and thus increases your modem's data throughput.

DATA FLOW CONTROL LAYER

The fifth layer of the SNA model. The data flow control layer manages sessions and provides error recovery.

DATA HIGHWAY/DATA HIGHWAY PLUS

(DH/DH+) Allen-Bradley proprietary, industrial, local-area networks designed for a manufacturing plant environment.

DATA LINK CONNECTION IDENTIFIER

Located in the frame relay header, this 10-bit field is the MAC address that identifies the PVC between the user and frame relay device.

DATA LINK CONTROL

(DLC) The native protocol of the IBM token-ring card.

DATA LINK CONTROL LAYER

The second layer of the SNA model. The data link control layer constructs frames from a bit stream.

DATA LINK LAYER

The second layer of the ISO OSI model. The data link layer handles the translation between the electrical signals transmitted by the physical layer and the bit patterns required by the higher layers.

DATA LINK TRACE

In the SNA Gateway, a diagnostic test in which the gateway simulates the host to check for proper LAN functioning.

DATA RATE

The speed of a data transfer process, normally expressed in bits per second or bytes per second.

DATA SET READY

See DSR

DATA STREAM

Data that is flowing between points on a network. A data stream is usually a succession of messages.

DATA TERMINAL EQUIPMENT

(DTE) To establish communications, a DTE prepares and controls data for transmittal. A DTE may be a PC or a communications server in a WAN

DATA TERMINAL READY

See DTR

DATABASE SERVER

Software such as Microsoft SQL Server that provides high-performance database access by splitting the DBMS function into a front-end component (where data is manipulated by users or applications) and a database-intelligent, back-end component (where data is stored, retrieved, and managed). In PC networks, the front-end component often resides on a personal computer controlled by a single user, while the back-end component resides on a high-performance PC that services requests for data submitted over the network by users.

DATAGRAM SOCKET

A communications channel that is not necessarily sequenced, reliable, or unduplicated. Datagram sockets allow data packets, called messages, to flow bi-directionally. Because datagram sockets do not need to be connected to a peer, messages are sent to a destination address. See also socket.

DATAPHONE DIGITAL SERVICE

An AT&T communications service in which data is transmitted in digital rather than analog form.

DAYTIME

Daytime Protocol

DB

dB = Decibel

DB-15

RS-422/RS-423 - 15 conductor EIA cable used as drop cable for Ethernet local area networks, among other uses.

DB-25

RS-232 - 25 conductor round or flat EIA cable assembly

DB-37

RS-449 -37 conductor EIA cable assembly. Connects DTE (Data Terminal Equip.) and DCE (Data Circuit Terminating Equip.) equipment employing binary serial data exchange.

DB-50

RS-422/RS-423 - 50 pins - Same standards as RS-232, meets interface needs of all high volume circuit systems.

DB-9

RS-449 - 9 conductor EIA cable

DBM

dBm = Decibels above or below a reference power of 0.001 Watt (1mW) in a 600 ohm resistor. 0dBm equals 0.78 volts.

DBMS

Database Management System.

DBS

Direct Broadcast Satellite

DBX

Source level debugger for C, Fortran 77, and Pascal programs.

DBXTOOL

Window/mouse based source level debugger for C, Fortran 77, and Pascal programs.

DCA

See Defense Communications Agency

DCC

Digital Control Channel

DCD

Data Carrier Detect signal in RS-232-C. A signal from the modem to a device to indicate that the carrier from another modem is being sensed on the phone line. It will not be asserted unless the phone is off-hook. Data will not be received at the asynchronous connector unless DCD is on. With full-duplex protocol, the module will not transmit unless DCD is on. If the modem does not properly control DCD, or if a modem is not being used, DCD must be jumpered to DTR at the device.

DCDB

See Domain Control Data Base

DCE

Data Circuit-Terminating Equipment OR See Distributed Computing Environment.

DCL

Data Carrier Level

DCN-MEAS

DCN Measurement Subsystems Protocol

DCP

Device Control Protocol

DCT

See Discrete Cosine Transform.

DDCMP

See Digital Data Communications Message Protocol

DDD

See Direct Distance Dialing

DDE

See Dynamic Data Exchange

DDLC

See Dual Data Link Controller.

DDN

See Defense Data Network

DDP

Datagram Delivery Protocol

DDS

See Digital Data Service. See Dataphone Digital Service.

DEBUGGING

The process of detecting, locating, and correcting errors in hardware or software.

DEC-423 SERIAL INTERFACE

A compact, high-performance wiring system for connections, such as local direct connections, that do not require modem control signals. The DEC-423 serial interface standard is a superset of the RS-423 A standard.

DECMCC

DEC Management Control Center

DECNET

The DEC networking software that runs on nodes in both local-area and wide-area networks. Digital Equipment Corporation's proprietary network architecture.

DECOMPRESSION

To reverse the procedure conducted by the compression software algorithm to return data to its original size and condition.

DEFENSE ADVANCED RESEARCH PROJECTS AGENCY

(DARPA) Defense Advanced Research Projects Agency. The U.S. government agency that funded the ARPANET.

DEFENSE COMMUNICATIONS AGENCY

(DCA) Defense Communications Agency. The government agency responsible for the Defense Data Network (DDN).

DEFENSE DATA NETWORK

(DDN) Comprises the MILNET and several other DoD networks.

DEFENSE DATA NETWORK

DDN refers to the MILNET and ARPANET, and the TCP/IP protocols they use. More literally, it is the MILNET and associated parts of the Internet that connect military installations.

DEFINITION

A type of hypertext link in Microsoft Help. A definition is a (usually) brief description of a term. Definitions appear when you press and hold the mouse button on a term that appears in Help as green text marked with a dotted underline. The description disappears as soon as you release the mouse button. Hence, it is not possible to access hypertext links within a definition. Some definitions contain hypertext links, however, because they are also available as cross-references. It is usually possible to reference a definition as a cross-reference via the Help Search button.

DELIVERY SYSTEM

The equipment used by end users to run or "play" an interactive program.

DELTA CHANNEL

See D Channel.

DELTA FRAME

Contains only the pixels different from the preceding Key Frame. Delta Frames reduce the overall size of the video clip to be stored on disk. Also called Difference Frame. See also Key Frame.

DEMAND PAGING

The common implementation of virtual memory, where pages of data are read into memory from storage in response to page faults. See also paging and virtual memory.

DEMPR

DEC Multiport Repeater

DENSITY

The degree of darkness of an image. Also, percent of screen used in an image.

DEREP

A device used to extend the length, topology, or interconnectivity of the physical network medium beyond the limits imposed by a single segment. Repeaters perform the basic actions of restoring signal amplitude, waveform, and timing applied to normal data and collision signals.

DESIGNATED ROUTER

(IS) The IS on a LAN that performs additional duties. A designated router generates link state PDUs on behalf of the LAN, treating the LAN as a psuedonode.

DESKTOP MANAGEMENT INTERFACE

A set of APIs and related code that is designed to provide component-level management of PCs across a network.

DESKTOP MANAGEMENT TASK FORCE

Group developing the Desktop Management Interface standard.

DESTINATION

A node designated as the intended receiver of data.

DESTINATION SERVICE ACCESS POINT

(DSAP) IEEE LLC protocol designates and uses a DSAP and SSAP to determine routes for LAN frames to be taken when entering a system of bridges. The DSAP designates the bridge to which the frame is to be forwarded. (Also see LLC, SSAP, spanning-tree protocol, source-routing protocol, and translating bridge.)

DETECTION

Scanning memory and disks for telltale marks or changes indicating that a virus might be present.

DEVICE DRIVER

The software that the system uses to communicate with a device such as a display, printer, mouse, or communications adapter.

DEVICE DRIVER REPLACEMENT

The technology, implemented by the LAN Distance product, in which a LAN MAC is replaced by a WAN adapter MAC to transmit data across wide area connections. The LAN Distance logical adapter and WAN adapter MAC handle the interface transition required to send and receive LAN data frames across a LAN Distance connection. This technology enables transparent support of LAN applications and LAN protocols.

DEVICE INDEPENDENT BITMAP

(DIB) Name for the new graphics engine in Windows 95.

DF1 HDX

The Allen-Bradley asynchronous half-duplex protocol.

DF1 PROTOCOL

A peer-to-peer link-layer protocol that combines features of ANSI X3.28-1976 specification subcategories D1 (data transparency) and F1 (two-way simultaneous transmission with embedded responses).

DFT

See Distributed Function Terminal.

DGP

Dissimilar Gateway Protocol

DH LINK

Data Highway link. An Allen-Bradley floating-master baseband link for a local area network.

DH+E LINK

Data Highway PlusE link. An Allen-Bradley token-passing baseband link for a local area network.

DH-485 LINK

Data Highway 485 link. An Allen-Bradley token-passing baseband link for a local area network based on the RS-485 standard.

DH/DH+

See Data Highway/Data Highway Plus.

DHIIE LINK

Data Highway IIE link. An Allen-Bradley token-passing carrier-band link for a local area network.

DIAL

To initiate a LAN Distance connection by placing a call to a target location. This term applies to connections over switched and nonswitched lines, even though the act of dialing a telephone number does not actually apply to nonswitched lines.

DIAL MODIFIERS

AT commands that you enter along with the AT command D (dial) to further instruct the modem when dialing the telephone. Dial modifiers specify things like whether to use pulse or tone dialing, when and how long to pause between numbers, and whether to dial a stored number.

DIAL SERVICES INTERFACE

(DSI) A program interface to the dialing services of the LAN Distance product.

DIAL TONE

A signal sent to a calling telephone indicating that digit dialing may begin. It usually consists of one or two tones between 350Hz and 480Hz. See Precise Dial Tone.

DIAL-IN

The action of establishing a LAN Distance connection to a LAN, by dialing a LAN Distance Connection Server workstation on the LAN.

DIAL-IN PORT

A LAN Distance communication port designated by the LAN Distance product to answer incoming calls.

DIAL-OUT CODE

A code (usually a 9) you must dial when using a System Access (SA) button to make an outside call on an AT&T Merlin Legend system.

DIALBACK

When a connection is requested, the system checks the username for validity, and then "dials back" the number associated with that username.

DIB

See Device Independent Bitmap.

DIB FIBER OPTIC CABLE

Tight buffered cable with 2 fibers

DIBIT

A group of two bits. In four-phase modulation, each possible dibit is encoded as one of four unique carrier phase shifts. The four possible states for a dibit are 00, 01, 10, 11.

DIFF

Show differences between the contents of two files or directories.

DIFF3

Same as DIFF but three way differential instead of two way.

DIFFERENCE FRAME

See Delta Frame.

DIGITAL

1. General: A method of signal representation by a set of discrete numerical values, as opposed to a continuously fluctuating current or voltage. Bits are relayed by signaling on and off in discrete variations (that is, on=+5 volts, and off=0 volts).2. Communications: Digital service requires special lines and equipment that support digital-instead of analog-service. Digital services are available at rates beginning at 24 kbps. LANs use digital data transmission, but long distance carriers normally offer lines with up to 1,544 kbps transmission rates (T-1s). Digital transmission uses various encoding methods, such as Manchester, Differential Manchester, 4B/5B, and Robbed-Bit Signaling (RBS) encoding. See also Analog.

DIGITAL AUDIO TAPE

A consumer recording and playback medium for high quality audio.

DIGITAL DATA COMMUNICATIONS MESSAGE PROTOCOL

(DD-CMP) Logic that controls the transmission of data between stations in a point-to-point or multi-point datacommunications system. The method of physical data transfer used may beparallel, series synchronous, or series asynchronous. DEC uses a DLC-type protocol very similar to SDLC, except that SDLC is bit-oriented, and DD-CMP is character count-oriented. The difference does not make it better, only different. DD-CMP is proprietary to DEC and used only in DEC equipment. For this reason, DEC equipment is not interoperable with other DLC protocols.

DIGITAL DATA SERVICE

(DDS) A special wide-bandwidth Private Leased Line (PLL) that uses digital techniques to transfer data at higher speeds and lower error rate than voice-band, analog PLLs. The line is available 24 hours a day.

DIGITAL VIDEO

A video signal represented by computer-readable binary numbers that describe a finite set of colors and luminance levels.

DIGITAL VIDEO INTERACTIVE

Intel's original name for its PC-based digital video technologies. This name has been replaced on the software side with Indeo™ video technology, on the retail side with Smart Video Recorder and on the hardware side with I750® Processors.

DIGITIZATION

Process of transforming analog video signal into digital information.

DIO

See Direct Input/Output.

DIP

Dual in-line package.

DIPLEX FILTER

A passive filter which combines and separates the inbound and outbound passbands in a broadband system.

DIR-2 VIRUS

This virus uses unused disk sectors in a unique way that places the virus in a class by itself. Dir-2 writes itself to an unused area on the disk, then changes the directory pointers on the disk so that all program file entries point to the disk sector containing the virus code. The virus saves a copy of the original directory pointers, so it can execute the program the user originally tried to run. The result is that the disk's directory structure is thoroughly scrambled, but this is not apparent until the computer is booted from a non-infected disk and the user attempts to access files on the infected disk.

DIRECT CONNECTION WEDGE

This wedge translates data from the input device to an input port on the host machine. The interface may be RS-232 or a proprietary formula.

DIRECT DISTANCE DIALING

A telephone service in North America tha tenables a subscriber to call other subscribers outside the local area without operator assistance.

DIRECT INPUT/OUTPUT

The direct reading or writing to the token-ring TMS380C16 chipset registers by the system processors.

DIRECT MEMORY ACCESS

Memory access that doesn't use the microprocessor, often used to transfer data directly between memory and a peripheral device such as a hard disk drive.

DIRECTED FRAME

A data frame that is intended to be delivered to a specific location and thus contains the address of its destination.

DIRECTORY SYSTEM AGENT

(DSA) The software that provides the X.500 Directory Service for a portion of the directory information base. Generally, each DSA is responsible for the directory information for a single organization or organizational unit.

DIRECTORY USER AGENT

(DUA) The software that accesses the X.500 Directory Service on behalf of the directory user. The directory user may be a person or another software element.

DISCARD

Discard Protocol

DISCARDABLE CODE SEGMENT

A Windows application code segment in memory, which can be discarded, overwritten, or reloaded from disk when necessary.

DISCRETE COSINE TRANSFORM

(DCT) A form of coding used in most of the current image compression systems for bit rate reduction.

DISINFECT

To eradicate a virus so that it can no longer spread or cause damage to a system.

DISK CACHE

A part of RAM that is set aside to temporarily hold data read from disk. A disk cache doesn't have to hold an entire file, as a RAM disk does, but can hold parts of running application software or parts of a data file. Disk-caching software such as SMARTDrive manages the process of swapping data to and from the disk cache.

DISK OPERATING SYSTEM

The operating system software that is loaded from a disk when you boot your computer (MS-DOS, PC-DOS, DR-DOS). The initial operating system used for System/360 mainframes; now a loose term for a personal computer operating system, short for PC-DOS or MS-DOS. See also MS-DOS.

DISPLAY MANAGER

The program that executes commands that start and stop processes, and commands that open, close, move, or modify windows and pads in DOMAIN/IX.

DISTINCTIVE RINGS

Different ways your phone rings to identify an inside, outside, transferred, or returning call.

DISTRIBUTED COMPUTING ENVIRONMENT

(DCE) Distributed Computing Environment. An architecture of standard programming interfaces, conventions, and server functionalities (e.g., naming, distributed file system, remote procedure call) for distributing applications transparently across networks of heterogeneous computers. Promoted and controlled by the Open Software Foundation (OSF), a consortium led by HP, DEC, and IBM. See ONC.

DISTRIBUTED FUNCTION TERMINAL

A coaxially-connected device that does not require cluster-controller interaction to respond to keystrokes.

DISTRIBUTED PROCESSING

A data processing type in which more than one processor shares the execution of an application. In a PC network, processing of data is conducted at each workstation instead of at one CPU. In SNA networks, distributed processing is implemented through APPC.

DISTRIBUTED VIAS

(Circuit board production) A term used to describe the use of plated through holes (vias) that are located ofer a larger area of the substrate as opposed to just the outer perimeter.

DITHERING

Blurred transition from one color to another in a computer picture.

DIW

See D-Inside Wire.

DIX CONNECTOR

The Ethernet II (Digital-Intel-Xerox) term for the connector that connects an external transceiver to the 15-pin connector on an Ethernet NIC. (See AUI.)

DL

Data Link

DLC

Data Link Control

DLCI

See Data Link Connection Identifier.

DLL

See Dynamic Link Library.

DM

See Display Manager

DM

National Semiconductor part prefix that indicates the device is in device family: Digital (Monolithic)

DMA

See Direct Memory Access.

DMI

See Desktop Management Interface.

DMS100

A digital central office switching system made by Northern Telecom.

DMTF

See Desktop Management Task Force.

DNIS

Dialed Number Identification System

DNS

See Domain Name System

DOD

Department of Defense

DOMAIN

In the Internet, a part of a naming hierarchy. Syntactically, an Internet domain name consists of a sequence of names (labels) separated by periods (dots), e.g., "tundra.mpk.ca.us." In OSI, "domain" is generally used as an administrative partition of a complex distributed system, as in MHS Private Management Domain (PRMD), and Directory Management Domain (DMD).

DOMAIN

See Workgroup

DOMAIN CONTROL DATA BASE

(DCDB) Contains the infomation that defines a domain's resources, valid Extended Services users, Extended Services work stations, and Extended Services servers.

DOMAIN NAME

A sequence of names (labels) in an IP network that are separated by periods to identify a network. The domain name is mapped to the networks as a 32-bit IP address.

DOMAIN NAME SYSTEM (DNS)

A DNS server has network addresses for each computer on a network. A PC must call a name server to get address information. A DNS server translates symbolic names (for example, Bronco) into IP addresses.

DOMAIN SPECIFIC PART

A part of an OSI NSAP address that determines the network addressing authority identified by the IDI.

DOS

See Disk Operating System.

DOS EXTENDER

See extended memory manager.

DOS PROTECTED MODE INTERFACE

Industry standard that allows MS-DOS applications to execute code in the protected operating mode of the 80286 or 80386 processor. The DPMI specification is available from Intel Corporation.

DOTTED DECIMAL NOTATION

The syntactic representation for the 32-bit IP address consisting of four 8-bit numbers written in base 10 with periods separating them. Used to represent IP addresses in the Internet as in: 192.10.54.3 or 190.82.10.2.

DOUBLE BUFFERING

The use of two buffers rather than one to temporarily hold data being moved to and from an I/O device. Double buffering increases data transfer speed because one buffer can be filled while the other is being emptied.

DOWNLOAD

The transfer of a file or information from one node to another. Generally refers to transferring a file from a "big" node, such as a computer. to a "small" node, such as a terminal server or printer.

DOWNLOADABLE FONTS

Fonts that reside on your hard disk and that are sent to the printer as needed.

DPA

Demand Protocol Architecture

DPMI

See DOS Protected Mode Interface.

DQDB

Distributed Queue Dual Bus

DRAM

See Dynamic Random Access Memory.

DRDA

Distributed Relational Database Architecture

DRILL DOWN

A capability common in EIS applications that allows a user to progressively move from summary data to more detailed data.

DRIVE ARRAY

A series of intelligent disk drives linked together, for the purpose of spanning data across the drives and/or improving reliability. (Also see RAID.)

DRP

DECnet Routing Protocol

DSA

See Directory System Agent.

DSAP

Destination Service Access Point

DSP

See Domain Specific Part.

DSR

Data Set Ready signal in RS-232-C. An RS-232C signal that tells the computer (or terminal) the modem is connected to the telephone line. A signal from the modem th a device that indicates the phone is off-hook. (It is the modem's answer to DTR) The device will not transmit or receive unless DSR is on. If the modem does not properly control DSR, or if no modem is used, DSR must be jumpered to a high signal (DTR can be used) at the device's asynchronous connector. See DTR.

DSR

See Data Set Ready.

DSS

Decision Support Systems

DSU

Data Service Unit

DSU/CSU

Data Service Unit/Channel Service Unit

DTE

See Data Terminal Equipment.

DTR

Data Terminal Ready signal in RS-232-C. An RS-232C signal that tells the modem the local computer (or terminal) is ready for data transmission. A signal from a device to the modem to connect to the phone line (pick up the phone). The device will assert DTR all the time except during the phone hang-up sequence. Modems built to American standards will not respond to DTR until the phone rings. Some European modems will always pick up the phone, whether it is ringing or not.

DTR

See Data Terminal Ready.

DU

Summarize disk usage (UNIX command)

DUA

See Directory User Agent.

DUAL DATA LINK CONTROLLER

A component of the serial interface card that handles port port interfacing by directly accessing data from local buffer memory via an internal DMA controller. The DDLC ensures that FWO overrun/underrun does not occur.

DUPLEX

The way modems exchange data: half duplex or full duplex. With half duplex transmissions, only one modem can send data at a time. Full duplex transmissions allow both modems to send data simultaneously.

DUPLICATE ADDRESS CHECKER

A LAN Distance feature to ensure that workstations dialing in to a LAN have a unique LAN adapter address.

DVI®

See Digital Video Interactive.

DYNAMIC DATA EXCHANGE

(DDE) The set of functions for interprocess communications (IPC) built into the Microsoft Windows operating system to support exchange of commands and data between two applications running simultaneously. In Windows 3.1, this capability is enhanced with Object Linking and Embedding.

DYNAMIC RANDOM ACCESS MEMORY

A type of storage the computer accesses at frequent intervals.

DYNAMIC-LINK LIBRARY

An executable code module for Microsoft Windows that can be loaded on demand and linked at run time, and then unloaded when the code is no longer needed.