supervisory frame (S-frame)
The Supervisory Frame (S-Frame) is a control format of the bit-oriented protocols High Level Data Link Control(HDLC) and Synchronous Data Link Control( SDLC) for control functions during data transfer, such as flow control and block acknowledgement.
The S- frame is used for acknowledgement of received information and flow control. If the data is received correctly, a positive acknowledgement is triggered as Receive Ready( RR), if the station is not ready to receive, a Receive Not Ready( RNR) is generated, and if the acknowledgement is negative, a Reject( REJ) is generated. S-frames have a receive sequence counter to indicate the number of the I-frame, but no own transmit sequence counter.