The term applied to the top portion of a protocol, typically at the Data Link Layer (Layer 2), that functions to format data originating in higher layers for processing by the lower layers.The CS adds a header or wraps the data in a header and trailer that contain information necessary to provide the necessary services. Typically, error control and priority information are added at this layer. In asynchronous transfer mode (ATM), for example, the CS functions established in the header (there is no trailer) are determined by the specifics of the service supported by a given ATM Adaptation Layer (AAL). Service classes are designated as Class A (AAL1), Class B (AAL2), Class C (AAL3/4 and AAL5), and Class D (AAL3/4). See also AAL1, AAL2, AAL3/4, AAL5, Class A ATM traffic, Class B ATM traffic, Class C ATM traffic, Class D ATM traffic, Data Link Layer, error control, header, protocol, and trailer.