Dedicated access line: a private line (analogue or digital/local or long distance) from the subscriber to the service provider.
Dark fibre
Optical fibre (cabling and repeaters) laid in anticipation of future usage but not in use. Dark fibre may be laid at the same time as active fibre in anticipation of future bandwidth demand. Since laying fibre cable is in itself, a major cost consideration, running additional dark fibre during optical fibre cable installation programmes provides for future network expansion plus additional network resilience and reduced downtime in the event of a cable failure.
Data centre
A central location where business-critical computing resources are maintained.
DC power
Direct current: considered as the constant flow of electrons in a single direction – from low to high potential.
Digital cross-connect system: a specialised, high-speed data channel switch for connecting various types of circuits. It provides all the functions of a traditional switch, but the connections are typically set up in advance of when the circuits are to be switched via an administrative process. DCS functions include de-multiplexing an incoming higher-rate, electronic digital line into its constituent signals and switching these signals to a different time slot on an outgoing higher-rate line.  The combination of these functions allows for aggregation, grooming and redistribution of traffic in order to increase network use and efficiency. A DCS is often used on a network backbone to re-arrange the trunk groups associated with physical facilities for routing purposes.
Demarcation point: the point at which the telco’s local loop network ends and connects with the telephone system or wiring at the customer’s premises.
A governmental process implemented to remove, reduce or simplify restrictions on businesses and individuals and encourage development by opening national markets to national/international competition. Deregulation by governments of PTT monopolies has speeded the growth of national and international communication infrastructures throughout the world, while at the  same time opening a minefield of regulatory controls, licence agreements and interconnectivity wrangles with regional fixed-line operators using the national trunk network that is often operated by the incumbent telco. With the advent of both national mobile networks and global satellite communications services, the number of licensed network operators and service providers in deregulated national environments has seen a sharp increase – with operating licence revenues providing an important source of treasury revenue and a means of controlled expansion of communications infrastructure via private enterprise.
Direct billing
Enables the costs associated with a call to be allocated by the originating operator directly among a number of organisations.
Digital loop carrier
Data local exchange carrier
Domain name system/service
Denial of service
Digital rights management: an umbrella term encompassing a number of technologies designed to give content providers control over redistribution and access to material for playback on computers, portable and networked (connected) devices. DRM is important because of the ease with which digital files can be copied and transmitted and enables operators to effectively “monetise” content by charging subscribers for access to protected digital content. Standardisation is being driven by a number of open organisations, including The Open Ebook Forum (OEBF) and the MPEG group (MPEG) in the ebook and multimedia sectors, as well as the Internet Engineering Task Force (IETF) and World Wide Web Consortium (W3C).
Digital signal: a hierarchy of digital signal speeds (DS-1 to DS-4) given to classify capacities of digital lines and trunks.
Digital subscriber line access multiplexer
Digital video broadcasting
Digital Video Broadcasting – Cable
Digital video broadcasting – handheld
Digital Video Broadcasting – Satellite
Digital Video Broadcasting – Terrestrial
Dense wavelength division multiplexing