y combined into the chan- radio frequencies in 27MHz band. nel by a multiplexer. CT2 Good quality domestic/business cordless telephony. Digital Depletion mode. The operation of a field effect device such that it radio technique. Operates on 40 channels in the 864-868MHz is conducting with zero bias voltage. band, using a single frequency time division duplex channel Depth of discharge. A measure of the state of charge of a power access. source, such as a battery. It is defined as the percentage of the Customer Usually refers to the line connecting the cus- rated capacity by which the source has been discharged. tomer's station to the central office. This is the same as an Deregulation. Refers to the removal of regulatory controls on access loop. telecommunication operations or practices. (Often used in con- Cutover. Usually refers to changing over, within a short time, from junction with liberalisation.) one system to another. For example cutting over from an old Destination field. The section of a message which contains infor- version of software to a new version. mation about its destination. Cyclic code. Coding system used to detect errors in transmitted Dial pulsing. Method of signalling, used in telephony, where the data. signal is obtained by interrupting the direct current path. Dial-up connection. The establishment of a circuit connection using the switched telephone network. Dark current. The current conducted through a photosensitive de- Die. An unencapsulated component, for example a semiconduc- vice, such as a reverse biased photodiode, when there is no light tor, before it is put into its package. falling on it. Diffusion. A technique for introducing impurities into a semicon- Dark fibre. Optoelectronic fibre cable which has been installed but ductor, during its production. Under the influence of high is not in use (e.g. it is a spare). temperatures these impurity atoms gradually work their way Data. A term loosely used to describe any information which is into the crystal lattice of the semiconducto