Glossary - D



Defense Advanced Research Projects Agency

Data Compression

Data compression removes redundancy in transmitted bit patterns and subsequently reduces transmission rates by 20% to 200%. For example, a modem that is designed to receive and send data at 1200 bps without data compression might be capable of supporting 2400 bps if it uses data compression, even if it used the same network and analog voice-grade channel.

Data Protection Register (DPR)

A register held by the Data Protection Registrar. It is a list of data users who hold personal data. It also registers computer bureauxs who provide services that are respective of personal data open to public inspection. Every user of data who holds personal data must apply for registration.

Data Service Units (DSU)

DSUs are channel service units (CSI's). Data service units (DSUs) are required in order to connect digital customer premises equipment (CPE ) to their carrier networks. They are hardware devices that provide an interface between a digital line and the unit of data terminal equipment. DSUs provide transmit and they receive control logic, synchronization and also timing recovery across the data circuits. DSUs may also act in converting ordinary binary signals that are generated by CPE to special bipolar signals. These bipolar signals are specifically designed to facilitate transmission at upto 1.544 Mbps rates over (UTP) cable which is a media originally intended for voice bandwidth signals(3 kHz) . See Channel Service Units (CSU).

Data Terminal Equipment (DTE)

DTE is any device that can send data and or receive data. (Note: At times DTE implies digital terminal equipment which is a type of CPE used with digital Service--see CSU and DSU.)


Data circuit terminating equipment.


Digital communications protocol.


Digital cross-connect system.


Direct distance dialing.


Defense Data Network.


See Digital Derived Services Network.


See Digital Enhanced Cordless Telecommunications.

DECT Handsets (Digital Enhanced Cordless Telecommunications)

Cordless handsets for domestic and business use.


Direct Exchange Line.

Dialled Number Identification Service (DNIS)

This is the ability to identify the number to which a call has been made. For example, in a call centre the call might come into a specific DDI number. The call is then answered by an agent from an ACD group, and then system uses the DNIS to give to the agent with a screen a script associated with that DDI number.

Dial-up Internet access

Internet access over an analogue or ISDN telephone line that uses a dial-up connection

Digit L

Digital L is another amplitude condition. Itrepresents a binary digit 0. See binary digit or bit.

Digital Access Signalling System (DASS)

This is protocol that is agreed as standard for digital signalling specifically between digital PBXs. This is done using digital public exchanges and PCM A-Law. Two versions are available: Either single channel connection which uses 80 Kbps links (see IDA) or alternatively multi-channel (30 channels) connection using 2.048 Mbps links

Digital Carrier Systems

Digital Carrier Systems exist as a system for digital signals that use regenerative as opposed to linear repeaters and also time division multiplexing.

Digital Cross Connect System (DCS)

DCS has arrived as a new generation of switching/multiplex equipment> It permits per-channel DS0 (64 kbps) electronic cross-connection. It operates from one T1 transmission facility to another facility, directly from the constituent DSl signals. It is commonly referred to as "DACS" (which stands for digital access and cross connect system). However, this is a trademark of AT&T.

Digital Derived Services Network (DDSN)

This is the overlay network used by BT r to provide interalia its Number Translation services.

Digital European Cordless Telecommunications (DECT)

A system used for office cordless networks and also suitable for more general cordless use (that includes Telepoint type networks)

Digital Local Exchange (DLE)

The telephone exchange that customers are directly connected to, often via a remote concentrator unit.

Digital Private Network Signalling System (DPNSS)

Allows the transfer of PBX facilities between PBXs, even if they are different makes.

Digital Signal

A digital signal (electrical or otherwise) is a signal in which information may be carried in a limited (two or more) number of different discrete states. Probably the most fundamental and actually widely used form of digital signal is binary. This is where one amplitude condition represents a binary.

Digital Subscriber Loop Access Multiplexer


Digital Termination Service (DTS)

DTS is a service that is provided by some carriers that permits the operators of private networks to use there digital microwave equipment in order to gain access to carrier networks. The FCC has allocated a specific microwave band for DTS.

Direct Dialling Inward (DDI)

Direct Dialling Inward uses a smaller group of exchange lines to gain access to a larger number of extensions. The first part of the number actually selects the line whilst the second part actually gives the extension number. This is a very economical method on exchange lines.

Direct Outward Dialing (DOD)

DOD is a PBX-to-central office trunk. It allows outgoing calls to be placed by PBX stations directly.

Directory enquiry service (DQ)

Operator assisted and involves the operator looking up entries on a database.


Digital International Switching Centre.


Dispersion (in dispersive media) are complex signals that are distorted because of the various frequency components that make up the signal and have different propagation characteristics and also paths. Because of the finite conductivity of copper, cable or wire media that deals with guided wave transmission is actually fundamentally dispersive. The upper bit rate that a medium can support is limited by dispersion because it distorts the signal waveforms all the way to the extent that transitions coming from one information state to a differnt one cannot actually be reliably detected by the receiving equipment, (for example, logical l logical 0 value changes).


Data link connection identifier.


Direct Marketing Association.


Data Protection Act 1984.


Distributed queue dual bus.


This is a directory status classification whereby a customer's directory information is actually available from directory enquiry services, however, it is not included in the phone book.

DSL (Digital Subscriber Line)

Capable of transforming ordinary phone lines, DSL is part of a family of technologies that are generically referred to as either DSL, or xDSL (its also known as "twisted copper pairs"). It can transform them into high-speed digital lines which are capable of supporting advanced services. An example of one of these would be fast Internet access and/or video-on-demand. ADSL (Asymmetric Digital Subscriber Line) or HDSL (High data rate Digital Subscriber Line) or VDSL (Very high data rate Digital Subscriber Line) are all variantions on xDSL.

DS"N" Digital Signal Hierarchy

A time division multiplexed hierarchy of standard digital signals used in telecommunications systems. DS1 level in the hierarchy corresponds to a 1.544 Mbps TDM signal which comprises 24 DS0 signals. DS0 refers to individual digital signals at channel rates of 64 kbps. Four DS1 signals digitally multiplexed produce a DS2 level signal, containing 96 DS0 channels, and requires a transmission medium that supports 6.312 Mbps. A DS3 level signal results from the digital multiplexing of 7 DS2 signals, supports 672 DS0 signals and requires a 44.736 Mbps transmission medium. Finally a DS4 level signal supports 6 DS3 level signals, 4032 DS0 signals and requires a 274.176 Mbps transmission medium. The DS hierarchy accounts for non synchronism in the multiplexing plan, hence the term "asynchronous digital hierarchy" and the use of overhead bits. Note that bit rates at higher levels are not integer multiples of 64 kbps.


Digital signal processor.


Direct station selection/busy lamp field.


Digital subscriber service.


Data service unit.


Dual tone multiple frequency.


Digital termination service.

Dual-Tone Multiple Frequency (DTMF)

The generic name that exists for the tone signaling scheme which is used to signal from telephones accross to switching equipment. Wihtih this 10 decimal digits and a further two auxiliary characters are represented by the selction of two frequencies from the following group: 697, 770, 852, 941,1209,1336,1447 Hz.


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