the signal conversion, and coding between the data terminal equipment and the common carrier's line; e.g.; data set, modem. Data switch. A location where an incoming data message is auto- matically or manually directed to one or more outgoing circuits, ac- cording to the intelligence contained in the message. Data terminal equipment (DTE). Equipment at which a data com- munications path begins or ends. Dedicated circuit. A circuit designated for exclusive use by a user. Demodulator. A component of a data set or modem which is re- sponsible for recovering data from received signals and converting to a form suitable for the receiving business machine. Digital to analog (D/A) converter. A device which converts a dig- ital input signal to an analog output signal carrying equivalent in- formation. Distribution frame. A structure with terminations for connecting the permanent wiring in such a manner that interconnection by "crossconnections" may be made readily. Downlink. The communications link from the satellite to the re- ceiving earth station. GLOSSARY 123 EIA interface. A standardized set of signal characteristics (time du- ration, voltage and current) specified by the Electronic Industries Association. Error detecting code. A code in which each data signal conforms to specific rules of construction so that departures from this can be automatically detected. Facsimile. Transmission of pictures, maps, diagrams, etc. The im- age is scanned at the transmitter, reconstructed at the receiving sta- tion, and duplicated on some form of paper of film. Four-wire circuit. A two-way circuit using two paths SO arranged that the electrical signals are transmitted in one direction only by one path and in the other direction only by the other path. Framing bit. A binary digit which is used for data synchronization. Frequency division multiplexing (FDM). A multiplex system in which the available transmission frequency range is divided into nar- rower bands, each used for a separate channel. Channels are derived by allocating or "