 coded, so that each element contains a different number of pulses and spaces. 126 GLOSSARY Ring network. A computer network in which each computer is in- terconnected to adjacent computers in a circular fashion. Simplex. A communication system capable of sending information in one direction only. Specialized common carrier. A common carrier offering limited services to customers in a particular geographic area. Star network. A communication system consisting of one central node with point-to-point links to several other nodes. Status channel. A channel to transmit status information. Store and forward switching. A method of transmission in which messages received at intermediate points are stored and then retrans- mitted to the intended destination. Synchronous data network. A data network in which the timing of all components of the network is controlled by a single timing source. T-Carrier. A series of highspeed transmission systems. Teletypewriter. A typewriter device capable of sending and receiv- ing alphanumeric information over communications channels. Also known as a teleprinter. Telex. An automatic dial-up teletypewriter switching service pro- vided on a worldwide basis by various common carriers. Time division multiplexing (TDM). A technique for combining sev- eral channels into one facility or transmission path in which each channel is allotted a specific position in the signal stream based upon time. Trunk. A large-capacity, long-distance channel used by a common carrier to transfer information between its customers. Two-wire circuit. A circuit formed by two metallic conductors in- sulated from each other. The term is also used, in contrast to a four- wire circuit, to indicate a circuit using one line or channel for com- munications in both directions. GLOSSARY 127 Uplink. The communications link from the transmitting earth sta- tion to the satellite. Voice grade. Refers to channels designed specifically for voice communication. Wide area telephone service (WATS). A flat rate long distance tele- phon