Glossary D

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D4 Framing: A T1 12-frame format in which the 193rd bit is used for framing & signal information; ESF is an equivalent but newer 24-frame technology.
Daisy Chain: To connect together in a series.
DAT: See Digital Audio Tape
Data: Information represented in digital form, including voice, text, facsimile, & video.
Data Communications: The reception, transmission & validation of data. The transfer of data between a data source & data link via one or more links.
Data Communications Equipment: A device that transfers data from one type of wire to another. A Modem is a DCE device that transfers data from the wire or cable to the phone line. Typically the wiring from a DTE to a DCE is straight.
Data Encryption Key: (DEK) Much like an actual key used for locking & re-opening doors, DEKs are used for the encryption & decoding of message text, sometimes in the form of a digital signature.
Data Integrity: A measure of accuracy based on error detection.
Data Link: Any serial data communications transmission path, generally between two adjacent nodes or devices & without intermediate switching nodes.
Data Link Control: The rules or procedure for reliable data transfer between two communicating DTE's A DLC makes specifications for Session Establishment, data Transfer, & error detection/recovery.
Data Link Layer: The network protocol module (defined as level 2 by OSI) which defines how packets for a specific type of LAN or network trunk line are addressed, structured, & delimited. The Data Link Layer has an interface downward to the Physical Layer, & upwards to the Network Layer.
Data Service Unit: (DSU) A component of customer premises equipment used to interface to a digital circuit combined with a channel service unit; converts a customer's data stream to bipolar format for transmission.
Data Set Ready: (DSR) A member of the control signal category of the RS232-C DTE-DCE interface. DSR is used to indicate that the DCE is operational.
Data Terminal Equipment: (DTE) A device which acts as the origin of, or destination for, data. Examples are a computer, terminal or printer.
Data Transfer Rate: The average number of bits, characters, or blocks per unit of time transferred from a data source to a data sink.
Database Server: A computer that stores data centrally for network users & managers & often uses client-server software to distribute the processing of that data between itself & nodes requesting information.
Datagram: A block of data that is "smart" enough (actually, which carries enough information) to travel from one Internet site to another without having to rely on earlier exchanges between the source & destination computers. In the Internet Protocols (IP), this refers to a packet containing destination address & data.
Dataphone Digital Service: (DDS) A private line digital service offered intraLATA buy BOCs & interLATA by AT&T communications, with data rates typically at 2.4, 4.8, 9.6 & 56 Kbps; part of the services listed by AT&T under the Accunet family.
dB: See Decibel
DB-15: A standardized connector with 15 pins for Ethernet transceivers
DB-9: A standardized connector with 9 pins for token ring & serial connections.
DC: See Direct Current
DCE: See Data Communications Equipment or see Distributed Computing Environment
DDD: See Direct Distance Dialing
DDN: See Defense Data Network
DDS: See Dataphone Digital Service
De facto Standard: An unofficial standard that exists because it is widely used by companies in the industry.
De jure Standard: A standard that has been officially approved by an organization established to define & develop standards.
Decentralize: The process of distributing responsibility from a central point to several local points of control.
Decibel: (dB) The unit for measuring the relative strength of a signal. One-tenth of a Bel. It is equal to 10 times the logarithm of the power ratio, 20 times the log of the voltage ratio, or 20 times the log of the current ratio. One decibel is the amount by which the pressure of a pure sine wave of sound must be varied in order for the change to be detected by the average human ear.
DECnet: Digital Equipment Corporation's proprietary network architecture that works across all of the company's machines; endowed with a peer-to-peer methodology.
Dedicated Line: Transmission medium, usually telephone lines, reserved for exclusive use of a customer. These lines are not routed through the switching equipment at the telephone company facilities.
Dedicated Server: A node on which applications are limited to maintaining network resources; no user applications are available.
Defense Data Network: (DDN) A global communications network that serves the US Department of Defense. It is made up of a network called NILNET
DEK: See Data Encryption Key
Delay: In communications, the time between two events. Typically the time it takes for circuits to react to an event.
Demarcation Point: A point where operations control or ownership changes.
Demodulation: The process of retrieving information (data) from a modulated carrier signal. Reverse of modulation. DES: (Data Encryption Standard) A scheme approved by the National Bureau of Standards that encrypts data for security purposes. DES is the data- communications encryption standard specified by Federal Information Processing Systems.
Destination Field: A field in a message header that contains the address of the station to which a message is being directed.
Detector (Fiber Optic): A device that picks up light from fiber & converts the information into an electrical signal.
Device: As related to workstation, a device is an item such as a telephone, personal computer or a graphic or video terminal. Dial-Up Line: A telephone line in which the connection between the DTE's is established by dialing. It is similar to dialing from a common telephone set.
DID: See Direct Inward Dialing
Dielectric: Non-metallic. All-dielectric cables are nonconductive, contain no metal & are lightning resistant.
Digital: Referring to communications procedures, techniques, & equipment by which information is encoded as either a binary one (1) or zero (0); the representation of the information in discrete binary form, discontinuous in time; compare with analog.
Digital Audio Tape: (DAT) Medium for storing digital rather than analog data.
Digital Network Architecture: (DNA) Digital Equipment Corporation's layered data communications protocol.
Digital Visual Interface: (DVI) An interface used with high resolution high speed displays. The video information is sent from the source to the display in digital format for more accuracy. The digital data is uncompressed data. Click here to see more information on DVI including connectors.
DIN: Deutsches Institiute Normung
Direct Burial Cable: A cable installed directly in the earth without a conduit.
Direct Current: (DC) An electrical current whose electrons flow in one direction only. It may be constant or pulsating as long as its movement is in the same direction.
Direct Distance Dialing: (DDD) A telephone service with which a subscriber can call someone outside the area without requiring operator assistance.
Direct Inward Dialing: (DID) A telephone service with which subscribers can have calls within a system routed to a specific telephone without switchboard assistance.
Direct Memory Access: (DMA) A method of moving data from a storage device to RAM.
Disk Duplexing: Fault tolerance method in which two disk controllers are used in a single server.
Disk Operating System: (DOS) Developed by Microsoft to run on IBM computers.
Disk/File Server: A mass storage device that can be accessed by several computers; enables the creation, storage, & sharing of files.
Dispersion: The cause of bandwidth limitations in an optical fiber. Dispersion causes a broadening of input pulses along the length of the fiber. Two major types are (a) mode dispersion caused by differential optical path lengths in a multimode fiber, & (b) material dispersion caused by a differential delay of various wavelengths of light in a wave guide material.
Distortion: Any undesired change in a wave form or signal.
Distributed Computing Environment: (DCE) An architecture based on standard programming interfaces, conventions & server functionalities used for distributing applications transparently across networks. The DCE is controlled & promoted by the Open Software Foundation (OSF) a consortium of vendors including DEC, IBM and Hewlett Packard.
Distributed Queue Dual Bus: (DQDB) A communication technique proposed for MANs (Metropolitan Area Networks).
Distribution Cable: In a CATV system, the transmission cable between the distribution amplifier & the drop cable.
Distribution Frame: A wall mounted structure for terminating telephone wiring, usually the permanent wires from or at the telephone central office where cross-connections are readily made to extensions; also called distribution block.
DLC Characters: The special characters specified by interchange codes to implement the Data Link Control (DLC) functions.
DMA: See Direct Memory Access
DNA: See Digital Network Architecture
DNS: See Domain Name Service
Domain: A collection of network devices that create a subgroup of a network. A domain can be created for logical, functional or geographic needs.
Domain Name Service: (DNS) A static, hierarchical name service used with TCP/IP hosts, & is housed on a number of servers on the Internet. Basically, it maintains a database for figuring out & finding (or resolving) host names & OP addresses on the Internet. this allows users to specify remote computers by host names rather than numerical IP addresses. The advantage of the DNS is that you don't have to remember numerical OP addresses for all the Internet sites available to access.
DOS: See Disk Operating System
Dot Address: The common notation for IP addresses of the form 1.2.3.4. where each number represents one byte in the four-byte IP address. Also knows as Dotted Decimal Notation.
Downline Loading: The process of sending programs (or other "nondata" type information) from a central source to individual station(s).
Download: The transfer of either a system or program from a disk file at one node to the memory of another computer. It is the activity of loading software into the nodes of a network from one node or device over the network media. Typically to Download is to load a file from a remote server to your computer.
Downtime: The period during which computer or network resources are unavailable to users because of failure.
DQDB: See Distributed Queue Dual Bus
Drain Wire: An uninsulated wire in contact with a shield through its length. Used for terminating the shield.
Driver: A software module that, under control of the processor, manages an I/O port to an external device, such as serial RS232C port to a modem.
Drop Cable: In a local area networks, a cable that connects the main network cable, or bus, & the data terminal equipment (DTE).
DSR: See Data Set Ready
DSU: See Data Service Unit
DTE: See Data Terminal Equipment
DTMF: See Dual-Tone Multiple-Frequency
DTS: Data Terminal Service
Dual Homing: A method of connecting devices in which there is a primary connection & a standby connection.
Dual-Tone Multiple-Frequency: (DTMF) The audio signaling frequency on touch-tone, push-button phones.
Duct: An underground or overhead tube for carrying electrical conductors.
Duplex: Two way data transmission.
DVI: See Digital Visual Interface

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