BT glossary - D

This glossary is here to help you understand some of the acronyms you may encounter in this site. Select a letter below that your word begins with.

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Abbreviation Term Definition


An automated computer process that runs itself at a set time or when triggered by a specified event. Mailer daemons report failed e-mail messages.


Dark fibre

Unconfigured optical fibre laid ready for users to install their own transmission equipment. Because no light source has been applied, it is 'dark'.


Data Encryption Standard

An encryption algorithm, developed by the US government, that allows the use of variable-length keys. The longer the key, the more difficult it is to break the algorithm.


Data warehousing

A technique for mining valuable hidden information concealed in existing databases. It involves creating a separate repository for trading data, sales reports, financial information and a whole load more. Once created, this data can be analysed for trends, patterns and other useful information that might not be noticed normally.



An Industry-standard middleware platform. DCE provides a number of security services, such as access control based on Access Control Lists (ACLs), a Kerberos-based authentication system, an interface to the GSS-API, delegation and audit.



An ISDN channel primarily used for call set-up and management, providing 16kbit/s in basic rate service or 64kbit/s in primary rate service. It removes the signalling overhead from traffic channels and where allowed, may also be employed for user applications.



The act of restoring an encrypted file to its original state.


Denial of service

The process by which legitimate users, customers, clients or other computers are prevented from accessing resources on a computer by an unauthorised party. This is usually accomplished by overwhelming a computer with bogus requests, or by tampering with legitimate requests.


Destination address

The network address to which a data packet or cell is addressed.


Dial-up access

Internet access that is not 'always on'. A narrowband (56Kbps) service where you have to dial up to log in to establish a connection every time you want to use the internet.



An electrical signal converted into a form consisting purely of on-off pulses, similar to those found within computers. The nature of this transmission system eliminates most errors and distortion, providing a more reliable means of sending information.


Digital Central Office

American term for digital exchange.


Digital certificate

See Certificate


Digital Enhanced Cordless Telecommunication system

Digitally Enhanced Cordless Technology (DECT), also known as digital cordless and provides enhanced call clarity for clear communication.
A standard developed by ETSI for domestic cordless telephone service but now extended to other uses as well such as telepoint and data transmission in office environments (wireless LAN). Operates in 1.88-1.90GHz band and uses TDMA coding.





Digital signature

A means of verifying the sender and integrity of a message. The sender calculates a message digest of the message and then encrypts it with his private key. This is the signature, which is sent with the message. The recipient is able to decrypt the signature with the senders public key, and compare the result with a message digest generated by the recipient.


Digital to Analogue

An interface at which digital signals carrying voice, fax or other analogue calls are converted back into analogue form.


Digital transmission

The sharp edge to the transmission wave enables its binary value to be reliably recovered.


Digital Versatile Disk

A disk the same size as a CD but which holds seven to fourteen times the amount of data or music.


Direct Dialling-In

A facility enabling telephone users to dial direct into a firm's PABX extensions as if they were normal public telephone numbers. Known as DID in the USA, not to be confused with DIA.


Direct Distance Dialling

The American term for STD.


Disk Operating System

Often used to denote MS-DOS (q.v.).


Document Image Processing

The electronic indexing and storage of images of documents, usually taken from paper via a scanner. DIP documents cannot be understood, re-used or processed, except by the human eye.



A domain, in computer file transfer systems, is an administrative entity, while a network is a technological one. A domain may include parts of many networks, and a network may include parts of many domains.


Domain name

The unique name that identifies an internet site. Domain names always have two or more parts, separated by dots. The part on the left is the most specific, and the part on the right is the most general. A given machine may have more than one domain name but a given domain name points to only one machine. For example, the domain names:,, can all refer to the same machine, but each domain name can refer to no more than one machine. Usually, all of the machines on a given network will have the same thing as the right-hand portion of their domain names ( in the examples above). It is also possible for a domain name to exist but not be connected to an actual machine. This is often done so that a group or business can have an internet e-mail address without having to establish a real internet site. In these cases, some real internet machine must handle the mail on behalf of the listed domain name.


Domain Name System

Domain name system. Domain names are linked to internet protocol (IP) addresses so you only have to type in, eg, "" and the DNS computer finds the correct IP address.



Bringing information from anywhere in the online world onto your computer at your home or office. You can download almost anything from the Internet - information, documents, maps, music, video - though you may have to pay the information provider for the privilege.


Distributed Queue Dual Bus

An IEEE standard for metropolitan area networks.


Digital subscriber line

Digital subscriber line is the technology that brings high bandwidth broadband services over an ordinary copper wire phone line.


Dual-Tone Multi-Frequency

A system which uses combinations to two tones to signify digits. Commonly known as Touch-Tone (q.v.), as used on all modern telephones.


Dumb terminal

A keyboard and monitor screen combination, connected over a network to a larger computer (such as a mainframe), for which it acts as a workstation (q.v.).


Dynamic IP address

Each computer or device connected to the internet needs its own internet protocol (IP) address. Devices and sites that are constantly connected to the internet usually have a static IP address so the Domain Name System (DNS) can reliably route traffic to the right place. When you connect as an individual to the internet you are usually assigned an IP address dynamically - it is your computer's address on the internet for the duration of that connection and ceases to be your address when you log out.