Glossary - D
- Digital Access and Cross-Connect System (network, T1)
Disk And Execution MONitor
From the mythological meaning, later rationalised as the acronym "Disk And Execution
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
- 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.
- 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).
Directory Access Protocol (standard, X.500)
An X.500 based protocol used between a Directory User Agent (DUA) and a
Directory System Agent (DSA).
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
- 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
- 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.
Deterministic Bit Rate (ATM)
Acronym used by ITU-T I.371 "ATM Transfert Capability". Equivalent
of CBR (ATM Forum).
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
- Digital Cross-connect System (network)
Discard Eligible (communications)
Windows Internal file exchange standard. Allow dynamic data exchanges between applications,
one being the client and the other the server.
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.
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
- 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.
Data Encryption Standard
Standard cryptographic algorithm developed by the US National Bureau of Standards.
- Destination End Station (network)
- DXI Frame Address (network)
- Dynamic GCRA (ATM)
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).
Directory Information Base (network)
Directory database used in the X.500 directory standard.
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.
- Document Image Processing (data interchange)
Directory Information Shadowing Protocol (standard, X.500)
Protocol used for X.500 to duplicate a directory.
Data Link Control (network)
The SNA layer responsible for transmission of data between
two nodes over a physical link.
- Data Link Connection Identifier (network, ATM)
Dynamic Link Library (communications)
Windows library that contains functions shared between several applications.
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
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.
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/.
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).
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).
- Departement Of Defense (network, Internet)
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.
Software that controls a device. It gives a standard API to applications for them to use
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.
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.
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.
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,
Directory System Protocol (norme, X.500)
Exchange protocol between DSA in the X.500 standard.
- Data Set Ready (physical layer, network)
- Digital Subscriber systems Signaling
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
Also DSUs will fragment frames into cells as the information is mapped to the digital
transmission facility. See CPE, DCE.
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.
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
Dual Tone Multifrequency Signaling (communications)
Signaling system used to send numeration in classical analogical switching systems.
- Data Terminal Ready (physical layer, network)
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.
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
DXI interfaces to the ATM UNI and has been chosen by the
ATM Forum as an affordable solution for providing ATM
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.