 its replace- individual light sources arranged in a matrix format. ment. Double sideband modulation. A method for modulating a signal in Encoding. Processing for formatting information in a form suit- which a baseband signal, having no d.c. energy, directly modu- able for transmission. Decoding involves converting it back to lates a carrier. The output is a signal with no carrier energy but its original form at the received end. with both upper and lower sidebands. Encryption. The coding of a signal so that it cannot be read by an Down conversion. A device which converts frequency from one unauthorised receiver, without use of a decoder. value to a lower value. End to end signalling. Signalling method in which signals pass Down-link. The radio transmission link used to transmit informa- right through from end to end without processing within inter- tion from a satellite, usually to Earth. (See also up-link.) mediate nodes. See also link by link signalling. Downtime. The period for which a resource is not available to a End office. Refers to the US Class 5 telephone central office, used user due to a fault. to terminate a subscriber's loop. Drop cable. The cable which connects the subscriber's premises to Energy dispersal. A process for spreading the power in a signal the distribution cable. over the transmission bandwidth, so as to reduce inter-channel Drop out. Termination or break in a call. Usually caused in mobile interference. radio systems, due to a mobile moving out of range of its Engineering. Usually refers to the design of a system, such as transmitter. determining the combination of cards needed in a shelf of Dual parenting. Term applied to situations where a local exchange equipment to perform a certain function, or the arrangement of within a network connects to two trunk exchanges. the network in terms of links and nodes in order to meet a Duobinary. A coding technique where a two level binary signal is certain level of performance. converted to one using three levels. Entity. Usually refers to an 