High-Level Data Link Control (HDLC) is a Data-Link layer (layer 2 of the OSI and TCP/IP model) protocol, used for communication between two nodes. In this, it being a data-link layer protocol, data is organised in the form of frames. It enables both point-to-point and point-to-multipoint communications. It is a bit-oriented protocol. Every HDLC frame contains up to 6 fields – flag (it is an 8-bit sequence 01111110), address (destination address), control (1 or 2 bytes, for flow and error control information), information (data, may or may not be present), and FCS (frame check sequence, for error detection).
HDLC is a successor to SDLC (Synchronous Data Link Control) and adds flow control and error detection capabilities to the already existing SDLC. It is an ISO standard and hence can be employed anywhere. HDLC is used to maintain the integrity of the data and to ensure the error-free transmission of the data over the network, to the intended recipient and it also controls the speed of the data transmission. HDLC can be helpful in more than one way. Its I-frames, also called information frames, carry information, along with flow and error control, over the network. The S-frames (supervisory frames) does not carry any payload, and are used for flow and error control when piggybacking is not required. The U-frames, or unnumbered frames, may or may not contain information, are used for miscellaneous functions, like link set-up and disconnection, link management, etc. It supports both half-duplex and full-duplex communication over the network. HDLC uses three response modes, namely – normal response mode (NRM), asynchronous response mode (ARM), and asynchronous balanced mode (ABM). NRM is used for point-to-point or point-to-multipoint communication between a primary node, which sends the data, and a secondary node, which responds to the data. In ARM, a secondary node can also initiate communication without first receiving explicit permission from the primary node. ABM is used only for point-to-point communication in which both nodes can send and respond to the commands. By using HDLC, we can acknowledge the receipt of multiple frames at once, which helps in improving the flow control of data.
In conclusion, it can be said HDCL is a very reliable protocol for flow control and keeping the integrity of the data intact. It can provide service to a variety of network configurations, simple point-to-point, or more complex point-to-multipoint. HDLC has even become a base for other data link protocols that incorporates the same mechanism as HDLC.
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