A term for a program that sits in the background and waits for requests. When it receives a request, it wakes up, processes the requests, and returns to its slumber. Web servers are HTTP Daemons.
daisy chain
To connect end to end. EG: In Mac machines, the mouse can connect to the keyboard which can connect to the computer. On the other hand PCs usually don't daisy chain and the mouse and the keyboard must be individually connected to the computer.
Data Access Objects. A set of COM objects that use JET to communicate with databases.
Digital Audio Tape. See tape storage.
(1) The plural of datum, a single piece of information. In general people use the word with no sense of plurality.

(2) The set of static values stored in databases and such. This is in contrast to information, which is data that is retrieved and organized in a meaningful way for the viewer. Some say there are three levels of information. The first is data, the raw figures. The second is information, the interpretation. The third is knowledge, utilization of information.

(3) The files that hold the actual data of the database as opposed to meta data files (eg index files and data dictionaries) which store administrative information about the data.

(4) A term used to distinguish files that are machine readable from files that are human readable, eg binary data files might be distinguished from ASCII text files.

(5) In software, there is data and programs. The programs manipulate data.
Data Access Objects
See DAO.
See DB.
database diagram
A graphical representation of the schema (or part of the schema) and relationships between tables. The schema of a database includes all of its tables, columns, and properties.
DataBase Management System
Data Communications Equipment
See DCE.
Data Encryption Standard
See DES.
Data Flow Diagram
See DFD.
A chunk or packet of information that consists of a header followed by the data. The header provides basic information about the included data such as the name, protocol, and a checksum. EG: TCP chunks data into datagrams for internet use.
data mining
Software that automatically looks through a database and automatically comes up with conclusions. EG: It might look for statistical patterns that are not usually discerned.
data sharing
Data that is accessed by multiple users. There are roughly three kinds:
  • Storage Sharing. Utilizes the same HW & SW but users access different data.
  • Data Exchange. Aka Data Copy Sharing. Data is replicated across different servers so there is no problem with simultaneous access although there are problems with the freshness of data.
  • True Data Sharing. Multiple servers full access to the same data.
Data Source Name
See DSN.
Data Terminal Equipment
See DTE.
data type
The kind of data the data is. In programming, variables are usually declared as being of a particular data type. This is done so that the program can tell the computer to reserve a customized amount of memory. In fact some languages require that all variables be declared.

The most common data types are integer (whole numbers without fractions, eg 3), floating point (number with a decimal portion, eg 3.14), and character or text. Others data types include be variant, financial, date, scientific notation, percentage, and so on. There are even user defined data types which is usually an object collecting various other data types. In OOP (Object Oriented Programming) the distinction between data types and object types gets blurry.
data warehouse
Aka information warehouse. A database of analytical information culled from an organization's operational information. A good data warehouse has the following characteristics:
  • The warehouse separates the analytical information from the operational information.
  • The warehouse is based on a comprehensive data model followed by the entire organization.
  • The warehouse is automatically maintained.
  • The warehouse is updated at regular intervals.
  • The warehouse stores data in a consistent format.
  • The warehouse can be accessed by standard query tools such as SQL and ODBC.
  • The warehouse is read only and cannot be rewritten.
  • The warehouse is replicated.
  • The warehouse is self contained.
Data warehouses are often accessed by managers with DSS (Decision Support System) software, and by executives with EIS (Executive Information System) software.
daughter board
A circuit board on the computer that is removable. It is either the main circuit board or an accessory board.
(1) DataBase. An organized information repository or data store. A DB can be accessed via query language or a database API (Application Programming Interface), i.e. via instructions typed at a command line or graphical interfaces specifically designed for the database.

Bits make bytes which often make characters which often make fields which are often collected into records; an organized bunch of records is usually considered to be a database.

The data in a database is collected by subject into tables (aka files, relations, or entities). The columns are a particular field in the subject. The rows are records of each instance in the subject. The intersection of column with the row is the cell that holds the attribute of that field for that instance in that subject. EG: In a database for a bookstore, the tables are entitled books, customers, and invoices. For the table called customers, the fields are entitled name, address, and phone number, and the records are Sue, George, and Amy. The cell for George's phone is 773-588-2300.

The way data in one table is related to another table is described as a relationship. The process of relating the data is called joining the data. The process of fetching desired data from a DB is called querying.

The different kinds of DBs include relational, flat, hierarchical, network, and object-oriented. DBs can also be passive or active data stores. Passive DBs just store data whereas active DBs are programmable and can enforce business rules by utilizing constraints, triggers, procedures, rules, and methods. Active DBs can react to events and prevent operations that could disrupt data integrity.

The backend of the repository is handled by the server or mainframe and the DBMS.

The repository is accessed at the front end by clients via PC-based DB programs (Borland's Paradox, Microsoft's Access, Microsoft's FoxPro, Lotus' Approach, Ashton Tate's dBASE, etc.), end-user access tools (Trinzic's Forest & Trees, Crystal Services' Crystal Reports Professional, Centura's Quest, etc.) Windows DB programs (written in C, C++, etc.), visual DB programs (written in Visual Basic, Power Builder, SQLWindows, etc.) or web DB programs (written in Java, JavaScript, VBScript, Active X, etc.).

Over the Web, DBs can be accessed via DB Web server products (WebDBC, WebBase, etc.), via DB gateways (Subperl, Oraperl, W3-mSQL, WDB, GSQL, dbWeb, etc.), or via customized client APIs.

See also normalization and distributed databases.

(2) Dual Boot. A computer system set up such that it can boot up as one of several operating systems. EG: A system can be set up to boot in either  Windows XP or Linux. The term "dual boot" can also loosely cover multi-boot scenarios such as machine with a triple boot.
IBM's SQL based DBMS for its mainframes.
IBM's SQL based DBMS for its minicomputers.
DataBase Administrator.
DataBase Management System. The software which creates and maintains databases.
Large database systems include Oracle Server, Sybase SQL Server, Informix Online, IBM's DB/2, Ingres, Microsoft SQL Server, etc.
Desktop database systems include Microsoft Access, Microsoft FoxPro, Borland Paradox, Borland dBase, Claris FileMaker, etc.
Shareware database systems include Postgres95, Mini SQL (aka mSQL), etc.
Data Communications Equipment. A standard class for serial communication, typically for equipment for transmitting data. A DCE will connect to a DTE.
See COM.
Desktop Color Separation. First implemented by CyberResearch, ca. 1980s. Converts a composite data graphic file into color separated master graphic file in EPS format. This file will contain an image preview for on-screen viewing, a low resolution PostScript data for composite proofing, and links to 4 pre-separated high resolution files containing CMYK data.
Dynamic Data Exchange. An IPC system utilized by the various OSes including Macintosh, OS/2, and Windows. DDE is being supplanted by OLE.
(1) Digital Data Service. See DSO.
(2) Digital Data Storage. The industry standard for DAT. A common format, DDS-3, can store 24 GB at a rate of 2 MB/s. See also tape storage.
dead key
A dead key is a key on a keyboard that provides additional Shift States. When you hit a dead key, nothing happens initially.
  • If you then hit a key that wants to wear that accent, then a combination appears. EG: If you have a US English International keyboard, then the following characters are dead keys.
    • ` makes: ÀàÈèÌìÒòÙù
    • ' makes: ÁáÇçÉéÍíÓóÚúÝý
    • ^ makes: ÂãÊêÎîÔôÛû
    • ~ makes: ÃãÑñÕõ
    • " makes: ÄäËëÏïÖöÜüÿ (not Ÿ which isn't ISO Latin 1 compliant anyway)
  • If you then hit a key that doesn't combine with the accent, then you get the accent followed by the character. EG: ~h.
  • If you then hit the space bar, the accent on the dead key appears alone. EG: ~.

See also Character Set Shortcuts.
DEBra IAN. A project created by Ian Murdock at Purdue University. It is most famous for its open-source and no-cost version of the UNIX operating system. Since the Debian OS and its bundled packages are based upon the Linux kernel and GNU, it is referred to as Debian GNU/Linux.
Digital Equipment Corporation. Manufacturers of the Alpha line of RISC processors, the VAX line of high-end servers and minicomputers, and the VMS OS. DEC also developed Alta Vista, a popular Internet search engine. DEC was purchased by Compaq in 1998.
Relating to a numbering system based on 10 and starts with 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,.... Machine languages are written in binary. See also octal and hexadecimal.
Decision Support System
See DSS.
dedicated line
A phone or data line for data communications that is available 24 hours and only for one computer or individual. This is sometimes referred to as a leased line. See also dial up line.
deep Web
Aka: Deepnet; invisible Web; hidden Web.The part of the Web that is not indexed by search engines. Deep Web content is usually unlinked to or dynamically created or requires a login or is othewise unavailable. The deep Web is said to be several magnitudes larger than the surface web.
A character or group of characters that separate pairs of names or data, or marks the start/end of a piece of code, or separates rows/columns of data. EG: UNIX uses the forward slash (/) to separate directories and filenames, whereas DOS uses the backward slash (\). Common delimiters also include the comma (,), colon (:), semicolon (;), single quote ('), double quote ("), braces ({}),  tab (char(9)), and carriage returns (char(13)). See also columnated.
Delimiter Separated Values
DEMUX. See multiplex.
DEMUltipleX. See multiplex.
Data Encryption Standard. A symmetric key encryption method developed in 1975 and standardize by the ANSI in 1981 as X.3.92. DES uses a 56-bit key. A sender uses the key to encrypt data before sending it. The receiver uses the same key to decrypt the data.
The part of a character that sticks out below the baseline of letters.
Desktop Color Separation
See DCS.
Deutsche Industri Norm
See DIN.
development life cycle
The steps in developing a computer program: Planning, Building, Testing, Compiling, and Distributing.
Data Flow Diagram. A chart showing the major processes in a computer system and how data flows from process to process. These are often used ins system design and analysis.
Dynamic Host Configuration Protocol. A protocol that handles the static and dynamic assignment of IP addresses to devices on a network. DHCP is built into Windows. See also DNS.
Dynamic HyperText Markup Language. DHTML turns all the HTML tags into objects (called elements) for use in client-side scripting. DHTML also uses other objects for client-side scripting. Some of those objects are pseudo-similar to some JavaScript objects such as window and document. The object interfaces (properties, methods, and events), as well as attributes and styles, enable the rendering and a content of a document to be changed and shown instantly/dynamically (without the page reloading).

DHTML is generally only available to browsers of version 4 and above. DHTML concepts have evolved into DOM.

Here are some of the things DHTML can do:
  • Dynamic Styles. DHTML can change styles and style sheets effecting elements without adding or removing elements. This can apply inline on specific elements or more generally across the document. The styles can change colors, fonts, visibility, position, etc. [Many of these can be applied without DHTML for some nice static HTML effects.]
    • Animation. Since you can modify position with script, then you can do animation on the fly.
    • Filters. CSS allows visual effects to be applied to elements as they change, eg: filter: FilterName(parameter1,...).
    • Transitions. CSS allows slideshow type transitions as whole pages or specific elements change.
    • Font Download. CSS allows fonts to be downloaded, used on that page, and then discarded when the page is no longer displayed, eg: <style>@font-face {font-family:comic;0 src:url(URLToFontToLoad)</style>
  • Dynamic Content. DHTML can add elements or change element content without reloading.
  • ActiveX Controls or Java Applets. Oodles and oodles of things can be done with add on controls or applets. One of the big things is to bind an ActiveX control to data (using the <object> HTML element) and then use DHTML to display portions of the acquired data as needed.
Aka diacritical mark; accent mark. A mark added to a letter. In Unicode x0300 (769) through x036F (879) are combining diacritics. EG: The Unicode x301 = 769, is a combining code point and many apps will combine it with a letter preceding it. EG: The HTML of a&#x301; may be rendered as the letter a with an acute accent: .

There are many diacritics including the following:
  • grave accent x300 = 768 = ̀. A left-leaning line on top. A stand-alone version is part of ASCII at x60 = 96. In English the grave accent is only used in poetry to indicate pronouncing a normally silent vowel. EG LOOKÈD.
  • acute accent x301 = 769 = ́. A right-leaning line on top. A stand-alone version is part of ISO Latin 1 at xB4 = 180. EG: SAUTÉ .
  • circumflex x302 = 770 = ̂. Aka caret. An up arrow on top. A stand-alone version is part of ASCII at x5E = 94. EG: "Thorough" is pronounced [thurô]".
  • tilde x303 = 771 = ̃. A wavy line on top. A stand-alone version is part of ASCII at x73 = 126. EG: In Spanish "tomorrow" is MAÑANA.
  • macron x304 = 772 = ̄. A horizontal line on top. A stand-alone version is part of ISO Latin 1 at xAF = 175. Commonly indicates a long vowel. EG: TŌNE.
  • breve x306 = 774 = ̆. A bowl on top. Commonly indicates a short vowel. EG: TŎN.
  • dot x307 = 775 = ̇. A dot on top. EG: Ċ.
  • diaeresis x308 = 776 = ̈. Aka dieresis; umlaut. Two dots on top. A stand-alone version is part of ISO Latin 1 at xA8 = 168. The combiner is the same code point as the umlaut.
    • In English, diaeresis indicates a separate pronunciation. EG: "naive" is pronounced [naïve] instead of [knave].
    • The umlaut is a very German diacritic. EG: If no umlaut is available, the Schröder can be spelled Schroeder.
  • hook x309 = 777 = ̉. A question-mark shape on top. Indicates a rising then falling tone in Vietnamese.
  • ring x30A = 778 = ̊. A circle on top. A stand-alone version is part of ISO Latin 1 at xB0 = 176. EG: The Angstrom symbol is Å.
  • caron x30C = 780 = ̌. Aka háček. A down arrow on top.
  • cedilla x327 = 807 = ̧. A "3" on bottom. A stand-alone version is part of ISO Latin 1 at xB8 = 184. Frequently adds a [ts] sound. EG: BARÇA.
  • double acute accent x30B = 779 = ̋. Two right-leaning lines on top. Chiefly Hungarian. EG: Ő .
dialog box
A window that pops up when a program needs to gather information from or give information to the user.
dial up line
A phone or data line for data communications that is available only for the duration of the communication and can be accessed by many users. See also dedicated line.
Digital Audio Tape
See tape storage.
digital certificate
An attachment to a message which can be used to verify the identity of the sender by the public key encryption method and a third party such as the CA (Certificate Authority). A popular standard for digital certificates is X.509.
Digital Data Service
See DSO.
digital rights management
See DRM.
digital signature
A digital code attached to a message that can be used to verify the identity of a sender.
Digital Subscriber Line.
See DSL.
Digital Video Disc
See DVD.
Dual In-line Memory Module. A circuit board that holds groups of RAM chips. DIMMs plug into sockets on the computer motherboard. It has 168 pins and is roughly 5 inches long. The bus for a DIMM is 64 bytes wide, this is twice that of a SIMM.
Deutsche Industri Norm. A German organizations that sets industrial standards. A very common serial connector is the DIN keyboard connector which has a similar appearance to the Mac ADB connector: round with pins.
DIP switch
Dual In-line Package. DIP switches are built right into circuit boards and are often used to configure a particular device, computer, or program. The physical switches are little rectangular blocks that can slide onto a pair of pins to form a physical circuit.
An Application Programming Interface (API) developed by Microsoft for 3D graphics. It allows the programmer to utilize most of the 3D graphics accelerator cards. See also OpenGL and DirectX.
direct access file
A file that has information that can be accessed in non-sequential order.
A software interface standard by Microsoft for controlling whether the CPU or the video adapter processes the video. Normally the Windows Graphics Display Interface (GDI) handles the video but if the CPU is busy, then DirectDraw allows the video information to go directly to the video adapter. DirectDraw also allows applications to access features of particular devices. Intel developed DCI (Display Control Interface) which is DirectDraw supports.
Direct Memory Access
See DMA.
directory server
A server that responds to client requests for network resources and services. In Windows NT, this is called a domain controller.
An Application Programming Interface (API) developed by Microsoft to enable programmers to access hardware features that might be on a users machine. DirectX does this by generating generic commands into commands for specific hardware. The DirectX 2 (1996) release supported Direct3D. The DirectX 5 (1998) release included capability for multimedia objects and streams, as well as support for MMX, USB, IEEE 1394 (Firewire), and AGP.
A measure of how easy it is for the user (reader, viewer, developer, etc.) to stumble upon the functions, uses, information, fun, interfaces, parameters etc. of a thing (program, tool, document, presentation, website, etc.). Discoverability is a measure of the design of the thing. An application may be powerful but if its users can't use it, then it actually isn't powerful is it? EG:
  • The high discoverability of the browser made the Internet boom.
  • Windows has better discoverability than DOS.
  • A website with a good navigation system has more discoverability than a site with a poor navigation system.
  • Visual programming languages often have better discoverability than entirely textual languages.
discussion group
See message board.
Disk Operating System
See DOS.
dispatch table
See interrupt vector table.
distributed database
A collection of database tables distributed over two or more servers. See also DRDA. See also my article on Distributed DBs.
distributed objects
Objects that can be located anywhere on the network but can interact with each other as if they were on the same machine. Fully developed objects are called components or plug-ins. See component software.
Distributed Relational Database Architecture
Distributed Queue Dual Bus
To choose the next best available color if the computer does not have the color requested by an image file.
DIgital Video eXpress. See DVD.
.dll. Dynamic Link Library. A common file which is a library of data or executable functions that can be called upon as needed, i.e. dynamically.  When a DLL is loaded, it is mapped into the address space of the calling process. A DLL can provide functionality to one or more programs, sometimes at the same time.

DLLs can be accessed by the Windows OS or applications; the Mac equivalents are called extensions and preferences. Windows itself is composed of DLLs. DLLs are often placed in the WINDOWS or SYSTEM directories. In addition to the .dll extension, sometimes DLL files end with .exe, .drv, or .fon.

Unix and Linux now support shared libraries but they have a tendency to use static (aka private) libraries instead. Each app that uses a static library has its own library that is not shared by anyone else. Static libraries that are actually also system libraries are also called side-by-side. Side-by-side libraries are sometimes used to ensure that an app uses a specific version of a library.

See also WFP.
DLL hell
Problems related to DLL conflicts. There are generally three kinds of DLL problems:
  • An older or improper version of a DLL replaces a newer or proper version.
  • A new DLL fixes a DLL bug that some old apps relied on.
  • A new DLL introduces a new bug.
Direct Memory Access. A system resource which provides a device with a direct line to memory, i.e. bypassing the microprocessor.
Drag aNd Drop.
Domain Name System. A DNS is a system that resolves/translates user-friendly names (such as host names or domain names) into computer-friendly names (such as IP addresses). A DNS can be a single server on a LAN or the multitude of DNS servers distributed all over the Internet. If one DNS server can't resolve a domain name it can ask other DNS servers as needed. See also my section on DNS.
Document Object Model
See DOM.
Document Type Definition
See DTD.
See alpha.
Document Object Model. DOM is an API (set of interfaces and objects) designed for managing valid HTML and well-formed XML documents. DOM is language independent. DOM may be implemented by language independent systems like CORBA or COM; DOM may also be implemented by language-specific bindings like Java, ECMAScript, or JavaScript.

Each DOM document has one root element node (object), zero or one doctype nodes, and zero or more comments or processing instructions.

DOM originated from DHTML but has broader application than DHTML. DOM Level 2 and its14 modules became an official W3C Recommendation on 2000/11/13. Here are the 14 modules: Core, XML, HTML, Views, Style Sheets, CSS, CSS2, Events, User Interface Events, Mouse Events, Mutation Events, HTML Events, Range, and Traversal. 
(1) Given a set of things, a domain is a defined sub-set of a set.

(2) A logical administrative unit for Windows networks. A domain consists of a group of computers and peripheral devices. A domain may be broken up into multiple Workgroups. The following are examples of domains:
  • banana
  • banana.com
  • sales.banana.com

(3) A logical administrative unit in TCP/IP networks. It is very much like domains in Windows networks but these must follow the DNS convention (Domain Name System).  A TCP/IP domain must also have a TLD portion (Top Level Domain), such as .com, .org, etc. A maximum of 63 characters are allowed, including the TLD. Letters and numbers are allowed. Hyphens are allowed too but not at the beginning or the end. Spaces are not allowed. Special characters, like these, are not allowed: @ # $ % ^ & * ( ) ? ".
domain controller
The Windows NT name for a directory server which provides access to control over users, accounts, groups, computers, and other network resources. For a domain, there will be one PDC (Primary Domain Controller) and possibly one or more BDCs (Backup Domain Controllers). All other servers on the network may be categorized as Member Servers.
domain name
(1) The name of a particular computer on the internet which is equivalent to an IP address. These names often have the following endings: .com, .org, .net, .edu, .gov, .SomeCountryAbbreviation.

(2) The name of a group of computers and peripheral devices with a common security database, eg a user's host name may be Sue and her domain may be Marketing.

See also DNS.
Domain Name Service
See DNS.
Domain Name System
See DNS.
To treat a semiconductor with additional elements to increase its conductivity. The additives, called dopants, are typically arsenic, antimony, bismuth and phosphorous. The type and level of doping determines whether the semiconductor is N-type (current is conducted by excess free electrons) or P-type (current is conducted by electron vacancies).
Disk Operating System. A text-based, single-user, preemptive-multitasking OS. It is completely backwards compatible, even with the 8088 CPUs that were limited to running programs in a RAM space no larger than 640 kB.

Different DOS versions and their release dates:
  • 1.0=1981
  • 1.1=1982
  • 2.0=1983
  • 3.0=1984
  • 3.1=1985
  • 3.2=1986
  • 3.3=1987
  • 4.0=1988
  • 5.0=1990
  • 6.0=1993
  • 6.22=1995
dot matrix printer
A printer that pushes pinpoints onto ink filled ribbon so as to transfer shapes of letters and other characters onto paper. Early models had 9 pins but today's models have 24 pins, producing improved images.
dots per inch
See dpi.
double click
(1) To use the mouse or pointing device to click on something on the screen twice in a sufficiently short time span. The program running will usually do something, like open up or execute.

(2) To explore something.
dots per inch. Same as ppi. See resolution.
Distributed Queue Dual Bus. A standard developed by the IEEE 802.6 committee for connecting LANs to make MANs. Its media consists of two cables of fiber optics connecting each node and has a rate of roughly 100 Mb/s. DQDB is typically maintained by the local telephone company.
To click and hold down the mouse button when the pointer is at the first desired item or area, and then move the pointer to the second desired item or area, and then let go of the button. In Windows, this is usually done with the left button.
To drag an icon on top of another icon or area to execute an action.
Dynamic Random Access Memory. A volatile RAM chip that has to be refreshed thousands of times per second. DRAM is cheaper bur slower than SRAM.
Distributed Relational Database Architecture. IBM's ideas on setting up distributed databases. It makes four kinds of transactions, each progressively more sophisticated.
  1. Remote requests. A single transaction makes a one request to one server.
  2. Remote transactions. A single transaction makes multiple requests to the same server.
  3. Distributed transactions. A single transaction makes multiple requests, each of which can go to separate servers.
  4. Distributed requests. A single transaction makes one request that can go to separate servers.
(1) device drivers. Files containing code that extends the capability of control memory or hardware devices.
(2) A program that tests a subprogram.
Digital rights management; digital restrictions management. Technical methods used to handle the description, layering, analysis, valuation, trading and monitoring of the rights held over a digital work. The most famous example of DRM is the iTunes system by Apple which uses technology to try to enforce copyright laws for music consumers buy from their iTunes Music Store.
Data Service 0. Aka DDS. Aka circuit. The basic unit of ISDN transmission, i.e. a single channel carrying voice or data information at a rate of 64 kb/s. If it is taken into account that each 8th bit is used for a check, then the rate is 56 kb/s.
Data Service 01 Aka T1. 24 DS0s aggregated together in a time slice fashion. A fractional DS1 often has some of its DSOs marked as unused and commonly has a rate transfer of 128 kb/s or 384 kb/s. If operating in frame relay mode the DS1 may have a CIR of 0 but might reach its maximum data rate transmission of 1.5 Mb/s. If operating in clear channel mode, the rate may be 1.536 or 1.544 Mb/s. This is often shortened to 1.5 Mb/s. DS1 uses B8ZS line coding over clear channel circuits.

Most telephone companies also allow the purchase of some of the channels of a DS1. This is known as a fractional T1. See also E1.
Data Service 3. Aka T3. 29 DS1s aggregated together to make it roughly 45 Mb/s.
(1) Digital still camera. As opposed to a DVC (digital video camera).

(2) Document structure convention. A set of standards for making comments in a PostScript document.
Digital Subscriber Line. Internet service that send digital data over a regular phone line as opposed to a modem which sends digital data by analog ("sound") transmission. DSL data moves over regular phone lines by utilizing frequencies higher than normal analog transmissions, i.e. voice uses the first 4 kHz, whereas DSL transmits at up to 2 MHz.

A DSL modem typically connects at 144 kb/s, compare that to the usual analog modem of 33.6 kb/s. An additional benefit of DSL is that the connection is always on, there is no dialing up involved. Another benefit is that the phone can be used for other things even while the DSL connection is on.  A major limitation is that DSL modems must have no more than 18,000 feet of wiring distance to the local phone company's central office.

There are many different kinds of DSL. Here are the most common:
  • ADSL. Asymmetric DSL. Has greater downstream than upstream rates. Receives at 8 Mb/s and transmits at 0.8 Mb/s. As of 2001, this is the most popular version, esp. for home users.
  • SDSL. Symmetric DSL. Downstream and upstream rates are the same. Available at speeds up to 1.5 Mb/s.
  • HDSL. High-bit-rate DSL. SDSL that uses one wire for downstream and the other wire for upstream. 1.5 Mb/s.
  • VDSL. Very-high-bit-rate DSL. HDSL that is limited to distances of 1,000 feet but can transmit up to 56 Mb/s.
  • IDSL. ISDN DSL. This is pseudo-DSL. It's actually DSL over an ISDN line, so it can achieve up to 144 kb/s instead of maxing at 128 kb/s.
  • RADSL. Rate Adaptive DSL. ADSL that automatically adjusts transmission rate. It allows DSL service at greater distances.
  • G.SHDSL. Like SDSL and HDSL combined but with distances of 18,000 feet (3 miles) and 2.3 or 4.6 Mb/s.
Data Source Name. Aka ODBC Data Source Name. A centralized location per computer that defines the type, location, and other parameters of an ODBC database. This is helpful since if multiple applications point a DSN, and if the database changes, then the apps do not need to be changed, just the central DSN.

DSNs are usually managed at the Control Panel, ODBC Data Sources applet. Follow the wizard!

There are two kinds of DSN: user and system. User DSN only applies to a particular user on the system. System DSN applies to all users and background process on the system.
Decision Support System. Software that accesses a data warehouse to provide a manager with reports that can help them make decisions.
Delimiter Separated Values. A data format where values are separated from each other with some sort of delimiter. The most common DSV format is the Comma Separated Value (CSV) format where the delimeter is a comma. EG: ape,boy,cat.
Document Type Definition. A document that defines an SGML compliant markup language.  Different versions of HTML are different DTDs of SGML.
Data Terminal Equipment. A standard class for serial communication, typically for equipment for generating data. A DTE will connect to a DCE.
Dual In-Line Memory Module
dual port memory
Shared memory. A portion of RAM set aside for the device and the CPU, no other device may use it.
duck typing
A style of dynamic typing where you examine properties and methods of an object to determine what it is rather than knowing the class of the object. The concept is based on the phrase "If it walks like a duck, swims like a duck, and quacks like a duck, the in probably is a duck".
An image with two color channels, usually grayscale accented with an additional color. It is also possible to have monotones, tritones, and quadtones.
dummy variable
A variable that is used only temporarily in order to read desired data values.
(1) The ability to send data either in one direction at a time (half duplex) or in both directions simultaneously (full duplex or bi-directional data transfer). Ethernet, Fast Ethernet, and FDDI use half-duplex. Phones, modems, and some high end Ethernet systems use full duplex. See also multiplex.
(2) Two systems that function identically but cooperatively in tandem.
Digital video camera. As opposed to a DSC (digital still camera).
Digital Video Disc. Digital Versatile Disc. A storage media that looks like a CD and can run CDs but can store 4.7 GB (in single-sided and single-layer format), 8.5 GB (in single-sided and double-layer format), 9.4 GB (in double-sided and single-layer format), or 17 GB (in double-sided and double-layer format). Compare this to the measly 0.65 GB CD. DVD has a data transfer rate ranging from 600 kb/s to 1.3 kb/s.

DVD is backwards compatible with CD-ROM and CD-I. DVD-2 can also read CD-R and CD-RW.

DVDs are also now coming out in different flavors:
  • DVD-Video. Just for video viewing. Includes CSS (Content Scrambling System) to prevent the copying of the contents.
  • Divx. DIgital Video eXpress. Another DVD format just for video viewing. However Divx players have a timer that starts once the disc is accessed; once the timer expires, the disc is no longer viewable. It is intended to replace VHS video rentals but the darn thing is not compatible with regular DVD-ROM!
  • DVD-ROM. Read Only Memory. For video and computers. Is only Read Only Memory, i.e. it cannot be written on.
  • DVD-R. Recordable. For video and computers, records one time.
  • DVD-RAM. For video and computers, records and rewrites multiple times. Can read all varieties of CDs and DVDs except for DVD+RW. This format by a DVD consortium can record 2.6 GB per side.
  • DVD+RW. Read Write. For video and computers, records and rewrites multiple times. Can read all varieties of CDs and DVDs except for DVD-RAM. DVD+RW can also write to CD-Rs and CD-RWs. This format by Hewlett-Packard, Phillips, and Sony can record 3 GB per side.
See also removable storage and MPG.
.dxf. Data eXchange File. Originally created boy AutoDesk for its AutoCAD system, it is a 2D graphic file format supported by nearly all CAD software.
Dynamic Data Exchange
See DDE.
Dynamic Host Configuration Protocol
Dynamic HTML
Dynamic Link Library
See DLL.
Dynamic Random Access Memory

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