Modes of Operation
HDLC has 2 main modes of operation:
NRM: Normal response mode. This is used in unbalanced configuration. Slave stations only transmit when instructed by the master. Used in point-to-point or multipoint networks
ABM: Asynchronous balanced mode. Declares both partners as equals. Allows one partner to initiate transmission without the permission of the other. Used normally with point-to-point and duplex links