HDLC (High-Level Data Link Control) is a bit-oriented protocol that is used for connection over the point-to-point and multipoint connection. This protocol executes the structure of ARQ (Automatic Repeat Request).
The HDLC procedure is controlled by ISO. This is applicable for high-speed communication of a high amount of data, something the essential control process cannot provide.
The HDLC process has been regulated and created on the SDLC. It can include characters, bit strings of the needed length can also be shared through this process. The unit of data communication is referred to as a frame.
A mode in HDLC is the realigning between two devices included in a network. The mode defines who controls the connection. HDLC decides three types of stations including primary, secondary, and combined.
A basic station is a controlling station on a connection. It controls the secondary stations on the connection and manages data flow and error recovery. A secondary station is actually on a link where there is a primary station. The secondary station is included by the primary station and has no specific authority for supporting the connection.
There are three different modes of operations which are as follows −
Normal Response Mode (NRM)
This is the mode in which the primary station launch sends to the secondary station. The secondary station must receive proper permission from the primary station to change a response. After creating permission from the primary station, the secondary station opens its transmission. The secondary station must continue once again for definitive permission to message all from the primary station.
Asynchronous Response Mode (ARM)
In this mode, the secondary station does not have to delay to get definitive permission from the primary station to eliminate any frames. This mode is asynchronous. The secondary station must wait until it can send any frames. This is when the ARM connection is implemented at half-duplex. In ARM mode, the duplex link features are used than in Normal Response Mode (NRM). In this control mode, the slave station can send unsolicited DCE blocks. The control station launches this operating mode with a control block.
Asynchronous Balanced Mode (ABM)
In this mode, all stations are equal and therefore combined stations linked in point-to-point. There is no need for permission on the techniques of any station in this mode. This is because combined stations do not set data to implement any function on the connection.