In ITU-T standards for asynchronous transfer mode (ATM), a class of traffic supported by ATM Adaptation Layer 1 (AAL1). Such traffic is connection-oriented constant bit rate (CBR) Class 1 Class 2 Class 3 Class 4 Class 5 High-Usage Group Regional Center Sectional Center Primary Center Toll Center End Office traffic that must be timed between the source and the sink. Class A traffic is stream-oriented and intolerant of latency. See also AAL, AAL1, ATM, CBR, connection-oriented, ITU-T, latency, sink, source, and stream-oriented.