The control field is an 8-bit long data field in the High Level Data Link Control (HDLC) and contains commands
and messages. It is used by the master control to command certain functional sequences to the addressed slave control.Block numbering in the control field is used by the master control as well as the slave control to monitor the correct sequence of the data blocks
. In HDLC there are three different frame types that are specified in the control field: The I-frame or Information Frame, which is used for data transmission, the S-frame or Supervisory Frame, which controls the data flow, and the U-frame or Unnumbered Frame, which controls the data connection.