Glossary - D

DACS

Digital Access and Cross-Connect System (network, T1)

Daemon

Disk And Execution MONitor
(operating system)
From the mythological meaning, later rationalised as the acronym "Disk And Execution MONitor".
A program that is not invoked explicitly, but lies dormant waiting for some condition(s) to occur. The idea is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon).
For example, under ITS writing a file on the LPT spooler's directory would invoke the spooling daemon, which would then print the file. The advantage is that programs wanting files printed need neither compete for access to, nor understand any idiosyncrasies of, the LPT. They simply enter their implicit requests and let the daemon decide what to do with them. Daemons are usually spawned automatically by the system, and may either live forever or be regenerated at intervals.
Unix systems run many daemons, chiefly to handle requests for services from other hosts on a network. Most of these are now started as required by a single real daemon, inetd, rather than running continuously. Examples are cron (local timed command execution), rshd (remote command execution), rlogind and telnetd (remote login), ftpd, nfsd (file transfer), lpd (printing).

Daisy-Chain (networking)

A method of connecting devices in a series. Computer signals are passed through the chain from one device to the next: device A is wired to device B, device B is wired to device C, etc. The last device is normally wired to a resistor or terminator.
All devices may receive identical signals or, in contrast to a simple bus, each device in the chain may modify one or more signals before passing them on.
Often used to describe Ethernet 10Base2.

DAN

Departemental Area Network (standard)

Desk/Desktop Area Network
A group of interconnected desktop computing devices. Usually implies fewer than five computers, operating peer-to-peer, in a relatively small area (such as two adjacent desktops).

DAP

Directory Access Protocol (standard, X.500)
An X.500 based protocol used between a Directory User Agent (DUA) and a Directory System Agent (DSA).

DARPA

Defense Advanced Research Projects Agency (network, Internet)
Agency of the Department of Defense responsible for managing research projects and coordinating activities among participating agencies, universities, research institutions, etc.

Data Flow Control Layer (network)

Layer 5 of the SNA architectural model.

Datagram (network)

A packet transport mode where packets are routed independently and may follow different paths, thus there is no guarantee of sequence delivery. See also VC.

Data Link Control Layer (network)

Layer 2 in the SNA architectural model.

Data Warehouse - Datawarehouse (database)

A generic term for a system for storing, retrieving and managing large amounts of any type of data. Data warehouse software often includes sophisticated compression and hashing techniques for fast searches, as well as advanced filtering.
A database, often remote, containing recent snapshots of corporate data. Planners and researchers can use this database freely without worrying about slowing down day-to-day operations of the production database.

DBT

Deterministic Bit Rate (ATM)
Acronym used by ITU-T I.371 "ATM Transfert Capability". Equivalent of CBR (ATM Forum).

DCC

Data Country Code (ATM)
Type of ATM Address format whose AFI is 39. The DCC is a two-octet field after the first octet in a DCC ATM address. The DCC is expressed in BCD and identifies the country in which an address is registered per ISO 3166.

DCTE - DCE

Data Circuit-Terminating Equipment or Data Communication Equipment
Equipment that resides at the customer end of a transmission link and provides all necessary termination functions for that link, typically a modem or other communications device,which acts as an access point to the transmission medium. May be owned by the customer or by the service provider. See DTE, CPE, CSU, DSU.

Distributed Computer Environment

DCS

Digital Cross-connect System (network)

DDE

Discard Eligible (communications)
Windows Internal file exchange standard. Allow dynamic data exchanges between applications, one being the client and the other the server.

DE

Discard Eligible (network, frame-relay)
A 1-bit field in a frame relay header that provides a two level priority indicator. Used to bias discards of frames in the event of congestion toward lower priority frames. Similar to the CLP bit in ATM.

DECT

Digital European Cordless Telecommunications (communications, standard)
A standard governing pan-European digital mobile telephony. DECT is based on advanced TDMA, and covers cordless PBXs, telepoint and residential cordless telephony.

Default Route (routing, network)

A routing table entry which is used to direct packets addressed to networks not explicitly listed in the routing table.

Defeaturing (product, hardware)

Demultiplexing (communications)

A function performed by a layer entity that identifies and separates SDUs from a single connection to more than one connection.

DES

Data Encryption Standard
Standard cryptographic algorithm developed by the US National Bureau of Standards.

Destination End Station (network)

DFA

DXI Frame Address (network)

DGCRA

Dynamic GCRA (ATM)

DHCP

Dynamic Host Configuration Protocol (security, protocol)
Protocol used on a local network to dynamically give a network configuration to local hosts. This is an extention of BOOTP.

Dial-Up (communications, Internet)

A temporary, as opposed to dedicated, connection between machines established over a phone line (analog or ISDN).

DIB

Directory Information Base (network)
Directory database used in the X.500 directory standard.

DGXIII-E

European Commission - Directorate General XIII
The work of Directorate-General XIII currently encompasses the following main activities:

• Telecommunications and postal services policy development, including liberalisation and regulation aspects, interconnection and interoperability of networks and services, universal services and communication security.
• Research and development programmes (advanced communications and telematics applications), and support for the exploitation of results.
• Supporting the establishment of a European single market in electronic information creation, retrieval and storage.

Digital Signal (communications)

The transmission method for sending voice, data, image or video by translating the information to binary code.

Digital Switching

Switching facility that establishes and maintains a connection, under stored-program control, to route binary-encoded information between an input port and an output port. Using time-division multiplexing, a virtual through-circuit is derived from a series of time slots, which is more efficient than dedicated circuits for the period of time that connections are set up.

DIP

Document Image Processing (data interchange)

DISP

Directory Information Shadowing Protocol (standard, X.500)
Protocol used for X.500 to duplicate a directory.

DLC

Data Link Control (network)
The SNA layer responsible for transmission of data between two nodes over a physical link.

DLCI

Data Link Connection Identifier (network, ATM)

DLL

Dynamic Link Library (communications)
Windows library that contains functions shared between several applications.

DMI

Desktop Management Interface (network, management, interface)
A set of APIs giving PCs a standard means of reporting information about themselves to management applications DMI outlined by the DMTF comprises three components:

• service layer
• component interface
• management interface

DMT

Digital/Discrete Multi Tone (communications)
A modulation technique developed by Bell Labs and specified in the ANSI T1E1.4 standard. DMT technology uses a large number, typically 256, of equally spaced subcarriers, each of which can be individually modulated by QAM with a variable number of levels. With DMT, the number of bits per carrier is assigned based upon channel capacity.

DMTF

Desktop Management Task Force (standard, vendor consortium)
Vendor consortium organisation on networking standards.
Focus: PC management. Membership: March 1997: 100 companies, nearly all vendors.
DMTF site: http://www.dtmf.org/.

DMZ

Demilitarizes Zone (security, Firewall)
subset of a network, located between the Internet and the local network (intranet). This subset is the public part of the local network. Communications between the local and the external networks are controled by security devices (routers, firewalls).

DNS

Domain Name System (network, Internet)
The DNS is a general purpose distributed, replicated, data query service. The principal use is the lookup of host IP addresses based on host names. The style of host names now used in the Internet is called "domain name", because they are the style of names used to look up anything in the DNS. Some important domains are: .com (commercial), .edu (educational), .net (network operations), .gov (U.S. government), and .mil (U.S. military). Most countries also have a domain. The country domain names are based on ISO 3166. For example, .us (United States), .uk (United Kingdom), .au (Australia).

DOD

Departement Of Defense (network, Internet)

DQDB

Distributed Queue Dual Bus
The IEEE 802.6 standard is a MAN, protocol based on 53-byte packets that can support connectionless and connection-oriented, isochronous integrated services. It is implemented as two unidirectional buses configured in a physical ring topology. Protocol used in SMDS between the end user and the SMDS network.

Driver

Software that controls a device. It gives a standard API to applications for them to use the device.

DS

Digital Signal
Signal level for the digital hierarchie use by North American telecommunication companies to carry digitized voice. See PDH.

DS-0 (Digital Signal 0)
Physical interface for digital transmission at the rate of 64 Kbps.

DS-1 (Digital Signal 1)
Physical interface for digital transmission at the rate of 1.544 Mbps. Also, known as a T-1 standard, it can simultaneously support 24 DS-0 circuits.

DS-2 (Digital Signal 2)
Physical interface for digital transmission at the rate of 6.312 Mbps.

DS-3 (Digital Signal 3)
Physical interface for digital transmission at the rate of 44.736 Mbps.

DSA

Directory System Agent (standard, X.500)
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. See RFC1208.

DSI

Digital Speech Interpolation
A means of transmitting telephony. Two and One half to three times more efficiently based on the principle that people are talking only about 40% of the time.

DSL

Digital Subscriber Line/Loop (communications, protocol)
A digital telecommunications protocol for use over existing copper telephone lines, as opposed to optical fibre.
See also ADSL, HDSL, SDSL, VDSL.

DSP

Directory System Protocol (norme, X.500)
Exchange protocol between DSA in the X.500 standard.

DSR

Data Set Ready (physical layer, network)

DSS1

Digital Subscriber systems Signaling

DSU

Data Service Unit
Device on the customer end of a digital circuit that provides framing of sub-rate (under 64 or 56 Kbps) customer access channels(s) onto higher rate data circuits. May be combined with a CSU in a single device (http://www.astrocorp.com/univers/introcsudsu.html). Also DSUs will fragment frames into cells as the information is mapped to the digital transmission facility. See CPE, DCE.

DTD

Document Type Definition
Document description used in SGML and XML. This allows users to transform a formal document into an HTML or postscript document, for instance.

DTE

Data Terminal Equipment
The host computer (PC or workstation) to provide the end-user with access to a communications network. The DTE is connected to a DCE which performs the signaling operation. See also CPE.

DTMF

Dual Tone Multifrequency Signaling (communications)
Signaling system used to send numeration in classical analogical switching systems.

DTR

Data Terminal Ready (physical layer, network)

DUA

Directory User Agent (standard, X.500)
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. See RFC1208.

DXI

Data Exchange Interface (ATM)
A variable-length frame-based ATM interface between a DTE (such as a router or a local switch) and a special ATM SDU/CSU (DCE). The ATM DSU/CSU converts between the variable-length DXI frames and the fixed-length ATM cells.
Defines a format for passing data that has gone through the ATM convergence sublayer (a CS-PDU) between a router and a CSU/DSU or other device with ATM SAR capability.
DXI interfaces to the ATM UNI and has been chosen by the ATM Forum as an affordable solution for providing ATM capabilities overWAN.

Data Exchange Interface (SMDS)
A variable-length frame-based SMDS interface between a DTE and either a special SMDS DSU/CSU or an SMDS network node. In the former case the SMDS DSU/CSU would convert between the variable-length DXI frames and the fixed-length SMDS cells. In the latter case, this conversion would occur in the SMDS network node and only a normal DSU/CSU would be required.

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