command/response (C/R)
The C/R bit is a bit in the control field of the frame relay header and in the header ofHigh Level Data Link Control ( HDLC) LAP-D. It is used to distinguish between command and response frames. Since the C/R bit is neither interpreted nor modified by the network, this bit can be used by the service user in an application-specific manner. It indicates whether the frame is a command or the response to a command.