ission line are assigned on a fixed, predetermined basis. synchronous transmission In data communications, a method of transmission in which the sending and receiving of characters is controlled by timing signals. The sending and receiving terminals operate continuously in step with each other. system A computer and its associated devices and programs. system test A complete simulation of an actual running configuration for purposes of ensur- ing the adequacy of the configuration. TA (terminal adapter) A protocol converter that connects a non-ISDN device to the ISDN network. TACS (Total Access Communication System) A standard for analog telephony that was deployed widely in Europe before GSM became the most popular approach. tandem (1) The connection of networks or circuits in series (i.e., the connection of the out- put of one circuit to the input of another). (2) An intermediate switch used in a tandem network, which connects only to other switches instead of to customers. tandem data circuit A data circuit that contains two or more pieces of DCE in series. tandem switch tandem switch A switch used to connect local exchanges throughout the metropolitan area. Also called a junction exchange or a tandem office. tap An open point on a cable bundle that is left SO that technicians can easily splice off a pair to bring new or additional service to a home. TAPI (Telephony Applications Programming Interface) An API that enables PCs use telephone services. TAPI is a call-processing protocol that resides between the signaling and application layers. tariff (1) The published rate for the use of a specific unit of equipment, facility, or type of ser- vice provided by a communications common carrier. (2) The vehicle by which regulating agen- cies approve or disapprove such facilities or services. TASI (Time-Assignment Speech Interpolation) Specialized switching equipment connects a party to an idle circuit while speech is taking place and disconnects the party when speech stops SO that a different party can use the same ci