r. presented in digital form, whether it is text, voice or video. Digital cellular radio. Cellular system which uses both digital sig- Data channel. A communication channel used to carry data be- nalling and digitised speech transmission. tween two points. Digital system. A system in which information is encoded in Data communications. The transmission of data between a source discrete binary form, i.e. logical 0 or 1. and a sink, using appropriate protocols. Digroup. Term sometimes applied to the USA digital group of Datagram. A message consisting of a short packet which is sent 1.544Mbit/s (DS1) signal. through the network, usually without first setting up the path. Direct broadcast. Usually refers to satellite systems which broad- Data link layer. The layer in the ISO OSI model which estab- cast information direct into the home or office. lishes, maintains and releases the data connection between two Direct dialling. Refers to direct inward dialling (DID), where a elements in a network. PABX extension may be called by an external user without dBm. A measure of power in decibels, referred to a power of one going through an operator, and to direct outward dialling milliwatt. (DOD), where the extension on the PABX can make a PSTN dBmO. A measure of power in decibels, referred to a point of zero call without going through an operator. transmission level. Dispersion. The broadening and hence distortion of a pulse within Deadlock. Network congestion which is SO severe as to cause the an optical fibre due to multipath wave propagation. throughput to fall to zero. Distortion. Signal corruption resulting in the received signal Debt capital. US term for money raise by a company by selling being different from the transmitted signal. Distortion may be bonds. See also Equity capital. in various forms, e.g. frequency, time, amplitude. Glossary of terms 62/23 Distributed data processing. Usually refers to distributed comput- Echo. In communication terms it refers to some of the transmitted ing system where several nodes sha