gital switching and digital transmission. digital signal A discrete or discontinuous signal, one whose various states are identified with discrete levels or values. Glossary digital switching digital switching The process of establishing and maintaining a connection, under stored program control, where binary-encoded information is routed between an input port and an out- put port. Generally, a virtual circuit is derived from a series of time slots (TDM), which is more efficient than requiring dedicated physical circuits for the period of time for which connections are set up. Dijkstra algorithm An algorithm that determines routes based on path length. It is used in OSPF. directory service A service that provides a white pages-like directory of the users and resources located on an enterprise network. Instead of having to know a device's or user's specific network address, a directory service provides an English-like listing for a user. The directory is being standardized collaboratively by the ITU (X.500 standards) and ISO. distance-vector routing Routing in which a router is aware only of routers directly con- nected to it. Each router sends its routing table to each of its neighbors; they in turn merge this routing table with their own. distortion The modification of the waveform or shape of a signal caused by an outside inter- ference or by imperfections of the transmission system. Most forms of distortion are the result of the varying responses of the transmission system to the different frequency components of the transmission signal. distributed computing environment An architecture in which portions of the applica- tions and the data are broken up and distributed among the server and client computers. distributed data processing Data processing in which some or all of the processing, stor- age, and control functions, in addition to I/O functions, are situated in different places and con- nected by transmission facilities. distributed database An application in which there are many clients as well as many 