re common facilities, such signal being reflected back to the sender, with a delay depend- as memory, database, printer. ent on the transmission medium and distance. Distribution frame. Equipment which terminates telephone wires Echo cancellation. Transmission technique for attenuating or fil- (usually in the telephone central office), where crossconnection tering out unwanted echo. can occur. Echo distortion. Distortion caused by a delay in a part of the Diversification. Refers to the act of moving into different areas. transmitted signal, which later interferes with the main signal. For example the RBOCs diversifying from traditional tele- Elastic store. A memory unit in which data can be clocked in and phony (in USA) to cable television (in Europe). out at different rates. It therefore buffers the output from vari- Diversity. The use of alternative paths, frequencies or polarisation ations of the input signal, such as caused by jitter. in order to reduce the fading effect of microwave radio relay Electroluminescence. A phenomenon based on the emission of systems. light from a semiconductor under the influence of an electrical Disvestiture. The breakup of AT&T, on 1 January 1984, which field. created the seven Regional Bell Operating Companies Electronic mail. A facility which allows text to be sent to one or (RBOCs), and also required AT&T to run its manufacturing more remote users via a computer based system. (Also called operations as a separate subsidiary. E-mail.) Domain. Usually refers to the sphere of responsibility of an agent, Elevation. Usually refers to the angle between an antenna and the such as the management domain of a network manager. horizon. Dopant. An additive to a semiconductor designed to give it certain Emulation. Mimicing of one device by another, such that the electrical properties. This is also called an impurity. second device can perform some or all of the functions of the Dot matrix display. A display which is made up of an array of first device and may be used within the network as