Glossary of Terms

D

Data packet
Although your computer and modem can send data one character at a time, when you're surfing the Internet, downloading files, or sending email, it's more efficient to send information in larger blocks called data packets. Modems generally send packets of around 64 characters along with some extras for error checking. When downloading files using a protocol like Xmodem, however, the packets are larger. And when using Internet protocols such as TCP/IP, the packets are larger still--around 1,500 characters.

DASS II
UK ISDN signalling protocol.

DCE
Data Communication Equipment. E.g. a modem.

DCT
Discrete Cosine Transform. The basic mathematical algorithm employed by most lossy video compression techniques such as MPEG, JPEG, and H.26x. Used to convert data between the spatial domain and the frequency domain.

DDI
Direct Dialling In (digital direct dial to internal subscribers).

DDRAM
Double data rate SDRAM.

DECT
Digitally Enhanced Cordless Telecommunications. This is the standard for cordless communications. Using advanced digital radio techniques, it makes very efficient use of the available radio spectrum, provides high quality sound (as good as a corded phone) and extremely high speech security.

Default settings
When you call up your email service, for example, your modem can be set to automatically dial the same access number every time. This is called a default setting.

Defragment
Process through which your computer rearranges data stored on your hard disk into a logical order.

DHCP
DHCP / (Dynamic Host Configuration Protocol) is used to make the allocation of IP addresses automatic, and simple to administer. A network server uses this protocol to dynamically assign IP addresses to networked devices. The DHCP server waits for a device to connect to it, then assigns it an IP address from a range of addresses on the server. DHCP can also assign the subnet mask, Default Gateway, and Domain Name Server IP addresses (DNSs).

Digital communications
Digitally transmitted information gets to where it's going faster than information that's transmitted in an analogue fashion. That's because it's fired along a digital service pipe in a straight line - a bit like bullets from a machine gun - as opposed to an undulating wave. Digital transmission results in clearer lines, less interference and fewer dropped lines. Connecting to the Internet is much faster, too.

Digital rain
A type of video compression artefact found when motion estimation fails to produce correct results in areas of high entropy. The effect is of a grainy moving field in these areas of the image.

DNS
Domain Name Service. Maps IP addresses to the names assigned to devices on a network.

Dolby 5.1
Six channel digital surround sound system.

DPNSS
Digital Private Network Signalling System. 2mb circuit which can provide upto 30 digital circuits for network telephone and data systems.

DRC
Data Router Cassette. Allows voice and data to be split over ISDN

DSL
Digital Subscriber Line. Modems using a single twisted pair wire to deliver ISDN Basic Rate Access.

DSLAM
Digital Subscriber Line Access Multiplexer. A device which takes a number of ADSL lines and multiplexes them into a single ATM line.

DSP
Digital Signal Processor. Microprocessor optimized for efficient processing of communication signals.

DTE
Data Terminal Equipment. E.g. a terminal device that talks to a DCE.

DTMF
Dual-Tone Multi-Frequency. A mechanism for sending simple alphanumeric data over an audio path.

DTX
Discontinuous transmission.

Dual band
A phone that can work on two network frequencies, eg O2 (900Mhz) and One2One (1800Mhz).

Duplex mode
Duplex mode is a special form of announcement whereby the called system terminal immediately transforms the announcement into an internal call.

Duplex operation
Two channels available allowing two parties to speak and listen simultaneously. Used for handsfree speech.

DVD
High capacity CD-ROM that will only work with a computer fitted with a DVD drive. Holds up to 4.7 Gigabytes of information, the equivalent of 7 CD-ROMs.

Dynamic channel allocation
Automatically selects the best quality, interference-free reception from the radio channels available.

DHCP
Dynamic Host Configuration Protocol. A communications protocol. DHCP automates the assignment of IP addresses in an organization's network. DHCP lets a network administrator supervise and distribute IP addresses from a central point and automatically sends a new IP address when a computer is plugged into a different place in the network.

DWDM
Dense Wave - Division Multiplexing.

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