. Also called switching center. Central processing unit (CPU). A unit of a computer that includes circuits controlling the interpretation and execution of instruc- tions. Centrex. A type of private branch exchange service where incoming calls may be dialed direct to extensions without operator assistance. Out-going and intercom calls are dialed by extension users. Circuit grade. The grades of circuits are broadband, voice, sub- voice, and telegraph. Circuits are graded on the basic line speed ex- pressed in characters per second, bits per second, or words per sec- Circuit switching. A method of communications where an electri- cal connection between calling and called stations is established on demand for exclusive use of the circuit until the connection is re- leased. Cluster. Refers to a group of terminal devices grouped in one spe- cific location. Codec. An assembly comprising an encoder and a decoder in the same equipment. Common carrier. A company which dedicates its facilities to a pub- lic offering of universal communications services and which is sub- ject to public utility regulations. Conditioned circuit. A circuit which has conditioning equipment to obtain the desired characteristics for voice or data transmission. Converter. A device which translates from one speed, code, or me- dium to another speed, code, or medium. 122 GLOSSARY Crossbar. A type of common control switching system using the crossbar or coordinate switch. Cross talk. An unwanted signal from one transmission circuit de- tected on another circuit. Data access arrangement (DAA). A device provided by the tele- phone company, used to connect privately owned or customer-pro- vided equipment (data sets) to the switched telephone network. Data circuit. A means of two-way transmission between any two data terminating devices such as teleprinters or computers. Data circuit-terminating equipment (DCE). The equipment in- stalled at the user's premises which provides all the functions re- quired to establish, maintain, and terminate a connection, 