HDLC [High-level Data Link Control] is a group of protocols for transmitting [synchronous] data [Packets] between [Point-to-Point] nodes. In HDLC, data is organized into a frame. HDLC protocol resides with Layer 2 of the OSI model, the data link layer.
HDLC uses zero insertion/deletion process [bit stuffing] to ensure that the bit pattern of the delimiter flag does not occur in the fields between flags. The HDLC frame is synchronous and therefore relies on the physical layer to provide method of clocking and synchronizing the transmission and reception of frames.

Features:

• 8/16 Bit Data.
• 16/32 Bit CRC Provision.
• Utilization of FIFO’s (16 Bit)
• Transfer Modes: Normal Response Mode (NRM), Asynchronous Balanced Mode (ABM), Asynchronous Response Mode (ARM).
• Here we are using FIFO at both ends (Transmitter and receiver) for reducing the interrupts presented to the CPU.

Applications:

• Ethernet
• FDDI
• Broadband ISDN
• ATM